Hybrid Division Duplex System for Next-Generation ... - IEEE Xplore

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IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 56, NO. 5, SEPTEMBER 2007

Hybrid Division Duplex System for Next-Generation Cellular Services Sangboh Yun, Member, IEEE, Seung Young Park, Member, IEEE, Yeonwoo Lee, Daeyoung Park, Member, IEEE, Yungsoo Kim, Member, IEEE, Kiho Kim, Senior Member, IEEE, and Chung Gu Kang, Member, IEEE

Abstract—The paper proposes a new duplexing scheme, which is called the hybrid division duplex (HDD), that is suitable for fourth-generation mobile communication systems. The proposed mobile communication system is much more flexible and efficient in providing asymmetric data service and managing intercell interference by exploiting the advantages of both time division duplex (TDD) and frequency division duplex (FDD) schemes. The HDD scheme has a pair of frequency bands such as the FDD, performing a TDD operation using one of the bands in such a manner that allows for simultaneous FDD and TDD operations. Considering the properties of the HDD system architecture, frequency hopping orthogonal frequency division multiple (OFDM) access is adopted in one band for the TDD operation and code division multiple access (CDMA) in the other band for the FDD uplink operation. The important advantage of the HDD scheme is the robustness against cross time slot interference that is inherent to the TDD system, which is caused by the asynchronous downlink/uplink switching boundaries among all neighbor cells. From the simulation results, the proposed system can achieve approximately 7% and 30% improvement with regard to the downlink and uplink throughput, respectively, as compared to the conventional TDD system under cell-independent downlink/uplink traffic asymmetries. It demonstrates that the HDD scheme is a viable solution for future communication systems that are projected to have a cell-independent asymmetric-traffic-supported hierarchical cell structure. Index Terms—Code division multiple access (CDMA), frequency division duplex (FDD), frequency hopping orthogonal frequency division multiple access (FH-OFDMA), hybrid division duplex (HDD), time division duplex (TDD).

Manuscript received July 28, 2005; revised March 12, 2006, July 3, 2006, September 24, 2006, and January 14, 2007. The review of this paper was coordinated by Prof. Y.-B. Lin. S. Yun is with Samsung Electronics Co., Ltd., Suwon 442-600, Korea, and also with the Department of Radio Communication and Engineering, Korea University, Seoul 136-701, Korea. S. Y. Park was with the Samsung Advanced Institute of Technology, Kiheung 440-600, Korea. He is now with the School of Information Technology, Kangwon National University, Chuncheon, 200-701, Korea (e-mail: [email protected]). Y. Lee was with the Samsung Advanced Institute of Technology, Kiheung 440-600, Korea. He is now with the School of Information Engineering, Mokpo National University, Mokpo 534-729, Korea. D. Park was with Samsung Electronics Co., Ltd., Suwon 442-600, Korea. He is now with the Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089 USA. Y. Kim and K. Kim are with Samsung Electronics Co., Ltd., Suwon 442-600, Korea. C. G. Kang is with the Department of Radio Communication and Engineering, Korea University, Seoul 136-701, Korea. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TVT.2007.900389

I. I NTRODUCTION

I

N THE third-generation (3G) mobile communication systems such as high-speed packet data access systems that are based on wideband code division multiple access (CDMA), the demand for multimedia services that support a high data rate and an asymmetric traffic service has accelerated the requirement for more efficient transmission technologies, system architectures, and resource management algorithms. Thus, an efficient and flexible duplexing technique is expected to be one of the key technologies that will enable the realization of flexible management of various traffic requirements and maximal utilization of radio resources in a system [1], [2]. Existing duplex schemes in the second-generation and 3G systems include time division duplexing (TDD) and frequency division duplexing (FDD). In the TDD-based systems, the uplink (UL) and downlink (DL) signals are transmitted over the same frequency band; furthermore, separate (multiple) time slots are assigned for UL and DL transmissions. The TDD scheme can support the asymmetric traffic demand between the UL and the DL by adjusting the number of time slots that are assigned to each link. In addition, another major feature of the TDD system is channel reciprocity, which allows the system to adopt adaptive modulation and coding (AMC) and multiple-input–multiple-output (MIMO) technologies without resorting to the overhead of feedback information. Furthermore, the TDD system can provide higher trunk efficiency because the system utilizes the entire bandwidth. In the TDD scheme, however, a guard time is required between the UL and DL intervals to prevent interference caused by the differing roundtrip delays among users that are distributed throughout the cell. In fact, a longer guard time is required as the cell size increases, which reduces bandwidth efficiency. Therefore, the TDD system is better suited to data services in the shortrange communication systems (microcell or picocell). When the frames of the TDD scheme are not synchronized with one another, and/or each cell has a different UL/DL transmission timing in a multicell environment, the TDD system suffers from severe cochannel interference (CCI) between mobile stations (MSs) and base stations (BSs). CCI appears in crossed time slots (CTSs) in which some cells are active in the DL, whereas other cells are active in the UL. To solve this particular type of interference in a multicell environment, the initial work in [3] proposed a dynamic channel assignment approach with a time-slot-opposing technique, which offers a capacity that is at least equivalent to an FDD system in a multicell environment.

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YUN et al.: HYBRID DIVISION DUPLEX SYSTEM FOR NEXT-GENERATION CELLULAR SERVICES

Hypothetically, there is a simple way to avoid CCI in the TDD system—by synchronizing the frames and setting the UL/DL transmission period to be identical. However, this results in loss of flexibility and efficiency in the TDD scheme. Meanwhile, with regard to the FDD scheme, the UL and DL signals are transmitted using different frequency bands, which are separated by a guard band. Since the frequency bandwidth is fixed, however, the FDD system is basically unable to support the traffic asymmetry between the UL and the DL in a dynamic manner. In contrast to the TDD scheme, the FDD scheme has no round-trip delay problem. For these reasons, it would seem evident that the FDD should cover the macrocell range, whereas the TDD should cover the picocell/microcell range, which was also discussed as a topical issue in the 3G hierarchical cellular networks [4]–[6]. A widely used hierarchical TDD/FDD cell structure consists of an FDD underlay macrocell and a TDD overlay microcell. In such a system, most previous studies focused on the adjacent channel interference effect and its tradeoff, depending on the optimal location deployment of the TDD BS and the FDD BS of the Universal Mobile Telecommunications System (UMTS) [7]. Another interesting study is the TDD/FDD underlay system that is proposed in [8], which exploits underused CDMA-FDD resources for CDMA-TDD picocellular users, thus increasing the flexibility of the CDMAbased TDD/FDD hierarchical cell structure. However, these previous studies are constrained to CDMA-based systems, with careful positioning of the TDD BS and the FDD BS. Next-generation mobile communication systems will be required to provide multimedia services over a wide coverage area with a flexible TDD/FDD hierarchical cell structure [9]. However, none of the current duplexing techniques can adequately support such demanding requirements. Recently, a band switching duplexing technique, which enables the TDD and the FDD to be operable in a single system, has been proposed in [10], where a pair of spectrum blocks alternates their DL/UL use for every fixed period (e.g., half of the time for the DL and half of the time for the UL). However, this cannot support an asymmetric traffic service since both DL and UL are always active. Another approach is to use the dual bandwidth that provides FDD and TDD modes by adopting wideband and narrowband spectrum blocks [11]. However, a solution to the inherent TDD CCI problem is not provided, and the continuity of the FDD operation is not entirely supported. Although these related studies are dealing with a fundamental framework, comprehensive performance evaluations are not yet presented. The main contribution of this paper is to present a new type of duplexing scheme, which is called the hybrid division duplex (HDD), as an enabling technology for flexible operation of mobile cellular systems. The focus is on the operation of the TDD/FDD hierarchical cell structure that is working in a single system, which supports the asymmetric traffic service in the different cells, namely, cell-independent asymmetric service. It exploits the attractive features of both the TDD and the FDD, combining flexibly these schemes into a single system [12], [13]. To support a cell-independent asymmetric traffic service, the TDD system should adopt different UL/DL time slot ratios among adjacent cells. It inherently entails the conventional

