On QAM Speech Transmission Schemes for Microcellular Mobile PCNs

Abstract

Abstract. On the way towards third generation personal communication networks (PCN) the deployment of 16-level quadrature amplitude nlodulation ( Q A W schemes is investigated via flat microcellular Rayleiph-fading mobile radio channels having mild co-channel interference with and without diversity. Our proposed differentially coded non-coherent 16-QAM scheme requires no pilot symbol assistance. no automatic gain control (AGC) and no carrier recovery. High speech quality associated with a mean opinion score of about four (MOS = 4.0) is ensured at a signalling rate of 6.1 kBd for channel signal to noise ratios tSNR) above 30 -25 dB and signal to interference ratios (SIR) above 75 -30 dB by the 13.4 kbit/s medium complexity resular pulse excited (RPE) speech codec, when carefully matched twin-class low complexity binary Bose-ChaudhuriHocquenghem (BCHI error correction coding and 16-QAM combined with second order diversity are deployed. The overall speech delay is about 10 Ins and the user bandwidth is 9.3 kHz. which allows us to accommodate 2 1 users in a 300 kHz channel slot.A few years after the definition of the Pan-European digital mobile radio system known as GSM [ I ] other srcond generation mobile systems such as the American IS-54 [2] and the Japanese system [3] followed. The key features of these systems are summarised in Table 1, including the half-rate GSM system. While the GSM system is a complex scheme for hostile, dispersive land mobile channels. the other two systems were contrived for more benign channels and represent a different design philosophy. Indeed. the IS-54 and the Japanese digital mobile systems provide a seamless conceptual transition towards the third peneration personal communications network (PCN) concept. where the friendly micro-or pico-cellulu environment guarantees high signal-to-noise ratios (SIVR) and low signal dispersion. The typically high SNR can be exploi- ted by deploying 16-level quadrature amplitude modulation (IBQAM), which lowers the signalling rate by a factor of four and hence quadruples the signalling interval's duration. Therefore the relative effects of signal dispersion are mitigated and the need for channel equalization can be removed.The paper is organised as follows. Section 2 gives a brief overview of QAM research for both Gaussian and Rayleigh fiiding channels and highlights the key features of two previously proposed QAM-based mobile communication schemes for PCN in contrast to the presently suggested system. Section 3 hishlights our proposed non-coherent differentially coded 16-QAM modem. Section 4 concentrates on the RPE speech codec and after a theoretical treatise it provides a rigourous bit sensitivity analysis based on a sensitivity measure derived from the amalgamation of segmental SNR (SEG-SNR) and cepstral distance (CD) measures. Section 5 deals with aspects of channel coding. while Section 6 describes the combined transceiver scheme proposed. Lastly. Section 7 summarises our results and provides some discussions.