C. H. Wong

Upper-bound performance of a wide-band adaptive modem

Adaptive modulation is applied in conjunction with a decision-feedback equalizer (DFE) in order to mitigate the effects of the slowly varying wide-band multipath Rayleigh fading channel in a noise-limited environment. An upper-bound mean bit-error rate and bits per symbol performance is introduced for this scheme by utilizing the pseudo signal-to-noise ratio at the output of the DFE in order to switch the modulation schemes on a burst-by-burst basis.

Reduced complexity in-phase/quadrature-phase M-QAM turbo equalization using iterative channel estimation

A reduced complexity trellis-based turbo equalizer known as the in-phase (I)/quadrature-phase (Q) turbo equalizer (TEQ-IQ) invoking iterative channel impulse response (CIR) estimation is proposed. The underlying principle of TEQ-IQ is based on equalizing the I and Q component of the transmitted signal independently. This requires the equalization of a reduced set of separate I and Q signal components in comparison to all of the possible I/Q phasor combinations considered by the conventional trellis-based equalizer. It was observed that the TEQ-IQ operating in conjunction with iterative CIR estimation was capable of achieving the same performance as the full-complexity conventional turbo equalizer (TEQ-CT) benefiting from perfect CIR information for both 4- and 16-quadrature amplitude modulation (QAM) transmissions, while attaining a complexity reduction factor of 1.1 and 12.2, respectively. For 64-QAM, the TEQ-CT receiver was too complex to be investigated by simulation. However, by assuming that only two turbo equalization iterations were required, which is the lowest possible number of iterations, the complexity of the TEQ-IQ was estimated to be a factor of 51.5 lower than that of the TEQ-CT. Furthermore, at BER = 10/sup -3/ the performance of the TEQ-IQ 64-QAM receiver using iterative CIR estimation was only 1.5 dB away from the associated decoding performance curve of the nondispersive Gaussian channel.

Burst-by-burst adaptive joint detection CDMA

Adaptive quadrature amplitude modulation (AQAM) is combined with joint detection code division multiple access (JD-CDMA) in order to increase the throughput of a communications system, while maintaining a target BER. The QAM modes are adaptively switched, by estimating the signal to interference plus noise ratio (SINR) at the output of the joint detection receiver. For an eight-user AQAM/JD-CDMA scheme, a bits per symbol (BPS) performance of about 3.5 bits/symbol is achieved at an average SNR of about 18 dB, with a BER of approximately 0.02%, when using three different modulation modes, namely BPSK, 4-QAM and 16-QAM.

Turbo- and BCH-coded wide-band burst-by-burst adaptive H.263-assisted wireless video telephony

The video performance benefits of burst-by-burst adaptive modulation are studied, employing a higher-order modulation scheme when the channel is favorable, in order to increase the systems bits per symbol capacity and conversely, invoking a more robust lower order modulation scheme when the channel exhibits inferior channel quality. It is shown that due to the proposed adaptive modem mode switching regime, a seamless video quality versus channel quality relationship can be established, resulting in error-free video quality right actress the operating channel signal-to-noise ratio (SNR) range. The main advantage of the proposed burst-by-burst adaptive transceiver technique is that irrespective of the prevailing channel conditions, the transceiver achieves always the best possible source-signal representation quality-such as video, speech, or audio quality-by automatically adjusting the achievable bit rate and the associated multimedia source-signal representation quality in order to match the channel quality experienced. This is achieved on a near-instantaneous basis under given propagation conditions in order to cater for the effects of path loss, fast-fading, slow-fading, dispersion, co-channel interference, etc. Furthermore, when the mobile is roaming in a hostile out-doors-or even hilly terrain-propagation environment, typically low-order low-rate modem modes are invoked, while in benign indoor environments, predominantly the high-rate high source-signal representation quality modes are employed.

Wideband burst-by-burst adaptive modulation with turbo equalization and iterative channel estimation

The performance of adaptive modulation-applied in conjunction with turbo equalization (TE)-is characterized in a noise limited environment over a slowly varying wideband multi-path Rayleigh fading channel. The iterative structure of the turbo equalizer was also exploited in order to invoke an iterative least mean square (LMS) channel estimator. Finally, the throughput performance of the adaptive modulation scheme was compared to that of its constituent modulation modes, where a gain of 1.5 dB to 1.7 dB was recorded.

