通信技术专业外文翻译--关于cdma2000的发射分集方案.doc

英文原文：Transmit Diversity Schemes for cdma-2000Dinesh Rajan，Steven D. GrayAbstract - Transmit diversity offers an advantage in the forward link of the cdma2000 system by balancing the spectrum efficiency in the uplink and downlink. Three schemes for performing transmit diversity are examined in this paperorthogonal transmit diversity, time switched transmit diversity, and selection transmit diversity for vehicular, indoor-outdoor pedestrian and indoor office environments. Also considered are some issues related to implementation complexity in the mobile handset. Under certain channel conditions, we show that low rate feedback of antenna selection and no forward link power control offers performance advantages over transmit diversity schemes with forward link power control and no antenna selection.I. INTRODUCTIONThird Generation cellular systems are expected to provide increased throughput, both for voice and packet data transmission. To achieve this goal, we must overcome the limitations inherent in a wireless channel. One of the biggest hurdles of mobile cellular communications is that of multipath fading, implying that, multiple copies of the transmitted signal are received with varying amounts of attenuation and delay. Diversity techniques are usually employed at either the transmitter and/or receiver to overcome the problem of fading. There are many techniques for providing diversity including temporal, frequency and spatial.In a cdma2000 system the bottleneck in capacity occurs in the forward link. This is due to the fact that the reverse link employs two antennas at the Base Station (BS) and maximal-ratio-combining of the received signals. As a result, transmit diversity schemes aid in balancing the forward and reverse links.Three schemes for transmit diversity are Orthogonal Transmit Diversity (OTD)1, Time-Switched Transmit Diversity (TSTD)4, and Selection Transmit Diversity (STD)3. It is unclear as to which of these schemes achieves the best trade of performance improvement vs implementation complexity. In this paper we present the results of simulations of the above schemes under the context of a cdma2000 3x physical layer. The remainder of the paper is organized as follows. Section 2 gives a description of the transmitter and receiver for the three diversity schemes. The results of our simulations are given in Section 3 followed by conclusions in Section 4.2. TRANSMIT DIVERSITY SCHEMESTable 1 gives the values for different operational environments for a low and median delay spread given by channels A and B respectively 2. The RMS delay spread increases in going from an indoor office environment to outdoor pedestrian environment and finally to the vehicular environment. The cdma2000 system considered has a nominal operating bandwidth of a 3x system (3.6864Mcps); the resolvability of the multipaths is approximately 270 nanoseconds. A frequency selective channel exists for the vehicular environment whereas the pedestrian and indoor environments are mostly characterized by flat fading. With a suitable rake receiver, the expectation is that the performance gain from transmit diversity is less for the vehicular environment when compared to the pedestrian and indoor environments.2.1 Orthogonal Transmit DiversityOrthogonal Transmit Diversity (OTD) is implemented in the transmitter as shown in Fig.1.The coded and interleaved bits are split into two different streams for simultaneous transmission over different transmit antennas. Different Walsh spreading codes are used for both streams to maintain the orthogonality. In order to maintain the effective number of Walsh codes per user as in single antenna transmission, the spreading length is doubled.Although we have considered the case of two transmit antennas, the scheme can be readily applied to any number of transmitting antenna. In addition to the normal pilot on one antenna, an auxiliary pilot is transmitted on the second antenna to aid in coherent detection at the receiver.2.2 Time Switched Transmit DiversityTime Switched Transmit Diversity (TSTD) is implemented in the transmitter as shown in Fig. 1. Unlike OTD where both the antennas are used all the time, in