KWON LAYOUT
11/17/05
12:07 PM
Page 136
CROSS-LAYER DESIGN
Design and Implementation of a Simulator Based on a Cross-Layer Protocol between MAC and PHY Layers in a WiBro Compatible IEEE 802.16e OFDMA System Taesoo Kwon, Howon Lee, Sik Choi, Juyeop Kim, and Dong-Ho Cho, Korea Advanced Institute of Science and Technology Sunghyun Cho, Sangboh Yun, Won-Hyoung Park, and Kiho Kim, Samsung Advanced Institute of Technology
ABSTRACT In this article we propose cross-layer design frameworks for 802.16e OFDMA systems that are compatible with WiBro based on various kinds of cross-layer protocols for performance improvement: a cross-layer adaptation framework and a design example of primitives for cross-layer operation between its MAC and PHY layers. In addition, we provide a simulation framework for cross-layer analysis between the MAC and PHY layers in 802.16e systems. Through this cross-layer simulator, we show that average cell throughput can be improved by 25–60 percent by applying careful cross-layer adaptation schemes.
INTRODUCTION The IEEE 802.16 family of standards specifies the air interface of fixed and mobile broadband wireless access (BWA) systems that support multimedia services. A WiMAX system is one based on technologies of this family, sponsored by an industry consortium called the WiMAX Forum. The IEEE 802.16-2004 standard, which was also previously called 802.16d or 802.16-REVd, was published for fixed access in October 2004. Good overviews of the standard can be found in [1, 2]. The standard has now been updated and extended to the 802.16e standard for mobile access, Mobile WiMAX, as of October 2005. In Korea, HPi, which means high-speed portable Internet, was first conceived as a Korean technology standard to provide better data handling than that of the third-generation (3G) cellular system before fourth-generation (4G) systems
136
0163-6804/05/$20.00 © 2005 IEEE
arrive, but was renamed WiBro, which means wireless broadband, once it adopted 802.16e as its standard for global harmonization. WiBro is designed to provide all-IP and packet services, such as streaming video and music, video and music on demand, online gaming, and broadcasting, over the 2.3 GHz spectrum at ground speeds up to about 60 km/h, and is currently compatible with the 802.16e orthogonal frequency-division multiple access (OFDMA) system with a 1024 fast Fourier transform (FFT) size. WiBro services are expected to be launched commercially by two Korean operators in the middle of 2006. Once the launch is successful in Korea, it may prompt the successful launching of WiMAX services worldwide. The 802.16 standard defines the specifications related to the convergence sublayer (CS), medium access control (MAC) layer, and physical layer (PHY). Its PHY supports four physical modes: WirelessMAN-SC for any applicable frequencies between 10 and 66 GHz, WirelessMAN-SCa for licensed frequencies below 11 GHz, WirelessMAN-OFDM for orthogonal frequency-division multiplexing, and WirelessMAN-OFDMA. In this article we mainly discuss the WirelessMAN-OFDMA system because it is compatible with the WiBro system. With respect to implementation, we need to design interlayer operations carefully. With this in mind, we present various cross-layer protocols for performance improvement and a design example of primitives for cross-layer operation between MAC and PHY layers, after first introducing a logical PHY frame structure for 802.16 OFDMA systems. Moreover, to verify the performance improvement of the cross-layer
IEEE Communications Magazine • December 2005
KWON LAYOUT
11/17/05
12:07 PM
Page 137
DL PUSC (mandatory)
FUSC OFUSC, or AMC
UL PUSC (mandatory)
OPUSC or AMC Time
k+M
k+3
k+2
k
k+M+1
k+N Burst 3
UL-MAP
CQICH, ACK CH fast feedback CH
FCH
s s+1
Burst 4 Burst 2 Burst 3
DL-MAP
Preamble
Burst 1
Burst 2
Burst 5
Burst 2 Ranging
Subchannel logical number
k+1
OFDMA symbol number
Burst 1 s+L DL subframe TTG Slot
Slot
Slot
Slot
PUSC: Partial usage of subchannels FUSC: Full usage of subchannels OP(F)USC: Optional P(F)USC AMC: Adaptive modulation and coding
Slot
Burst 2
UL subframe
RTG
One subchannel DL PUSC: two OFDMA symbols DL FUSC: one OFDMA symbol UL PUSC: three OFDMA symbols DL/UL AMC: two, three or six OFDMA symbols
FCH: Frame control header CQICH: Channel quality information channel TTG: Transmit/receive transition gap RTG: Receive/transmit transition gap
n Figure 1. Example of IEEE 802.16 TDD frame structure. framework presented in this article, we present a framework for designing a simulator for crosslayer analysis between the MAC layer and PHY in a WiBro or 802.16e system, and discuss its performance.
FRAME STRUCTURE OF 802.16 OFDMA SYSTEMS IEEE 802.16 systems can support time-division duplexing (TDD) and frequency- division duplexing (FDD). The frame can be composed of several zones that are divided according to subcarrier allocation methods or MIMO modes. Figure 1 shows an example of an IEEE 802.16 TDD frame structure, which is also a model frequently referred to when constructing WiBro systems that support only TDD. In an FDD frame structure, the downlink (DL) and uplink (UL) subframes are allocated in a different frequency band without guard time such as TTG and RTG. The frame structure consists of the following: a preamble using the first symbol, FCH with fixed size for resource allocation of the DL PUSC zone and DL_MAP, DL_MAP and UL_MAP messages for resource allocation of DL and UL data bursts, DL/UL data bursts for data or control messages, and UL control channels for ranging, UL acknowledgment (ACK), and CQI feedback.
