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
Trang 1is 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
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)
Trang 2See Figure 4.12 and Typical Urban Rayleigh-faded Weights
Channel Parameters:
8 Number of RKCE taps
0 < g 1 1 Measurement error covariance Matrix, R ( k ) = g1
Recursive Kalman Channel Estimator Parameters:
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
Trang 37.2 CHANNEL QUALITY ESTIMATION LATENCY 259
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
Trang 4Figure 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
Trang 57.2 CHANNEL OUALITY ESTIMATION LATENCY 261
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
Trang 6Figure 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 1 3 3 3 d s 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 7 3 b ) 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
Trang 77.2 CHANNEL QUALITY ESTIMATION LATENCY 263
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 8 9 d s in the Pan European FRAMES Proposal [ 15 1 1
Trang 8(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
(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
Trang 97.2 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 l o p 2 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
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
Trang 101"
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
(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 7 2 p s 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
Trang 117.2 CHANNEL OUALITY ESTIMATION LATENCY 267
(a) Performance at a target BER 01 1% for different channel quality esti-
mation delays in terms of time-slots (TS), where each time-slot is of 72ps
duration
(b) Performance at a target BER of 0.01% for different channel quality es-
timation delays in terms of time-slots (TS), where each time-slot is of 72ps
duration
Figure 7.7: Impact of channel quality estimation latency upon the wideband AQAM scheme for two
different normalized Doppler frequencies of 3.25 x (at 13.3ds) and 2.17 x 1OP6(at 0.89m/s), where the modem mode switching thresholds were set according to Table 4.8
The average channel SNR was set to 20dB and the other simulation parameters are listed
in Table 7.1
Trang 12(a) Performance at a target BER of 1% for different channel quality esti-
mation delays in terms of time-slots (TS), where each time-slot is of 7 2 ~ s
(b) Performance at a target BER of 0.01% for different channel quality es-
timation delays in terms of time-slots (TS), where each time-slot is of 7 2 ~ s
duration
Figure 7.8: Impact of channel quality estimation latency upon the wideband AQAM scheme for two
different normalized Doppler frequencies of 3.25 x (at 1 3 3 d s ) and 2.17 x lO-'(at 0.89ds), where the switching thresholds were set according to Table 4.8 The performance utilizing the linear prediction technique (denoted by Linear Prediction) was compared to the conventional non-predicted technique (denoted by Past Estimate) The average channel SNR was set to 20dB and the other simulation parameters are listed in Table 7.1
Trang 137.2 CHANNEL QUALITY ESTIMATION LATENCY 269
4
12
8
16 0.89
Past(TS) Linear(TS) Past(TS)
Linear(TS)
Table 7.2: The channel quality estimation delays in an AQAM wideband scheme in order to achieve
target BERs of 1% and 0.01%, which were extracted from Figure 7.8 The delays were measured for different normalized Doppler frequencies of 3.25 x lop5 (at a vehicular speed
of 13.3ds) and 2.17 x 1OP6(at a vehicular speed of 0 8 9 d s ) Further comparisons were made between the performance achieved by utilizing the linear prediction technique of Sec- tion 7.2.4 (denoted by Linear) and the conventional non-predicted scheme (denoted by Past) The delays were measured in terms of time-slots (TS), where each time-slot was of 72ps du- ration
Having considered the implications of channel quality estimation latency, we will now in- vestigate the performance of wideband AQAM in a sub-frame based TĐÁDMA scheme, which was discussed in Section 7.2.1 The channel quality estimation latency incurred was equivalent to half of a TDMA frame, which was set to 4.615ms according to the Pan Euro- pean FRAMES proposal [ 1511 Hence the channel quality estimation latency incurred was 2.3075ms or 32 time-slots, where each time-slot was of 72ps duration The linear prediction technique of Section 7.2.4 was invoked, in order to predict the next channel qualitỵ The modem mode switching thresholds - which are shown in Table 7.3 - were experimentally determined in order to achieve the target BERs of 1% and 0.01%, since the impact of delay prohibited the utilization of the Powell optimization technique discussed in Section 4.3.5 The normalized Doppler frequency was set to 2.17 x l o p 6 in order to create a slowly varying propagation environment and the other simulation parameters were set according to Table 7.1 The associated wideband AQAM performances for target BERs of 1% and 0.01% are shown in Figures 7.9 and 7.10, respectivelỵ In both of these figures, the corresponding upper bound performance was also included for benchmarking