TSTD each user uses only one of the antennas at any instant of time. The same Walsh code is used for transmission on both the antennas and the length of these codes is the same as in single antenna transmission.The different users could be forced to shift between antennas using identical or different pseudo random switching patterns. Switching them pseudo-randomly means that on the average half the users will use one antenna and hence the capacity and crest factor of the power amplifiers are reduced. As in the case of OTD, two different pilots are needed for coherent detection.2.3 Selection Transmit Diversity (STD)Selective Transmit Diversity (STD) is implemented as shown in Fig. 1. The instantaneous transmit antenna used in TSTD may not yield the highest SNR at the MS receiver. Ideally, we want the transmitter to choose that antenna that yields the highest received SNR.However this is not possible because the transmitter does not know the state of the channel between the BS and MS.Hence a feedback channel is used from the MS to the BS,indicating which of the two antennas has a higher SNR.To prevent a large reduction in the reverse link capacity, a one-bit antenna selection (AS) message is sent from the mobile to the base station. An important consideration is how the performance is affected by the AS rate, the delay in implementing the AS command and error in the AS feedback. Ideally, the rate of feedback should be matched to the channel variations, the delay is zero and the feedback channel error rate is also zero. However using a variable rate feedback scheme is complicated and a fixed rate scheme must be used for ease of implementation.The basic idea behind these schemes is that even if the channel between one of the antennas and the receiver goes through a deep fade, the channel code is strong enough to recover the lost information. This conjecture assumes that the fading between the antennas is uncorrelated. In reality, experimental data has shown that the correlation coefficient as seen at the mobile from the two antennas may be as large as 0.7.The correlation between transmit antennas is affected by antenna spacing, orientation and polarization.The trade-offs of BS antenna design are such that to get maximum diversity a large antenna separation is required. However, large antenna separation may cause a time difference in multipath arrivals from each antenna causing an increase in searcher hardware at the mobile. Also in the case of TSTD and STD, if the time delay from the two antennas is large, an increase in multiuser interference will occur.2.4 Receiver IssuesThe structure of a typical Rake receiver is shown in Fig. 2. The following steps are taken in detecting the signal. 1. The received signal is despread by the pilot sequence to obtain a channel estimate, 2. each finger is adjusted in delay to match the multipath profile, 3. the data channel is despread and 4. the output of each finger is weighted and combined. Considering each of the diversity schemes, modifications must be made to the mobiles RAKE receiver. The appropriate modifications shown in Fig. 3 are additional integrators for the pilot and data channels for OTD and an additional integrator for the auxiliary pilot for TSTD and STD.Based on a first order approximation, the complexity of OTD and TSTD are comparable. The switch in TSTD replaces the combiner in OTD. Also there is an additional multiplier and traffic integrator needed in OTD.We feel there is not much difference in hardware complexity between TSTD and STD. We use the AS message in STD to control the switch at the transmitter.3. SIMULATION RESULTSThe parameters used in the simulations are given in Table II. All simulations were done in COSSAP.We carried out the simulations for the indoor-outdoor pedestrian channel A and vehicular channel A as specified in 1. First, for the pedestrian channel A, we compared the difference in performance between OTD, TSTD and STD. The Frame error rates (FER) given in Fig. 4 indicate that TSTD has about 0.10.2 dB gain over OTD. STD is about 0.3-0.5 dB better than TSTD.Fig.4 also shows that for antenna correlation of 0.75, OTD and TSTD suffer a loss between 0.7 0.9dB at antenna