The wideband AQAM BPS throughput performance was then compared to that of the constituent fixed modulation modes, where BPS/SNR gains of approximately 1 - 3dB and 7 - 9dB were observed f
Trang 1L 1 5 1
Conclusions and Suggestions for
Further Research
In this brief chapter a summary of this monograph is presented and the corresponding con-
clusions that can be drawn are presented This will be followed by a range of ideas set aside
for future research
15.1 Book Summary and Conclusions
In Part I of this monograph initially the performance of the DFE was analysed using multi-
level modulation modes over static multi-path Gaussian channels, as shown in Figure 2.10
These discussions were further developed in the context of a multi-path fading channel envi-
ronment, where the recursive Kalman algorithm was invoked in order to track and equalize
the received linearly distorted data, as evidenced by Figure 3.16 Explicitly, an adaptive CIR
estimator and DFE were implemented in two different receiver structures, as shown in Figure
3.14, while their performances were compared in Figure 2.10 In this respect, Structure 1,
which utilized the adaptive CIR estimator provided a better performance, when compared to
that of Structure 2 , which involved the adaptive DFE, as evidenced by Table 3.10 Further-
more, the complexity of Structure 2 was higher than that of Structure 1, which was studied
in Section 3.4 However, these experiments were conducted in a fast start-up environment,
where adaptation was restricted to the duration of the training sequence length By contrast,
if the adaptation was invoked over the entire transmission frame using a decision directed
scheme, the complexity advantage of Structure 1 was eroded, as discussed in Section 3.5
The application of these fast adapting and accurate CIR estimators was crucial in a wideband
AQAM scheme, where the CIR variation across the transmission frame was slow In these
experiments valuable insights were obtained with regards to the design of the equalizer and
to the characteristics of the adaptive algorithm itself This provided a firm foundation for the
further investigation of the proposed wideband AQAM scheme
Following our introductory chapters, in Chapter 4 the concept of adaptive modulation
cast in the context of a narrow-band environment was introduced in conjunction with the
639
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 2640 CHAPTER 15 CONCLUSIONS AND SUGGESTIONS FOR FURTHER RESEARCH
application of threshold-based power control In this respect, power control was applied in the vicinity of the switching thresholds of the AQAM scheme The associated performance was recorded in Table 4.4, where the trade-off between the BER and BPS performance was highlighted The relative frequency of modulation mode switching was also reduced at the cost of a slight BER degradation However, the complexity of the scheme increased due to the implementation of the power control regime Moreover, the performance gains portrayed
at this stage represented an upper-bound estimate, since perfect power control was applied Consequently, the introduction of threshold-based power control in a narrow-band AQAM
did not offer an attractive complexity versus performance gain trade-off
The concept of AQAM was subsequently invoked in the context of a wideband channel, where the DFE was utilized in conjunction with the AQAM modem mode switching regime Due to the dispersive multi-path characteristics of the wideband channel, a metric based on the output SNR of the DFE was proposed in order to quantify the channel’s quality This ensured that the wideband channel effects were mitigated by the employment of AQAM
and equalization techniques Subsequently a numerical model based on this criterion was established for the wideband AQAM scheme, as evidenced by Figures 4.16 and 4.17 The wideband AQAM switching thresholds were optimised for maintaining a certain target BER and BPS performance, as shown in Figure 4.3.5 The wideband AQAM BPS throughput performance was then compared to that of the constituent fixed modulation modes, where BPS/SNR gains of approximately 1 - 3dB and 7 - 9dB were observed for target BERs of 1% and 0.01%, respectively However, as a result of the assumption made in Section 4.3 l , these gains constituted an upper bound estimate Nevertheless, the considerable gains achieved provided further motivation for the research of wideband AQAM schemes
The concept of wideband coded AQAM was presented in Chapter 5, where turbo block coding was invoked in the switching regime for different wideband AQAM schemes The key characteristics of these four schemes, namely those of the FCFI-TBCH-AQAM, FCVI- TBCH-AQAM, P-TBCH-AQAM and VR-TBCH-AQAM arrangements, were highlighted