IEEE Communications Magazine • December 2005
The DL_MAP can present resource allocation information for each burst or each user. Presenting the MAP information for each burst decreases the number of DL_MAP IE messages, but causes processing overhead because a mobile station (MS) needs to find its own packet among many packets concatenated into a burst. Presenting for each user can allocate resources to each user effectively, but causes considerable overhead due to the transmission of many DL_MAP IE messages. So the 802.16 system defines various MAP messages, such as compressed and compact MAP, that reduce the size of MAP messages or allocate resources effectively for each user. To support various types of physical channel condition, IEEE 802.16 OFDMA systems define two types of subchannel building method: the distributed subcarrier permutation mode (PUSC, OPUSC, FUSC, or OFUSC mode in Fig. 1) and the adjacent subcarrier permutation mode (AMC mode in Fig. 1). The ratio of these modes can be flexible in the IEEE 802.16 standard. However, one burst for data transmission consists of several slots, and one slot is the minimum possible data allocation unit. In addition, the definition of this slot depends on the OFDMA symbol structure, which varies for DL and UL, for FUSC and PUSC, and for distributed subcarrier permutations and adjacent subcarrier permutation, as shown in Fig. 1.
137
KWON LAYOUT
11/17/05
12:07 PM
Page 138
DIVERSITY SUBCHANNELS In terms of maximizing the system throughput, max CIR is the best scheduling scheme such that subcarriers are allocated for only users with the best channel conditions. However, for the MAC layer, required QoS as well as system throughput should be satisfied.
The distributed subcarrier permutation mode is a very useful scheme for averaging intercell interference and avoiding deep fading by selecting subcarriers pseudo-randomly. Therefore, it is expected to be suitable for users with high velocity and/or low signal-to-interference-plus-noise ratio (SINR). Basic resource units in the frequency domain of this mode are called diversity subchannels.
BAND AMC SUBCHANNELS In adjacent subcarrier permutation mode, adjacent subcarriers are grouped into clusters and are allocated to users. In this channel structure, the channel response can be seen as a flat fading channel. Thus, frequency selectivity of the channel cannot be exploited. Due to the flat fading nature of this sub-channel, the system can make better use of multiuser diversity as long as the channel state does not change significantly during the scheduling process. Therefore, it is expected to be suitable for users with low velocity and/or high SINR. Basic resource units in the frequency domain of this mode are called band AMC subchannels.
CROSS-LAYER PROTOCOL DESIGN BETWEEN 802.16E MAC LAYER AND PHY The 802.16e standard provides a number of UL control channels for the fast exchange of information for cross-layer operation. We first introduce some UL control channels, and then present: • A cross-layer adaptation framework for interlayer operation between MAC and PHY layers • A design example of primitives for exchanging PHY layer information for cross-layer protocol operation We also discuss cross-layer issues regarding the hybrid automatic repeat request (HARQ) protocol as another important link adaptation technique for 802.16e systems.
UPLINK CONTROL CHANNELS FOR A CROSS-LAYER PROTOCOL We can design cross-layer protocols efficiently and improve system performance by carefully utilizing the uplink control channels to exchange cross-layer information such as physical channel information and ACK/negative ACK (NACK) for HARQ. CQICH — The channel quality information channel (CQICH) is allocated to an MS using a CQICH control IE, and is used to report the DL carrier-to-interference-plus-noise ratio (CINR) for either diversity subchannels or band AMC subchannels. This channel occupies one UL slot in the FAST-FEEDBACK region allocated through UL_MAP message. For diversity subchannels, the MS reports the average CINR of the BS preamble from which the BS is able to
138
determine the DL modulation and coding scheme (MCS) level. Here, a CINR measurement is quantized into 32 levels and encoded into five information bits. On the other hand, for band AMC subchannels, a mobile station (MS) can report the differential of CINR values of five selected frequency bands (increment: 1 and decrement: 0 with a step of 1 dB) on this CQICH after reporting the CINR measurements of the five best bands using a MAC management message such as REP-RSP. Fast Feedback Channels — Fast feedback channels may be allocated individually to MSs for the transmission of PHY-related information that requires a fast response from the MS. One fast feedback channel occupies one UL slot in the FAST-FEEDBACK region allocated through a UL_MAP message. Using these fast feedback channels, the MS can report the followings: • Variable information for MAC operation, such as the anchor BS selection information for macro diversity handover and the request for UL rate adaptation of VoIP service • PHY-related information, such as DL channel measurement information for multipleinput multiple-output (MIMO )operation, the MIMO coefficient for the best DL reception (e.g., antenna weight), and MIMO mode selection (e.g., space-time transmit diversity [STTD], spatial multiplexing [SM], and beamforming). UL ACK Channel — A HARQ ACK channel region for the inclusion of one or more ACK channel(s) for HARQ support of MSs is allocated using a HARQ ACK region allocation IE. The UL ACK channel occupies one half-slot in this HARQ ACK channel region, which may override the fast feedback region. This UL ACK channel is implicitly assigned to each HARQenabled burst according to the order of the HARQ-enabled DL bursts in the DL MAP. Thus, the MS can quickly transmit ACK or NACK feedback for DL HARQ-enabled packet data using this UL ACK channel. UL Sounding — The 802.16e OFDMA system defines UL sounding to support smart antenna or MIMO, and this UL sounding is a kind of UL pilot signal. The BS measures the UL channel response from UL sounding waveforms transmitted by each MS, and translates the measured UL channel response to an estimated DL channel response under the assumption of TDD reciprocity. In order to allocate resources for the transmission of UL channel sounding, the BS allocates a sounding zone through a UL_MAP message. In this sounding zone each MS can transmit its UL sounding signal, maintaining signal orthogonality among multiple multiplexed MS sounding transmissions.
CROSS-LAYER ADAPTATION FOR EFFICIENT RESOURCE ALLOCATION In OFDM systems [3] the channel gains of subcarriers are quite different, due to the frequency selectivity of the channel. In terms of maximizing
IEEE Communications Magazine • December 2005
KWON LAYOUT
11/17/05
12:07 PM
Page 139
In the proposed model, the MAC layer contains a user User 1
User 2
User 3
User N-1
grouper, scheduler,
User N
and resource
MAC-d flows MAC-c flow (MAC control information)
User grouper
controller. Each functional entity exploits PHY information to
Diversity user group
increase system
AMC user group
throughput. The physical layer consists
Scheduler
of a diversity channel PPDU controller,
Diversity user data QoS classes
AMC channel PPDU
AMC user data
Best effort
QoS classes
information PHY control information
Resource controller Diversity channel Resource controller
controller, control
Best effort
AMC channel Resource controller
controller, and HARQ functional blocks.
