Link Adaptation Techniques for High-Speed Packet Data ... - CiteSeerX

Link Adaptation Techniques for High-Speed Packet Data in Third Generation Cellular Systems Eduardo Esteves, Peter J. Black and Mehmet I. Gurelli Corporate R&D QUALCOMM Incorporated {eesteves, pblack, mgurelli}@qualcomm.com ABSTRACT In this paper, we review link adaptation techniques used in third generation wireless systems. In particular, we discuss the cdma2000 high rate packet data air interface, also known as IS-856. An overview of the forward and reverse link channel is presented, including physical layer features allowing the implementation of link adaptation techniques. Such advanced techniques provide up to 2.4 Mbps throughput on the forward link over 1.25 MHz bandwidth. Simulation results are presented, demonstrating the effectiveness of such techniques in providing a high spectral efficient cellular system under a variety of channel environments and including the effects of errors on feedback channels.

3G system optimized for packet data services with forward link peak rates up to 2.4Mbps. The IS-856 forward link transmissions are based on a shared channel using 100% of power and code space achieving the highest data rate that a terminal can receive at a given time. This is accomplished by a set of side information channels providing the system with a mechanism for fast adaptation of modulation and coding schemes to the mobile channel environment experienced by a given user. In addition to this per user adaptation, a scheme that adapts the transmissions on the forward shared channel to the channel quality of all users provided by the side information channels will be referred to as macro link adaptation. 2

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INTRODUCTION Third generation wireless cellular systems require techniques that provide reliable and spectrally efficient packet data services. These requirements are a consequence of user demand for economical, high-speed wireless Internet services. Typically, bandwidth efficient wireless systems dynamically allocate resources such as power, code space and time slots, adapting to the time varying nature of shadowing and fast fading processes. For CDMA downlink, a practical and spectrally efficient system is one that allocates a fixed amount of power and code space while adapting the transmission rate, modulation and coding to the varying conditions of the radio channel [1]. In fact, allocating all power and code space to a single terminal at a time maximizes the achievable peak rate, thus enhancing user’s experience. In addition, using information theoretic arguments, it was concluded in [2] that once the transmission data rate is adapted to the fading channel, adapting the transmit power as well yields negligible capacity gains. The idea of adaptive modulation and coding schemes, though known for decades, has recently regained attention in the literature [3][4] and involves maximizing spectral efficiency by selecting the combination among a variety of modulation schemes (e.g. QPSK, 8PSK, 16QAM) and code rates that best matches the channel environment. In order to fully exploit these concepts, a system should include a collection of techniques referred to as link adaptation, which consists of fast feedback channel state information, adaptive modulation, incremental redundancy and repetition coding, time diversity adaptation, hybrid ARQ, selection diversity, and multiuser diversity. In this paper, we review the link adaptation techniques used in the cdma2000 high rate packet data air interface [5], also known as IS-856, which is the first

IS-856 OVERVIEW In this section a general overview of the IS-856 air interface is provided. In particular, the physical and MAC layer aspects relevant for the link adaptation are discussed in details. A more overall description of the standard as well as several performance results may be found in the literature [7],[8],[9]. The IS-856 system consists of an access network (AN), which by means of its distributed base stations or sectors, provides widearea high-rate data access on a wireless CDMA channel to a collection of static or mobile access terminals (AT). The IS-856 system provides an “always on” wireless experience since an AT may maintain an open session1 with the network without the need for establishing a connection, i.e. without radio resources being allocated to the user. Whenever the AT establishes a connection to the IS-856 network, it maintains a radio link with one or more sectors constituting its active set. If the active set of a terminal consists of more than one sector, the terminal is said to be in soft handoff. The special case where all sectors in the active set are collocated in the same base station is referred to as softer-handoff. In IS856, the AT receives data packets on the forward link from only one sector in its active set while it transmits data on the reverse link to all members of its active set. Similarly to the procedure in IS-95-B, the active set is determined based on signal to interference plus noise ratio (SINR) measurements of the forward link signals received at the terminal. As shown in Figure 1, the fundamental timing unit for forward link transmissions2 is a 1.67 ms slot (2048 chips). A slot contains the Pilot, medium access control (MAC) channels and a data portion that may contain 1 An open session includes an assigned IP address and a PPP state. 2 Both forward and reverse link waveforms are directsequence spread at the rate of 1.2288 Mcps in an IS-95 compatible bandwidth of 1.25MHz.