in Table 5.10 in terms of the respective turbo interleaver size and the coding rate utilized The general aim of using turbo block coding in conjunction with a high code rates was to increase the effective BPS transmission throughput, which was achieved, as shown in Table 5.1 1 for the arrangement that we referred to as the Low-BER scheme In this respect, all the schemes produced gains in terms of their BER and BPS performance, when compared to the uncoded
AQAM scheme, which was optimised for a BER of 0.01% This comparison was recorded in Table 5.1 1 for the four different turbo coded AQAM schemes studied
The FCFI-TBCH-AQAM scheme exhibited a better throughput gain, when compared
to the other schemes This was achieved as a result of the larger turbo interleaver used
in this scheme, which also incurred a higher delay The size of the turbo interleaver was then varied, while retaining identical coding rate for each modulation mode, resulting in the
FCVI-TBCH-AQAM scheme, where burst-by-burst decoding was achieved at the receiver The BPS throughput performance of this scheme was also compared to that of the constituent fixed modulation modes, which utilized different channel interleaver sizes, as shown in Fig- ure 5.1 1 SNR gains of approximately 1.5 and 5.0dB were achieved by the adaptive scheme for a target BERs of 0.01%, when compared to the fixed modulation modes using the small- and large-channel interleavers, respectively By contrast, for a target BER of 1% only modest gains were achieved by the wideband AQAM scheme These apparently low gains were the consequence of an ’unfair’ comparison, since sibnificantly larger turbo interleaver and chan-
Trang 3ne1 interleaver sizes were utilized by the fixed modulation modes Naturally, his resulted in
a high transmission delay for the fixed modulation modes By contrast, the FCVI-TBCH- AQAM scheme employed low-latency instantaneous burst-by-burst decoding, which is im- portant in real-time interactive communications
The size of the turbo interleaver and the coding rate was then varied according to the modulation mode, in order to ensure burst-by-burst decoding at the receiver This resulted
in the P-TBCH-AQAM scheme, which also incorporated un-coded modes for the sake of increasing the achievable throughput Finally, the VR-TBCH-AQAM scheme activated dif- ferent code rates in conjunction with the different modulation modes These schemes pro- duced a higher maximum throughput due to the utilization of higher code rates However, the SNR gains in terms of both the BER and BPS performance degraded, when compared to the
FCFI-TBCH-AQAM scheme as a result of the reduced-size turbo interleaver used Further- more, the utilization of higher code rates for the VR-TBCH-AQAM arrangement resulted
in a higher decoding complexity Once again, these comparisons are recorded in Table 5.7 Similar characteristics were also observed in the context of the High-BER candidate scheme and in conjunction with the near-error-free schemes However, the performance gains of the
High-BER schemes were less than those of the Low-BER schemes This was primarily due to the lower channel coding gain achieved at higher BERs and due to the smaller turbo interleaver size used
The advantages of burst-by-burst decoding were also exploited in the context of blindly detecting the modulation modes In this respect, the channel coding information and the mean square phasor error was utilized in the hybrid SD-MSE modulation mode detection algorithm of Section 5.6.2 characterized by Equation 5.6 Furthermore, concatenated m- sequences [ 1691 were used in order to detect the NO TX mode while also estimating the channel’s quality The performance of this algorithm was shown in Figure 5.16, where a modulation mode detection error rate (DER) of lop4 was achieved at an average channel SNR
of 15dB However, the complexity incurred by this algorithm was high due to the multiple channel decoding processes required for each individual modulation mode
Turbo convolutional coding was then introduced and its performance using fixed modu- lation modes was compared to that of turbo block coding, as shown in Figure 5.23 A BER versus SNR degradation of approximately 1 - 2dB was observed for the turbo convolutional
coded performance at a BER of 10W4 However, the complexity was significantly reduced,
namely by a factor of seven, when compared to the previously studied turbo block coded schemes Turbo convolutional coding was then incorporated in our wideband AQAM scheme and its performance was compared to that of the turbo block coded AQAM schemes, where the results were similar, as evidenced by Figure 5.24 Consequently the complexity versus performance gain trade-off was more attractive for our turbo convolutional coded AQAM schemes