t l ( d B ) l 2 ( d B ) t 3 ( W t4(dB)
1%
25.00 18.80
13.56 11.25
0.01%
18.68 13.65 8.00 5.64
Table 7.3: The switching thresholds that were manually optimised in order to achieve target BERs
of 1% and 0.01% The wideband AQAM regime was implemented in a sub-frame
based TĐiTDMA system having a channel quality estimation latency of 32 time-slots or 2.3075ms as shown i n Figures 7.9 and 7.10
Referring to Figures 7.9 and 7.10, the target BERs of 1% and 0.01% were achieved with slight degradation in terms of its throughput performance, when compared to the upper bound performancẹ Explicitly, a BPS/SNR degradation of 0.9dB and 1.8dB was observed for tar- get BERs of 1% and 0.01%, respectivelỵ The BPS throughput performance of the latency- impaired wideband AQAM scheme was also compared to that of the fixed modulation modes
Trang 14Channel SNR(dB)
Figure 7.9: The performance of a sub-frame TDD/TDMA based wideband AQAM scheme having a
channel quality estimation latency of 32 time-slots or 2.3075ms, as described in Section
7.2.1 The switching thresholds are set according to Table 7.3 for a target BER of 1 % and
the simulation parameters are listed in Table 7.1 The upper-bound performance without
channel quality estimation delay was also displayed for comparison
of Figure 7.1 for target BERs of 1% and 0.01% The results are shown in Figure 7.1 1 exhib-
ited the same characteristics as Figure 4.25 and hence can be justified similarly The gains
achieved by the latency-impaired wideband AQAM are tabulated in Table 7.4 for a through-
put of 1, 2 and 4 bits per symbol, corresponding to the throughput of BPSK, 4QAM and
16QAM modes, respectively
BPS
5.70 25.70 32.00 1.40
22.40 21.00
4
5.40 29.00
23.60 0.70
13.70 13.00
2
6.40 26.70 20.30 1.40
Table 7.4: The channel SNR gain achieved by the sub-frame based TDDiTDMA wideband AQAM
scheme with a channel quality estimation latency of 32 time-slots or 2.3075ms, when com-
pared to the fixed modulation modes, at throughputs of 1, 2 and 4 bits per symbol (BPS)
The values were extracted from Figure 7.1 1
Trang 157.3 EFFECT OF CO-CHANNEL INTERFERENCE ON AOAM 271
Figure 7.10: The performance of a sub-frame TDDiTDMA based wideband AQAM scheme with a
channel quality estimation latency of 32 time-slots or 2.3075ms, as described in Section 7.2.1 The switching thresholds are set according to Table 7.3 for a target BER of 0.01% and the simulation parameters are listed in Table 7.1 The upper-bound performance with- out channel quality estimation delay was also displayed for comparison
In this section, we have analysed and recorded the impact of channel quality estimation latency over channels having different Doppler frequencies upon a wideband AQAM scheme,
We invoked a simple linear prediction technique, in order to predict the next channel quality estimate, which allowed the wideband AQAM scheme to be more robust against channel quality estimation delay Subsequently, the maximum channel quality estimation delay that can be tolerated by a wideband AQAM scheme was recorded in Table 7.2 for target BERs
of 1% and 0.01% Finally, we characterized a realistic and practical sub-frame TDD/TDMA based AQAM system, which was robust up to delays of 2.3ms and still achieved substantial BEWSNR gains over fixed modulation modes, as evidenced by Figure 7.11 and Table 7.4
7.3 Effect of Co-channel Interference on AQAM
In all our previous experiments our work has been restricted to a noise limited environ- ment However, in a cellular mobile environment the impact of interference - in particular co-channel interference - has to be considered in a wideband AQAM scheme In order to increase the capacity of a cellular mobile environment, tight frequency reuse techniques are frequently utilized [222] This is a technique, whereby a particular radio channel of a cell can
Trang 16Figure 7.11: The BPS throughput of the sub-frame TDDRDMA based wideband AQAM scheme with
