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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
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