In our continued investigations of coded AQAM schemes, turbo equalization was invoked where BPS/SNR gains of approximately 1 - 2dB were achieved by our AQAM scheme In achieving this performance, iterative CIR estimation was implemented based on the LMS algorithm, which approached the perfect CIR estimation assisted AQAM performance, as shown in Figure 5.31 However, the implementation of this scheme was severely hindered by the high complexity incurred, which increased exponentially in conjunction with higher-order modulation modes and longer CIR memory
The chapter was concluded with a system design example cast in the context of TCM,
Trang 4642 CHAPTER 15 CONCLUSIONS AND SUGGESTIONS FOR FURTHER RESEARCH
TTCM and BICM based AQAM schemes, which were studied under the constraint of a sim- ilar implementational complexity The BbB adaptive TCM and TTCM schemes were inves- tigated when communicating over wideband fading channels both with and without channel interleaving and they were characterised in performance terms over the COST 207 TU fading channel When observing the associated BPS curves, adaptive TTCM exhibited up to 2.5 dB
SNR-gain for a channel interleaver length of four transmission bursts in comparison to the non-interleaved scenario, as it was evidenced in Figure 5.40 Upon comparing the associated BPS curves, adaptive TTCM also exhibited up to 0.7 dB SNR-gain compared to adaptive TCM of the same complexity in the context of System 11, while maintaining a target BER of less than 0.01 %, as it was shown in Figure 5.44 Finally, adaptive TCM performed better, than the adaptive BICM benchmarker in the context of System I, while the adaptive BICM-
ID scheme performed marginally worse, than adaptive TTCM in the context of System 11, as
it was discussed in Section 5.1 1 S
In Chapter 6 following a brief introduction to several fading counter-measures, a general model was used for describing various adaptive modulation schemes employing various con- stituent modulation modes, such as PSK, Star QAM and Square QAM, as one of the attractive fading counter-measures In Section 6.3.3.1, the closed form expressions were derived for the average BER, the average BPS throughput and the mode selection probability of the adap- tive modulation schemes, which were shown to be dependent on the mode-switching levels
as well as on the average SNR In Sections 6.4.1, 6.4.2 and 6.4.3 we reviewed the existing techniques devised for determining the mode-switching levels Furthermore, in Section 6.4.4 the optimum switching levels achieving the highest possible BPS throughput were studied, while maintaining the average target BER These switching levels were developed based on the Lagrangian optimization method
Then, in Section 6.5.1 the performance of uncoded adaptive PSK, Star QAM and Square QAM was characterised, when the underlying channel was a Nakagami fading channel It was found that an adaptive scheme employing a k-BPS fixed-mode as the highest throughput constituent modulation mode was sufficient for attaining all the benefits of adaptive mod- ulation, while achieving an average throughput of up to k - 1 BPS For example, a three- mode adaptive PSK scheme employing No-Tx, l-BPS BPSK and 2-BPS QPSK modes at- tained the maximum possible average BPS throughput of 1 BPS and hence adding higher- throughput modes, such as 3-BPS 8-PSK to the three-mode adaptive PSK scheme resulting
in a four-mode adaptive PSK scheme did not achieved a better performance across the 1 BPS throughput range Instead, this four-mode adaptive PSK scheme extended the maximum BPS throughput achievable by any adaptive PSK scheme to 2 BPS, while asymptotically achieving
a throughput of 3 BPS, as the average SNR increases
On the other hand, the relative SNR advantage of adaptive schemes in comparison to fixed-mode schemes increased as the target average BER became lower and decreased as the fading became less severe More explicitly, less severe fading corresponds to an increased
Nakagami fading parameter m, to an increased number of diversity antennas, or to an in-
creased number of multi-path components encountered in wide-band fading channels As the fading becomes less severe, the average BPS throughput curves of our adaptive Square QAM schemes exhibit undulations owing to the absence of 3-BPS, 5-BPS and 7-BPS square QAM modes
The comparisons between fixed-mode MC-CDMA and adaptive OFDM (AOFDM) were made based on different channel models In Section 6.5.4 it was found that fixed-mode MC-