MAC PHY HARQ
HARQ
Diversity channel PPDU controller
AMC channel PPDU controller
Control information controller
HARQ: Hybrid automatic repeat request PPDU: Physical layer protocol data unit
n Figure 2. Cross layer adaptation scheme for efficient resource allocation. the system throughput, max carrier-to-interference ratio (CIR) is the best scheduling scheme such that subcarriers are allocated for only users with the best channel conditions. However, for the MAC layer, required quality of service (QoS) as well as system throughput should be satisfied. To maintain high system performance of both layers, it is necessary to design a MAC-PHY cross-layer optimized resource allocation scheme. Scheduling and subchannel allocation algorithms are two such widely accepted key functions in the MAC layer to exploit the crosslayer information and improve performances. Recently, many ideas have been proposed to address the problem of cross-layer resource allocation; see [4–8]. The proportional fair (PF) algorithm [4] also considers MAC-PHY issues, such as channel condition and fairness. In this section we propose a MAC-PHY cross-layer adaptation model for efficient resource allocation in 802.16e systems. Figure 2 shows the proposed MAC-PHY cross-layer adaptation
IEEE Communications Magazine • December 2005
functions and corresponding information flows. In the proposed model the MAC layer contains a user grouper, scheduler, and resource controller. Each functional entity exploits physical layer information to increase system throughput. The physical layer consists of a diversity channel PPDU controller, AMC channel PPDU controller, control information controller, and HARQ functional blocks. In the proposed model AMC subchannel users and diversity subchannel users are classified by the user grouper. Since the properties of AMC subchannels and diversity subchannels are quite different, the grouping of users into two channel types is essential if system throughput is to be increased. The scheduler determines the scheduling of users and the quantity of packets that should be scheduled in the current frame. For cross-layer optimization, the scheduler should be designed to exploit not only PHY information but also application layer information. Utility-function-based scheduling [6, 7] is
139
KWON LAYOUT
11/17/05
12:07 PM
Application-layer information is gathered by the MAC-c controller and used in the scheduler. With
Page 140
one of the representative methods for resolving this problem. Consider a system consisting of N users, and let Uk(x) denote the utility function of user k. In addition, let Rk,i,j denote the transmission rate of user k if it is scheduled on the jth subchannel of the ith frame. Then the scheduler can select the users to be served on the ith frame based on the following rule: k * = arg
respect to implementation, primitives between the PHY, MAC, and application layers should also be defined to support efficient cross-layer integration.
max
k ∈ {1,…, N }
U k′ ( x ) Rk , i, j ,
(1)
where |Uk′(x)| can be considered the priority of user k in the current scheduling frame [6]. We can define U k(x) based on our own scheduling criteria, which rely on not only MAC and PHY requirements, but also application layer requirements. For example, let Uk(x) = log x where x = Tk and Tk denotes the mean data rate of user k over a certain time period; then the right term of Eq. 1 can be rewritten as arg max k∈{1,…,N} (Rk,i,j/Tk), which becomes equivalent to the proportional fair scheduling rule. If Uk(x) = –aix2, where x means the packet delay and ai denotes a positive weighting factor, the scheduling rule becomes equivalent to the modified largest weighted delay first (M-LWDF) rule [6]. Similarly, we can define a novel utility-function-based scheduling rule that can satisfy the QoS requirements of the application layer and optimize MAC-PHY cross-layer performance. Once the scheduler determines the scheduled users, the resource controller assigns frequency bands to each selected user in order to maximize the throughput. A subcarrier allocation algorithm can be applied to this resource controller. A subcarrier allocation algorithm performs an important role in optimizing MAC-PHY cross-layer performance. Therefore, it should be able to exploit physical layer information such as SINR, MCS level, and velocity. If we also consider the cross-layer optimization between the MAC and application layers, the subcarrier allocation algorithm should satisfy such application layer requirements as delay bound and minimum data rate. However, the scheduler already considers the application layer requirements, and determines the users and the amount of resource that should be allocated for each of them in the current frame. Therefore, the problem that should be solved in the subcarrier allocation algorithm can be reduced to the MAC-PHY cross-layer optimization problem. We can model this optimization problem simply as follows. We assume that each frame consists of M time slots whose length is T ms, and each timeslot is divided into S subchannels in the frequency domain. Then we can obtain the following optimization problem: maximize subject to
∑ kN=1 ∑ Sj =1 Rk , j mk , j T ∑ Sj =1 Rk, j mk, j T ≥ Bk , ∑ kN=1mk , j = M mk , j ∈ {0,1, 2,L, M }
for ∀k for ∀j for ∀k , ∀j ,
(2)
where Rk,j denotes the maximum achievable rate of user k on the jth subchannel and mk,j denotes the number of slots that are assigned to user k on the jth subchannel. In addition, Bk denotes the amount of data user k should transmit in the current frame. In the proposed model the resource controller
140
determines the value of R k,j based on the CQI feedback information, such as average SINR or MCS level. When the minimum packet requirement is too strict compared to the current channel condition, there may exist no solutions that meet the minimum packet requirements. In this case we can choose the user whose channel condition is worst or priority is lowest, and exclude this user from the scheduling list in the current frame. The MAC layer functional entities, such as user grouper, scheduler, and resource controller, gather PHY and MAC layer control information from the control information controller and MAC-c controller, respectively. The control information controller generates and manages such PHY control information as channel matrix, SINR, MCS level, velocity, and location. The MAC-c controller is a MAC layer function block and controls MAC layer control information (e.g., fairness) and QoS. In addition, application layer information is gathered by the MAC-c controller and used in the scheduler. With respect to implementation, primitives between the PHY, MAC, and application layers should also be defined to support efficient cross-layer integration.
