Implementation Issues for OFDM-Based Multihop Cellular ... - CiteSeerX

SURAWEERA LAYOUT

8/21/07

3:45 PM

Page 74

TECHNOLOGIES IN MULTIHOP CELLULAR NETWORKS

Implementation Issues for OFDM-Based Multihop Cellular Networks Basak Can, Maciej Portalski, Hugo Simon Denis Lebreton, and Simone Frattasi, Aalborg University Himal A. Suraweera, Victoria University

ABSTRACT In this article we present various issues that need careful design for the successful implementation of OFDMA-based multihop cellular networks which need incorporation of relay terminals. The first issue we present is synchronization. We show that it is not a problematic issue for infrastructure-based relaying, where the relay is deployed by a system operator at strategic points in the cell. Second, we focus on the advantage of adaptive relaying and provide a frame structure to enable adaptive relaying in a cellular network operating according to the IEEE 802.16e standard. The third issue we present is related to hardware implementation aspects. Hardware performance and resource usage analysis will show that cooperative diversity schemes increase hardware resource usage and power/energy consumption at mobile terminals. The last issue we present is within the context of link layer ARQ, where we propose a novel retransmission method, named local retransmissionARQ (LR-ARQ), which is designed to take advantage of the multihop nature of the cellular network. Practically, we show that LRARQ improves performance with respect to its single-hop counterpart in terms of cell latency, goodput, and throughput.

INTRODUCTION

This work was supported by Telecommunication R&D Center, Samsung Electronics Co. Ltd., Suwon, Republic of Korea.

74

Cooperative wireless communication systems require the incorporation of relay terminals into conventional cellular networks and need multihop transmission due to the half duplex nature of wireless terminals. Such systems are referred to as multihop cellular networks and require careful design for successful implementation. Cooperative communication schemes can provide enhancements in terms of end-to-end throughput even if they require additional expenditure of radio resources arising from the need for multihop transmissions. All these schemes necessitate a two-phased (i.e., two-hop) communications as the relay station (RS) needs to be informed of the signals that are transmitted by a source terminal. In this study we consider downlink transmissions assisted by a given RS that is assumed to be deployed by a system operator.

0163-6804/07/$20.00 © 2007 IEEE

The cooperative schemes adopted in multihop cellular networks include[1]: Cooperative multiple-input multiple-output (MIMO): The mobile station (MS) and RS listen to the transmission of the base station (BS) during the first phase. In the second phase, both BS and RS transmit simultaneously using the same radio resource. Hence, cooperative space-time coding can be used. If the MS can combine the received signals during the first and second phase appropriately (e.g., with space-time decoding [STD]), it can benefit from cooperative diversity. Cooperative multiple-input single-output (MISO): Only the RS listens to the transmission of the BS during the first phase. In the second phase, both BS and RS transmit simultaneously using the same radio resource. Hence, cooperative space-time coding can be used. If the MS can combine the received signals from the BS and RS appropriately (e.g., with STD), it can benefit from cooperative transmit diversity. Cooperative single-input multiple-output (SIMO): The MS and RS listen to the transmission of the BS during the first phase. In the second phase, only the RS transmits (i.e., it relays the signals it has received during the first phase). If the MS can combine the received signals during the first and second phase appropriately (e.g., with maximum ratio combining [MRC]), it can benefit from cooperative receive diversity. Conventional relaying: In the first phase of conventional relaying, the RS receives the transmissions of the BS that are destined to a given MS. In the second phase, the RS simply forwards to the MS the signals it has received during the first phase. This scheme provides only path loss savings, whereas cooperative schemes providediversity gain as well. Spectral efficiency and robustness against multipath impairments are two major advantages of orthogonal frequency-division multiplexing (OFDM). When applied in OFDM-based wireless networks, cooperative schemes can be used at each subchannel comprising several contiguous or frequency diverse subcarriers. Direct communication without relay intervention is referred to as the w/o relay scheme. In frequency selective environments, subcarriers of a properly designed OFDM network experience fiat fading with different amplitudes. Therefore, it would be