a channel quality estimation latency of 32 time-slots or 2.3075ms and that of the individ- ual fixed modulation modes of BPSK, 4QAM, 16QAM The BPS throughput values were extracted from Figures 7.10 and 7.2 and the simulation parameters are listed in Table 7.1
be reused in another cell, which is separated by a certain distance As a result of the utiliza-
tion of a common radio channel in these two cells, transmission in one cell can propagate to and distort the co-channel transmissions in the other cell Hence the interference is termed as CO-Channel Interference (CCI)
In our subsequent experiments the interferer was assumed to be temporally synchronous,
in other words the signals transmitted by the interferer and the reference user were perfectly synchronous at the receiver This approach was also adopted by - amongst others - Torrance and Webb [61,223] The signal of the interferer was also assumed to be phase non-coherent with the reference signal at the receiver In this respect, we have assumed that the independent fading nature of each user’s channel resulted in a phase non-coherent scenario The channel model of the interferer and desired user was assumed identical, as described by Table 4.5 and Figure 4.12 The Signal to Interference Ratio (SIR) is a parameter which characterizes an interference-limited environment and is defined as follows [222,224] :
Trang 177.3 EFFECT OF CO-CHANNEL INTERFERENCE ON AQAM 273
c
where K 1 is the number of interferers, S is the signal power of the reference user and P p t f
is the power transmitted by the kth interferer
Channel Quality Estimation
In a wideband AQAM scheme the presence of CC1 can potentially degrade the accuracy of the demodulation process and the channel quality estimation, which is needed for AQAM mode selection The issues associated with the impact of CC1 upon the demodulation pro-
cess is discussed in Section 7.3.2 in more depth, while here we focus our attention on the
performance degradation inflicted by the channel quality estimation in this section
In Section 7.2 we have discussed the importance of channel quality estimation, in order to ensure that the selected modulation mode was optimum However, in an interference-limited environment the performance of a wideband AQAM scheme is degraded due to two factors:
0 The presence of CC1 degrades the ability of the receiver to accurately estimate the
channel quality on a burst-by-burst basis Consequently the modulation mode selection errors increase, yielding a degraded BER performance
0 In a TDD/TDMA AQAM scheme, the channel’s reciprocity is exploited in the uplink and downlink transmission, in order to estimate the channel quality, as highlighted in Section 7.2 However, this reciprocity is not applicable in estimating the CCI, since the uplink and downlink CC1 possess different propagation paths and different transmitted powers Consequently the modulation mode selection regime of the receiver - which is subjected to uncorrelated uplink and downlink CC1 - may not be optimum
By assuming that statistically speaking the impact of CC1 on the receiver is identical in the uplink and downlink transmission, we can focus our investigations on the downlink per- formance for simplicity In order to isolate and study the impact of CC1 on the modem mode switching regime, we assumed that the CC1 was only present during the uplink transmission, but not during the downlink transmission Again, this was a hypothetical situation, but it was necessary to stipulate these conditions, in order to analyse the impact of CC1 on the switching regime With this assumption in place, the reception at the BS was contaminated with CC1 while the MS experienced a interference-free demodulation conditions
In our subsequent discussion, we considered only a single-interferer scenario and the
modulation mode of the interferer in the wideband AQAM scheme was chosen randomly
from the set of permissible modes of the AQAM regime The uplink channel quality was
estimated using the CIR estimator based on the Kalman algorithm of Section 3.2.1 with the aid of the mid-amble sequence of the FRAMES non-spread speech burst of Figure 4.13 The switching thresholds were set according to Table 4.8 for target BERs of 1% and 0.01% The other simulation parameters are listed in Table 7 l
An informative insight into the impact of CC1 on the AQAM switching regime can be
obtained by observing the average channel quality estimation errors for different average
Trang 180 2 ' I
-
Average SIR (dB)
Figure 7.12: The downlink average channel quality error performance defined by Equation 7.2 for a