CROSS-LAYER PROTOCOL FOR CQI FEEDBACK In OFDMA-based systems, the condition of UL and DL channels should be considered when scheduling for the purpose of increasing throughput. For this reason, IEEE 802.16 defines variable UL control channels, as described earlier. Based on these channels, we propose a crosslayer protocol for CQI feedback, as shown in Fig. 3. Figure 3 shows MAC-PHY primitives, the cross-layer protocol sequence of the CQI feedback for DL channel measurement, and the UL sounding signal for UL channel measurement. All AMC subchannel users that have a transport connection identifier (CID) should periodically transmit a DL channel measurement report on CQICH. To construct a CQI feedback message, the MAC layer needs to receive channel measurement results from the physical layer. Primitives such as CQI-MSG.request and CQI-MSG. response in the DL CQI feedback of Fig. 3 are used for this purpose. Once a CQI feedback message is constructed in an MS MAC layer, it is transmitted to a BS MAC layer through CQICH. This information is exploited during scheduling and resource allocation. The UL sounding in Fig. 3 shows the transmission sequence of the UL sounding signal. UL sounding is a kind of UL pilot signal and is defined to support smart antenna or MIMO in 802.16e. If an MS confirms its sounding channel allocation in a UL_MAP message, an MS MAC layer sends a SOUNDING.request primitive to an MS PHY. Then an MS PHY sends a sounding signal on the allocated UL sounding region. A BS can use the received sounding signal to measure the quality of the UL channel and translate the measured UL channel quality to an estimated DL channel quality under the assumption of TDD reciprocity.
CROSS-LAYER PROTOCOL FOR HARQ The 802.16e system optionally provides the combining gain by incremental redundancy. HARQ is a very important technique for link adapta-
IEEE Communications Magazine • November 2005
KWON LAYOUT
11/17/05
12:07 PM
Page 141
DL CQI feedback
In order to model a MS
MAC layer
PHY layer
BS
PHY layer
MAC layer
physical channel for cross-layer analysis between 802.16e
MAP message
MAP message
MAP message
MAC and PHY layers, we consider
Identify CQI channel
path loss, log-normal
CQI-MSG.request (CQI message mode)
shadowing and
CQI-MSG.response (CQI message mode)
frequency-selective Rayleigh fading,
CQI message (CQI message mode)
according to each
CQI message (CQI channel)
CQI message
user’s mobility
UL sounding MS
MAC layer
MAP message
PHY layer
BS
PHY layer
MAP message
MAC layer
MAP message
Identify CQI channel CQI-SOUNDING.request (channel sounding mode)
Sounding signal (sounding region)
CQI-SOUNDING.indication
n Figure 3. Cross-layer protocol for DL CQI feedback and UL sounding. tion, and makes aggressive decisions possible at the MCS level. Thus, the use of HARQ can result in considerably increased throughput [10]. However, it is a critical issue to decide the MCS level and packet size for the original transmission of a HARQ-enabled connection. Extensive retransmissions cause considerable overhead, because HARQ retransmission requires control messages such as Compact_DL/UL MAP_IE in 802.16e systems. Thus, it is necessary to consider a trade-off between overhead caused by control messages and efficiency of link adaptation. Recently, various schemes have been studied that attempt to optimize ARQ performance by applying a channel-aware scheduling algorithm to the HARQ retransmission packet. System performance can be improved through a HARQ-aware scheduler design [11]. However, when HARQ is applied to real-time service, we may carefully design retransmission strategies that consider service delay bound as well as channel quality.
DESIGN AND IMPLEMENTATION OF A SIMULATOR BASED ON CROSS-LAYER OPTIMIZATION OF 802.16E SYSTEMS In this section we present a cross-layer simulation framework to evaluate the performance of the cross-layer framework for WiBro or 802.16e OFDMA systems, presented earlier. We need to simplify the operation of the PHY layer for effi-
IEEE Communications Magazine • December 2005
cient cross-layer analysis of 802.16e MAC performance, because we cannot implement all operations of the MAC and PHY layers due to practical problems, such as simulation time and simulator complexity, when cross-layer operation is simulated. So we introduce the simulation environment considered in our simulator for cross-layer analysis, how to apply PHY simulation results to a MAC simulator, and how the MAC layer interworks with the PHY or upper layer. We use the MATLAB tool for PHY simulation, and OPNET as an event-driven simulator for cross-layer analysis of the MAC and PHY layers.
SIMULATION ENVIRONMENT In order to model a physical channel for crosslayer analysis between 802.16e MAC and PHY layers, we consider path loss, log-normal shadowing, and frequency-selective Rayleigh fading, according to each user’s mobility [12]. We consider an 802.16e OFDMA system that uses 9 MHz system bandwidth with 1024 FFT size and 5 ms TDD frame structure. This 802.16e OFDMA system supports mobility and a multicell environment. Seven hexagonal cells for this multicell environment are considered, and each cell experiences interference from the first- and second-tier cells. Mobility affects link-level performance such as packet error rate (PER), and is divided into stationary, pedestrian, and vehicular according to user velocity. Pedestrians can move in arbitrary directions at velocities from 1 to 10 km/h, while vehicles can move in only four
141
KWON LAYOUT
11/17/05
12:07 PM
Because the simulation of many cells requires too long a simulation time and great computing power, we implement only seven central cells in reality and model the remaining 30 cells virtually by the seven central cells, through wrap-around cell configuration.
Page 142
directions at velocities of 10–60 km/h. Here, each user’s direction and velocity changes periodically during the total simulation time. This 802.16e system supports five QoS scheduling types: unsolicited grant service (UGS) for constant bit rate service, real-time polling service (rtPS) for variable bit rate service, extended real-time polling service (ertPS) for voice over IP (VoIP) service with silence suppression, non-real-time polling service (nrtPS) for non-real-time variable bit rate, and best effort (BE) for service with no rate or delay requirements. We use the following traffic models: VoIP using a Markov source model with activity factor 0.403 (full rate: 9.6 kb/s), video streaming with a source video rate of 32 kb/s and 10 frames/s, FTP with exponentially distributed reading time (mean: 180 s) and truncated-lognormally distributed file size (mean: 2 Mbytes), and HTTP divided into ON/OFF periods representing Web page downloads and the intermediate reading times [13]. VoIP traffic is supported by the UGS, rtPS, or ertPS scheduling type, while video streaming, FTP, and HTTP are supported by the rtPS, nrtPS, and BE scheduling types, respectively.