Reduced complexity in-phase/quadrature-phase turbo equalisation using iterative channel estimation

A novel reduced complexity trellis-based equaliser, referred to as the in-phase/quadrature-phase equaliser (I/Q-EQ), is proposed. The I/Q-EQ is employed in the context of turbo equalisation (TEQ-IQ) and with the aid of iterative channel estimation. The performance of the TEQ-IQ is characterized in a noise limited environment over an equally-weighted, symbol-spaced three-path Rayleigh fading channel. The TEQ-IQ achieved the same performance as the conventional turbo equaliser, while achieving a complexity reduction by a factor of 1.25 and 7.17 for 4-QAM and 16-QAM, respectively.

Burst-by-burst adaptive wideband wireless video telephony

The design trade-offs of interactive wireless video systems are discussed and performance comparisons are provided both in the context of second- and third-generation wireless videophone systems. We commence our discussions by a comparative study of arbitrarily programmable, but fixed-rate, videophone codecs using quarter common intermediate format (QCIF) video sequences scanned at 10 frames/s. These proprietary codecs were designed to allow direct replacement of mobile radio voice codecs in second generation wireless systems, such as the Pan-European GSM, the American IS-54 and IS-95 as well as the Japanese systems, operating at 13, 8, 9.6 and 6.7 kbps, respectively, although better video quality is maintained over higher-rate, 32 kbps cordless systems, such as the Japanese PHS and the European DECT and CT2 systems. From the range of codecs investigated, best overall performance was achieved by our vector-quantised codecs, followed by the discrete cosine transformed and the quadtree-based schemes. The associated video peak signal-to-noise ratio (PSNR) was around 30 dB, while the subjective video quality can be assessed under http://www-mobile.ecs.soton.ac.uk. A range of multimode wireless transceivers is also proposed. The second part of the paper is dedicated to burst-by-burst (BbB) adaptive wireless video transceivers employing the standard H.263 codec. It is demonstrated that the proposed BbB adaptive transceivers provide an improved video performance in comparison to their statically reconfigured counterparts in the context of both wideband BbB adaptive quadrature amplitude modulation (AQAM) transceivers and the joint-detection based code division multiple access (CDMA) transceivers of the third generation systems.

Channel-adaptive wideband wireless video telephony

The fundamental advantage of burst-by-burst (BbB) adaptive intelligent multimode multimedia transceivers (IMMTs) is that-irrespective of the propagation environment encountered-when the mobile roams across different environments subject to path loss; shadow- and fast-fading; co-channel-, intersymbol-, and multiuser interference, while experiencing power control errors, the system will always be able to configure itself in the highest possible throughput mode, while maintaining the required transmission integrity. Finding a specific solution to a distributive or interactive video communications problem has to be based on a compromise in terms of the inherently contradictory constraints of video quality, bit rate, delay, robustness against channel errors, and the associated implementational complexity. Considering some of these tradeoffs and proposing a range of attractive solutions to various video communications problems is the basic aim of this overview. The article portrays a range of proprietary video codecs and compares them to some of the existing standard video codecs. A number of multimode video transceivers are also characterized. Systems employing the standard H.263 video codec in the context of wideband BbB adaptive video transceivers are examined, and the concept of BbB-adaptive video transceivers is then extended to CDMA-based systems.

Upper-bound performance of a wideband burst-by-burst adaptive modem

In this article, adaptive modulation is applied in conjunction with a decision feedback equalizer (DFE) in order to mitigate the effects of the slowly varying wideband multipath Rayleigh fading channel. The upper-bound mean BER and bits per symbol (BPS) performance of this scheme is determined by utilizing the pseudo-SNR at the output of the DFE, in order to switch the modulation schemes on a burst-by-burst basis. The performances of each individual modulation scheme and their amalgamated adaptive scheme were compared in terms of the information throughput for a low- and a high-BER arrangement. The results indicate both SNR gains of approximately 1-3 dB and 7-9 dB, for the high- and low-BER arrangement, respectively using the proposed burst-by-burst adaptive wideband scheme in a noise limited environment.

Burst-by-burst adaptive decision feedback equalized TCM, TTCM, BICM and BICM-ID

Decision feedback equalizer (DFE) aided wideband burst-by-burst (BbB) adaptive trellis coded modulation (TCM), turbo trellis coded modulation (TTCM), bit-interleaved coded modulation (BICM) and BICM with iterative decoding (BICM-ID) are proposed and characterised in performance terms over the COST 207 typical urban (TU) wideband fading channel. These schemes are evaluated using a practical modem mode switching regime. System I represents schemes without channel interleaving, while System II invokes channel interleaving over four transmission bursts. A substantially improved bit per symbol (BPS) and bit error rate (BER) performance was achieved by System II in comparison to System I. BbB-adaptive TTCM was found to give the best performance, when aiming for a target BER of below 0.01%.