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TDD interferences problem [6], [14]. To solve this problem, the proposed HDD system employs an interference-reducing mechanism by splitting a cell into two areas and allocating resource based on the location of MS, thereby enhancing the overall system performance. To address a question of how to divide the cell areas and allocate resource, we investigate the impact of the system design parameters on the overall performance and its associated performance tradeoff. Moreover, such a hybrid combination of FDD and TDD operations creates an efficient system that can have an intrinsic joint radio resource management (RRM) functionality of handling geometrically unbalanced traffic that is offered between the TDD and the FDD in a single management framework, with a lower cost of intermode signaling overhead between different radio access technologies than that of intersystem handover overhead in a heterogeneous radio environment. Note that a recent work [15] has proposed a joint RRM algorithm for UMTSbased TDD/FDD hierarchical cell systems and reported that with joint RRM policies, the signaling overhead can be reduced by virtue of employing a centralized interoperator, which can reduce the blocking and dropping probabilities, while improving a level of the quality of service. Thus, the proposed HDDbased system is expected to reduce the signaling overhead that is caused by an intermode handover between two modes. This paper is organized as follows. In Section II, the distinct features of the HDD system are described. In Section III, the underlying system architecture such as cell structure, multiple access, and frame structure is explained. In Section IV, an interference analysis and signal-to-interference-plus-noise ratio (SINR) performance are presented with regard to various cell-independent UL/DL traffic asymmetries. In Section V, the system level simulation results are presented in terms of throughput performance and effectiveness of the location-based resource allocation algorithm. Finally, Section VI presents the concluding remarks. II. M AIN A SPECTS OF HDD The HDD scheme has its own unique properties compared to those of existing duplexing schemes. This section describes the concept of the proposed HDD scheme and the properties, which include a new cell structure combined with a novel frequency planning and an interference-reducing mechanism. A. Frequency Planning and Cell Structure The core feature of the HDD is the capability of enabling both TDD operations and FDD operations in a single system. To support the conventional FDD mode, a system must have a pair of frequency bands for the UL and the DL. However, the HDD scheme utilizes the FDD-DL spectrum as the TDD operating spectrum in such a manner that a simultaneous FDD and TDD operation is achievable without an extra spectrum band in a single system. As such, this is the major difference from the conventional FDD mode. The deployed frequency bands of the HDD scheme are depicted in Fig. 1, where the t-band is used for the TDD-UL and TDD-DL operations, whereas the f-band is dedicated to the FDD-UL operation. Due to this structure, an MS can operate in either the TDD mode or the FDD mode: An

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Fig. 1. Frequency planning of the HDD-based system.

Fig. 2. Cellular structure and conceptual transmission scheme of the HDDbased system.

MS in the TDD mode utilizes the resource of the t-band for the DL and the resource of the t-band for the UL. Meanwhile, an MS in the FDD mode utilizes the resource of the t-band for the DL and the resource of the f-band for the UL. When considering the fact that the DL traffic will require a much larger bandwidth than the UL in emerging service requirements [16], [17], the t-band of the HDD system should be much wider than the f-band. With this HDD band structure in mind, each cell is divided into two regions, employing the different duplexing mode in each region. As shown in Fig. 2, the proposed HDD system is given by a TDD/FDD hierarchical cell structure, which supports a cell-independent asymmetric traffic service. Each MS is operated under the more suitable duplexing mode according to its location and level of mobility. In determining the inner radius, there is a tradeoff between the inner-zone radius and the bandwidth allocation in each band when uniformly distributed users are assumed. In other words, the ratio of the inner and outer radii to the cell radius affects various aspects of the system. The effect of this ratio on the system performance will be investigated in Section IV. When the conventional TDD system that is deployed in the macrocell area is considered, a large amount of the resource is wasted due to the need of a large guard time that is required between the UL and DL intervals to prevent interference caused by the differing round-trip delays. Moreover, the portion of the guard time increases as the frame length decreases. However,

in the HDD system, this resource wastage can be reversed by confining the TDD operation area into an inner zone of which the radius is less than that of the macrocell. The system parameter for the TDD operation is then determined, such as the guard time after the setting of the inner-zone radius of the HDD system. Due to this design approach, a noticeable amount of resources are reserved for this guard time compared to that of the conventional TDD system. Meanwhile, a particular interference problem may arise. It is interesting to consider the case where the resources for the DL users at the cell boundary are allocated at the end of the DL interval. Since the guard time length is shorter than that of the conventional TDD system, the TDD-UL users may transmit their signals to the BS, whereas the DL users at the cell boundary receive their signal with a large propagation delay. This means that the signal of the TDDUL users may degrade the signal qualities of the DL users at the cell edges. To avoid this problem, the location-based resource allocation scheme introduced in Section III-B can be applied. When applying this scheme, the resources for the DL users at the cell boundary are mostly allocated at the beginning part of the DL interval. Therefore, this interference problem can be avoided. Consequently, the TDD operation parameters can be determined, considering only the inner-zone radius, as long as the location-based resource allocation algorithm is applied. A different duplexing mode is applied for each MS according to its location and mobility. To implement this operation, it is imperative that the BS can track the location of each MS. Although this location tracking is challenging, there is literature regarding location and mobility tracking using multiple BSs [18]–[20]. When the Global Positioning System is employed in the location estimation, its accuracy is further improved [21]. Therefore, it is assumed that MS location and mobility can be estimated. B. Interference-Reducing Mechanism Adopting the different UL/DL time slot ratios between neighbor cells to support an asymmetric traffic service in the different cells, a TDD-based system suffers from the intercell interferences, as discussed in previous literature [6], [14]. Fig. 3(a) depicts how this interference occurs in the TDD system. When cell 0 is in the DL transmission period while the adjacent cell k is in the UL period, each cell must transmit and receive the signal through an asynchronous time slot, i.e., CTS. In such a case, various types of CTS interferences are incurred, such as MS-to-MS interference, BS-to-BS interference, or MS-to-BS interference, which degrade the overall system performance. As one of the solutions to mitigating the CTS interference, the dynamic allocation of time slots can be employed in a centralized or a distributed manner [3]. The HDD system can be considered to be a special case of dynamic time slot allocation since the HDD system mitigates this effect by combining the time slot allocation mechanism with the frequency allocation. Meanwhile, the transmitter power control has been known as one of the solutions to minimize UL interference [22], [23]. There is no known suitable power control mechanism that considers this MS-to-MS interference problem. Although this is one of the issues that require careful consideration, the power


control mechanism is simply applied to the UL users near the cell boundary, i.e., in the outer zone, since the main source of MS-to-MS interference comes from them. The following is mentioned in the interference-reducing mechanism as a major advantage of the HDD system. In the HDD system, MS-to-MS interferences can be considerably reduced by employing an FDD-UL outer zone, which enables most of the CTS interference victims to be isolated from the interference coming from MSs in adjacent cells. It means that the inner zones are separated from each other through the out in FDD-UL outer zone. In Fig. 3(b), MSin 0 , MS0 , MSk , and out MSk denote the MS in the inner and outer zones of reference cell 0 and the MS in the inner and outer zones of cell k, respectively. In the DL from BS0 to MS0 s during the CTS period, MS0 s suffer from the interference that is caused by both out MSin k and MSk in the conventional TDD scheme. In the HDD scheme, however, MSin 0 only suffers from the interference that is caused by MSin k . For a UL case from MSk to BSk during the CTS period, BS0 generates interference that affects BSk in both the TDD and HDD schemes. In the HDD scheme, however, only MSs in the inner zone of the adjacent UL cells generate UL interference; therefore, MS-to-BS interference can be reduced compared to that of the TDD scheme. The effect of interference reduction mechanism will be mathematically analyzed in Section IV-A. It implies that the HDD scheme can avoid the worst CTS interference while creating a TDD interference-free area by employing an FDD-UL region in the outer-cell area. Therefore, this structural concept significantly reduces the CCI between MSs. Consequently, it enables each BS in the HDD system to operate with a variable asymmetric traffic ratio without suffering from serious outage in the outercell area [12]–[14]. The HDD system can further achieve interference reduction by exploiting MS location information in a proactive manner. As shown in Fig. 4, a location-based resource allocation can be considered for the HDD system. Here, MSs are numbered in the order of the distance from BS to MS. Fig. 4 shows the time slot that is allocated to MSs for UL and DL transmissions with the t-band and the f-band. In the DL, a basic principle is to allocate time slots to MS3 first, which is located farthest away from BS, then to MS2 and MS1 in a descending order in terms of their distances. In particular, time slots that are closest to the guard time (or switching point) are allocated to MS1 so that the residual CTS interference can be further reduced, as they are most susceptible to the CTS interference. Thereafter, the next closest time slot to the guard time is allocated to MS2 . The TDD-UL time slots for the t-band are allocated in order of ascending distance. Note that MS3 in the outer zone uses the fband resource for its UL while using a former part of the t-band resource for its DL. Therefore, it is hardly likely that the DL transmission of MS3 will be affected by CTS interference. III. HDD-B ASED S YSTEM A RCHITECTURE Based on the properties of the proposed HDD scheme, this section presents the system architecture and the air interface structure such as UL/DL channel design, multiple-access scheme, frame structure, and AMC operation.