IEEE Communications Magazine • September 2007

SURAWEERA LAYOUT

8/21/07

3:45 PM

Page 75

beneficial for relay terminals operating in OFDM-based networks to use the best forwarding and relaying scheme at each subcarrier. Identification of the best scheme can be based on channel state information(CSI). Incorporation of OFDM into wireless relaying brings the following advantages: • To relay or not can be decided for each subchannel. • The best forwarding and relaying scheme can be chosen for each subchannel. Such a scheme is referred to as adaptive relaying. Multihop networks, also referred to as wireless relay networks, involve three main links that constitute the end-to-end path: source to relay (S → R), relay to destination (R → D), and source to destination (S → D). Hence, the endto-end performance should be the key criterion considered in the design of wireless relay networks. Relaying should be used only if it can improve the end-to-end throughput. Adaptive relaying can further improve the performance of conventional cellular and multihop cellular networks where one type of relaying scheme is always used [2, 3]. The orthogonal frequency-division multiple access (OFDMA)-based IEEE 802.16e standard is developed for providing broadband coverage for mobile users in single-hop wireless metropolitan area networks [4]. This standard is also referred to as mobile WiMAX. The emerging IEEE 802.16j standard will extend the IEEE 802.16e standard by enabling multihop transmissions. In this study the critical design issues for successful extension of IEEE 802.16e to IEEE 802.16j are presented. These design issues are: Synchronization: This issue is critical in order to achieve the potential throughput offered by the system. Adaptive relaying: The careful design of adaptive relaying is critical in order to efficiently use the radio resources available in the system. If adaptive relaying is designed properly, the throughput delivered by the system can be optimized. Hardware implementation aspects: This issue is critical for real-time operability and the hardware requirements of the mobile terminals served by a multihop cellular network. Hardware related issues should be jointly considered for the design of adaptive relaying. Local retransmission automatic repeat request (LR-ARQ): The design of LR-ARQ is crucial to take advantage of the multihop nature of the cellular network. Once designed properly, LRARQ can improve performance over that of its single-hop counterpart in terms of cell latency, goodput, and throughput.

RELATED WORK In this section related work on OFDM-based multihop cellular networks is presented. In [5] a new hybrid forwarding scheme for OFDM-based relaying is proposed. At the relay, this scheme adaptively chooses to switch between amplify and forward (AF), decode and forward (DF), or “no relaying” modes depending on the instantaneous signal-to-noise ratio (SNR) conditions observed in the source-to-relay, relay-to-

IEEE Communications Magazine • September 2007

destination, and source-to-destination links. In order to minimize the bit error rate (BER), the decisions on the modes are made on a per subcarrier basis. Subcarrier rearrangement at the relay has the potential to increase the OFDM system capacity. In [6] such an adaptive a relaying technique suitable for implementation in future-generation mobile networks is presented. In this technique the relay estimates the S → R and R → D channels, and then rearranges the strongest received subcarrier to the strongest subcarrier of the (R → D) link, the second strongest received subcarrier to the second strongest subcarrier of the (R → D) link, and so on. In [7] a two-way DF-based relay protocol for the emerging IEEE 802.11n standard is developed in the form of network coding. The authors conclude that their two-way DF scheme is capable of improving the capacity of IEEE 802.11n systems. In [8] a new equalization scheme based on space-time block codes for distributed MIMOOFDM has been developed and diversity order analysis is provided. As the related work shows, critical design issues for the successful implementation of OFDM-based multihop cellular networks are not well covered in the literature. Such issues are presented in this article.

Relaying should be used only if it can improve the end-toend throughput. Adaptive relaying can further improve the performance of conventional cellular networks and conventional multi-hop cellular networks where one type of relaying scheme is always used.