Traffic or Control channel. All channels are transmitted at full sector power, except that the data portions of the waveform are gated off when the Traffic channel is idle. The Pilot channel, identified by a sector specific PN offset, is always transmitted during 96 chips every halfslot (1200Hz) providing a reference for coherent demodulation, channel estimation and SINR prediction. Groups of 16 consecutive slots, referred to as frames, are synchronously transmitted from all sectors according to CDMA system time. This results in overlapping pilot bursts at the AT, allowing the SINR estimation to accurately account for the effects of co-channel interference. In addition, even when the data portion is gated off, the MAC channel surrounding each pilot burst guarantees that worst-case co-channel interference is accounted for up to 64 chips of differential propagation delay between sectors (≈15.6 km).

Data 400 Chips

MAC Pilot MAC 64 96 64 Chips Chips Chips

Table 1: Parameters of different packet types in IS-856

1 slot 1.67ms

1/2 slot 1,024 Chips Data 400 Chips

modulation size and preamble length. If the number of allocated code symbols in the first slot is less than the total number of code symbols in the packet, a higher effective code rate is achieved by puncturing some parity bits (e.g. 2/3 code rate packets in Table 1). If the packet is transmitted over multiple slots, all code symbols are first transmitted once, followed by as many repetitions needed to satisfy the number of code symbols allocated to the packet. Partial truncation of code symbols in the last repetition may occur if an integer number of repetitions is not possible. In this design, at higher SINR, significant coding gains are achieved by incremental transmission of parity bits, while at lower SINR powerful coding is achieved by simple repetition of a basic rate 1/5 encoded packet. Further details on the coding and interleaver construction can be found in [6].

1/2 slot 1,024 Chips Data 400 Chips

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Figure 1: Forward Link Slot Structure The MAC channels consist of code-division multiplexed (CDM) reverse activity (RA) channel and up to 59 reverse power control (RPC) channels. The RPC channel is used for reverse link power control at a 600Hz update rate. The common RA channel is used to indicate reverse loading for the reverse link MAC algorithm. Each of the MAC channels is spread using a 64-ary Walsh function and repeated four times, resulting in the allocated 256 chips per slot. For each sector in the active set a unique MAC index is assigned to address information to the specific terminal. The MAC index identifies, for example, the Walsh channel used to spread the corresponding RPC channel. The Forward Traffic channel (FTC) is transmitted during the 1600 data chips allocated in each slot as depicted in Figure 1. The access terminal also uses the assigned MAC index to identify transmissions on the FTC, which is a time division multiplexed (TDM) channel shared by all terminals communicating with a given sector. A data scheduler allocates slots to a user depending on the data rate information specified by the data rate control (DRC) channel on the reverse link. As will become clear in the next section, the data rate is adapted to reflect the channel conditions seen by the terminal. In addition, a TDM structure is a simple and efficient way of achieving macro diversity gains (multiuser gain) with a dynamic data scheduler that takes channel quality information into account (DRC channels). The possible packet data types are described in Table 1. The packets are encoded by a parallel, concatenated convolutional code (PCCC) based on either a rate r=1/3 or r=1/5 code. Some packets are transmitted over multiple slots introducing time diversity, which is particularly useful in fast fading channels. The total number of code symbols allocated for transmission is determined by the number of slots,

Data Slots Packet Turbo rate per size code (kbps) packet (bits) rate 38.4 16 1024 1/5 76.8 8 1024 1/5 153.6 4 1024 1/5 307.2 2 1024 1/5 307.2 4 2048 1/3 614.4 1 1024 1/3 614.4 2 2048 1/3 921.6 2 3072 1/3 1228.8 1 2048 1/3 1228.8 2 4096 1/3 1843.2 1 3072 1/3 2457.6 1 4096 1/3