correlation of 0.75. Howver, STD is only about 0.3-0.5dB worse. Hence at a correlation of 0.75, STD is about 1dB better than TSTD, which in turn is about 0.2dB better than OTD. However for antenna correlation of 1, the performance of STD is the same as that of having no diversity. Hence the performance of STD deteriorates rapidly for correlations higher than 0.75. In practical situations, the antennas can be placed such that the antenna correlation is less than 0.75.The performance of OTD, TSTD and STD in the vehicular A channel, for a mobile speed of 120.0 kmph is given in Fig. 5. The results indicate that there is not much difference between OTD and TSTD. Further more their performance is close to the no diversity case. The performance of STD is much worse than the no diversity case. This is because the channel changes so fast that the AS message is outdated and the inferior antenna is often selected.OTD and TSTD are relatively insensitive to antenna correlation, in the vehicular case as evidenced from Fig. 5. However, as shown in Fig. 5, the performance of STD improves with increasing correlation. This is because for zero correlation, the performance of STD is worse than the no diversity case and for a correlation of one, the performance of the two are identical. Although counter intuitive at first, this can be explained by the fact that whenever the AS message is outdated, if the wrong antenna goes through a fading state that is closer to that of the better antenna, then the performance is better.Next, we studied the effects of switching frequency on the performance of TSTD. We found that the optimum switching frequency in terms of FER for TSTD depends on the Doppler rate of the channel, which is highly dependent on the speed of the mobile. Due to the difficulties in dynamically changing the switching frequency, a nominal frequency of 800 Hz was chosen due to good average performance. (Note - we chose 800 Hz because it is the same as one Power Control Group period)Fig. 6 shows the effect of improving the quality of the feedback channel on STD performance. For purposes of simulation the feedback channel is modeled as a Binary Symmetric Channel (BSC) with 4% crossover probability, under all conditions (outdoor and vehicular). As the error rate on the feedback channel is reduced from 4% to 1%, approximately 1dB improvement in STD performance is observed. One possible way of improving the feedback rate is by using channel coding. For reducing delay, we suggest the use of a simple repetition code.We found that the trends exhibited by TSTD and OTD with power control are similar to that without power control. For a fair comparison between STD and the other schemes, we fixed the rate of AS commands and compared TSTD with power control information at 800 Hz and STD without power control. Since STD is more sensitive to errors in the feedback channel, we use 400 Hz feedback with rate repetitive coding, rather than 800 Hz feedback and no coding. The comparison is given in Fig. 7. We see that STD performs about 1dB better than TSTD, with power control at an FER of 10%. Also in the range of FERs that we are interested in (between 10 % and 1 %) STD performance is better. We noted that going to lower information rate and higher code rates will not give much further gain in STD performance.A different approach to improve the performance of STD is to implement a verification process. Verification is achieved by distinguishing the signal on the two antennas by suitable means (ex. using different pilot symbols). The receiver determines which antenna was used for transmission, and uses this information for decoding purposes, rather than assume that the AS message that it sent to the transmitter was error free. We feel that if the AS messages are channel coded, resulting in very low error rates, then the gain from antenna verification is minimal. In addition it increases the software and hardware complexity at the mobile station.Another interesting result of this simulation study is that there appears to be no monotonic relationship between the FER and BER. This may be due to correlated bit errors and block interleaver used was designed for a single antenna