SYNCHRONIZATION ISSUES FOR OFDMA-BASED MULTIHOP CELLULAR NETWORKS In this section the synchronization issues for downlink (DL) transmissions in OFDMA-based two-hop cellular networks are presented. Infrastructure-based relay terminals deployed by a system operator to be used exclusively for relaying are considered. In conventional multi-antenna schemes where the transmit and receive antennas are collocated at a given terminal, signals that are simultaneously transmitted by the transmitter antennas arrive at the receiver simultaneously since they are transmitted from the same terminal. However, in cooperative transmission schemes where the signals transmitted by the BS and RS are supposed to be received simultaneously at the receiving terminal, the receiver might experience a time offset between the signals received from the BS and RS. In order to have the most robustness to this time offset problem, the MS should align its discrete Fourier transform (DFT) window with the earliest arriving link when the data transmissions start simultaneously from the BS and RS. Such alignment is considered in this section. Let ∆ represent the time offset in seconds. For OFDM-based networks, the time offset does not cause any throughput degradation if |∆| +5σ rms < τ CP , where τ CP represents the cyclic prefix (CP) duration and σrms represents the root mean square (rms) delay spread of the channel [9]. If |∆| + 5σ rms > τ CP, intersymbol interference (ISI) occurs with power directly proportional to: • The signal-to-interference-plus-noise ratio

75

SURAWEERA LAYOUT

8/21/07

3:45 PM

Page 76

1 OFDM symbol

The time offset problem does not cause significant CP

degradation on the

Original downlink transmission timing of the BS and RS

system throughput achieved with

Downlink transmission timing of the BS

adaptive relaying scheme provided that the RS

Downlink transmission timing of the RS

introduces an δ, the delay introduced by the relay

appropriate delay in its transmissions.

Received signal at the MS from the RS (RS ->MS)

∆RD

Received signal at the MS from the BS (BS ->MS)

∆SD

DFT window timing at the MS: aligned from the early arriving link

∆, timing offset ∆ < 0 case: signals from the BS arrive first

■ Figure 1. The time offset problem and the alignment of the DFT window at the MS in order to achieve the most resilience against the time offset problem. (SINR) condition in the RS → MS link (if the signals from the RS arrive after the signals from the BS) or in the BS → MS link (if the signals from the BS arrive after the signals from the RS) •

∆ + 5σ rms −τ CP Tsymb

1

64-QAM is the highest rate modulation mode used in IEEE 802.16e.

76

,

where we assume that the DFT window is aligned with the earliest arriving signal. T symb represents the OFDM symbol duration. If only one terminal is allowed to transmit in the second phase (either the BS or the RS), the time offset problem does not occur; however, in this case the radio resources are not used efficiently; for example, the relay is always used in the second phase even when it does not bring any performance improvement. These time offset issues are visualized in Fig. 1, where ∆RD and ∆SD represent the propagation delay in the R → D and S → D links, respectively. For the analysis presented in this section, the following system setup has been considered. The effective isotropic radiated power (EIRP) transmitted from the BS and RS are PBS = 57.3 dBm [4] and PRS = 47.3 dBm, respectively. The BS is at the center of a cell with radius 9.8 km. The RS is placed at a distance of 4.3 km from the BS (i.e., d SR = 4.3 km).With the current system setup, the reasons for this placement are the following. First of all, at this position it is possible to detect 64-quadrature amplitude modulation (QAM) symbols1 with negligible error rate at the RS. Hence, the benefits of relaying can be

exploited efficiently. Second, this is a far enough distance such that within the coverage of a given relay, the transmissions with the relay improve the performance over w/o relay. The time offset effect is analyzed within the coverage of the relay, which has a radius of 5.5 km. A single relay is assumed for the analysis presented in this section. We observe in Fig. 2 that when the RS introduces a delay (i.e., δ samples) to its transmissions, the difference between SINR (including the interference caused by ISI, i.e., time offset) and SNR (when the interference caused by the time offset problem is not considered) is less than 1 dB.To achieve this value, the MS should align its DFT window from the earliest arriving link as depicted in Fig. 1. Consequently, the gain in throughput comes at the cost of a loss of as much as 1 dB in SNR only at very close distances to the RS. Indeed, over this region, conventional relaying can be used since the SNRs of the RS → MS links are very strong compared to BS → MS links. Hence, a loss of just 1 dB in SNR does not cause significant degradation. In summary, the time offset problem does not cause significant degradation of the system throughput achieved with adaptive relaying provided the RS introduces an appropriate delay in its transmissions. Evaluations of cell throughput while considering the time offset problem should be investigated. The amount of necessary delay depends on the position of the RS in the cell and the transmit power difference between the BS and RS. In conventional multi-antenna schemes, the