Modulation Preamble Effective length in code rate chips QPSK 1024 1/48 QPSK 512 1/24 QPSK 256 1/12 QPSK 128 1/6 QPSK 128 16/49 QPSK 64 1/3 QPSK 64 16/49 8PSK 64 16/49 QPSK 64 2/3 16QAM 64 16/49 8PSK 64 2/3 16QAM 64 2/3

Data packet transmissions on the FTC are identified by a preamble sent at the beginning of the first slot of the packets. The preamble consists of a certain number of repetitions of a 32-chip bi-orthogonal sequence identified by the desired terminal’s MAC index. As a result, the preamble identifies the start of a new packet and the intended user but not the transmit data rate, which is uniquely determined by the information sent on the DRC channel. This design simplifies and increases reliability of the preamble demodulation process3 since a reduced amount of information needs to be conveyed. As shown in Table , the preamble length increases as the requested data rate reduces in order to maintain a low miss probability (compared to the target packet error rate) in a variety of channel conditions. The tradeoff between preamble overhead and reliability is strongly dependent on a good a-priori channel state feedback. For example, a system that overestimates the initial data rate relying on an ARQ scheme to reduce the packet error rate (PER) may require a large side information channel (e.g. preamble) overhead in order to guarantee the packet can be detected in the first place. Multi-slot packet transmissions on the FTC follow a 4-slot interlacing structure as illustrated in Figure 2. That is, three time slots separate successive slot 3

Since transmission slots are not pre-scheduled, preamble detection has to be performed all the time. However, only packets addressed to the AT need to be decoded.

transmissions of a multi-slot packet. The 4-slot interlacing allows time for the receiving access terminal to decode the partially received packet and to return an indication to the transmitting sector via the ACK channel on the reverse link. If a positive acknowledgement is received, the remaining slots are not transmitted and may be allocated for the transmission of another data packet(s), possibly to different users. The early termination of multi-slot packets, or equivalently a type II Hybrid-ARQ procedure, increases the effective throughput of the system. In Figure 2, this is exemplified for a requested data rate of 153.6 kbps (4-slot packet). After the third slot of the packet, an ACK is received indicating the terminal has correctly decoded the packet. In this case, an effective 33% higher transmission rate (204.8 kbps) is achieved by the Hybrid-ARQ procedure. First Slot for the Next Physical Layer Packet Transmission Forward Traffic Channel Physical Layer Packet Transmissions with 153.6 kbps

DRC Channel Slot Transmission Requesting 153.6 kbps

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Figure 2: Timing of FTC, DRC and ACK channels Note that the 4-slot interlacing structure creates four independent ARQ streams that can be used to transmit packets to a single or multiple terminals at the same time. Sequential delivery of packets to higher layer protocols is achieved by simply following the time of arrival associated with each packet’s preamble detection. In addition, there is an implicit timing relation between a DRC and ACK received on the reverse link and their corresponding effects on the forward link transmissions. This significantly simplifies the design of the feedback channels requiring the minimum amount of information to be transmitted (reduced overhead) on the reverse link. In fact, the Reverse Traffic channel (RTC) transmission consists of four code division multiplexed channels: Pilot channel, Reverse Rate Indicator (RRI) channel, Data Rate Control (DRC) channel, Acknowledgement (ACK) channel, and the Data channel. The details of the IS-856 reverse link can be found in [5],[7]. Next we discuss only the aspects related to the feedback channels used for forward link adaptation. The DRC symbols are encoded using 16-ary biorthogonal code. Each symbol comprises of 4 bits that indicate one of the 12 possible packet types shown in Table 1. Each code symbol is further spread by one of the 8-ary Walsh functions in order to indicate the desired transmitting sector on the forward link. The DRC message is transmitted at half-slot offsets with respect to a slot boundary as depicted in Figure 2. The reason is to minimize prediction delay, while providing enough time for processing at the desired sector before transmission on the forward link starts on the next slot.