SIMULATOR DESIGN TO EFFECTIVELY APPLY PHY SIMULATION RESULTS TO A MAC SIMULATOR As mentioned above, we need to simplify PHY operations to simulate cross-layer operation between MAC and PHY layers, and so describe how to apply PHY simulation results, such as a SINR-PER table to a MAC simulator. In order to model the first- and second-tier cells for a hexagonal cell structure, 37 cells are needed in total. Because the simulation of many cells requires too long a simulation time and great computing power, we implement only seven central cells in reality and model the remaining 30 cells virtually by the seven central cells, through wraparound cell configuration. The 802.16e OFDMA system supports both distributed subcarrier permutation mode and adjacent subcarrier permutation mode. We present a simple method for calculating the SINR for each mode. We assume that one burst, which is transmitted by one serving BS and experiences intercell interference from the first- and secondtier cells, occupies N b,i subchannels in the ith symbol, and that one BS can maximally use N subchannels for DL burst transmission. For distributed subcarrier permutation mode, since the effect of intercell interference averaging is obtained, the DL SINR for one burst can simply be modeled as SINRDL , dist =
∑i Pi ⋅ ( N b, i / N i ) ∑ j ∑i Pj , i ⋅ ( N b, i / N ) + ηb
,
(3)
where N i denotes the number of subchannels that the serving BS uses for DL burst transmission in the ith symbol, and ηb is the background noise power that the corresponding burst experiences. However, j denotes one of the indices of neighbor BSs that are responsible for the firstand second-tier cells that affect the serving cell. Thus, Pi and Pj,i denote the received power from the serving BS and the neighbor BSs in the ith
142
symbol, respectively. By contrast, in adjacent subcarrier permutation mode, the serving BS examines whether or not the neighbor BSs use the same subchannels as they use for DL burst transmission. If so, power received on these subchannels is directly added to the interference power. The UL SINR can be modeled in the same way as the DL SINR. The error of non-HARQ bursts can be modeled simply by referring to the SINR-PER mapping table according to mobility and MCS level, which is prepared in advance through the linklevel simulation in the PHY layer. However, the error of HARQ-bursts should consider the combined gain of the original transmission and incremental redundancy (IR). We present two methods to reflect this combined gain of the HARQ scheme. Method 1. We prepare all results that can show SINR-PER curves for the original transmission, the first retransmission, the second retransmission, and so on, according to each MCS level and mobile speed, through extensive link-level simulation of the PHY layer. Method 2. We consider only the approximate combined gain of the current retransmission to the previous (re)transmission. For example, the PER of a burst with the first retransmitted IR is decided by referring to the SINR-PER mapping table for non-HARQ with SINR org + α 1 when the SINR of the original transmission is SINRorg and the combined gain by the first IR is α1. This method cannot reflect the exact combined gain of HARQ in a MAC simulator, but makes the link-level simulation of the PHY layer simpler than method 1. Using these design methods for interference modeling, SINR calculation, and burst error, we can analyze effectively the performance of crosslayer protocols, such as a channel-aware scheduler, band AMC, and HARQ, in a MAC simulator.
IMPLEMENTATION OF A MAC SIMULATOR Figure 4 shows the architecture of a MAC simulator for the performance analysis of cross-layer protocols between 802.16e OFDMA MAC and PHY layers. Upper-layer protocols are considered simply a link-layer level traffic generator that generates VoIP, video streaming, FTP, and HTTP traffic. In addition, the CS protocol is responsible for the simple mapping of the service flow identifier (SFID) and CID in a MAC simulator. Upper-layer protocol data units (PDUs), which are created by the traffic generator, are inserted into queues in the MAC layer after SFID-CID mapping, and data packets in these queues are treated as MAC service data units (SDUs). These MAC SDUs are fragmented into various sizes according to the MAC scheduling operations, and are processed by a selective repeat ARQ mechanism for ARQ-enabled connections. The MAC scheduler classifies connections into five QoS classes: UGS, rtPS, ertPS, nrtPS, and BE. These QoS classes are associated with certain predefined sets of QoS-related service flow parameters, and the MAC scheduler supports the appropriate data handling mechanisms for data transport according to each QoS
IEEE Communications Magazine • December 2005
KWON LAYOUT
11/17/05
12:07 PM
Page 143
Upper layer
Trafficdata data Traffic Traffic data
Traffic generator
SFID-CID mapping
CS
MACSDU SDU MAC MAC SDU
Fragmentation/defragmentation Control flow Message flow
Handover module (backbone communication: BS only)
Ranging MAC layer PHY layer
MAC management
ARQ (selective repeat) Scheduler (UGS, rtPS, ertPS, nrtPS, BE) Diversity Band AMC scheduler scheduler
Block Block Block Block
Non-ARQ queue
ARQ queue
Scheduled Fragment Fragment MACSDU SDU ofofMAC
MAC PDU
Concatenation/deconcatenation
SINR/PER modeling
Burst 1
UL control channel
PHY PDU
Burst 4 Burst 3
CDMA code
CQI
Burst 2
Resource allocation
MAC PDU
MAC PDU
Burst Burst Burst
HARQ UL-MAP FCH DL-MAP Preamble
Mobility (MS only)
Block Block
SubHeader header Payload SubHeader header Payload
MAC PDU
DL/ ULMAP
Fragment Fragment Fragment of MAC SDU Fragment of MACSDU SDU ofMAC MAC of SDU
Burst Burst 3 1 Burst Burst 2 5 Burst 2 Burst 2
n Figure 4. MAC simulator architecture for cross-layer analysis. class. First, the UGS is designed to support real-time data streams that generate fixed-rate data such as T1/E1 and VoIP without silence suppression. For UL, this service offers fixedsize grants for data transport on a real-time periodic basis. Second, the rtPS is designed to support real-time data streams consisting of variable-sized data packets that are issued at periodic intervals, such as MPEG video. For UL, this service offers periodic unicast request opportunities. Third, the ertPS is designed to support real-time data streams that generate variable size data packets on a periodic basis, such as VoIP services with silence suppression. For UL, this service offers a mechanism for periodic UL allocations, which may be used for requesting the bandwidth as well as for data transfer, considering the traffic characteristics of VoIP with silence suppression. Fourth, the nrtPS is designed to support delay-tolerant data streams consisting of variable-sized data packets for which a minimum data rate is required, such as FTP. For UL, this service offers unicast polls on a regular basis; typically, in an interval on the order of 1 s or less. Fifth, the BE service is designed to support data streams for which no minimum service level is required and therefore may be handled on a space-available basis. For UL, this service may offer contention request opportunities.
