L7/ Practical Considerations of Wideband AQAM In deriving the upper bound performance of wideband AQAM portrayed in Figure 4.21(b), various assumptions were made and stated in Section 4.3.1. However, in order to provide a more accurate comparison between AQAM and its constituent fixed modulation modes, those assumptions must be justified and their effects have to be investigated. Specifically, perfect, i.e. error-free feedback was assumed for the DFE, while in practice erroneous decision can be fed back, which results in error propagation. Consequently the impact of error propagation is studied in the context of both fixed and adaptive QAM schemes. Furthermore, as stated in Section 4.3.1, perfect modulation mode selection was assumed, whereby the output SNR of the DFE was estimated perfectly prior to transmission. However, in stipulating this assump- tion, the delay incurred between channel quality estimation and the actual utilization of the estimate was neglected in the wideband AQAM scheme. In this chapter the impact of co-channel interference on the wideband AQAM scheme is also investigated. In this respect, interference compensation techniques are invoked in order to reduce the degradation resulting from the co-channel interference. Let us now commence our investigations by studying the error propagation phenomenon in the DFE. 7.1 Impact of Error Propagation Error propagation is a phenomenon that occurs, whenever an erroneous decision is fed back into the feedback filter of the DFE. When a wrong decision is fed back, the feedback filter produces an output estimate which is erroneous. The incorrect estimate precipitates further errors at the output of the equalizer. This leads to another erroneous decision being fed back into the feedback filter. Consequently, this recursive phenomenon degrades the BER performance of the DFE. Intuitively, the effects of this error will last throughout the memory span of the feedback filter. This causes an error propagation throughout the feedback filter, until the memory of the feedback filter is cleared of any erroneous feedback inputs. The performance of the fixed modulation modes of our AQAM scheme in conjunction 257 Adaptive Wireless Tranceivers L. Hanzo, C.H. Wong, M.S. Yee Copyright © 2002 John Wiley & Sons Ltd ISBNs: 0-470-84689-5 (Hardback); 0-470-84776-X (Electronic) 258 CHAPTER 7. PRACTICAL CONSIDERATIONS OF WIDEBAND AQAM Transmission Burst type: Non-Spread Speech Burst of Figure 4.13. DFE Parameters: No. of feedforward taps, Nf 7 No. of feedback taps,Nb 35 See Figure 4.12 and Typical Urban Rayleigh-faded Weights Channel Parameters: 8 Number of RKCE taps 0 Initial Channel Estimate Vector h(o) 2 300 6 (see Equation 3.47) 0.011 System error covariance Matrix, Q(k) 0<g11 Measurement error covariance Matrix, R(k) = g1 Recursive Kalman Channel Estimator Parameters: Past Decision Decision Feedback Normalized Doppler Frequency: 3.25 x 10-5 Table 4.5 Table 7.1: Generic simulation parameters that were utilized in our experiments. with error propagation is depicted in Figure 7.1, where the corresponding curve of the error- free feedback scenario is also displayed for comparison. Perfect channel compensation was applied at the receiver and the other simulation parameters are listed in Table 7.1. There was only a slight degradation in the BER performance of the BPSK and 4QAM modes, as evidenced by Figure 7.1. However, for the higher-order modulation modes of 16QAM and 64QAM, a more severe degradation of approximately 1.5 and 3.0dB was recorded, respec- tively. These results were expected, since the higher-order modulation modes were more susceptible to feedback errors due to the smaller Euclidean distance of their constellation points. The impact of error propagation on the wideband AQAM scheme over a TU Rayleigh fad- ing channel was also investigated and the results are shown in Figure 7.2. The corresponding curve of the wideband AQAM scheme with error-free decision feedback was also shown for comparison and the switching thresholds of the wideband AQAM scheme were set according to Table 4.8 for target BERs of 1% and 0.01%. At low to medium average channel SNRs the BER performance of the wideband AQAM scheme exposed to error propagation was similar to that of the AQAM scheme with error-free decision feedback. However, at higher average channel SNRs, as a result of error propagation, a BEWSNR degradation of approximately 3dB was observed. These results were consistent with the results shown for the fixed mod- ulation modes of Figure 7.1. At low to medium average channel SNRs, the impact of error propagation was negligible due to two factors. Firstly, at those channel SNRs the lower-order modulation modes, which were more robust against error propagation were utilized more frequently. Secondly, the higher-order modulation modes were only utilized, when the chan- nel quality was favourable, which resulted in low instantaneous BERs. Consequently, less erroneous decisions were made, which reduced the impact of error propagation. However, at higher average channel SNRs, the probability of modulation mode switching 7.2. CHANNEL QUALITY ESTIMATION LATENCY 259 10.' W p! m 1 o-~ 1 rr-6 Channel SNR(dB) Figure 7.1: Impact of error propagation on the modulation modes of BPSK, 4QAM, 16QAM and 64QAM over the TU Rayleigh fading channel of Figure 4.12. Perfect channel compen- sation was applied and the simulation parameters are listed in Table 7.1. was low, where the 64QAM mode was frequently chosen. Consequently, the impact of error propagation was more apparent, as it was observed the case of the fixed modulation mode of 64QAM in Figure 7.1. Nevertheless, the target performance of 1% and 0.01% was achieved even in the presence of erroneous decision feedback. 