The ACK channel is BPSK modulated in the first half-slot (1024 chips) of an active slot. A '0' bit is transmitted on the ACK channel if a data packet has been successfully received on the Forward Traffic channel, otherwise a '1' bit is transmitted. Transmissions on the ACK Channel only occur if the access terminal detects the preamble of a data packet directed to it. Therefore, the ACK channel has negligible impact on reverse link capacity since practically only one ACK channel is active at any given time. As shown in Figure 2, for a data packet transmitted in slot n, the corresponding ACK channel bit is transmitted in slot n+3 on the reverse link. The 3 slots of delay allows the terminal to demodulate and decode the received packet before transmitting on the ACK channel. 3

LINK ADAPTATION IN IS-856 Link adaptation in IS-856 is achieved by a combination of several mechanisms designed to improve spectral efficiency while achieving the required simplicity and robustness for effective operation in a wireless cellular environment. As discussed in the last section, an adaptive forward link packet data modem is employed by the base station (BS) to optimize spectral efficiency by matching the transmit data rate, modulation and coding to the time-varying received SINR at the terminal. This is achieved by a combination of a rate control mechanism at the mobile terminal and the feedback channel state information (CSI) provided by both the DRC and ACK channels on the reverse link. Access Terminal

96 chips of Pilot every half-slot from the serving sector

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Figure 3: Adaptive Rate Control A general rate control mechanism is described in Figure 3, where the pilot samples received on the forward link of the serving sector are used to estimate the received SINR. Due to the small delay between CSI feedback and the actual transmission on the forward link, prediction of the channel quality over the duration of the next packet is necessary for an accurate packet type selection. The packet type selection (DRC value) is achieved by determining the highest data rate for which the predicted SINR is greater than a given threshold. Once a new slot of the requested packet is received, the packet is decoded and the appropriate ACK information is provided to the serving sector4. The a-posteriori CSI provided by the ACK channel allows the data rate of a 4

A packet error event is detected by a cyclic redundant check (CRC) code inserted in each packet

SINR estimation, prediction and data rate selection

Preamble detection and packet decoding attempt.If CRC pass sends an ACK, otherwise NAK

Continue decoding/ACK procedure until CRC pass or last slot of the packet is received

Pilot bursts transmission Data rate control (DRC ) channel

mission First slot of packet trans ACK/NAK transmission

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Full power pilot bursts are transmitted every 0.834ms (overhead=6.25%) Adaptive data scheduling using DRCs and fairness criteria

If ACK or single slot packet schedule new data packet, else continue transmitting second slot

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Continue transmission until ACK is received or last slot of the packet is transmitted

Figure 4: Per User Link Adaptation The link level performance of an IS-856 variable rate modem given 100% of the forward link time slots is shown in Figure 5. The average throughput is plotted against geometry (signal to thermal noise ratio) for two different channel conditions at 1.9GHz: the ITU Pedestrian A model (1 path Rayleigh channel at 3km/hr) and 3 path, 100km/hr Rayleigh fading channel. For the slow fading 3km/hr channel the link adaptation is primarily based on the a-priori DRC information and for the fast fading 100km/hr channel the link adaptation is primarily based on the a-posteriori ACK (ARQ) information. At low geometry the received SINR is dominated by the No interference (independent of the numbers of paths) and hence is approximately equal for the two channels. Consistent with the SINR, 5

In cellular systems employing frequency division duplexing, it is common to encounter areas where the stronger forward link does not correspond to the stronger reverse link signal.