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A. Hierarchical Cell Structure The HDD system has an implicit hierarchical cell structure in a single system, which forms the TDD inner zone and the FDD outer zone. With such a cell structure, a new air interface can be designed to exploit the advantages of the TDD and the FDD at the same time. The TDD-employing inner zone can guarantee a higher SINR by increasing the distance between adjacent TDD inner zones. In the outer zone, meanwhile, the SINR level is relatively lower due to higher adjacent-cell interference; thus, an interference-immune FDD mode will be useful. As such, the communication scheme must be differently tailored for the inner zone and the outer zone according to the respective needs and characteristics. A viable air interface option that is applicable to an HDDbased system is shown in Fig. 2, in which the UL/DL channels are classified into four different types: 1) TDD-DL for the inner zone, 2) TDD-UL for the inner zone, 3) TDD-DL for the outer zone, and 4) FDD-UL. Links 1 and 2 are designed to achieve a high spectral efficiency, so that the MSs that are operated in the TDD mode using the t-band can transmit at a high data rate. Meanwhile, links 3 and 4 are designed for combating a worse channel quality due to higher adjacent-cell interference.

B. Multiple Access There may be various combinations of duplexing and multiple-access schemes. In this paper, the multiple-access technique that is suitable for each duplexing mode is chosen, considering different properties of the interference due to the cell structure. Frequency hopping orthogonal frequency division multiple access (FH-OFDMA) for the TDD [24], [25] is adopted. In the case of FDD, FH-OFDMA is adopted for FDDDL since the DLs of TDD and FDD share the same band, whereas CDMA is adopted for the FDD-UL. The reason for this is as follows. In the case of TDD, the multiple-access scheme should support all the MSs for the DL and only the inner-zone MSs for the UL. In other words, it should support two types of MSs, with different conditions, within the same band (i.e., the t-band): the inner-zone MSs of UL/DL that require a high data rate and the outer-zone MSs of the DL, which are susceptible to the interference caused by neighbor cells. Thus, to simultaneously support these two requirements of high spectral efficiency and interference robustness, the FHOFDMA with AMC is employed. In the FDD, the multipleaccess scheme should support all the MSs for the DL and only the outer-zone MSs for the UL. Since the DL of the FDD is the same as that of the TDD, FH-OFDMA should be adopted for the FDD-DL operation. However, for the FDD-UL operation, more interference-robust techniques rather than FH-OFDMA should be considered. The reason for this is that the outer-zone MSs are more susceptible to interference than those of the inner zone due to the cell-independent UL/DL asymmetry. Thus, the FDD-UL employs CDMA techniques. For these reasons, FHOFDMA and CDMA are applied to the UL of the t-band and the UL of the f-band, respectively. Despite the use of frequency hopping or interference averaging techniques, the outage probability at the cell boundary

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Fig. 3. Interference mechanism in the HDD-based system. (a) Interference scenarios in the TDD system. (b) Interference scenarios in the HDD system.

Fig. 4. Resource allocation algorithm based on user location.

is still sensitive to the system loading. However, the HDDbased system with location-based resource allocation can be applied to mitigate this drawback. The modulation and coding scheme (MCS) level is determined by the SINR that is measured for each subchannel that satisfies a 1% frame error rate (FER). The CDMA system employs an open-loop power control, whereas the FH-OFDMA system employs a full power transmission with a rate control. With this control, path loss and long-term fading are compensated while mitigating the socalled near–far problem. The transmission rate of the CDMA system is controlled by dynamic allocation of different codes. In other words, the maximum allowable number of codes for the multicode transmission is determined to satisfy a 1% FER, depending on the level of the SINR in the CDMA system. C. Frame Structure The frame structure is shown in Fig. 5, where the HDD frame consists of a TDD frame in the t-band and an FDD-

UL frame in the f-band. For the t-band, DL transmission begins with preambles followed by traffic time slots. To allow for the BS turnaround time, a transmit-to-receive guard time (TTG) and a receive-to-transmit guard time (RTG) are inserted between the DL and the UL, in the center of the frame and the end of the frame, respectively. The duration of TTG and RTG is determined by considering propagation and processing delays. Note that TTG is allocated only once within one frame; however, its location can be dynamically changed according to the UL/DL asymmetric ratio. The frame length is denoted by Lframe , depending on various aspects of system design such as coherence time of the channel, channel quality information (CQI) feedback mechanism, and application properties. It is known that the frame lengths of WCDMA and CDMA-2000, which operate at approximately 2 GHz, are 10 and 20 ms, respectively. Note that the carrier frequency that is considered in this paper is 5 GHz, and its coherence time is less than one half of that in CDMA systems. Furthermore, when AMC is employed, at least two frames are


Fig. 5.

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Frame structure of the HDD-based system.

required to receive the CQI feedback [26]. To operate the AMC scheme in a stable manner, therefore, the shorter frame size will be more helpful. The numbers of the UL and the DL slots in the t-band are NUL and NDL , respectively, and they can be changed according to the UL/DL time slot ratio. Each time slot extends over three OFDMA symbols. A detailed construction of the subchannel with randomly selected subcarriers for frequency diversity can be found in [27]. For the f-band, NF_UL time slots exist for the FDD-UL frame. The CDMA-based transmission scheme is employed for the UL in the f-band. Each slot is assigned for one user only with multiple orthogonal codes for multicode transmission. More detailed system configurations are provided in Sections IV and V. D. AMC Operation For efficient use of resources, AMC is employed as a means of link adaptation. To implement this, determination of a valid MCS level is crucial. The MCS level is determined by the SINR that is measured at each subchannel that satisfies the specified FER. For the DL, the BS determines the DL adaptive MCS level using CQI feedback that is measured using the DL pilot signal at the MS. However, MCS level determination of the UL is quite different since there is no common pilot signal. When a particular user transmits its UL signal in the previous frame, the BS can easily determine the MCS level using this signal. To continuously determine the MCS level of the UL, all UL users are permitted to transmit their UL pilot signals. In such a case, however, the BS underestimates the SINR due to an increase in the interference signal power. To solve this problem, the following method is considered. The UL MCS active user level is always updated using the previously measured SINR of the desired user. Each time the UL resource is left unallocated to the desired user during the previous ten frames, its MCS level is decreased by one level. This updated information is used for the UL scheduling algorithm at the BS. For example, let the MCS level of a particular UL user be set at “4” at frame no. 20. If any UL resource has not been allocated to users during the previous ten frames, the MCS level after frame no. 30 is

calculated at “3.” Otherwise, its MCS level is updated based on its previous UL signal at every update interval. The effect of CQI feedback will be investigated and compared with the TDD and HDD systems in Section V. IV. I NTERFERENCE A NALYSIS As discussed in Section III, the effects of CCIs due to cellindependent UL/DL asymmetries between neighboring cells are expected to be mitigated in the HDD system. This interference mitigation is the key performance improvement characteristic of the HDD system. Thus, evaluating CTS interference scenarios due to cell-independent asymmetries is essential in discussing the merits of the HDD system. Therefore, in this section, we investigate whether the structure of the HDD helps to mitigate CTS interference under the same evaluation conditions. In this section, we first analyze the signal-to-interference ratio (SIR) performance. This analysis tells us how interference in the HDD system can be mitigated and which system parameter affects the system performance. Then, the observations from this analysis are verified through SINR distribution analysis. It will be demonstrated that the HDD system has the potential to improve system performance by reducing the outage probability due to the asymmetries. A. SIR Performance Analysis The level of the received signal is determined by various fading factors such as path loss, shadowing (i.e., slow fading), and fast fading. Among these factors, path loss is dominant in determining the signal level. In addition, it has been known that the statistical distribution of the received signal is overwhelmed by that of path loss [28]. Therefore, it is expected that the analysis with regard to the effect of path loss is sufficient to investigate the system-level SIR performance characteristics. In this subsection, therefore, we focus on the SIR performance with regard to path loss. Fig. 6 shows the cellular structure we consider in this section. The BS-to-BS distance and the cell radius are given as D and

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Fig. 7.

MS–BS interference.

2 ρ = µInner MS PMS /πR . The average interference from the MS in the nearest neighboring cell can be expressed as

Fig. 6. Cellular structure.

2π MS→BS IUL

R, respectively. One reference cell and its first tier cells, i.e., six nearest neighboring cells, with omnidirectional antenna in the BS for a simple interference analysis are considered because the interference from second tier cells is not significant compared to that from the first tier cells. The power received from the BS at distance r can be modeled as αr−γ PBS , where γ is the path loss exponent, PBS is the transmitted power of the BS, and α is a constant [29]. Similarly, the power received from the MS is given by αr−γ PMS , where PMS is the transmitted power of the MS. In this paper, we set γ = 4 for simple analysis [29]. As demonstrated in Fig. 3(a), UL/DL slot ratio asymmetries between neighbor cells in the TDD and HDD systems simultaneously impact on MS-to-MS/BS-to-BS interference and MS-to-BS interference. It means that, at a given time instant, some of the cells are in the UL transmission, whereas the others are in the DL transmission. This is modeled by a CTS ratio τ , which is defined as the ratio of the average number of cells operated in the DL to the total number of cells within the CTS period. Note that the CTS does not occur in the FDD scheme. In the TDD and HDD schemes, however, τ can be nonzero due to independent UL/DL slot ratio asymmetries between neighboring cells. In the UL, some interferences are caused by cells that operate in the TDD-UL, and others are caused by cells that operate in the TDD-DL. The interference from one of other BSs can be expressed as I BS→BS = αµBS PBS D−4

(1)

where µBS denotes the cell loading factors of the BS. Note that the loading factor is defined as the ratio of the number of the used subcarriers to the total number of subcarriers in the OFDMA system. Fig. 7 illustrates the interference from mobile in other cells. We assume that mobiles uniformly distribute in one cell, and denote the cell loading factors of the MSs in the let µInner MS inner cell. The inner-cell radius is λR, where R denotes the cell radius, and 0 < λ < 1. The interference power per unit space is

= αρ

λR dθ (r2 + D2 − 2rD cos θ)−2 dr.

