Wireless speech and audio communications
The limited applicability of Shannon's separation theorem in practical speech/audio systems motivates the employment of joint source and channel coding techniques. Thus, considerable efforts have been invested in designing various types of joint source and channel coding schemes. This thesis discusses two different types of Joint Source and Channel Coding (JSCC) schemes, namely Unequal Error Protection (UEP) aided turbo transceivers as well as Iterative Source and Channel Decoding (ISCD) exploiting the residual redundancy inherent in the source encoded parameters. More specifically, in Chapter 2, two different UEP JSCC philosophies were designed for wireless audio and speech transmissions, namely a turbo detected UEP scheme using twin class convolutional codes and another turbo detector using more sophisticated Irregular Convolutional Codes (IRCC) In our investigations, the MPEG 4 Advanced Audio Coding (AAC) the MPEG 4 Transform Domain Weighted Interleaved Vector Quantization (TwinVQ) and the Adaptive MultiRate WideBand (AMR WB) audio/speech codecs were incorporated in the sophisticated UEP turbo transceiver, which consisted of a threestage serially concatenated scheme constituted by Space Time Trellis Coding (STTC) Trellis Coded Modulation (TCM) and two different rate Non Systematic Convolutional codes (NSCs) used for UEP. Explicitly, both the twin class UEP turbo transceiver assisted MPEG 4 TwinVQ and the AMR WB audio/speech schemes outperformed their corresponding single class audio/speech benchmarkers by approximately 0.5 dB, in terms of the required Eb/N0, when communicating over uncorrelated Rayleigh fading channels. By contrast, when employing the MPEG 4 AAC audio codec and protecting the class 1 audio bits using a 2/3 rate NSC code, a more substantial Eb/N0 gain of about 2 dB was achieved. As a further design alternative, we also proposed a turbo transceiver employing IRCCs for the sake of providing UEP for the AMR WB speech codec. The resultant UEP schemes exhibited a better performance when compared to the corresponding Equal Error Protection (EEP) benchmark schemes, since the former protected the audio/speech bits according to their sensitivity. The proposed UEP aided system using IRCCs exhibits an Eb/N0 gain of about 0.4 dB over the EEP system employing regular convolutional codes, when communicating over AWGN channels, at the point of tolerating a SegSNR degradation of 1 dB. In Chapter 3, a novel system that invokes jointly optimised ISCD for enhancing the error resilience of the AMR WB speech codec was proposed and investigated. The resultant AMR WB coded speech signal is protected by a Recursive Systematic onvolutional (RSC) code and transmitted using a non coherently detected Multiple Input Multiple Output (MIMO) Differential Space Time Spreading (DSTS) scheme. To further enhance the attainable system performance and to maximise the coding advantage of the proposed transmission scheme, the system is also combined with multi dimensional Sphere Packing (SP) modulation. The AMR WB speech decoder was further developed for the sake of accepting the a priori information passed to it from the channel decoder as extrinsic information, where the residual redundancy inherent in the AMR WB encoded parameters was exploited. Moreover, the convergence behaviour of the proposed scheme was evaluated with the aid of both Three Dimensional (3D) and Two Dimenstional (2D) EXtrinsic Information Transfer (EXIT) charts. The proposed scheme benefitted from the exploitation of the residual redundancy inherent in the AMR WB encoded parameters, where an approximately 0.5 dB Eb/N0 gain was achieved in comparison to its corresponding hard speech decoding based counterpart. At the point of tolerating a SegSNR degradation of 1 dB, the advocated scheme exhibited an Eb/N0 gain of about 1.0 dB in comparison to the benchmark scheme carrying out joint channel decoding and DSTS aided SP demodulation in conjunction with separate AMR WB decoding, when communicating over narrowband temporally correlated Rayleigh fading channels. In Chapter 4, two jointly optimized ISCD schemes invoking the soft output AMRWB speech codec using DSTS assisted SP modulation were proposed. More specifically, the soft bit assisted iterative AMR WB decoder's convergence characteristics were further enhanced by using Over Complete source Mapping (OCM) as well as a recursive precoder. EXIT charts were used to analyse the convergence behaviour of the proposed turbo transceivers using the soft bit assisted AMR WB decoder. Explicitly, the OCM aided AMR WB MIMO transceiver exhibits an Eb/N0 gain of about 3.0 dB in comparison to the benchmark scheme also using ISCD as well as DSTS aided SP demodulation, but dispensing with the OCM scheme, when communicating over narrowband temporally correlated Rayleigh fading channels. Finally, the precoded soft bit AMR WB MIMO transceiver exhibits an Eb/N0 gain of about 1.5 dB in comparison to the benchmark scheme dispensing with the precoder, when communicating over narrowband temporally correlated Rayleigh fading channels.