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Figure 5: Performance of the Variable Rate Modem in Fading Channels The remaining simulation results presented in this paper are derived from a full network level simulation of 3 tiers of tri-sectored cells (111 sectors in total) based on a tri-cellular network layout. The modified Hata path loss model is used in addition to a time varying lognormal shadowing with 8dB sigma and 50% site-tosite correlation. Further details on the simulation assumptions can be found in [8]. The effectiveness of the early-termination feature of IS-856, or equivalently Hybrid ARQ, can be demonstrated by comparing the probability distribution function of the scheduled data rates based on the DRC information and the final data rates after the feedback on the ACK channel. This is shown in Figure 6 where the ITU’s vehicular A model (2 path Rayleigh fading at 120km/h) is considered. It is clear that due to the channel uncertainty, the terminal adapts by requesting conservative multi-slot packet types, which introduce enough margin and time diversity. However, after the aposteriori feedback on ACK channel, the final served rates are significantly higher than the originally requested rates. 0.4 0.35

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approximately equal throughput was achieved over this wide range of Doppler speeds showing the efficiency of the link adaptation. At higher geometry the lower throughput of the fast fading case is due to the lower SINR, which in this case is dominated by multi-path (self) interference.

0.25 0.2 0.15 0.1 0.05 0 Nu l * 3 lRa 8. t e 4 40 { 1 . 9 6} 6 43 { 1 . 8 5} 8 47 { 1 . 2 4} 6 { 51 13 .2 } 55 { 1 . 8 2} 5 61 { 1 . 4 1} 4 68 { 10 .2 } 6 * 7 {9 6. } 8 87 { 8 .7 } 7 10 { 7 2 } 12 . 4 { 2. 6} 8 *1 8{ 53 5} .6 20 {4 4 } * 3 .8 { 07 3} * 3 .2 { 07 2} .2 40 {4 9 } * 6 .6 { 14 3} * 6 .4 { 14 1} * 9 .4 { 2 2} * 1 1.6 22 {2 } * 1 8. 8 22 { 1 } * 1 8. 8 84 { 2 } * 2 3. 2 45 { 1 7. } 6 {1 }

given multi-slot packet transmission to be optimized based on the actual wireless channel condition. This is particularly useful in the case of low SINR and/or fast fading channels. The final packet error statistics are used to adapt the selection thresholds to different mobile and interference environments. The target packet error rate (PER) is selected low enough not to cause excessive retransmissions delays that can have a severe impact on the performance at higher layers. The overall sequence of events during the link adaptation procedure is summarized in Figure 4. Note that if the quality of the ACK channel decisions at the serving sector is not sufficient for robust operation, the data scheduler can implicitly assume a NAK decision and carry to completion the transmission of the requested packet type5. This is possible because the packet type indicated by the DRC message has an associated maximum number of slots to be transmitted. Moreover, robustness is preserved by committing the sector to serve the terminal up to a maximum number of slots at the cost of not realizing the potential gains of the a-posteriori link adaptation. Finally, an erased ACK (implicit NAK) though degrading the potential ARQ gain does not increase the PER as the packet has decoded correctly. In addition, the IS-856 standard allows the terminal to transmit a redundant ACK, four slots after the first, in an attempt to improve ACK detection probability.

Rate {Number of Slots} * Requestable Data Rates

Figure 6: Comparison between Scheduled and Final Rate Distribution

periods of time where the ATs see strong signal levels6. This is accomplished by selecting the AT, among the ones with data in their queues, that has the largest metric defined as DRC i (t ) mi (t ) = Ri (t ) where DRCi (t ) is the current DRC from AT i at slot t.

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The average rate received by AT i, Ri (t ) , is updated every slot by Ri (t + 1) = (1 − α ) ⋅ Ri (t ) + α ⋅ Ri (t )