0

(2)

0

If we use the following equation: 2π 0

R dθ (r2 + D2 − 2rD cos θ)−2 dr = 0

πR2 (D − R)2 (D + R)2 (3)

we can evaluate the average interference from mobiles in one nearest neighboring cell as MS→BS = IUL

αµInner MS PMS . (D − λR)2 (D + λR)2

(4)

For 0 < λ < 1 and D > R, the following equation is satisfied: (D − λR)2 (D + λR)2 > (D − R)2 (D + R)2

(5)

which means that the interference from mobiles of (4) decreases as the inner-cell radius λR decreases. The overall average UL SIR of the mobile whose distance from the reference BS is r is given as αPMS r−4 BS→BS + 6(1 − τ )I MS→BS 6τ IUL UL αPMS r−4 /6 . = MS→BS BS→BS MS→BS τ IUL + IUL − IUL

SIRUL (r) =

(6)

In (6), we can see that the SIR performance is improved as the MS→BS inner-cell radius is decreased. The reason is that IUL in (6) BS→BS is is a decreasing function of λ due to (5), although IUL constant, regardless of the value of λ. For the DL, we consider an MS whose distance from the reference BS is r (0 < r ≤ λR) in Fig. 6. The angle of location of the MS with respect to the interfering kth BS, i.e., θk , is expressed as θk = θ + (π/3)k, k = 0, 1, . . . , 5. The interference from the kth BS is BS→MS IDL,k (r) =

(r2

αµBS PBS . + D2 − 2rD cos θk )2

(7)


In addition, the average interference from mobiles in the kth cell is MS→MS (r) = IDL,k

(r2

+

D2

αµInner MS PMS . − 2rD cos θk − λ2 R2 )2

(8)

Note that (8) is an increasing function of λ. As the inner-zone MS→MS (r) decreases for radius λR decreases, we can see that IDL,k fixed r. From these equations, we can see that interferences in the DL depend on the location of mobile, whereas those in the UL are independent of the location. The overall average DL SIR of the mobile whose distance from the reference BS is r is given as SIRDL (r) =

τ

k

αPBS r−4 MS→MS . BS→MS + (1 − τ ) IDL,k IDL,k

(9)

k

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√ typical values, i.e., λ = 0.7, R = 1 km, D = 3 km, and the worst estimation error of δ = 100 m [20], [21],1 the factor is approximately given as 1.11. In other words, at most 11% MS→BS is expected. Since the interference consists increase in IUL MS→BS BS→BS and IUL , and the factor is an upper-bounded of IUL value, the SIR performance degradation is definitely far less than the upper bound of 11%. Therefore, the effect of the location estimation on the performance can be considered as negligible. This analysis result is verified from the simulated SIR distribution in Section IV-E. BS→MS of (7) is not affected by the locaFor the DL, IDL,k tion estimation error since the position of interfering BS is fixed. Similar to the derivation of (11), the upper bound of MS→MS is IDL,k αµInner MS PMS 2 2 (r + D − 2rD cos θk − (λR + δ)2 )2 αµInner MS PMS ≈ (r2 + D2 − 2rD cos θk − λ2 R2 )2 4λR × 1+ 2 δ . r + D2 − 2rD cos θk − λ2 R2

MS→MS (r) < IDL,k

In (9), we can see that the SIR performance is improved as the MS→MS inner-cell radius is decreased. The reason is that IDL,k in BS→MS (9) is a decreasing function of λ, although IDL,k is constant, regardless of the value of λ. B. Location Estimation Error

≡LDL

In the HDD system, permitting only the mobiles within the inner cell to transmit their signal is the key performance improvement. To determine whether the mobiles are within the inner cell or not, it is required that the BS can track the location of each MS [18], [19]. Since there exists the location estimation error, we need to investigate the effect of this estimation error on the performance. First, let us consider the effect of the estimation error on MS→BS of (4) the UL SIR performance. In this case, only IUL BS→BS of (1) is is affected by the estimation error, whereas IUL independent. When the maximum location estimation error is given as δ, it is assumed that the MS with distance of r (λR ≤ r < λR + δ) from its reference BS is in the inner region of the cell, and the MS with r (λR − δ ≤ r < λR) is in the outer MS→BS is bounded as region. Then, IUL αµInner αµInner MS→BS MS PMS MS PMS < I < . UL 2 2 2 2 (D − (λR − δ) ) (D − (λR + δ)2 )2 (10) To get the maximum SIR performance reduction due to the MS→BS is location estimation error, the upper bound of IUL rewritten as αµInner MS PMS 2 (D − (λR + δ)2 )2 αµInner MS PMS = 2 2δλR+δ 2 (D2 − λ2 R2 )2 1 − D 2 −λ2 R2 Inner αµMS PMS 4λR ≈ δ 1+ 2 (D2 − λ2 R2 )2 D − λ2 R 2

(12) Note that the maximum interference upper-bound factor LDL is a function of the position from the BS, i.e., r and θk , and the value of LDL is bounded as 4λR 4λR δ ≤ LDL ≤ 1+ δ . 1+ (D+r)2 −λ2 R2 (D−r)2 −λ2 R2 (13) For example, with the same typical values of the UL case at r = λR, it ranges from about 5% to 50%, depending on the MS position. Therefore, it is hard to tell whether the location error is a critical fact to the SIR performance from the result of (13). Besides (13), we consider an indirect method to determine whether the location error is critical. The lower bound on the SIR performance due to the location error can be considered as that with the inner-zone radius of (λR + δ). The reason is that in this case, TDD-DL resources of the interfering cells are allocated to the MSs within the distance of (λR + δ) in the interfering cell. If the performance difference between the case of the inner-zone radius λR and that of (λR + δ) is not significant, the location error may not be critical for the performance. C. Derivation of SINR CDF

MS→BS < IUL

(11)

≡LUL

which means that the maximum interference upper-bound factor LUL is given as (1 + (4λR/D2 − λ2 R2 )δ). With

To evaluate outage probability, the cumulative distribution function (CDF) of SINR, considering the effect of cellindependent UL/DL asymmetries, needs to be derived. Since the TDD-DL and TDD-UL in the t-band of the HDD scheme causes this effect, the corresponding CDFs of the band are 1 In [21], it is said that the GPS location estimation error ranges from about 20 to 100 m. On the other hand, in [20], without GPS, the error is about 70 m. In this paper, we employ the worst estimation error. Using the worst error, we will show that the location error does not significantly affect the system performance.

3048


The conditional CDF averaging over ρ, ρi , and ρj can be obtained as that in (16), shown at the bottom of the page. Similarly, the instantaneous SINR of the DL can be expressed as that in (17), shown at the bottom of the next page. The conditional CDF is found in a similar manner [see (18) at the bottom of the next page]. To obtain the CDF of the SINR, multidimensional integration of the above conditional CDF is necessary with respect to r, ri , rj , x, xi , xj , φ, φi , and φj . In general, it is impossible to obtain the distribution of SINR in a closed form. Herein, the Monte Carlo computation is used. The computation methodology is presented in the following section. In the Monte Carlo computation, at each trial, we uniformly drop the MSs in the cell areas, and each MS is allocated to the BS with the largest channel gain. Each BS randomly selects multiple MSs among the allocated MSs and then collects the SINR information. The detailed assumptions will be presented in the following section.

derived. Note that these results can be applied to both the TDD and HDD schemes. The power received from the BS at a distance r can be modeled as αr−γ 10x/10 ρGBS (φ)PBS , where x is Gaussian √ distributed with mean 0 and standard deviation σ dB, ρ is a Rayleigh-distributed random variable, and GBS (φ) is the antenna gain function for the arrival angle. Similarly, the power received from the MS is given by αr−γ 10x/10 ρGMS (φ)PMS . In the UL, some interference is caused by cells that operate in the TDD-UL, and others are caused by cells that operate in the TDD-DL. Let SMS and SBS be the set of interfering MSs in neighbor cells and the set of interfering BSs, respectively. Thus, given the normalized CTS ratio τ , the instantaneous SINR of the UL is expressed as that in (14), shown at the bottom of the page, where N0 denotes the thermal noise power. Then, the CDF of the UL can be computed as that in (15), shown at the bottom of the page.