IEEE Communications Magazine • December 2005
The MAC scheduler then classifies users into an AMC group and diversity group according to QoS and channel state, and finally, it applies the scheduling algorithm in either the distributed or adjacent subcarrier permutation mode. The BS can apply variable channel-aware scheduling schemes using the SINR information reported by MSs on UL control channels. If we only consider data services without delay bound, throughput maximizing scheduling algorithms, such as PF or max CIR schemes, can be used. Otherwise, scheduling algorithms that consider QoS as well as cell throughput should be considered; for example, the M-LWDF scheme [4, 9]. However, the scheduler in the adjacent subcarrier permutation mode (or band AMC scheduler) can improve throughput performance considerably by optimally or suboptimally assigning subcarriers to users according to their frequency selectivity [14, 15]. MAC payloads are built by the scheduler, and then MAC PDUs are built by adding the generic header and inserting subheaders such as fragmentation subheader, packing subheader, grant management subheader, and fast feedback subheader, if necessary. MAC PDUs that use the same MCS level can be concatenated and build a burst. Also, HARQ can be applied in the case of HARQ-enabled connections. All the above operations, such as scheduling, decision of MCS level, HARQ, and resource
143
KWON LAYOUT
11/17/05
0.30
Page 144
DL delay UL UGS delay UL rtPS delay UL ertPS delay
0.28 0.26 0.24 Packet transmission delay (s)
12:07 PM
ertPS
0.22
UGS
0.20 0.18 0.16 rtPS
0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0
20
40
60
80
100
Number of VoIP users
n Figure 5. Packet transmission delay of MAC SDUs vs. number of VoIP sessions for UGS, rtPS and ertPS.
allocation, can be optimized by cross-layer mechanisms using PHY information. DL/UL-MAP is implemented for the delivery of general MAC resource allocation information, and compact DL/UL MAP is implemented for the operation of HARQ and band AMC. Moreover, some MAC management messages are implemented for variable MAC management operations, such as ranging, dynamic session management, ARQ, and handover. A backbone network of BSs is built for handover operation. We can analyze variable cross-layer performance between MAC and PHY layers of such operations as HARQ and band AMC, through this MAC simulator for cross-layer analysis. In addition, this simulator can be utilized for the efficient design of cross-layer protocols such as scheduling, resource allocation, and determination of PDU size.
PERFORMANCE ANALYSIS AND DISCUSSION In this section we introduce the method of derivation of VoIP capacity through the crosslayer simulator presented earlier, and analyze the VoIP capacity for UGS, rtPS, and ertPS, respectively. Moreover, we analyze and discuss the capacity enhancement of 802.16 OFDMA systems through cross-layer protocol designs, such as channel-aware scheduling schemes and band AMC scheduling. The simulation results were obtained using the following MCS levels: quaternary phase shift keying (QPSK) 1/12, QPSK 1/8, QPSK 1/4, QPSK 1/2, 16-quadrature amplitude modulation (QAM) 1/2, 16-QAM 3/4, 64-QAM 2/3, and 64QAM 5/6. Low density parity check code (LDPC) was used, and coding rates lower than 1/2, which are needed to solve intercell interference problem of cell boundary users, were obtained by repetition. Users were distributed uniformly in seven cells and experienced inter-
144
ference from the first- and second-tier cells. The ratio of DL to UL subframe length was 2:1, and the simulation environments described previously were considered. Figure 5 shows the packet transmission delay of MAC SDUs of VoIP sessions for UGS, rtPS, and ertPS, respectively. The DL scheduler uses a round-robin (RR) scheduling scheme, while the UL scheduler uses UGS, rtPS, or ertPS scheduling types that do not utilize multi-user diversity by fading channel. Here, the DL and UL MCS levels of each user are decided from the CINR value reported by the MSs or measured by the BS. In other words, the DL or UL scheduler in this simulation utilizes physical cross-layer information such as average CINR. Moreover, the ertPS provides efficient UL cross-layer resource allocation mechanisms that utilize the traffic characteristics of VoIP with silence suppression. Because the level of interference increases with the number of VoIP sessions, and thus transmitters select more robust MCS levels in order to meet the requirement of packet error rate for VoIP service, throughput is saturated, many packets are queued, and thus packet transmission delay rapidly increases with the number of VoIP sessions due to this large queuing delay. Here, we can decide the maximum number of VoIP users from these results, considering the requirement of packet transmission delay for VoIP service on air interface. For example, when its delay requirement is 60 ms, the maximum supportable number of VoIP users is limited by UL rather than DL, and the voice capacity is obtained as 76 VoIP users/cell, when using the UGS scheduling type. In this simulator the UGS scheduler aggregates the VoIP packets within the UGS intergrant time, which may adaptively increase with system load. Then the aggregated packet is transmitted. Thus, this UGS with adaptive intergrant time can support many more VoIP users than can UGS with a fixed intergrant time. However, about 68 and 92 VoIP users/cell can be supported when using rtPS and ertPS, respectively. Hence, the ertPS can increase the VoIP capacity by 21 and 35 percent compared to UGS and rtPS, respectively. The ertPS offers a scheduling mechanism that builds on the efficiency of both UGS and rtPS, by operating like UGS with variable grant size adaptive to the VoIP codec rate during the VoIP ON-period while operating like rtPS during the VoIP OFF-period. Figure 6 shows average cell throughput for non-real-time data traffic when using nonopportunistic scheduling such as RR considering only average CINR and channel-aware opportunistic scheduling schemes such as PF scheduling and band AMC scheduling. The PF scheduler utilizes the channel variation in the time domain, while the band AMC scheduler utilizes the channel variation in both frequency domain and time domains. Here, PHY throughput means the total size of MAC PDUs transmitted per second in the PHY. MAC throughput means the total size of MAC SDUs successfully received per second in the MAC layer. We can calculate the MAC overhead due to retransmission, MAC header or subheaders, and MAC management messages, based on the ratio of MAC throughput to PHY throughput. However, MAP overhead means the
IEEE Communications Magazine • December 2005
11/17/05
Average DL cell throughput (Mb/s)
5.5 5.0
12:07 PM
Page 145
MAC throughput PHY throughput
Average UL cell throughput (Mb/s)
KWON LAYOUT
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5
4.0
MAC throughput PHY throughput
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
0.0 DL diversity only1) DL diversity only2) (RR scheduling) (PF scheduling)
DL diversity (PF)2) + band AMC3)
UL diversity only1) UL diversity only2) (RR scheduling) (PF scheduling)
UL diversity (PF)2) + band AMC3)
RR scheduling: Round robin scheduling PF scheduling: Proportional fair scheduling 1) Using average channel quality information 2) Using instantaneous channel quality information in the time domain 3) Using instantaneous channel quality information in the frequency domain as well as the time domain
n Figure 6. Average cell throughput of 802.16e OFDMA system under various cross-layer protocol designs: a) DL average cell throughput vs. scheduling type; b) UL average cell throughput vs. scheduling type.