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Figure 7: Distribution of Final rates for a 76.8kbps Requested Rate in Fast Fading Another way of looking at the ACK gain is shown in Figure 7 where the probability distribution of the final data rates given the requested rate (DRC) of 76.8kbps is shown. In the scenario simulated, 1-path 120 km/h Rayleigh Fading, almost 80% of the packets are successfully decoded at an effective data rate higher than 200 Kbps. 3.1 Adaptive Data Scheduler From the base station point of view, macro diversity can also be exploited in IS-856 since a single shared channel is used to transmit data packets to all terminals served by a given sector. Typically a data scheduler allocates time slots to different users based on their priority, amount of data to transmit and some degree of fairness. In IS-856, a smart data scheduler may also adapt to the varying channel conditions experienced by different users based on the a-priori channel state information provided by the DRC channels. Because different users experience independent fading processes, an adaptive data scheduler may attempt to serve a terminal near its peak SINR. This can be viewed as a form of macro link adaptation (selection diversity) from the data scheduler point of view. The key aspect is the delay tolerant characteristic of packet data communications. For example, suppose 10 terminals equally share a common wireless channel. In average, a data scheduler allocates 10% of the time to each of the terminals. Even though it is possible to guarantee a fixed delay between transmissions to a single user by using a round robin scheduler, it is clear that a much higher throughput can be achieved in certain fading channels if the serving time (10%) is judiciously chosen to be near the terminal’s maximum received SINR. In this case, the time between data scheduling for a given terminal typically varies from less than 10ms to a few hundreds of milliseconds. Such time variation, however, can be easily absorbed by most transport protocols and applications [9]. In this paper we discuss the proportional fairness scheduler that takes advantage of temporal variations of the channel by scheduling transmissions to ATs during

where Ri (t ) is the data rate being transmitted to AT i during slot t. If AT i is not being served, then Ri (t ) is set to zero. Fairness and maximum packet delays are controlled by selecting the averaging filter time constant, ≈ 1 / α , which basically sets the tradeoff between throughput optimization and the amount of latency an AT experiences when its channel condition degrades abruptly. In Figure 8, the embedded sector throughput results are shown as a function of the number of users for slow and fast fading channels with and without antenna diversity reception. It is clear that sector capacity can be a high as 1.6 Mbps for slow fading with dual receive antennas. Moreover, the multiuser gains7 achieved by the proportional fairness scheduler running with a time constant of 1.7 seconds vary from 2 to 3.1 dB for single antenna receivers and from 1.7 to 1.9 dB for dual antenna. The multiuser diversity gain in slow fading case is a consequence of the time variation in a-priori CSI provided by the DRC channel. For the fast fading case, the primary source of a-priori SINR variation is the time-varying shadowing process. Sector Throughput vs. Number of Users 1800000 1600000 1400000

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Figure 8: Sector Throughput versus Number of Active Users 3.2 Effects of DRC Length and Channel Reliability While a fast and reliable CSI provided by the DRC channel improves forward link throughput, it constitutes an overhead channel for the reverse link. As a result, it is important to carefully tradeoff reverse link capacity 6 In IS-856, a variety of data schedulers may be implemented since the air interface does not specify a particular algorithm. 7 Here multiuser gain is defined as the ratio of throughput of a sector with 16 users to the throughput achieved with a single user.

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Figure 10: DRC Erasure Rate versus User Throughput for Several Locations in the Embedded Sector Serving 16 Simulteneous Users In Figure 11, the sector throughput as a function of number of users is shown for the three erasure decoder settings. The throughput is essentially the same for both the nominal and 2dB margin cases. Moreover for the 4 dB margin case, which corresponds to erasure rates as high as 25 to 60%, the sector throughput is reduced by only 28% and 9% with respect to nominal for 1 and 16 users, respectively. Not only does multi-user diversity yield capacity gains as the number of users increases but the diversity also provides robustness against DRC erasures, hence the relative lower degradation in throughput as the number of users increases.

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In Figure 10, it is also shown the results for two erasure decoder implementations with 2 and 4 dB of additional margin, respectively. Despite the fact that the erasure rate significantly increases for each user, the corresponding average throughput is only slightly reduced. This is a consequence of the scheduling times (non-erased DRC) still being chosen near the peak received SINR of each user.