7.2 Channel Quality Estimation Latency The estimation of the channel quality prior to transmission is vital in the implementation of the wideband AQAM scheme, since it is used in the selection of the appropriate modulation mode for the next transmission burst. In generating the upper bound performance curves depicted in Figure 4.21(b), we assumed that the required modulation mode was selected per- fectly prior to transmission, as stated in Section 4.3.1. However, in a realistic and practical wideband AQAM scheme this assumption must be discarded as a result of the inherent chan- nel quality estimation delay incurred by the scheme. Nevertheless, it must be stressed that 260 CHAPTER 7. PRACTICAL CONSIDERATIONS OF WIDEBAND AQAM 1 o-2 IO-^ S 2 5 2 d p32 IO-^ 1 o-6 W5 S 2 5 0 1 % BER, 0 0.01 % BER - Error Prop. . . . . . . . Perfect 0 5 10 15 20 25 30 35 40 Channel SNR(dB) Figure 7.2: Impact of error propagation on the wideband AQAM scheme over a TU Rayleigh fading channel, where the switching thresholds were set according to Table 4.8 for target BERs of 1% and 0.01%. Perfect channel compensation was applied and the simulation parameters are listed in Table 7.1. the assumption was essential in order to record the upper bound performance of the AQAM scheme. The channel quality estimation latency is defined as the delay incurred between the event of estimating the channel quality to the actual moment of transmission using the modem mode deemed optimum at the instant of the channel quality estimation. During this delay, the fad- ing channel quality varies according to the Doppler frequency and consequently, the channel quality estimates perceived prior to transmission may become obsolete. Consequently, the chosen modulation mode is not optimum with regards to the actual channel quality and this degrades the BER performance of the wideband AQAM scheme. This degradation is de- pendent on the amount of delay incurred and the rate at which the fading channel quality fluctuates, as quantified by its Doppler frequency. Before we proceed to investigate the per- formance degradation as a result of the channel quality estimation latency, let us present two possible time-frame structures, where wideband AQAM can be implemented. This will pro- vide us with a clearer understanding concerning the amount of delay incurred by the scheme. 7.2. CHANNEL OUALITY ESTIMATION LATENCY 261 7.2.1 Sub-frame Based Time Division Dupleflime Division Multiple Access System In this sub-frame based Time Division Duplexmime Division Multiple Access (TDD / TDMA) system, the uplink and downlink time-slots are separated equally into two halves of the TDMA frame, as shown in Figure 7.3. In this respect the time-slot is defined as the window in time, in which the transmission burst is received or transmitted. By utilizing the time-frame configuration shown in Figure 7.3, we will explain the operation of the wideband AQAM scheme and the corresponding channel quality estimation latency that is incurred. In the up- link transmission, shown in Figure 7.3, the channel quality was estimated at the Base Station (BS) and subsequently an appropriate modulation mode was selected for its next downlink transmission. This was achieved by exploiting the channel’s reciprocity during the uplink and downlink transmissions, since the transmission frequencies for both links were identical in a TDD system. Having selected the modulation mode, a delay of half a TDMA frame was incurred at the BS before the downlink transmission was activated as shown in Figure 7.3. We refer to this regime as open-loop controlled AQAM. Let us now in the next section consider closed-loop control. 7.2.2 Closed-Loop Time Division Multiple Access System The corresponding closed-loop TDMA construction was similar to that of the sub-frame TDDRDMA with the exception that the uplink and downlink transmission frequencies were different. Hence this was a Frequency Division Duplex (FDD) system. Consequently, the assumed channel reciprocity - which was invoked in the sub-frame based TDDmDMA sys- tem - was less applicable. Hence a closed-loop signalling system was required in order to implement the wideband AQAM scheme, which is shown in Figure 7.4. In the uplink trans- mission, the channel quality was estimated at the BS, in order to select the next uplink modu- lation mode. Subsequently, the selected uplink modulation mode was conveyed to the Mobile Station (MS) with the aid of control symbols during the next downlink transmission. Conse- quently, the selected modulation mode was utilized by the MS in its next uplink transmission. As a result of the closed-loop signalling regime, the delay incurred by the system was equal to the duration of one TDMA time-frame. Consequently, the open-loop system described in Section 7.2. l was more applicable to AQAM transmission as a result of its lower delay, when compared to the close-loop system. This latency can be substantially reduced using slot-by- slot TDDEDMA, where the uplink and downlink slots are adjacent, which is also supported by the third-generation Universal Mobile Telecommunication System (UMTS) [221]. 