Trang 5CDMA might outperform adaptive OFDM, when the underlying channel provides sufficient diversity However, a definite conclusion could not be drawn since in practice MC-CDMA
might suffer from MU1 and AOFDM might suffer from imperfect channel quality estimation and feedback delays
systems were investigated in Section 6.5.5 The coded schemes reduced the required average SNR by about 6dB-7dB at a throughput of 1 BPS, achieving near error-free transmission It
was also observed in Section 6.5.5 that increasing the number of transmit antennas in adap-
tive schemes was not very effective, achieving less than 1 dB SNR gain, due the fact that the
transmit power per antenna had to be reduced in order to limit the total transmit power for the
sake of a fair comparison
The practical issues regarding the implementation of the advocated wideband AQAM
scheme was analysed in Chapter 7 The impact of error propagation in the DFE was high-
lighted in Figure 7.2, where the BER degradation was minimal and the target BERs were
achieved without any degradation to the transmission throughput performance The impact of
channel quality estimation latency was also studied, where the sub frame based TDD/TDMA system of Section 7.2.1 was implemented In this system, a channel quality estimation delay
of 2.3075ms was imposed and the channel quality estimates were predicted using a linear pre-
diction technique In this practical wideband AQAM scheme, SNR gains of approximately
1.4dB and 6.4dB were achieved for target BERs of 1% and 0.01%, when compared to the per-
formance of the constituent fixed modulation modes This was shown graphically in Figure
7.1 1
CC1 was then subsequently introduced in Section 7.3, where in terms of channel quality
estimation, the minimum average SIR that can be tolerated by the wideband AQAM scheme
was approximately lOdB, as evidenced by Figure 7.14 In order to mitigate the impact of
CC1 on the demodulation process, the JD-MMSE-BDFE scheme using an embedded con-
volutional encoder was invoked, where the performance was shown in Figure 7.21 and 7.22
for the fixed modulation modes and for the wideband AQAM scheme, respectively The per-
formance gains achieved by the wideband AQAM scheme were approximately 2 - 4dB and
7 - 9dB for the target BERs of 1% and 0.01%, when compared to the performance of the as-
sociated fixed modulation modes However these gains constituted an upper bound estimate,
since perfect channel estimation of the reference user and the interferer was assumed
The concept of segmented wideband AQAM was then introduced, in order to reduce
the impact of CCI In this scheme, the inner and outer switching thresholds were developed
based on a noise and interference limited environment, respectively, for any given average
channel SNR and average SIR A channel quality estimation delay of 2.3075ms was imposed
in estimating the instantaneous SIR and the associated performance was displayed in Figure
7.24 By employing this segmented AQAM scheme, accurate estimation of the instantaneous
SIR was needed, which was provided by the Kalman-filter based mid-amble assisted CIR
estimation However, information regarding the interferer was not required for reducing the
impact of CCI, which was a substantial advantage
In Part I1 of the book we investigated the application of neural networks in the context
of channel equalization As an introduction, the family of established neural network based
equalizer structures was reviewed We opted for studying RBF network based equalizers
in detail and investigated their implementation in conjunction with adaptive modulation and
turbo channel coding, in order to improve the performance of the transceivers investigated
Concatenated space-time block coded and turbo convolutional-coded adaptive multi-carrier
Trang 6644 CHAPTER 15 CONCLUSIONS AND SUGGESTIONS FOR FURTHER RESEARCH
More explicitly, Chapter 8 provided a brief overview of neural networks and augmented, why channel equalization can also be viewed as a classification problem, namely that of clas- sifying the received phasor into one of the M phasors associated with an M-ary modulation scheme We studied the performance of the RBF equalizer assisted QAM schemes and their adaptive convergence performance in conjunction with both clustering algorithms and LMS channel estimators The RBF equalizer provided superior performance compared to the linear MSE equalizer using an equivalent equalizer order at the expense of a higher computational complexity, as it was shown in Figure 8.28 and 8.29 According to Figure 8.28 and 8.29, the RBF equalizer ( m = 9) provided performance improvements of