SINRUL =

(1 − τ )αµMS PMS

i∈SMS

αµMS PMS r−γ 10x/10 ρGMS (φ) −γ x/10 −γ ri 10x/10 ρi GMS (φi ) + τ αµBS PBS rj 10 ρj GBS (φj ) j∈SBS

Pr(SINRUL ≤ s|r, ri , rj , x, xi , xj , φ, φi , φj , ρ, ρi , ρj )   = Pr 

(1 − τ )αµMS PMS

i∈SMS

 = Pr ρ ≤ (1 − τ )

(14)

+ N0

 −γ

x/10

αµMS PMS r 10 ρGMS (φ) −γ x/10 −γ x/10 ri 10 ρi GMS (φi ) + τ αµBS PBS rj 10 ρj GBS (φj ) j∈SBS

+ N0

 ≤ s

ri −γ GMS (φi ) s 10(xi −x)/10 ρi r GMS (φ)

i∈SMS

 µBS PBS rj −γ (xj −x)/10 GBS (φj ) N0 s  +τ s+ 10 ρj −γ 10x/10 G µMS PMS r GMS (φ) αµ P r (φ) MS MS MS j∈S

(15)

BS

Pr(SINRUL ≤ s|r, ri , rj , x, xi , xj , φ, φi , φj ) N0 s −ρi e = 1−exp − dρ dρj e−ρj i αµMS PMS r−γ 10x/10 GMS (φ) i∈S j∈SBS MS   r −γ ri −γ µ G (φ ) P G (φ ) MS i BS BS j BS j s−τ s × exp −(1−τ ) 10(xi−x)/10 ρi 10(xj−x)/10 ρj r GMS (φ) µMS PMS r GMS (φ) i∈SMS j∈SBS N0 s = 1−exp − αµMS PMS r−γ 10x/10 GMS (φ) −1 −1 r −γ µBS PBS rj −γ (xj−x)/10 GBS (φj ) i (xi−x)/10 GMS (φi ) × s s 10 10 1 + (1−τ ) 1+τ r GMS (φ) µMS PMS r GMS (φ) i∈SMS

j∈SBS

(16)


√ It is known that the Monte Carlo error is in the order of 1/ n, where n is the number of trials. Therefore, if the number of trials is sufficiently high, the error can be made arbitrarily small. As the effects of the Monte Carlo computation in terms of convergence and error have been well analyzed and understood in classical books on numerical integration [30], [31], a more detailed discussion regarding the Monte Carlo computation is not addressed in this paper due to space limitations. D. Evaluation Conditions To derive the SINR distributions in the previous subsection, the Monte Carlo computation, which is referred to as a snapshot analysis, is performed [32]. From this integration, the outage probability can be evaluated. Note that the evaluation conditions described in this subsection will be reused in the system level simulation of Section V. For evaluation, a wraparound structure with 27 hexagonal cells is considered, with each cell having three sectors. Each sector employs a directional antenna with a horizontal 90◦ 3-dB beam width and a 15-dB antenna gain in each cell with a transmitting power of 36 dBm as in [26]. The MS antenna is assumed to be omnidirectional with a transmitting power of 23 dBm [26]. The carrier frequency is set to 5 GHz, and the noise spectral density is −174 dBm/Hz with a 6-dB noise figure. The system parameters for the multiple access that is employed for the current system design are as follows. The total system bandwidth is 25 MHz out of which 20-MHz bandwidth and 5-MHz bandwidth are allocated to the t-band and f-band, respectively. In FH-OFDMA, each cell employs a 20-MHz total bandwidth with 1552 subcarriers used for the t-band, and, unless otherwise stated, a loading factor is “1.” Ninety seven subchannels are used, and each subchannel consists of 16 subcarriers that are selected by an FH pattern that is uniformly distributed. The use of the 1552 subcarriers may incur a severe peak-to-average-power ratio problem. However, it is not unusual in practice. For example, a 2048-subcarrier option is typically considered in the current 802.16d Wireless Metropolitan Area Network standard [27] for a 20-MHz bandwidth, even if a large margin in power amplifier is required

SINRDL =

τ αµBS PBS

i∈SBS

3049

to maintain a linearity of the amplified signal. It employs an AMC with full power transmission. With the discussion of Section III-C in mind, the frame length is set at 5 ms, which seems to be one of the most typical options widely accepted in the broadband wireless access systems, e.g., Mobile Worldwide Interoperability for Microwave Access (WiMAX). For a fair comparison of the HDD and TDD systems, it is assumed that the channel bandwidth of the pure TDD system is 25 MHz, which is the sum of the bandwidth for the t-band and the f-band in the HDD system. Regardless of the cell size, the HDD system can be applied; however, the microcell size is considered due to the evaluation convenience. As long as an interference-limited system is assumed, the cell radius does not make any difference in the interference characteristics. Given a microcell radius of 1 km, the inner-zone radius for HDD purposes is set to 707 m (i.e., half of the cell area), unless otherwise stated. In this case, half of the users are located in each zone, assuming uniform user distribution. Due to limitations in computing power, the number of MSs per cell is required to be limited. Although this may affect the system performance (for instance, multiuser diversity gain due to the scheduling algorithm), it is expected that the compared performance trend between the HDD and TDD systems will not change. Thus, it is assumed that there are 16 MSs uniformly distributed in each sector (thus, a total of 48 MSs in a cell). In each inner- and outer-cell region, eight users are allocated when the inner zone is allocated to half of the cell area. The path loss is simulated using the log-distance model with a path loss exponent of 3.74 and lognormal fading with a 10-dB standard deviation [33]. It is assumed that the correlation between the sectors within a single cell is 1.0, and the correlation between BSs is 0.5 [26], [34]. In addition, an international telecommunication union (ITU) vehicular type A (Veh-A) channel model is employed to include the effect of frequency selective fading in the evaluation [33]. It is assumed that the MS speed is 3 km/h, and the corresponding channel variation is generated based on Jake’s model as in [26]. The MSs select their serving BS/sector with maximum received power [26]. A frequency reuse scheme factor of “1” is considered in this evaluation.

αµBS PBS r−γ 10x/10 ρGBS (φ) −γ x/10 ri−γ 10x/10 ρi GBS (φi ) + (1 − τ )αµMS PMS rj 10 ρj GMS (φj ) + N0

(17)

j∈SMS

N0 s Pr(SINRDL ≤ s|r, ri , rj , x, xi , xj , φ, φi , φj ) = 1−exp − αµBS PBS r−γ 10x/10 GBS (φ) −1 −1 r −γ µMS PMS rj −γ (xj−x)/10 GMS (φj ) i (xi−x)/10 GBS (φi ) × s s 10 10 1+τ 1+(1−τ ) r GBS (φ) µBS PBS r GBS (φ) i∈SBS

j∈SMS

(18)

3050


TABLE I MCS LEVEL PER SUBCHANNEL. (a) DL ; (b) UL

The DL loading factor for each sector in FH-OFDMA was “1,” unless otherwise stated. However, when the UL loading factor for each sector was “1,” outage performance was severely degraded. From the simulation study, it was determined that one of the main sources of UL interference was intersector interference of the same cell. This is mainly due to the use of the omnidirectional antenna of the MS. Intersector interference that occurs in the same cell can be eliminated by orthogonally allocating the resource within the sectors in the same cell. Due to this configuration, the UL loading factor for each sector results in “1/3”; therefore, the UL loading factor of each cell is “1.” The MCS level selection criteria that are applied per subchannel in the link-level simulation are given in Table I. To simulate link-level performance, the ITU Veh-A channel model with the MS speed of 3 km/h is employed [33]. Due to space limitations, FER performance will not be provided in this paper. Detailed conditions of the system-level simulation that is employed in this paper can be found in [26] and [34]. Table II summarizes the system configurations that are considered in this paper. E. Evaluation of SINR CDF First, in order to investigate the effect of UL/DL asymmetries, the CTS ratio τ is varied from 1/3, 1/2, to 2/3. As described earlier, reducing or eliminating CTS interference is the key performance improvement characteristic of the pro-