7.2.3 Impact of Channel Quality Estimation Latency Regardless of the type of wideband AQAM scheme that was implemented, we investigated the maximum delay that could be tolerated by the AQAM scheme by assuming that the per- formance degradation in the uplink and downlink transmission was identical. In our experi- ments, the delay was measured in terms of a time-slot duration of 72ps, as proposed in the Pan European FRAMES framework [l5 l]. Mid-amble associated CIR estimation based on the Kalman algorithm - which was discussed in Chapter 3 - was implemented, in order to es- timate the channel quality. The normalized Doppler frequency was set to 3.25 x lop5, which 262 CHAPTER 7. PRACTICAL CONSIDERATIONS OF WIDEBAND AOAM Downlink Transmission I TDMA Frame I W MS Uplink - - Downri~k Band Band '\\ Select Activate ,l' Mode Mode ,,' Modulation Modulation,' I W ,' Delay = Frame ._ __ __ Half aTDMA Figure 7.3: Sub-frame based TDD/TDMA system for the uplink and downlink transmission, as de- scribed in Section 7.2.1. Channel reciprocity was exploited in this system and the channel quality estimation latency was equivalent to half a TDMA frame. was equivalent to a TDMA system using a 1.9GHz in carrier frequency, transmission rate of 2.6 MSymbols/s and a vehicular speed of 13.33ds. The specific simulation parameters used in our subsequent experiments are listed in Table 7.1. The AQAM switching thresholds were set according to Table 4.8, which were optimised for maintaining target BERs of 1% and 0.01%. The results of our investigations are shown in Figures 7.5(a) and 73b) for target BERs of 1% and 0.01%, respectively. In these figures the wideband AQAM scheme was subjected to a delay of 8, 16 and 32 time-slots and the performance was compared to that of the zero- delay upper bound performance. For the target BER of 1% we can observe that the BER performance degradation increased, as delay was increased as evidenced by Figure 7.S(a). At high average channel SNRs, the BER degradation was minimal as a result of the reduction of modulation mode switching frequency, where the 64QAM mode was frequently selected. The BER degradation was more evident for the AQAM scheme designed for a low target BER of 0.01% as a result of its increased sensitivity to errors. By referring to Figure 7.S(b), at a 7.2. CHANNEL QUALITY ESTIMATION LATENCY 263 Uplink Transm:lssion ‘ _ . Downlin~ Transmission 1 TDMA Eqme * D Uplink Dowili.qk Activate Band = = Band ‘\, * Modulation ‘\ Mode t I1 II II l Select Signal Modulation Modulation.” Mode Mode ,,‘ 4 D Delay e~ 4- TDMA Frame _- Figure 7.4: Closed-loop FDD/TDMA system for the uplink and downlink transmission, as described in Section 7.2.2. Channel reciprocity was not assumed in this system in favour of a closed- loop signalling regime and the channel quality estimation latency was equivalent to the duration of one TDMA frame. channel SNR of 20dB and at a delay of 32 time-slots, the BER performance was degraded by approximately two orders of magnitude in comparison to the upper bound performance. In these experiments, the modulation mode selection regime was affected by the delay incurred by the system. The impact was especially significant, when the channel quality was low and a less robust higher-order modulation mode was utilized erroneously. The BPS performance in Figures 7.5(a) and 7.5(b) remained unchanged for different delays. This can be readily explained by observing that on average the throughput was the same even if the modulation mode selected was erroneous. As discussed previously, the performance of the wideband AQAM scheme depended on the channel quality estimation delay incurred, as well as on the Doppler frequency of the fading channel. In order to investigate the system’s performance dependency on the Doppler frequency, a slower fading channel having a normalized Doppler frequency of 2.17 x IOp6 was utilized. This corresponded to a carrier frequency of l.SGHz, transmission rate of 2.6 Msymbols/s and a pedestrian speed of 0.89ds in the Pan European FRAMES Proposal [ 15 1 1. 264 CHAPTER 7. PRACTICAL CONSIDERATIONS OF WIDEBAND AQAM IOU 6 10 ' 5 IO 4 e! 210' m 3a m IOJ 10 I IOh 0 0 5 10 15 20 25 30 35 40 Channel SNR(dB) (a) Performance at a target BER of 1% at channel quality estimation delays of 8, 16 and 32 time-slots, where each time-slot is of 72ps duration. 