IEEE Communications Magazine • September 2007

8/21/07

3:45 PM

Page 77

signals received at the receiving antennas suffer from a single carrier frequency offset (CFO): the CFO caused by the local oscillator (LO) mismatches between the transmitter and receiver terminals. For relaying schemes where simultaneous transmissions from the BS and RS are needed, the MS suffers from two distinct CFO effects simultaneously. 2 One results from the CFO in the BS → MS link and the other from the CFO in the RS → MS link. It is not possible for the MS to compensate for the CFO of both links at the same time. For infrastructure-based relay terminals, the CFO problem can be solved if the RS estimates its carrier frequency mismatch with the carrier frequency of the BS and compensates for this offset before its transmissions. 3 This way, the MS sees only one CFO caused by the LO mismatch between its own LO and the LO of the BS. Such offset can be compensated by using existing CFO compensation algorithms used in single-hop cellular networks. The following synchronization problems are identified for the UL transmissions in OFDMAbased two-hop cellular networks. Assuming that the infrastructure-based relay terminals can align their carrier frequency with that of the BS, the CFO problem in the UL of two-hop cellular networks converges to the problems in single-hop cellular networks. Regarding the time offset problem in the UL, the relays can be treated as users. Hence, the existing solutions for the time and CFO problems in the UL of single-hop cellular networks can be used for the synchronization problems in the UL of two-hop cellular networks.

A FRAME STRUCTURE TO ENABLE RELAYING IN OFDMA-TDD-BASED CELLULAR NETWORKS In Fig. 3 a frame structure is presented as a possible solution to enable adaptive relaying per sub-channel in an OFDMA time-division duplex (TDD)-based cellular network, where the users are within the coverage area of the BS, and some of them are in the coverage areas of both the BS and RS. IEEE 802.16e is taken as a reference [4]. Adaptive relaying necessitates the CSI for each subchannel (if adaptation is made for each subchannel) and for each user. In order to have centralized control, this information should be fed back by the users to the BS. The CSI is obtained at the BS at the end of each UL subframe via the fast feedback channel (i.e., channel quality indicator channel [CQICH]). At the end of the preamble transmission, the BS schedules the users per subchannel while considering the end-to-end throughput. Afterward, the BS broadcasts in the DL-MAP which user is scheduled on which subchannel and, for each subchannel, which scheme (with or w/o relay) is used. At the end of the transmission of this control information, the data transmissions start. Since we need two phases in order to enable relaying, a guard interval (GI) is needed between the first phase (where the RS is overhearing) and the second (where the RS is in transmission mode if relaying is decided on at least one subchannel). The earliest arriving signal from one of the two links, BS → MS and RS → MS, can be determined via the help of the proposed pream-

IEEE Communications Magazine • September 2007

PBS-PRS=10 dB, RBS=9.8 km, RRS=5.5km, dSR=4.3km SINR(dB)-SNR(dB) per position where σrms=22 samples, CP=256 samples, δ=80 samples 10 8

-0.1

6

-0.2

4 -0.3 Distance, km

SURAWEERA LAYOUT

2 -0.4

0

-0.5

-2 -4

-0.6

-6

-0.7

-8

-0.8

-10 -10

-5

0 Distance, km

5

10

■ Figure 2. The SINR (dB) – SNR (dB) per position (over the region where time offset causes ISI) within the coverage area of the BS and RS where the RS transmits its signals with a delay of 80 samples as compared to the instant when the BS starts its transmissions.

ble structure, which is depicted in Fig. 3. The preamble transmissions of the BS and RS are done on different OFDM symbols. This way, each MS can determine the relative arrival time difference (i.e., the time offset) of the signals transmitted simultaneously by the BS and RS by measuring the relative arrival times of the preamble symbols. Each MS can align the DFT window from the earliest arriving link. Furthermore, the change in the received signal power when both signals transmitted by the BS and RS arrive at the MS can help determine the earliest arriving signal at a given MS.