posed HDD-based system. Thus, evaluating CTS interference scenarios with differing UL and DL ratios is essential in discussing the advantages of the HDD-based system compared to a TDD- or an FDD-only system. Therefore, from the results of these evaluations, the duplexing option with or without a CTS environment can provide improved link quality under the same evaluation conditions. Fig. 8(a) and (b) presents the CDF of the SINR in the DL and the UL for the TDD and HDD schemes, respectively. Note that all the cases except one consider the SINR distributions in the CTS period. For comparison, the TDD scheme is considered without CTS, where all the cells have the perfect UL/DL transmission timing synchronization and the same UL/DL asymmetry ratio; therefore, there is no overlapped (crossed) time slot between the UL and DL transmission periods (i.e., τ = 0). The core interference source for the cell boundary DL users at the CTS point came from the UL users in adjacent cells. When the CTS period is known a priori to the BSs under consideration, a simple resource allocation policy can be applied where the resources during the CTS period are only allocated to the DL users in the inner zone, as described in Section III-B. Unlike the DL, the interference from the adjacent BS does not depend on the allocation policy, and, thus, the average UL performance is the same, regardless of the DL resource allocation policy. The effect of interference from the MS in an adjacent cell on the DL performance can be reduced by means of a DL resource allocation. In this case, it is important to note that only the MSs in the inner region are considered since this analysis is performed only during the CTS period. The overall performance including all the periods is investigated in Section V. From Fig. 8(a), it can be noted that the HDD scheme with the CTS and the allocation algorithm at 100% loading produces a similar performance result to that of the TDD scheme without the CTS at 50% loading. The core reason for this performance improvement is that, in the HDD scheme, users located in the outer zone are not affected by other-cell interference during the CTS period. This indicates that the proposed HDD scheme supports various asymmetric traffic services without any interference penalty, which is a clear distinction from the conventional FDD and TDD schemes. As shown in Fig. 8(b), a similar type of performance improvement is observed for the UL. This is because the HDD scheme uses either the UL of the f-band or the UL of the t-band depending on the UL user location, as described previously in Section II. Note that there is a large improvement on the SINR performance of the UL compared to that of the DL. The reason for this is as follows. The resource of the UL is physically divided into FDD-UL and TDD-UL bands. This not only mitigates the effect of DL interference on the TDD-UL users as discussed in Section III but also reduces the TDD-UL interference on the DL. In other words, this bandwidth division itself increases the average distance between the TDD-UL users among adjacent cells. Thus, it results in a larger reduction of TDD-UL interference from the TDD-UL users. Fig. 9(a) and (b) demonstrates the effect of the size of the inner-zone area on the SINR distributions of the DL and the UL for the HDD scheme, respectively. As the proportion of


3051

TABLE II SYSTEM CONFIGURATIONS

the inner-zone area versus the whole cell area is increased, the SINR distribution curve approaches that of the TDD system. The reason for this is that there are TDD-UL users near the cell boundary at large inner-zone radius. It may be misinterpreted that system performance is improved the smaller the inner-zone radius. In this case, however, the number of FDD-UL users increases so that the bandwidth of the f-band is required to be increased in order to support them. This results in the reduction of the t-band bandwidth. This means that the capability of supporting various asymmetry ratios will be degraded. Consequently, it is clear that there is a tradeoff between the innerzone radius and the bandwidth allocation in each band. In other words, the ratio of the inner and outer radii of the cell affects various system aspects such as the UL bandwidth allocation to each zone and the interference characteristics. Although it is one of the key aspects in the HDD system, this will be left as a future research topic. Fig. 10(a) and (b) shows the effect of the location estimation error on the SINR performance. It is assumed that the estimation error is uniformly distributed on ±100 m centered

on the exact position. As expected in Section IV-B, the effect of the location error is negligible. The reason is that the effect of the location error on the performance is much lesser than that of other fading factors such as path loss and shadowing. Therefore, the addition of the estimation error to the composite signal does not considerably change the resultant SINR distribution. Table III summarizes the outage probability per cell under varying values of the system parameters and allocation schemes. To evaluate outage performance, the minimum required SINRs, which are obtained through the link-level simulations as shown in Table I, are used. The required SINR was −1.8 dB with a coding rate of 1/12 and quaternary phase-shift keying (QPSK) modulation for the UL, whereas the required SINR was −3.46 dB with a coding rate of 1/12 and QPSK modulation for the DL. Table III considers slot performance during the CTS period. In the case of a CTS ratio of 1/2, the DL outage probability was reduced to 18.9% and 9.4%, respectively, without/with the allocation policy as compared to 32.6% of the conventional TDD-DL. For UL outage performance,

3052


Fig. 8. CDF of SINR. (a) DL. (b) UL.

considerable performance improvement was achieved. Accordingly, these results demonstrate that the proposed HDD scheme with the allocation algorithm would appear to effectively support asymmetric traffic service requirements as well as avoid and resolve TDD interference by efficiently combining the advantages of the TDD and FDD modes.

V. S YSTEM -L EVEL S IMULATIONS This section performs system-level simulations to evaluate the proposed system while also considering various other aspects such as scheduling, SINR measurement for MCS-level

determination, and various levels of traffic asymmetries. Thus, additional evaluation conditions are required. The following subsection explains these additional conditions.

A. System Conﬁguration A full buffer model is employed, i.e., data packets always waiting in the buffer. The proportional fair scheduling is performed on a subchannel-by-subchannel basis in every frame [35]. The CQI feedback mechanism is employed, as discussed in Section II. The corresponding feedback delay is set to two frames. This implies that after SINR measurement is


Fig. 9.

CDF of SINR with various inner zone areas (CTS ratio = 1/2). (a) DL. (b) UL. TABLE III OUTAGE PERFORMANCE COMPARISON

3053

3054


Fig. 10. CDF of SINR with distance estimation error (CTS ratio = 1/2). (a) DL. (b) UL.

performed, two additional frames are required before the measurement can be used at the BS. For example, SINR measurement made during one frame is transmitted to the BS and processed during the following two frames, and finally, it then can be used to select the MCS level for the next frame, i.e., the fourth frame. It is assumed that the CQI information is received without error at the BS. A chase-combining scheme is also applied, as described in [34]. To investigate the effect of UL/DL asymmetry, two types of cells are considered: a DL-dominant cell and a UL-dominant cell. The former represents a much larger level of DL traffic than UL traffic in the cell, whereas the latter represents a comparable amount of DL and UL traffic. To simplify the

simulation study, it is assumed that the DL/UL slot ratio of DL-dominant cells was 14/2 (14/6) in the HDD (TDD) system, whereas the DL/UL slot ratio of UL-dominant cells was 7/9 (7/13). Thus, CTSs only existed in the DL slots in the DLdominant cells, whereas they only existed in the UL slots in the UL-dominant cells. Half of the cells were randomly assigned as DL dominant and the other half as UL dominant. B. Throughput Performance For fair comparison in various system configurations, system performance was compared in terms of the spectral efficiency. It is defined as the ratio of aggregate throughput of each


3055

Fig. 11. Performance comparison of spectral efficiency (in bits per second per hertz). (a) DL performance in DL-dominant cell. (b) UL performance in UL-dominant cell.

band versus the corresponding system bandwidth. Fig. 11(a) and (b) shows the DL and UL spectral efficiency of the DL- and UL-dominant cells, respectively. In the case of full loading, the proposed system produced an approximately 15% (45%) improvement in the DL (UL) spectral efficiency of the DL-dominant (UL-dominant) cells during the CTS period when compared to the TDD system. A similar performance improvement was also found in the case of 50% loading. The core reason for the significant improvement in the UL was because the TDD-UL in the HDD system supports users that are within the inner zone of the cell. Table IV presents the throughput performance. In terms of the total throughput performance, there was a 7% improvement in the DL throughput of the DL-dominant cells and about a 30% improvement in the UL throughput of the UL-dominant cells under 100% loading.

C. Validity of CQI As discussed in Section V-A, data packets are always waiting in the buffer according to the full buffer model assumption. Thus, a packet scheduler selects the users, as well as their MCS levels that are listed in Table I based on the CQI measured in the previous frame. Note that its validity at the transmission instant is a crucial factor in determining link reliability. For instance, when the SINR at the transmission instant is smaller than the measured SINR, packet error becomes more likely. In particular, in the UL, the MSs that cause interference signals at one particular instant may be quite different from those at measurement instants. As such, this results in a high packet error probability, regardless of SINR measurement accuracy. Moreover, when the CTS effect is considered, the SINR measurement of the DL is also unreliable within the CTS period.