lOdB and 20dB over the linear MSE equalizer for transmission over two-path and three-path Gaussian channels, respectively, at a BER of lop3 We note that both the linear MSE equalizer and the RBF equalizer exhibited residual BER characteristics, if the channel states corresponding to dif- ferent transmitted symbols are inseparable in the channel’s output observation space, as it was shown in Figure 8.30 The adaptive performance of the RBF equalizer employing the LMS channel estimator of Section 8.9.4, the vector centre clustering algorithm of Section 8.9.5 and the scalar centre clustering algorithm of Section 8.10 was compared The convergence rate of the clustering algorithm depends on the number of channel coefficients to be adapted and therefore also on the modulation scheme used as well as on the CIR length However, the convergence of the LMS channel estimation technique only depends on the CIR length and therefore this technique is preferred for high-order modulation schemes and high CIR lengths
In Section 8.1 1 decision feedback was introduced into the RBF equalizer, in order to re-
duce its computational complexity and to improve its performance, since due to its employ- ment the Euclidean distance between the channel states corresponding to different transmit- ted symbols was increased The performance degradation due to decision error propagation increased as the BER increased, which became more significant for higher-order QAM con- stellations, as it was shown in Figure 8.42 The performance degradation for higher-order modulation schemes was higher for fading channel conditions, since they are more sensitive
to fades due to the reduced Euclidean distance between the neighbouring channel states
We note that even for relatively slow fading channels, the channel states value can change significantly on a symbol-by-symbol basis in a transmission burst duration Inseparable chan- nel state clusters were observed for symbol-invariant - but burst-variant - fading, as it was shown in Figure 8.48(b), which is due to the fading effects manifesting themselves across the burst duration These phenomena, together with the non-ideal learnt channel states, explain the residual BERs present in our simulations
Chapter 9 introduced the concept of adaptive modulation invoked for improving the throughput of the system, while maintaining a certain target BER performance The RBF DFE’s ’on-line’ BER estimation of the received data burst was used as the AQAM modem mode switching metric in order to quantify the channel’s quality Our simulation results
of Section 9.4.5 showed that the proposed RBF DFE-assisted BbB adaptive modem outper- formed the individual constituent fixed modulation modes in terms of the mean BER and BPS The AQAM scheme employing RBF DFE was compared to the AQAM scheme using a conventional DFE, in terms of mitigating the effects of the dispersive wideband channel Our results in Section 9.4.5 showed that the AQAM RBF DFE scheme was capable of performing
as well as the conventional AQAM DFE at a lower decision delay and lower feedforward as well as feedback order It is worth noting futhermore that the performance of the AQAM
Trang 7RBF DFE can be improved by increasing both the decision delay 7 and the feedforward or- der m, at the expense of increased computational complexity, while the performance of the conventional AQAM DFE cannot be improved significantly by increasing its equalizer or- der However, the computational complexity of the RBF DFE is dependent on the AQAM mode and increases significantly for higher-order modulation modes This is not so in the context of the conventional DFE, where the computational complexity is only dependent on the feedforward and feedback order
A practical method of obtaining the switching BER thresholds of the joint AQAM RBF DFE scheme was proposed in Section 9.5, which was shown to provide a near-identical per- formance in comparison to the achievable best-case performance for the target BER of lop2
However, for the lower target BER of lop4, the BER performance degradation in compar- ison to the best-case performance was more significant, since the RBF DFE was unable to provide a BER estimate of such high accuracy and also because of the spread nature of the BER estimates seen for example in Figure 9.14
Overall, we have shown that our proposed AQAM scheme improved the throughput per- formance compared to the fixed modulation modes On the whole, the RBF DFE provides a reliable channel quality measure for the AQAM scheme, which quantifies all channel impair- ments, irrespective of their source and at the same time improves the BER performance