HARDWARE IMPLEMENTATION ASPECTS OF COOPERATIVE RELAYING Adding cooperative diversity to a wireless communication system introduces additional signal processing at all network nodes involved in the transmission to the w/o relay case. This increases the overall hardware complexity of the utilized communication devices. A limited number of hardware implementations involving cooperative diversity can be found in the literature [10]. These examples show the practical feasibility of using relay nodes in single-carrier systems based on commodity hardware. However, in OFDMbased wireless relay networks the increase in processing workload and hardware complexity can present a greater challenge due to the need for processing multiple subcarriers/subchannels independently. This issue is extremely critical in MS design due to limited hardware resources and the need for high power efficiency at those terminals. In this section hardware requirements for cooperative diversity methods are evaluated by measuring the usage of hardware resources at

2

We assume that the effects of the CFO in the BS → RS link in DL and in the RS → BS link in uplink(UL) transmissions are compensated perfectly. This can be achieved with infrastructure-based relays and existing CFO compensation algorithms in single-hop cellular networks. 3

This estimation can be done with high accuracy since line-of-sight conditions in the BS → RS links can be achieved with infrastructure-based relay terminals.

77

SURAWEERA LAYOUT

8/21/07

3:45 PM

Page 78

OFDM symbol index

NS

BS->RS, Ms4

First phase Downlink subframe

w/o relay BS->Ms5 BS->Ms2 RS->MS2 BS->Ms4 RS->MS4 Second phase

UL transmissions

BS->Ms1 RS->MS1 BS->Ms3 RS->MS3

Guard

Preamble and control data of the RS

BS->RS, Ms2

Guard (needed for the BS to switch from Rx to Tx mode)

BS->Ms5

Guard (needed for the RS/BS to switch from Rx/Tx to Tx/Rx mode)

DL-MAP UL-MAP

w/o relay

FCH

s

Preamble BS

Sub-channel logical index

3

2

BS->RS, Ms3

1

2

Ndl

BS->RS, Ms1

1

2

1

1

Fast feedback (CQICH)

Uplink subframe

■ Figure 3. The frame structure as a possible solution to enable adaptive relaying in an OFDMA-TDDbased cellular relay network where the users are within the coverage of the BS and RS.

Symbol processing time (µs)

1200 1000

Sequential processing Parallel processing

800

354%

251%

Percentage of %% - processing time as compared to without relay case (separately for sequential and parallel execution)

262%

600 400 100% 200 102,9

100%

100% 100%

121%

74%

74%

IEEE 802.16e symbol duration

0 Without relay

Conventional relaying

Cooperative Cooperative Cooperative SIMO MIMO MISO

Scheme

■ Figure 4. Execution times of corresponding EQ, MRC, and STD algorithms implemented on an FPGA chip in a sequential version and one involving parallel computations.

the MS. Hardware performance is evaluated as well by estimating the time needed to process a single OFDM symbol (i.e., symbol processing time) for each considered scheme. Such a performance measure is independent of the frame structure and enables the analysis of real-time operability by comparing it to the OFDM symbol duration considered in the system. The system model considered in this section includes the reception of over 2048 subcarriers. This is the most demanding case in terms of hardware requirements for IEEE 802.16e-based systems. For all the transmission schemes presented earlier, Fig. 4 shows the execution times of the algorithms using signal equalization (EQ), MRC, and STD at the MS during DL reception.