system. To optimize performance for systems with antenna diversity, the interleaver design must be changed.4. CONCLUSIONSResults from this study indicate that the performance of STD is better than OTD and TSTD in the indoor-outdoor pedestrian environments. Although STD performance is poor in fast fading channels, our initial goal was to add diversity gain in slowly changing flat fading channels.The comparison of STD without power control and TSTD with power control, clearly shows that it is better to use the feedback channel to send antenna selection messages, rather than power control messages.We know that power control offers no performance gain in the vehicular case and hence even in that scenario, STD without power control is better than OTD or TSTD with power control. A first order hardware complexity analysis has not been conclusive in favor of any of these three schemes. Thus, from a pure performance perspective in slow fading, STD with 400Hz feedback and rate repetitive coding offers cdma2000 a performance boost over other transmit diversity schemes.REFERENCES1.“Orthogonal Transmit Diversity (OTD) for cdma Forward Link”, Motorola contribution to TIA,TR45.5.4/97.12.08.05, December 8-11, 1997.2. The cdma2000 ITU-R RTT Candidate Submission, Annex 2 Test environments and deployment models3. “Downlink Transmit Diversity”, Nokia contribution to ETSI SMG2, Bocholt, Germany, May 18-20, 1998.4.“TSTD (Time Switched Transmission Diversity)”,contribution by TTA to TIA- TTAJSG/01.98.05.13.05, May 13, 1998.5. “The cdma2000 ITU-R RTT Candidate Submission”,TIA TR45.5/98.04.03.03ACKNOWLEDGMENTSThe authors wish to thank Dr. Behnaam Aazhang and Lin Ma for their valuable comments.中文译文：关于cdma2000的发射分集方案摘要在通过cdma2000系统的前向链路时，发射分集提供了一个优势，那就是发射分集平衡了上行链路和下行链路的频谱效率。在车辆，行人和室内办公室环境下，本文检测了将执行的发射分集的三个方案正交发射分集，时间切换发射分集，选择发射分集。这些方案要实现移动手机的复杂度存在着一定的问题。在一定的信道条件下，我们证明，低速率反馈天线的选择和没有前向链路功率控制提供前向链路功率控制和无天线选择发射分集方案的性能优势。引言第三代蜂窝系统，预计可提供增加的吞吐，为语音和分组数据传输。为了实现这一目标，我们必须克服无线信道固有的局限性。多径衰落是蜂窝移动通信的最大的障碍之一，它意味着所接收的多个副本的传送信号的将导致大量的衰减和延迟。发射器/接收器通常采用分集技术克服衰落的问题。这里提供了各种各样的技术，包括时间和频率以及空间。cdma2000系统容量的瓶颈发生在前向链路中，这是由于一个事实，即反向链路在基站（BS）运用了两根天线和采用了信号所接收的最大合并集。因此，发射分集技术维持着正向和反向链路的平衡。发射分集的三个方案是正交发射分集（OTD），时间切换发送分集（TSTD），并选择发送分集（STD）。目前还没有一种方案能达到最佳的性能改善与实施的复杂性的平衡。在本文中，我们得到了上述方案中关于cdma20003x 物理层的的模拟结果。发射分集方案表1给出了不同的操作环境下由通道A和B各自得到的低时延扩展中值的结果。RMS时延扩展增加了室内办公环境和室外步行环境，以及车载环境。 cdma2000系统考虑一个名义来经营一3x 系统（3.6864Mcps）的带宽;RMS时延扩展可分解的多径约270纳秒。频率选择性信道有行车环境和行人以及室内环境，这些环境大多的特点是平坦衰落。用合适的分离多径接收器中，期望的是，从发送分集的性能增益是以下的车辆用环境时相比，行人和室内环境。实验恰当的运用了rake接收机，希望得到的结果是，车辆环境的发射分集所增加的性能比行人和室内环境差。正交发射分集正交发射分集（OTD）在发射机中实现，如图1所示。编码和交织后的比特被分成两个不同的流，在不同的发送天线同时发送。不同的两个数据流的都采用沃尔什扩频码来保持正交性。为了保持有效的沃尔什码数量，每个用户的单天线发送的扩展长度都是加倍的。虽然我们已经考虑的两条发射天线情况下，但是该方案可以很容易地应用到任何发送天线的数量。除了正常的一个天线的导频外，还有一个备用天线的辅助导频用来协助接收机的相干检测。时间切换发射分集（TSTD）在发射机中实现，如图1所示。与OTD使用两个天线不同，TSTD每个用户使用在任何时刻只有一个天线。在单天线发射的情况下，相同的沃尔什码应用于相同长度的天线和代码的传输。不同的用户可能会使用相同的或不同的伪随机开关模式进行天线的切换。一般情况下，伪随机开关意味着半数的用户将使用一个天线降低功率放大器的容量和波峰因数。对于OTD来讲，两个不同的导频需要进行相干检测。选择发送分集（STD）选择性发射分集（STD）的实现，如图1所示。在MS接收机情况下，瞬时发射天线用于TSTD可能不会产生最高的信噪比。理想情况下，我们要选择发射机，天线，产生最高的接收信噪比。因此，反馈信道用于MS与BS之间，表示两个天线中，具有更高SNR的一个。防止反向链路容量的大幅减少，一比特的天线选择（AS）的消息需从移动台发送为了到基站。一个考虑的重要因素是AS率影响性能的情况，而且AS命令和错误反馈将会产生延迟。理想情况下，应相匹配的信道变化率反馈的延迟是零，反馈信道误码率也为零。然而，使用一个浮息反馈计划是复杂的，为了便于实施，必须使用固定利率计划。这些计划背后的基本思路是，即使天线和接收器的通道经过快衰落，信道编码也足以恢复丢失的信息。这种推测假定是天线之间的衰落是不相关的。实际上，数据表明，所观察的手机来自两个天线的相关系数可能都是0.7。天线之间的相关性受到天线间距，方向和极化发射的影响。权衡基站天线的设计是这样的，最大的多样性需要大型天线分离。然而在移动的情况下，大天线分离可能会引起搜索器硬件来自每个天线在多径到达的时间差的增加。而且，在TSTD和STD的情况下，如果两个天线的时间延迟大，那么多用户干扰的增加会发生。接收器的问题一个典型的rake接收机的结构示，如图 2所示。他采取以下步骤检测信号，先对接收到的信号进行解扩，得到信道估计值的导频序列。接着，每个手指调整延迟相匹配的多路径配置文件。然后对数据信道进行解扩，以及每根手指的输出需进行加权和组合。考虑每个分集方案的不同，必须对移动站的rake接收机作出修改。图3做出了适当的修改，增加了TSTD和STD的额外集成商和OTD的数据通道和导频的额外集成商。 基于一阶近似的情况，OTD和TSTD的复杂性具有可比性。TSTD开关将取代OTD组合，也需要一个额外的乘数和交通集成OTD。我们觉得目前的工作，已经达到顶峰，TSTD和STD之间的硬件复杂度没有太大的区别。我们使用消息由STD用发射器来控制开关。仿真结果仿真中使用的参数列于表二。所有进行仿真在COSSAP。在表1中，我们指定了模拟室内室外的行人通道和车辆通道。首先，在行人通道中，我们比较了OTD，TSTD和STD之间的性能差异。图4中给出的帧错误率（FER） 4表明TSTD超过OTD约0.1-0.2 dB增益。STD约0.3-0.5分贝优于TSTD。仿真也显示，天线的相关性为0.75，OTD和TSTD在其间蒙受损失在0.7 - 0.9分贝7然而，STD仅约0.3-0.5分贝损失。因此，上述的相关性为0.75，STD优于TSTD1dB左右，比OTD优于0.2分贝。然而，当天线的相关系数为1时，STD的性能是一样的，不具有多样性。因为其相关性高于0.75，STD的性能大幅下降。在实际情况中，天线的放置可以使天线的相关性小于0.75。图5中给出的是车辆通道情况，移动速度为120.0 KMPH中OTD，TSTD和STD的表现。 结果表明，OTD和TSTD之间没有太大