Chapter 10 proposed the Jacobian RBF equalizer that invoked the Jacobian logarithmic approximation, in order to reduce the computational complexity of the original RBF equalizer discussed in Section 8.9.1, while providing a similar BER performance For example, the total complexity reduction was by a factor of about 2.1, when we considered a 16-QAM RBF DFE in conjunction with the equalizer parameters of m = 3, n = 1 and ‘T = 2 The performance of the RBF DFE was investigated using turbo coding and it was compared to the turbo-coded conventional DFE scheme in Section 10.4 Introducing BCH(31,26) turbo coding into the system improved the SNR-performance by 9.5dB for BPSK and by about 8dB for 4-QAM, 16-QAM and 64-QAM at a BER of lop4 The performance of the conventional DFE and RBF DFE schemes depends on their uncoded performance
We have also investigated the application of turbo BCH coding in conjunction with AQAM
in a wideband fading channel We observed in Section 10.6.2 that the performance of the switching mechanism depends on the fluctuation of the switching metric since the AQAM switching regime assumed that the channel quality was slowly varying This was demon- strated in Section 10.6.2, when we compared the performance of the AQAM scheme using the short-term BER before and after turbo decoding, as the switching metric The spurious nature of the short-term BER after turbo decoding was shown in Figure 10.22, which de- graded the performance of the AQAM scheme, as it assumed that the channel quality was slowly varying
The turbo-coded AQAM RBF DFE system exhibited a better BPS performance, when compared to the uncoded system at low to medium channel SNRs - in the range of 0dB to 26
dB - as evidenced by Figure 10.27 The same figure also showed an improved coded BER performance at higher channel SNRs - in the range above 30dB A virtually error-free turbo- coded AQAM scheme was also characterized in Figure 10.28 The BPS performance of the error-free coded system was better, than that of the uncoded AQAM system for the channel SNR range of 0dB to 15dB, as evidenced by Figure 10.28 Overall, we have presented the advantageous interactions of RBF-aided DFE and BbB AQAM in conjunction with turbo FEC
Trang 8646 CHAPTER 15 CONCLUSIONS AND SUGGESTIONS FOR FURTHER RESEARCH
Chapter 11 presented the Jacobian RBF DFE TEQ and comparatively analysed its asso- ciated performance and complexity in conjunction with the well-known Log-MAP TEQ in the context of BPSK, 4-QAM and 16-QAM The computational complexity of the Jacobian RBF DFE TEQ was shown in Section 1 1.4 to be dependent on the number of RBF centres, on the CIR length and on the modulation mode The associated ’per iteration’ implementational complexity of the Jacobian RBF DFE TEQ ( m = 3, n = 2, 7 = 2) was approximately a factor 2.5, 4.4 and 16.3 lower in the context of BPSK, 4-QAM and 16-QAM, respectively, for transmission over the three-path channel considered as seen in Table 1 1.1 The associated performance degradation compared to the Log-MAP TEQ was shown in Figures 1 1.8, 11.9 and 1 1.10 to be approximately 0.2dB, 0.2dB and lOdB for BPSK, 4-QAM and 16-QAM, respectively over the three-path, equal-weight, symbol-spaced Rayleigh fading channel envi- ronment considered The large performance degradation for the 16-QAM scheme was due to the error propagation effect of the DFE, which became more grave in conjunction with higher- order constellations Therefore, the Jacobian RBF DFE TEQ of Section 11.2 could only provide a practical performance versus complexity advantage for lower modulation modes
In terms of storage requirements, the Jacobian RBF DFE is less demanding, as it only has
to store the values of the RBF centres, while the Log-MAP equalizer has to store both the forward- and backward-recursively calculated metrics Our proposed reduced-complexity RBF DFE TEQ - where the RBF DFE skips evaluating the symbol LLRs in the current iter- ation when the symbol is sufficiently reliable after channel decoding in the previous iteration
- was shown in Section 1 1.6 to give significant computational complexity reductions, while providing an equivalent BER performance to the RBF DFE TEQ The complexity reduction was approximately 21% (at an SNR of 6dB) and 35% (at an SNR of 4dB) for dispersive Gaussian and Rayleigh channels, respectively
In Part I11 of the book adaptive CDMA, OFDM and space-time coded systems have been investigated by invoking the previously detailed burst-by-burst adaptive principles
Specifically, in Chapter 12 we commenced our discussions with the recent history of smart CDMA MUDS and the most promising schemes have been comparatively studied,