78

It is assumed that the cooperative MIMO and cooperative MISO schemes use space-time coding to provide spatial diversity to the MS. During signal detection at the MS, the MRC is used for the cooperative SIMO scheme and STD for the cooperative MIMO and MISO schemes. Channel EQ is used for all of the schemes. For all the schemes, evaluations are performed on hardware models implemented by field programmable gate array (FPGA) architecture with the system clock frequency fixed at 50 MHz. For sequential operation execution, a gradual increase in processing time can be observed with increasing computational complexity of the schemes. This can lead to difficulties in maintaining real-time operability of devices due to the symbol processing time significantly exceeding the symbol duration, as seen in Fig. 4. This challenge can easily be overcome by introducing efficient parallel computations (e.g., in complex number and matrix operations). In such a case symbol processing time can even be reduced for cooperative schemes compared to the w/o relay case. For example, for the cooperative MIMO and cooperative MISO schemes, which use space-time coding at the transmitters, OFDM symbols are processed in pairs at the receiver. This, in combination with arithmetical operations performed in parallel, results in lower symbol processing time than in the w/o relay case. However, high performance in this case is achieved at the cost of increased hardware complexity. Figure 5 shows the processing resource and random access memory (RAM) utilization for the EQ, MRC, and STD hardware blocks used for the realization of the transmission schemes described earlier. Parallel processing blocks are

IEEE Communications Magazine • September 2007

3:45 PM

Page 79

considered for this evaluation. Processing resource utilization is given by the total number of FPGA basic blocks and digital signal processing (DSP) blocks used. The total number of DSP blocks used is multiplied by 10 Such weighting is chosen due to the fact that the number of DSP blocks on a single FPGA chip is highly limited. Therefore, the utilization of these elements should be considered critically. As seen in Fig. 5, processing resource usage depends on the realized transmission scheme and increases with the complexity of the implemented relaying scheme. Initial estimates using FPGA vendor tools also indicate that due to the increased resource usage, dynamic power consumption is higher for transmission schemes involving more processing and RAM usage. On the other hand, this does not directly depend on the algorithm complexity, but rather on the specific properties of the transmission schemes. For example, to achieve cooperative SIMO and MIMO, buffering of the symbols received during the first transmission phase is needed. This results in increased RAM usage over the other schemes. The analysis of hardware resource usage shows that cooperative diversity can bring advantages at the cost of increased complexity. Consequently, hardware related metrics should be taken into consideration when selecting a transmission scheme suitable for a specific application. This is crucial especially with resource-constrained terminals.

LINK LAYER ARQ PROTOCOLS FOR MULTIHOP CELLULAR NETWORKS In order to enhance the reliability of the wireless link, retransmission protocols have been widely adopted in current wireless systems (third generation [3G], WiMAX, etc.). Usually, they assume that lost packets within a cell (i.e., packets that are not received correctly at an MS) have to be retransmitted from the corresponding BS based on the ARQ protocol. In this case MSs suffering from continuously bad channel conditions can experience large latencies to receive packets correctly because of the repeated retransmissions performed by the BS. The latter can dominate overall traffic, thus reducing the total throughput in the cell. Although the use of relays has been extensively proposed in the literature,4 retransmission protocols for multihop networks with cooperative fixed RSs have found little attention so far. Indeed, to the best of our knowledge, there has been no proposal for a procedure that would enable packets to be retransmitted not only from the BS but also locally from an RS. Note, in fact, that conventional retransmission protocols are designed for point-to-point (BSMS) links and thus cannot be applied as is to the multihop case. In this section we propose a retransmission method called LR-ARQ in which we assume that the RSs deployed by the operator in the cell support the BS in case of retransmissions. Basically, each RS attempts to decode the packets addressed to the MSs in its range. For instance, this can be done by making an RS overhear/ decode the subcarriers allocated to the MSs in its range during the firstst phase of the downlink

IEEE Communications Magazine • September 2007

7633%

4500

80,000

4000

6567% 70,000

3500 235% 3000 2500

40,000

132% 100%

60,000 50,000

172%

2000 1500

90,000

30,000

100%

1000

Total RAM usage (bytes)

8/21/07

Total processing resource usage

SURAWEERA LAYOUT

20,000

1233%

10,000

500 100%

100% 0

0 Without relay

Conventional Cooperative Cooperative Cooperative relaying SIMO MISO MIMO

Percentage of processing resources used as compared to without relay case

Percentage of RAM used as compared to without relay case

Scheme

Processing resource type: DSP blocks (scaled 10x) Registers ALUTs

RAM buffer content: Second phase symbols First phase symbols Channel coefficients