6 5 4 32 m 2 I ''.'l! 10 l5 20 25 30 35 y2 Channel SNR(dB) (b) Performance at a target BER of 0.01% at channel quality estimation delays of 8, 16 and 32 time-slots, where each time-slot is of 72ps duration. Figure 7.5: Impact of channel quality estimation latency upon the wideband AQAM scheme, where the modem mode switching thresholds were set according to Table 4.8. The normalized Doppler frequency was set to 3.25 x and the other simulation parameters are listed in Table 7.1. 7.2. CHANNEL OUALITY ESTIMATION LATENCY 265 The other simulation parameters were set according to Table 7.1. The BER and BPS perfor- mances of the AQAM scheme over this slower fading channel are shown in Figures 7.6(a) and 7.6(b) for a target BER of 1% and 0.01%, respectively. In these figures, the characteristics observed in Figures 7.5(a) and 7.5(b) were also evident and hence the associated trends can be explained similarly. However. in order to investigate the impact of the Doppler frequency, the BER performance at an average channel SNR over the two fading channels exhibiting differ- ent Doppler frequencies were recorded against different delays in Figures 7.7(a) and 7.7(b). For a target BER of 1% a higher BER degradation was experienced by the higher Doppler frequency scheme, where at a BER of 2 x lop2 the lower Doppler frequency scheme can tolerate an additional delay of 7 time-slots, as evidenced by Figure 7.7(a). Similarly, at a BER of 1 x for the scheme having a target BER of 0.01%, an additional 5 time-slots delay can be tolerated by the scheme with the lower Doppler frequency. From the above experiments, we can conclude that as the channel quality estimation de- lay and Doppler frequency increased, the performance degradation of the wideband AQAM scheme was higher. Furthermore, the impact of channel quality estimation latency was more evident at low target BERs due to its increased error sensitivity. In order to improve the ro- bustness of the AQAM scheme against channel quality estimation delay, in the next section we will invoke a simple channel quality prediction method and experimentally optimise the modem mode switching thresholds. 7.2.4 Linear Prediction of Channel Quality In order to mitigate the effects of channel quality estimation delay on the wideband AQAM scheme, the next channel quality estimate can be predicted using linear prediction. This sim- ple technique utilizes the previous channel estimates for linear prediction, in order to predict the next channel quality estimate. Subsequently, if the prediction is accurate, the modulation mode selection errors will decrease, yielding a more delay-robust wideband AQAM scheme. This linear prediction technique was applied to the wideband AQAM scheme in conjunction with two different Doppler frequencies and various time delays for target BERs of 1% and 0.01%. The results are depicted in Figures 7.8(a) and 7.8(b) for an average channel SNR of 20dB, where the performance without linear prediction is also shown for comparison. In these figures, the linearly predictive scheme exhibited a higher tolerance against channel quality es- timation delay. The maximum delays that can be tolerated for a target BER of 1% and 0.01% are tabulated in Table 7.2 for the schemes with and without linear prediction. From this table, channel quality estimation delay gains of approximately 8 time-slots can be achieved using the above linear predictive techniques for the lower Doppler frequency scheme. Similarly, delay gains of 6 time-slots were recorded for the higher Doppler frequency scheme. In these experiments we have highlighted that a simple channel quality prediction tech- nique can substantially improve the robustness of the wideband AQAM scheme against chan- nel quality estimation delay. However, it must be stressed that the AQAM scheme performed better in a slowly varying environment, which also facilitated a better channel prediction performance. 266 CHAPTER 7. PRACTICAL CONSIDERATIONS OF WIDEBAND AQAM 1" 0 5 IO 15 20 25 30 35 40' Channel SNR(dB) (a) Performance at a target BER of 1% at channel quality estimation delays of 8, 16 and 32 time-slots, where each time-slot is of 72ps duration. IO2 a: i& 10" IO4 32 m 2 1 n 0 5 IO 15 20 25 30 35 40' Channel SNR(dB) (b) Performance at a target BER of 0.01% at channel quality estimation delays of 8, 16 and 32 time-slots, where each time-slot is of 72ps duration. Figure 7.6: Impact of channel quality estimation latency upon the wideband AQAM scheme, where the modem mode switching thresholds were set according to Table 4.8. The normalized Doppler frequency was set to 2.17 x and the other simulation parameters are listed in Table 7.1. [...]... modem mode switching regime of the AQAM scheme in reducing the impact of CC1 on the demodulation process Before we invoke these two approaches, let us first quantify the impactof CC1 on both fixed and adaptive modulation modes without the aid of CC1 compensation techniques In the following fixed modulation mode based experiments, the modulation mode of the interferer and reference user was identical . the impact of error propagation is studied in the context of both fixed and adaptive QAM schemes. Furthermore, as stated in Section 4.3.1, perfect modulation. performance of the fixed modulation modes of our AQAM scheme in conjunction 257 Adaptive Wireless Tranceivers L. Hanzo, C.H. Wong, M.S. Yee Copyright © 2002