in order to assist in the design of powerful third- and fourth-generation receivers Future transceivers may become BbB-adaptive, in order to be able to accommodate the associ- ated channel quality fluctuations without disadvantageously affecting the system’s capacity Hence the methods studied in this monograph are advantageous, since they often assist in avoiding having to power up at the transmitter, which would result in inflicting increased levels of co-channel interference and power consumption Furthermore, the techniques char- acterized in this chapter support an increased throughput within a given bandwidth and will contribute towards reducing the constantly increasing demand for more bandwidth
Both SIC and PIC receivers were investigated in the context of AQAMKDMA schemes, which were outperformed by the JD based twin-mode AQAM/CDMA schemes for a spread- ing factor of Q = 64 and K = 8 users Both IC receivers were unable to provide good performances in the triple-mode AQAMKDMA arrangement, since the BER curves exhib- ited error floors In the VSFKDMA systems the employment of variable spreading factors
of Q = 32 and Q = 16 enabled the PIC and SIC receivers to provide a reasonable BER and throughput performance, which was nonetheless inferior to that of the JD When the number
of users in the system was increased, the PIC and SIC receivers were unable to exploit the variability in channel conditions in order to provide a higher information throughput, as op- posed to the JD scheme, which showed performance gains in both the adaptive-rate AQAM
Trang 9and VSF CDMA schemes However, the complexity of the IC receivers increased only lin- early with the number of CDMA users, K , compared to the joint detector, which exhibited a complexity proportional to 0 ( K 3 ) Tables 12.5, 12.6 and 12.7 summarize our performance comparisons for all three multiuser detectors in terms of the E,/N, required for achieving the target BER of 1% or less, the normalized throughput performance and the associated complexity in terms of the number of operations required per detected symbol, respectively Both the third and future fourth generation standardisation activities may benefit from this study We note, however that in conjunction with adaptive beam-steering [8 11 and space-time coding the preference order of the various receivers’ performance may change
In Chapter 13 we commenced our discussions with a historical perspective on OFDM
transmissions with reference to the literature of the past 30 years The advantages and disad-
vantages of various OFDM techniques were considered briefly and the expected performance was characterized for the sake of illustration in the context of WATM systems Our dis- cussions deepened, as we approached the subject of adaptive OFDM subcarrier modulation mode allocation and channel coding Here we would like conclude with a brief discussion comparing the various coded and uncoded, fixed and adaptive OFDM modems and identify future research issues
Specifically, Figure 13.29 compared the different adaptive modulation schemes discussed The comparison graph was split into two sets of curves, depicting the achievable data through- put for a data BER of lop4 highlighted for the fixed throughput systems in Figure 13.29(a), and for the time-variant-throughput systems in Figure 13.29(b)
The fixed throughput systems - highlighted in black in Figure 13.29(a) - comprise the non-adaptive BPSK, QPSK and 16QAM modems, as well as the fixed-throughput adaptive scheme, both for coded and uncoded applications The non-adaptive modems’ performance
is marked on the graph as diamonds, and it can be seen that the uncoded fixed schemes require the highest channel SNR of all reviewed transmission methods to achieve a data BER of l o p 4 Channel coding employing the turbo coding schemes considered dramatically improved the SNR requirements, at the expense of half the data throughput The uncoded fixed-throughput (FT) adaptive scheme, marked by the filled triangles, yielded consistently worse data throughput than the coded (C-) fixed modulation schemes C-BPSK, C-QPSK and C-16QAM, with its throughput being about half the coded fixed scheme’s at the same SNR values The coded FT-adaptive (C-FT) system, however, delivered a fairly similar throughput
to the C-BPSK and C-QPSK transmissions, and were capable of delivering a BER of lo-* for SNR values down to about 9 dB