ratio of the amount of resources occupied by DL/UL-MAP messages to the total amount of used resources. Because these MAP messages need to be received successfully by all MSs in a cell, they have to be transmitted using the most robust MCS level; this is one of the most serious problems in 802.16e systems. We observed from the simulation result that these MAP messages occupy up to 20–60 percent of DL resources. However, we can improve both DL and UL average cell throughput by about 25–60 percent, as shown in Fig. 6, due to careful cross-layer approaches (e.g., PF and band AMC scheduling) that apply the cross-layer adaptation framework and primitives presented earlier. In addition, we can observe that MAC efficiency is lowest, and MAC throughput is not much improved, when using DL band AMC scheduling. The reason is that this case needs a MAP message for each band AMC user, as well as for each burst. These results indicate that the number of band AMC users scheduled in a frame should be decided by considering MAC overhead caused by MAP messages. Many other factors affect system performance, and most of these factors pertain to the design of cross-layer protocols. For example, the determination of the MAC PDU size is a very important cross-layer issue. A MAC PDU that is large enough to obtain a forward error correction (FEC) effect by channel coding may be transmitted. However, this FEC effect cannot be utilized sufficiently for low-rate service such as voice with a small packet size. Hence, it would be wise to consider alternative mechanisms to obtain the FEC effect, such as packet aggregation within a delay bound. In addition, the system should consider a multicell environment, so naturally it is important to try to improve the performance of cell-boundary users. In particular, the intercell interference problem of the UL cell boundary is severe. It is possible to solve this problem through frequency reuse, but this
IEEE Communications Magazine • December 2005
method requires very careful cell planning and cannot provide good trunk efficiency. Hence, it may be advisable to search for a smart solution, such as intercell-interference-aware resource allocation, scheduling, power control, and adaptive frequency reuse. Moreover, in practical terms, the performance of band AMC subchannels is not much better than that of diversity subchannels because of the feedback delay of channel quality information and scheduling complexity. Thus, it will be necessary to develop smart band AMC scheduling algorithms. It should be noted that spatial multiplexing using an adaptive antenna system (AAS) is provided in 802.16 systems. For the improvement of system capacity through AAS, all MAC algorithms such as scheduling, resource allocation, and the HARQ scheme should be redesigned carefully considering MIMO channel information. All the issues mentioned above are very important for improving system performance, and a cross-layer approach is necessary. In the future we will study the protocol design methodology for these cross-layer issues mentioned above and analyze its system performance with a cross-layer simulator.
CONCLUSIONS Considering WiBro or 802.16e OFDMA systems, we present a cross-layer adaptation framework for interlayer operation between the MAC and PHY layers, and a design example of primitives to exchange PHY information for cross-layer protocol operation. A MAC-PHY cross-layer adaptation scheme for efficient resource allocation classifies users into AMC and diversity user groups according to QoS and channel state, and each user group is scheduled separately. This can improve the average cell throughput by 25–65 percent. That is, cross-layer protocol design has a large effect on system performance, and physical information should be utilized very
145
KWON LAYOUT
11/17/05
12:07 PM
For the improvement of system capacity through AAS, all MAC algorithms such as scheduling, resource allocation and the HARQ
Page 146
carefully for efficient cross-layer operation. In addition, we provide a simulation framework for this cross-layer analysis between the MAC and PHY layers. We have shown that this simulator can be utilized for the efficient design of crosslayer protocols such as scheduling, resource allocation, determination of PDU sizes, and ARQ window size. The design and implementation methods of a simulator based on cross-layer protocol operation can also be utilized in future systems based on OFDMA.
scheme should be redesigned by carefully considering MIMO channel information.