3056


TABLE IV THROUGHPUT PERFORMANCE COMPARISON (IN MEGABITS PER SECOND PER SECTOR)

The reason for this is as follows. The MS measures the SINR using the pilot signal of the DL, which means that the measured SINR value is no longer valid within the CTS period due to the UL interference that cannot be measured using the pilot signal. Thus, a large power margin is required to compensate for this effect; otherwise, a large number of retransmission attempts may be required. Consequently, another important aspect that needs to be investigated is the performance of the average number of retransmissions, which indirectly indicates the reliability of the SINR measurement. More specifically, as long as the SINR measurement is valid at the transmission instant, the average number of retransmissions should also be low. Moreover, a large power margin may not be required to compensate for this effect. Note that since the performances of the TDD and the HDD are not significantly different within the non-CTS period, the main focus was during the CTS period. Fig. 12(a) and (b) presents the average number of retransmissions in the CTS period with respect to the MCS levels that are listed in Table I. In this subsection, the cases without the allocation algorithm are considered. Note that the effect of the allocation algorithm will be investigated in the following subsection. For fair comparison, only the t-band (i.e., TDD band) of the HDD and the TDD was compared. As expected, the average number of retransmissions in the HDD system was smaller than that in the TDD system, regardless of the MCS levels. As previously discussed, this means that the SINR measurement with the HDD system was more reliable than that of the TDD system. Note that the number of retransmissions of the UL MCS level 1 was significantly reduced compared to that of the TDD system. It means that UL CQI measurement of the TDD system is unreliable, particularly for the lowest MCS level users. The reason is as follows. The UL users that are transmitting their signals are subject to be changed according to the scheduler at each frame; therefore, UL interference signals consist of different UL users’ signals. In particular, for a UL user at cell boundary, it is expected that the interference signal power is much stronger than that of the desired one. This also means that the SINR of the user is subject to be changed at each frame. It is natural to assume that there are UL users with low SINR values (i.e., low MCS level) at the cell boundary; therefore, the measured CQI is unreliable, particularly for the low MCS level users. As discussed in Section III, the HDD system can avoid this degradation by dividing the UL users into two bands. For these reasons, the number of retransmissions

of the UL MCS level 1 is significantly reduced compared to that of the TDD system. In addition, the DL performance of the HDD shows better than that of the TDD. The reason is as follows. In the TDD, the DL CQI difference between the measurement instant and the transmission instant in the CTS period can be quite large because there are interferences from adjacent BS and those from MSs in adjacent cell that are not observed in the measurement instant. Also, in the HDD, the interference signals consist of those from the BS and the MS; however, we note that the signal level of the interference from the MS is quite smaller than that of the TDD. The reason is that the HDD does not permit UL transmission from the MSs in the outer region of the cell. In other words, the average path loss of the interference in the HDD is much larger than that of the TDD. These simulation results demonstrate that the proposed HDD-based system can feasibly vary the UL/DL time slot ratio (i.e., asymmetric traffic support) in a multicell environment, successfully supporting reliable channel quality feedback information. D. Effect of Location-Based Resource Allocation In this subsection, the performance of the location-based resource allocation algorithm, as discussed in Section II, is investigated. First of all, the packet scheduler selects the users to be served in each frame [35]. Then, the DL-scheduled users are sorted with the BS-to-MS distance in a descending order. Meanwhile, the UL-scheduled users in the t-band are sorted in an ascending order. Referring to this order, the time slots are allocated to the users as shown in Fig. 4. Thus, it is expected that the users in the outer region of the cell are allocated with time slots in the non-CTS period. Fig. 12(a) and (b) presents the average number of retransmissions in the CTS period with respect to the MCS levels in Table I. As expected, the performance of the systems that implement location-based resource allocation is superior to that of the conventional approach, regardless of whether it is using the TDD or HDD systems. Note that when the t-band of the HDD system is being considered, the improvement of the UL by resource allocation is not significant compared to that of the DL. Since the HDD-UL is separated into two bands according to the distance between the MS and the BS, additional improvement due to the allocation is not significant.


3057

Fig. 12. Average number of retransmissions in the CTS period using location-based resource allocation algorithm. (a) DL. (b) UL.

VI. C ONCLUSION This paper proposed a 4G mobile communication system architecture that employs a new duplexing scheme, namely, the HDD. This scheme enables a mobile communication system to more flexibly and efficiently use its radio resources by exploiting the advantages of both the TDD and the FDD. In the HDD system, the FH-OFDMA for TDD operation is adopted, whereas the FH-OFDMA is adopted for the FDD-DL and the CDMA for the FDD-UL. From the simulation results, it is found that the proposed system can achieve an approximately 7% improvement in terms of the DL throughput of the DLdominant cell and an approximately 30% improvement for the UL throughput of the UL-dominant cell, compared to those

of the TDD system under cell-independent UL/DL asymmetries between neighboring cells. It is demonstrated that the proposed HDD system can effectively support an asymmetric traffic service as well as suppress TDD interference in multicell environments. It is shown that the proposed HDD system outperforms the conventional TDD system in the context of outage probability, total throughput performance, SNR distribution, and CQI reliability. Therefore, it is expected that the proposed HDD-based system can be an attractive alternative to duplexing technology to efficiently harmonize two dominant duplexing schemes and can play the role of a flexible wireless system enabler to build a cost-effective broadband access infrastructure for next-generation cellular services.

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R EFERENCES [1] D. G. Jeong and W. S. Jeon, “CDMA/TDD system for wireless multimedia services with traffic unbalance between uplink and downlink,” IEEE J. Sel. Areas Commun., vol. 17, no. 5, pp. 939–946, May 1999. [2] J. Chuang and N. Sollenberger, “Beyond 3G: Wideband wireless data access based on OFDM and dynamic packet assignment,” IEEE Commun. Mag., vol. 38, no. 7, pp. 78–87, Jul. 2000. [3] H. Haas and S. McLaughlin, “A dynamic channel assignment algorithm for a hybrid TDMA/CDMA-TDD interface using the novel TS-opposing technique,” IEEE J. Sel. Areas Commun., vol. 19, no. 10, pp. 1831–1846, Oct. 2001. [4] H. Haas, S. McLaughlin, and G. Povey, “Capacity-coverage analysis of TDD and FDD mode in UMTS at 1920 MHz,” Proc. Inst. Electr. Eng.—Commun., vol. 149, no. 1, pp. 51–57, Feb. 2002. [5] L. Ortigoza-Guerrero and A. H. Aghvami, “Overlay of WB-CDMAFDD on WB-TD/CDMA-TDD reverse link capacity assessment,” in Proc. IEEE VTC—Fall, Sep. 1999, pp. 2586–2590. [6] G. J. R. Povey, H. Holma, and A. Toskala, “TDD-CDMA extension to FDD-CDMA based third generation cellular system,” in Proc. ICUPC, Oct. 1997, vol. 2, pp. 813–817. [7] H. Haas, S. McLaughlin, and G. Povey, “The effect of interference between the TDD and FDD mode in UMTS at the boundary of 1920 MHz,” in Proc. IEEE 6th ISSSTA, Sep. 2000, pp. 486–490. [8] H. Haas and G. Povey, “Capacity analysis of a TDD underlay applicable for UMTS,” in Proc. IEEE Pers., Indoor, Mobile Radio Commun., Sep. 1999, pp. 167–171. [9] J. von Häfen, “WINNER-system concept overview,” in Proc. MOCCA/ WWI Workshop, Shanghai, China, Oct. 21, 2005. [10] A. Alexiou et al., “Duplexing, resource allocation and inter-cell coordination-design recommendations for next generation systems,” in Proc. 11th WWRF Meeting, Jun. 2004. [11] M. Rinne et al., “Dual bandwidth approach to new air interface,” in Proc. 11th WWRF Meeting, Jun. 2004. [12] S. B. Yun, S. Y. Park, Y. W. Lee et al., “Hybrid duplex technology: A flexible wireless system enabler for supporting time and frequency division duplex,” in Proc. 11th WWRF Meeting, Jun. 2004. [13] A. Alexiou, Ed., “White paper: Duplexing, resource allocation and intercell coordination-design recommendations for next generation systems,” in Proc. 13th WWRF Meeting, Nov. 2004. [14] W. S. Jeon and D. G. Jeong, “Comparison of time slot allocation strategies for CDMA/TDD systems,” IEEE J. Sel. Areas Commun., vol. 18, no. 7, pp. 1271–1278, Jul. 2000. [15] N. Motte, R. Rümmler, D. Grandblaise, L. Elicegui, D. Bourse, and E. Seidel, “Joint radio resource management and QoS implications of software downloading for SDR terminals,” in Proc. IST Mobile Wireless Telecommun. Summit, Jun. 2002. [16] W. Lu, Broadband Wireless Mobile: 3G and Beyond. Hoboken, NJ: Wiley, 2002. [17] S. Ohmori, Y. Yamao, and N. Nakajima, “The future generations of mobile communications based on broadband access technologies,” IEEE Commun. Mag., vol. 38, no. 12, pp. 134–142, Dec. 2000. [18] P. N. Pathirana, A. V. Savkin, and S. Jha, “Location estimation and trajectory prediction for cellular networks with mobile base stations,” IEEE Trans. Veh. Technol., vol. 53, no. 6, pp. 1903–1913, Nov. 2004. [19] P.-C. Chen, “A cellular based mobile location tracking system,” in Proc. Veh. Technol. Conf., 1999, pp. 1979–1983. [20] M. Hellebrandt and R. Mathar, “Location tracking of mobiles in cellular radio networks,” IEEE Trans. Veh. Technol., vol. 48, no. 5, pp. 1558–1562, Sep. 1999. [21] R. Bajaj, S. L. Ranaweera, and D. P. Agrawal, “GPS: Location-tracking technology,” Computer, vol. 35, no. 4, pp. 92–94, Apr. 2002. [22] R. D. Yates, “A framework for uplink power control in cellular radio systems,” IEEE J. Sel. Areas Commun., vol. 13, no. 7, pp. 1341–1347, Sep. 1995. [23] J. Zander, “Performance of optimum transmitter power control in cellular radio system,” IEEE Trans. Veh. Technol., vol. 41, no. 1, pp. 57–62, Feb. 1992. [24] R. Nee and R. Prasad, OFDM for Wireless Multimedia Communications. Norwood, MA: Artech House, 2000. [25] S. Hara and R. Prasad, “Design and performance of multicarrier CDMA system in frequency-selective Rayleigh fading channels,” IEEE Trans. Veh. Technol., vol. 48, no. 5, pp. 1584–1595, Sep. 1999. [26] 3GPP2 Technical Specification Group C R1002, 1xEV-DV Evaluation Methodology (v13.1), 2003.