Prob(DRC Erasure)

for a more reliable and effective (increased forward capacity) DRC channel. The air interface [5] allows the DRC channel gain and a repetition factor, namely DRC length in slots (1,2,4 or 8), to be specified for each user in the system. In addition, the receiver at the base station may reduce the error rate of the DRC decisions by erasing unreliable DRC messages. A DRC data rate error directly impacts forward link PER since the serving sector may schedule a packet at the wrong data rate causing it to fail at the terminal. On the other hand, a DRC erasure only prevents the scheduler to serve a terminal during that time, therefore only postponing the transmission rather than repeating it. First, we show in Figure 9 the effect of different DRC lengths on the sector throughput. For each case there are three values of DRC lengths depending on the number of different base stations in the active set, i.e. 1, 2 and 3+, respectively. Increasing the DRC length as the number of handoff legs increase, improves DRC reliability in imbalance scenarios without allocating more power to the DRC channel. Moreover, users in handoff are more likely to be requesting lower data rates that are longer in duration and, hence, less affected by DRC length. For example, it can be seen that the forward link throughput loss, with respect to the (1,1,1) case, for the (2,4,4) slots configuration is only 14% in slow fading while negligible in fast fading. In more unpredictable fast fading environments, the AT essentially requests a data rate based on long term channel statistics, causing it to be insensitive to DRC length.

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Assuming the (2,4,4) slots configuration with DRC channel gain (relative to Pilot) set to (-1.5, -3.0,-3.0) dB, a nominal erasure decoder is used at the base station in order to guarantee an error rate better than 0.1%. To characterize the DRC erasure process and the effect on sector throughput, a network simulation was run for 24 drops of 16 users – total of 384 user snap-shots. The channel was 1-path 3km/hr Rayleigh fading. In Figure 10, we show the DRC erasure rate versus the per user throughput for each of the 384 users. The nominal erasure rate ranges from less than 1% for non-handoff users (higher average rates) to 20% for users at the edge of the sector (lower average rates).

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Figure 11: Sector Throughput versus Number of Users for Different DRC Channel Margins 3.3 Effects of Antenna Correlation on Throughput of Dual-Receive Diversity Terminals

The results presented in subsection 3.1 for the dualreceive diversity terminals assumed uncorrelated signals at the two antenna connectors. Typically, uncorrelated signals are assumed only if the receive antennas are

separated by at least 10 wavelengths. This requirement would prevent the widespread use of receive diversity in future small form factor devices. Consequently, practical dual-receive diversity terminals will exhibit some degree of antenna correlation. In this section, sector throughput performance is evaluated as a function of antenna correlation for a dual-antenna receiver. We consider

model, however, the degradation of severe antenna correlation is about 40% regardless of the number of users. This is attributed to the fact that in this outdoor model with no building penetration loss, the thermal noise component is less significant allowing the total interference correlation to be high.

antenna correlation coefficients of 0, 0.5, 1 / 2 and 0.99. The results for the Pedestrian and Vehicular A models are shown in figures 12 and 13, respectively.

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CONCLUSIONS In this paper, an overview of the forward and reverse link channels in IS-856 was presented. In particular, the physical layer features allowing the implementation of link adaptation techniques were discussed in details. Simulation results were presented demonstrating the effectiveness of such techniques and showing that a high spectral efficient cellular system can be achieved under a variety of channel environments. In fact, in slow fading channels with dual antenna receivers, nearly 1.6 Mbps/sector or equivalently 3.8 bps/Hz/cell can be achieved by the combination of the techniques described in this paper.

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ACKNOWLEDGEMENTS The authors thank Roberto Padovani for comments that improve the quality of this paper.

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First, note that only small throughput degradation is observed for antenna correlations of up to 0.7, both in fast and slow fading channels. The fact that the forward link performance of IS-856 with dual-receive diversity is somewhat insensitive to antenna correlation is a consequence of both the link adaptation and multiuser diversity gains of the system. Though antenna correlation reduces the diversity order of the combiner output, the adaptive scheme takes advantage of the increased average combined SINR and those instances of time when the interference correlation coefficient is small. For very high antenna correlation, e.g. 0.99, the degradation with respect to the uncorrelated case in slow fading is 30% and 10% for 1 and 16 users, respectively. This result shows that multiuser diversity minimizes the effect of severe antenna correlation. For the vehicular A

[8]

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