wideband AQAM scheme at an average SNR of 20dB An interference-free scenario was assumed at the MS and the switching thresholds were set according to Table 4.8 for a
target BER of 0.01% The performance was averaged over 10000 transmission bursts and the simulation parameters were set according to Table 7.1
SIRs, which is shown in Figure 7.12 The average channel quality estimation error is defined
In Figure 7.12 the average difference between the actual and estimated SNR output of the DFE was recorded for different SIRs over the COST207 CIR of Table 4.5 and Figure 4.12, measured at an average channel SNR of 20dB As expected, at low average SIRs the average channel quality estimation error was high as a result of inaccurate channel estimation
of the reference user Conversely, at higher average SIRs, the magnitude of the CC1 was lower, resulting in better channel estimation of the reference user Consequently the average channel quality estimation error converged to an average minimum of 0.25dB, as evidenced
by Figure 7.12
The BER and BPS performances for target BERs of 1% and 0.01% for average uplink SIRs of 0, 10, 20 and 30dB are shown in Figures 7.13 and 7.14, respectively In terms of BER performance, as the average SIR increased, the CC1 induced BER and BPS degrada-
Trang 197.3 EFFECT OF CO-CHANNEL INTERFERENCE ON AQAM 275
Figure 7.13: The downlink performance of a wideband AQAM scheme contaminated with co-channel
interference at the BS and that of an interference-free scenario at the MS This hypothet- ical situation was necessary in order to study the impact of CC1 on the channel quality estimation process, as explained in Section 7.3.1 The modem mode switching thresholds were set according to Table 4.8 for a target BER of 1% and the simulation parameters are listed in Table 7.1 Midamble channel estimation was applied at both the BS and MS
tion decreased, as evidenced by Figures 7.13 and 7.14 The degradation was more evident in the context of the wideband AQAM scheme, which was optimised for a low BER of 0.01% due to its increased sensitivity to modulation mode selection errors At a low average SIR
of OdB, the estimation of the reference user's channel quality degraded and hence the effect
of modulation selection errors increased Consequently, both schemes encountered severe
degradation in terms of their BER performance However, at higher average SIRs - above lOdB - the BER performance approached the target BER, for which it was optimised This was achieved as a result of sufficiently accurate channel quality estimates The BPS perfor- mance did not change significantly for different average SIRs However, at an average SIR of OdB, the BPS throughput increased, which was consistent with the corresponding BER degra- dation In this respect, as a result of channel quality estimation errors, less robust modulation modes were erroneously selected, yielding a degraded BER performance and an increased
BPS throughput
By referring to Figures 7.13 and 7.14, the wideband AQAM scheme was sufficiently robust against CC1 in terms of its modem mode switching regime performance for average
Trang 20Figure 7.14: The downlink performance of a wideband AQAM scheme contaminated with co-channel
interference at the BS and an interference-free scenario at the MS This hypothetical situ- ation was necessary in order to study the impact of CC1 on the channel quality estimation process, as explained in Section 7.3.1 The modem mode switching thresholds were set according to Table 4.8 for a target BER of 0.01% and the simulation parameters are listed
in Table 7.1 Midamble channel estimation was applied at both the BS and MS
SIRs above 10dB Hence, the subsequent experiments incorporating CC1 will only consider average SIRs equal to or in excess of 10dB
In the last section we have quantified and investigated the impact of CC1 on the modem mode switching regime of the AQAM scheme Consequently, in this section we will investigate the impact of CC1 on the AQAM demodulation process in the presence of CCI In order to isolate and study the impact of CC1 on the demodulation process, we have considered an interference-free scenario at the BS and a CCI-impaired receiver at the MS in a downlink transmission scenario This is justified upon assuming that the average SIRs considered here are in excess of lOdB, which was shown to be sufficient for an AQAM scheme in terms of reliable channel quality estimation
In order to mitigate the impact of CC1 on the demodulation process, two different ap- proaches are presented here In the first approach we will utilize Joint Detection (JD) tech-