[27] IEEE Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Fixed Broadband Wireless Access Systems, Oct. 2004. [28] B. Hajek, A. Krishna, and R. O. LaMaire, “On the capture probability for a large number of stations,” IEEE Trans. Commun., vol. 45, no. 2, pp. 254–260, Feb. 1997. [29] J. S. Lee and L. E. Miller, CDMA System Engineering Handbook. Norwood, MA: Artech House, 1998. [30] W. H. Press et al., Numerical Recipes in C, 2nd ed. Cambridge, U.K.: Cambridge Univ. Press, 1992. [31] M. H. Kalos and P. A. Whitlock, Monte Carlo Methods. Hoboken, NJ: Wiley, 1986. [32] J. Zander and S. L. Kim, Radio Resource Management for Wireless Networks. Norwood, MA: Artech House, 2001. [33] Guidelines for Evaluation of Radio Transmission Technologies for IMT-2000, 1997. Recommendation ITU-R M.1225. [34] Feasibility Study for OFDM for UTRAN Enhancement (Release 6), May 2004. 3GPP TR 25.892 V1.1.0. [35] A. Wang et al., “Dynamic resource management in the fourth generation wireless systems,” in Proc. IEEE Int. Conf. Commun. Technol., 2003, pp. 1095–1098.

Sangboh Yun (M’97–S’99–M’02) received the B.S., M.S., and Ph.D. degrees in electronic engineering from Korea University, Seoul, Korea, in 1994, 1998, and 2006, respectively. From 1994 to 2000, he was with Daewoo Telecom, Inc., as a research engineer. From 2000 to 2001, he was with NeoSolution, Inc., as a co-founder and CTO. He joined Samsung Advanced Institute of Technology, Kiheung, Korea, in 2001 and then transferred to Telecommunication R&D Center, Samsung Electronics Co., Ltd., Suwon, Korea in 2006. His current research interests include wireless communication systems, multi-hop relay, and radio resource management.

Seung Young Park (S’97–M’03) received the B.S., M.S., and Ph.D. degrees in electrical engineering from Korea University, Seoul, Korea, in 1997, 1999, and 2002, respectively. From April 2003 to December 2005, he was with Samsung Advanced Institute of Technology, Kiheung, Korea, where he was a Senior Engineer, working on several projects in the field of nextgeneration wireless mobile communications. From January 2006 to February 2007, he was with the School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN, where he was a Postdoctoral Research Associate. Since March 2007, he has been with the School of Information Technology, Kangwon National University, Chuncheon, Korea, where he is an Assistant Professor. His research interests include iterative detection, multicarrier systems, multiuser communications, and radio resource management.

Yeonwoo Lee received the M.S. and Ph.D. degrees from the Department of Electronics Engineering, Korea University, Seoul, Korea, in 1994 and 2000, respectively. From October 2000 to December 2002, he joined the Core 2 work of Mobile VCE program in the U.K. From October 2000 to December 2003, he was with the School of Electronics and Engineering, University of Edinburgh, Edinburgh, U.K., as a Research Fellow. From January 2004 to August 2005, he was with the 4G Mobile Communication team at the Samsung Advanced Institute of Technology, Kiheung, Korea, as a Senior Researcher. Since September 2005, he has been with the School of Information Engineering, Mokpo National University, Mokpo, Korea, as a Professor. His research interests include wireless multimedia mobile telecommunication systems, radio resource management, (ad hoc) multihop relay systems, sensor networks, and particularly, their applicable issues to 4G mobile communication systems and cognitive radio systems.


Daeyoung Park (S’00–M’05) received the B.S. and M.E. degrees in electrical engineering and the Ph.D. degree in electrical engineering and computer science, all from Seoul National University, Seoul, Korea, in 1998, 2000, and 2004, respectively. He was with Samsung Electronics as a Senior Engineer from 2004 to 2007, contributing to the development of next-generation wireless systems based on MIMO-OFDM technology. He is currently a Postdoctoral Researcher at University of Southern California, Los Angeles. His research interests include communication systems, wireless networks, multiuser information theory, and resource allocation.

Yungsoo Kim (M’01) received the B.S. degree in electronic engineering from Yonsei University, Seoul, Korea, in 1987, the M.S. degree in electrical engineering from the University of Wisconsin, Madison, in 1989, and the Ph.D. degree in electrical engineering from Korea Advanced Institute of Science and Technology, Daejeon, Korea, in 2000. He joined Samsung Advanced Institute of Technology, Korea, in 1991 and then transferred to the Telecommunication R&D Center, Samsung Electronics Co., Ltd., Suwon, Korea, in 2006. His current research interests are in 4G mobile and wireless communications.

Kiho Kim (S’88–M’91–SM’03) received the B.S. degree from Hanyang University, Seoul, Korea, in 1980 and the M.S. degree from the Korea Advanced Institute of Science and Technology, Daejeon, Korea, in 1982, both in electrical engineering, and the Ph.D. degree in electrical and computer engineering from the University of Texas, Austin, in 1991. From 1982 to 1987, he was with Korean Broadcasting System, where he developed the Korean Teletext (Data Broadcasting) system. In 1991, he joined Samsung Advanced Institute of Technology [which belongs to Samsung Electronics Co., Ltd., (SEC)], where he had been engaged in R&D of HDTV transmission (OFDM), ADSL and VDSL modem (DMT), and HDD/DVD read channel (PRML) technologies. Since 2000, he has been in charge of R&D of B3G mobile and nomadic radio interface technologies (MIMO-OFDM). He is currently the Senior Vice President in charge of the Next-Generation Mobile System Team at SEC. He currently serves as the Vice Chair (Asia) of Wireless World Research Forum. His R&D interests include mobile and wireless communication system and signal processing. Mr. Kim is currently a Senior Member of the Korean Institute of Communication Sciences and the Korea Institute of Electronics Engineers.

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Chung Gu Kang (M’96) received the B.S. degree in electrical engineering from the University of California, San Diego, in 1987 and the M.S. and Ph.D. degrees in electrical and computer engineering from the University of California, Irvine, in 1989 and 1993, respectively. While working on the Ph.D. dissertation from June 1991 to May 1992, he was also with the Aerospace Corporation, El Segundo, CA, as a PartTime Member of technical staff (MTS). In 1993, he joined Rockwell International Inc., Anaheim, CA, where he worked on the signaling system no. 7 and other telecommunication systems development. Since March 1994, he has been with the Department of Radio Communication and Engineering, Korea University, Seoul, Korea, where he is currently a Full Professor. In the academic year 2000–2001, he was a Visiting Associate Professor at the University of California, San Diego, where he was also affiliated with the Center for Wireless Communications. He is currently serving as an Editor of the Journal of Communication and Network and a member of the technical program committee of various international conferences. He is also a Vice Chairman of 2.3-GHz Portable Internet Project Group (PG305) and a Chairman of PG305-WG2 service and network working group of the Telecommunications Technology Association of Korea. His research interests include next-generation mobile radio communication system and mobile WiMAX networks, with special emphasis on physical layer/medium access control layer design and performance analysis. He has over 100 refereed publications in international journals and conference proceedings in the areas of communications network, CDMA cellular systems, OFDM systems, and wireless local area/personal area networks. He has been a consultant to wireless industries, including cellular service and content providers. His recent research is focused on the cross-layer design issues for MIMO/multiple-access schemes for mobile broadband wireless access systems and MAC/routing protocols for mobile ad hoc networks. Mr. Kang is a member of IEEE Communications, Information Technology, and Vehicular Technology Societies, and a member of the Korean Institute of Communication Sciences (KICS) and of the Korean Institute of Telematics and Electronics, having served as a Chairman of the KICS Mobile Communication Technical Activity Group.