The variable throughput schemes, highlighted in Figure 13.29(b), outperformed the com- parable fixed throughput algorithms [420] For high SNR values, all uncoded schemes’ per- formance curves converged to a throughput of 4bits/symbol, which was equivalent to 16QAM transmission The coded schemes reached a maximal throughput of 2BPS Of the uncoded schemes, the “data” switching-level (SL) and target-BER adaptive modems delivered a sim- ilar BPS performance, with the target-BER scheme exhibiting slightly better throughput than the SL adaptive modem The adaptive modem employing pre-equalization (PE) significantly outperformed the other uncoded adaptive schemes and offered a throughput of 0.18 BPS at
an SNR of 0 dB, although its crest-factor PDF is slightly less attractive
The coded transmission schemes suffered from limited throughput at high SNR values, since the half-rate channel coding limited the data throughput to 2 BPS For low SNR values, however, the coded schemes offered better performance than the uncoded schemes, with the
Trang 10CHAPTER 15 CONCLUSIONS AND SUGGESTIONS FOR FURTHER RESEARCH
exception of the “speech” SL-adaptive coded scheme, which was outperformed by the un- coded PE-adaptive modem The poor performance of the coded SL-scheme can be explained
by the lower uncoded target BER of the “speech” scenario, which was l%, in contrast to the
10% uncoded target BER for the coded BER- and PE-adaptive schemes The coded PE- adaptive modem outperformed the target-BER adaptive scheme, thanks to its more accurate control of the uncoded BER, leading to a higher throughput for low SNR values
It is interesting to observe that for the given set of four modulation modes the uncoded PE-adaptive scheme was close in performance to the coded adaptive schemes, and that for SNR values of more than 14 dB it outperformed all other studied schemes It is clear, how- ever, that the coded schemes would benefit from higher order modulation modes or higher- rate channel codecs at high SNR, which would allow these modems to increase the data throughput further when the channel conditions allow
Based on these findings, adaptive-rate channel coding is an interesting area for future research in conjunction with adaptive OFDM, adaptive beam-stearing and interference can- cellation Provided that these OFDM enhancements reach a similar state of maturity to the OFDM components used in the current standard digital audio and video broadcasting, WATM, ADSL and power-line communications proposals, OFDM is likely to find a range of further attractive applications in wireless and wireline communications both for businesses and in the home
In our closing chapter, namely in Chapter 14 our discussions were centred around the various trade-offs in the context of adaptive transmission and space-time coding Space- time trellis codes and space-time block codes constitute state-of-the-art transmission schemes based on multiple transmitters and receivers Space-time trellis codes were introduced in Section 14.2 by utilising the simplest possible 4-state, 4PSK space-time trellis code as an example The state diagrams for other 4PSK and 8PSK space-time trellis codes were also given The branch metric of each trellis transition was derived, in order to facilitate their maximum likelihood (ML) decoding
In Section 14.3, we proposed to employ an OFDM modem for mitigating the effects of dispersive multipath channels due to its simplicity compared to other approachs [3 19,5231 Turbo codes and Reed-Solomon codes were invoked in Section 14.3.1 for concatenation with the space-time block code G2 and the various space-time trellis codes, respectively The estimated complexity of the various space-time trellis codes was derived in Section 14.3.3
We presented our simulation results for the proposed schemes in Section 14.4 The first scheme studied was the TC(2,1,3) coded space-time block code G2, whereas the second one was based on the family of space-time trellis codes It was found that the FER and BER per- formance of the TC(2,1,3) coded space-time block G2 was better than that of the investigated space-time trellis codes at a throughput of 2 and 3 BPS over the channel exhibiting two equal- power rays separated by a delay spread of 5 p s and having a maximum Doppler frequency
of 200 Hz Our comparison between the two schemes was performed by also considering the estimated complexity of both schemes It was found that the concatenated G2/TG(2,1,3) scheme still outperformed the space-time trellis codes using no channel coding, even though both schemes exhibited a similar complexity
The effect of the maximum Doppler frequency on both schemes was also investigated in Section 14.4.3 It was found that the maximum Doppler frequency had no significant impact
on the performance of both schemes By contrast, in Section 14.4.4 we investigated the effect
of the delay spread on the system Initially, the delay-spread dependent SIR of the space-