■ Figure 5. Processing resource and memory utilization for EQ, MRC, and STD hardware blocks realized in FPGA architecture to support various transmission schemes developed for OFDM-based wireless relay networks.

subframe (Fig. 3). When a packet is lost (e.g., a negative acknowledgment [NACK] is received at the BS from a certain MS in the uplink subframe), the BS may order a certain RS in the upcoming DL-MAP to perform the needed retransmission locally by exploiting the second phase of the downlink subframe. Due to the strategic placement of the RSs in the cell, it is in fact very likely that a short-range link (BS-RS) would have a better channel condition than a long-range one (BS-MS). Therefore, there is a higher probability that an RS receives a packet correctly even if it is lost at the destination MS. In general, we can expect the following advantages of local over conventional retransmission: • The higher modulation level to be possibly used on an RS-MS link can decrease the overall packet transmission time. • The higher reliability of a short-range link can diminish the number of iterated retransmissions to be performed. • The shorter range of an RS-MS transmission can reduce intracell interference. In conclusion, we can derive that a local retransmission has the potential to decrease cell latency while increasing cell goodput, throughput, and capacity. Computer simulations are carried out to compare the performance of LR-ARQ and conventional ARQ. Simulated cells of variable radius (1 km ≤ r1 ≤ 7 km) are considered, where a hexagonal test cell is surrounded by six neighboring cells. In each cell we consider six RSs located on the diagonals of the hexagons and having a coverage area of 100 m with respect to the cell edge. Specifically, the serving RS is placed on the right end of the x axis of the first cell and embraces 32 MSs uniformly generated around it.

4

The IEEE 802.16 standardization body has formed the IEEE 802.16 Relay Task Group, which is developing a draft under the PAR 802.16j: "Amendment to IEEE Std 802.16 on Mobile Multihop Relaying" [11].

79

1000 800 600 400 200 0

Outage users

Cell throughput (Mb/s)

Cell goodput (Mb/s)

Cell latency (ms)

SURAWEERA LAYOUT

8/21/07

3:45 PM

Page 80

ARQ LR-ARQ

1

2

3

6

4 5 Cell radius (km)

6

7

8 ARQ LR-ARQ

4 2 0

1

2

3

4 5 Cell radius (km)

6

7

ARQ LR-ARQ

10

25 20 15 10 5 0

8

FUTURE RESEARCH AREAS

15

5 0

1

2

3

4 5 Cell radius (km)

6

7

8

2

3

4 5 Cell radius (km)

6

7

8

ARQ LR-ARQ

1

■ Figure 6. Performance metrics comparison of ARQ and LR-ARQ vs. cell radius.

All MSs are assumed to move with a pedestrian speed v =3 km/h and have a best effort (BE) traffic type (traffic rate R = 25 kbytes/s and fixed packet size S = 66 bytes), which is modeled with a Poisson distribution with number of occurrences per time unit equal to λ = Simulation_Time/Average_Interarrival_Time, where Average_Interarrival_Time = S/R. Therefore, in the time period (t, t + τ], where τ is the frame duration, the packets being queued are derived from the Poisson distribution with parameter λτ. The OFDMA parameters, link adaptation table, and other system-specific parameters are taken from [4, 12, 13]. According to the current adaptive modulation and coding (AMC) level and the instantaneous SINR at the receiver, the packet error rate (PER) is calculated using the following formula: PER =1 – (1 – BER) b, where b is the total number of bits in the packet, and coded BER curves are recreated by curve fitting in MATLAB from the ones obtained using an additive white Gaussian noise (AWGN) channel in [14]. Specifically, the SINR is calculated by considering intercell interference Gaussian distributed. The propagation model, which includes the path loss and log-normal shadowing, is taken from [15, 16].Finally, we have used the tapped delay line model in [17] to simulate the multipath fading channel. From Fig. 6, it is observed that: • LR-ARQ outperforms standard ARQ. Indeed, since the PER (BS-MS) increases when the cell radius increases, in order to receive a certain packet correctly with standard ARQ, the BS has to retransmit it a crescent number of times. With LR-ARQ, instead, since PER (RS-MS)