REFERENCES [1] C. Eklund et al., “IEEE Standard 802.16: A Technical Overview of the WirelessMAN Air Interface for Broadband Wireless Access,” IEEE Commun. Mag., June 2002, pp. 98–107. [2] A. Ghosh et al., “Broadband Wireless Access with WiMAX/ 802.16: Current Performance Bandchmarks and Future Potential,” IEEE Commun. Mag., Feb. 2005, pp. 129–36. [3] I. Koffman and V. Roman, “Broadband Wireless Access Solutions based on OFDM Access in IEEE 802.16,” IEEE Commun. Mag., Apr. 2002, pp. 96–103. [4] A. Jalali, R. Padovani, and R. Pankaj, “Data Throughput of CDMA-HDR a High Efficiency-high Data Rate Personal Communication Wireless System,” Proc. IEEE VTC2000, vol. 3, May 2000 , pp. 1854–58. [5] S. Shakkottai and T. S. Rappaport, “Cross-Layer Design for Wireless Networks,” IEEE Commun. Mag., Oct. 2003, pp. 74–80. [6] P. Liu, R. Berry, and M. L. Honig, “Delay-Sensitive Packet Scheduling in Wireless Networks,” Proc. IEEE WCNC 2003, vol. 3, Mar. 2003, pp. 1627–32. [7] G. Song and Y. Li, “Adaptive Resource Allocation Based on Utility Optimization in OFDM,” Proc. IEEE GLOBECOM 2003, vol. 2, Dec. 2003, pp. 586–90. [8] J. Rhee, J. M. Holtzman, and D. Kim, “Scheduling of Real/Non-Real Time Services: Adaptive EXP/PF Algorithm,” Proc. IEEE VTC 2003-Spring, vol. 1, Apr. 2003, pp. 462–66. [9] M. Andrews et al., “Providing Quality of Service over a Shared Wireless Link,” IEEE Commun. Mag., Feb. 2001, pp. 150–54. [10] S. Kallel, “Analysis of a Type-II Hybrid ARQ Scheme with Code Combining,” IEEE Trans. Commun., vol. 38, Aug. 1990, pp. 1133–37. [11] H. Zheng and H. Viswanathan, “Optimizing the ARQ Performance in Downlink Packet Data Systems With Scheduling,” IEEE Trans. Commun., vol. 4, Mar. 2005, pp. 495–506. [12] ITU-R Rec. M.1225, “Guideline for Evaluation of Radio Transmission Technologies for IMT-2000,” 1997. [13] C50-20010820-02, “1xEV-DV Evaluation Methodology — Addendum,” July 2001. [14] C. Y. Wong et al., “ Multicarrier OFDM with Adaptive Subcarrier, Bit, and Power Allocation,” IEEE JSAC, vol. 17, no. 10, Oct. 1999, pp. 1747–58. [15] Z. Shen, J. G. Andrews, and B. L. Evans, “Optimal Power Allocation for Multiuser OFDM,” Proc. IEEE GLOBECOM 2003, Dec. 2003, pp. 337–41.
BIOGRAPHIES TAESOO KWON [S’01] (
[email protected]) received his B.S. and M.S. degrees in electrical engineering from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, in 2001 and 2003, respectively. He is currently working toward a Ph.D. degree in electrical engineering at KAIST. His research interests include radio resource management and multiple access protocols in wireless communication systems such as WiBro, 802.16, 3GPP LTE, and 4G systems and performance analysis of CDMA and OFDM mobile communication systems. SUNGHYUN CHO [S’97, M’05] (
[email protected]) received his B.S., M.S., and Ph.D. in computer science and engineer-
146
ing from Hanyang University, Korea, in 1995, 1997, and 2001, respectively. Since 2001 he has been with Samsung Advanced Institute of Technology, where he has been engaged in the design and standardization of MAC and upper layers of B3G, IEEE 802.16e, and WiBro systems. His research interests include radio resource management, cross-layer design, and handoff in wireless systems. H OWON L EE [S’04] (
[email protected]) received his B.S. and M.S. degree in electrical engineering from KAIST in 2003 and 2005, respectively. Currently, he is working toward his Ph.D. at KAIST. His current research interests include radio resource management, mobility management, and next-generation mobile communications SIK CHOI [S’05] (
[email protected]) received his B.S. and M.S. degrees in electrical engineering from KAIST in 2003 and 2005, respectively. Currently, he is working toward his Ph.D. at KAIST. His research interests include mobility management, adaptive modulation/coding, and B3G mobile communications. JUYEOP KIM [S’05] (
[email protected]) received his B.S. degree in electrical engineering from KAIST in 2004, and is currently working on his Master’s at KAIST. His research interests include MAC for multicasting and control overhead minimization in MAC. S ANGBOH Y UN [M’97] (
[email protected]) has been a senior research engineer with Samsung Advanced Institute of Technology, Korea, since August 2001. From 2000 to July 2001 he was with NeoSolution, Inc. as founder and CEO/CTO. From 1994 to 1999 he was with DAEWOO Telecom, Inc. as a research engineer. He received his B.S. and M.S. from the Department of Electronics Engineering of Korea University, Seoul, in 1994 and 1998, respectively. He is currently a Ph.D. candidate in the Department of Electronics Engineering of the same university. His research interests are wireless communication systems, radio resource management, MAC, and particularly their applicable issues to B3G mobile communication systems. WON-HYOUNG PARK [S’05] (
[email protected]) received B.S. and M.S. degrees in electrical engineering from Seoul National University, Korea, in 1998 and 2000, respectively. He is currently working toward a Ph.D. degree in electrical engineering and computer science at Seoul National University. Since 2000 he has been with Samsung Advanced Institute of Technology, Suwon, Korea. His research interests include radio resource management, cross-layer design, and quality of service in wireless network. D O N G -H O C H O [M’85, SM’00] (
[email protected]) received a B.S. degree in electrical engineering from Seoul National University in 1979, and M.S. and Ph.D. degrees in electrical engineering from KAIST in 1981 and 1985, respectively. From 1987 to 1997 he was a professor in the Department of Computer Engineering at Kyunghee University. Since 1998 he has been at KAIST, where he is a professor in the Department of Electrical Engineering and Computer Science. His research interests include wired/wireless communication networks, protocol, and services. KIHO KIM [M’91, SM’03] (
[email protected]) received a B.S. degree from Hanyang University, Korea, in 1980, an M.S. degree from KAIST in 1982, and a Ph.D. degree from the University of Texas at Austin in 1991, all in electrical engineering. From 1982 to 1987 he was with the Korean Broadcasting System, where he developed the Korea Teletext System. Since 1991 he has been with the Samsung Advanced Institute of Technology, Korea, where he has been engaged in research on HDTV, ADSL, and DVD read channel projects. He is currently vice president of the Communication Laboratory, Samsung Advanced Institute of Technology, and his research interests include mobile and wireless communication and signal processing.
IEEE Communications Magazine • December 2005
