1,720,968 research outputs found
Modern Quadrature Amplitude Modulation: Principles and applications for fixed and wireless channels: one
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A Low-Rate Multi-Level Voice/Video Transceiver for Personal Communications
No full textA personal communication system (PCS) transceiver is proposed and investigated. A 4.8 kbit/s transformed binary pulse excited (TBPE) linear predictive speech codec, embedded source sensitivity-matched binary Bose-Chaudhuri-Hocquenghem (BCH) block error correction codecs, non-coherent differentially coded 16-level quadrature amplitude modulation (16-QAM) modem and packet reservation multiple access (PRUA) are deployed. The 2.15 kBd transceiver requires a signal-to-noise ratio (SNR) and signal-to-interference ratio (SIR) in excess of about 24 dB over Rayleigh-fading channels in order to support 1&11 nearly un-impaired voice conversations within a bandwidth of 30 kHz. Additionally, by reserving two PRMA time slots for video telephony, an 8.52 kbps videophone user can also be supported. Key words: Wireless multimedia communications, mobile voice and video telephony, transformed binary pulse excited speech coding, multilevel modulation, packet reservation multiple access, the Pan-American IS-54 mobile system
Does 16-QAM Provide an Alternative to a Half-Rate GSM Speech Codec?
No full textThe computational complexity, speech-quality, spectral efficiency and robustness trade-offs of speech transmission schemes for personal communications networks (PCNs) are addressed. Differentially encoded star 16-QAM arrangements with and without oversampling and diversity are compared in terms of robustness to the parallel frequency division multiplex (FDM) 16-QAM system to find the most appropriate modern scheme. The 13.4 kbps regular pulse RPE-LTP speech codec is subjected to rigorous bit sensitivity analysis in terms of both cepstral distance (CD) and segmental signal to ratio (SEGSNR) degradations. A twin-class embedded binary Bose-Chaudhuri-Hocqunghem (BCH) bit protection scheme is devised to improve robustness. The 6.2 kBd system proposed guarantees low-complexity, high-quality speech transmissions in a bandwidth of 12 kHz for vehicular speeds of 30 mph and channel signal to ratios (SNRs) in excess of 25 dB, a value readily maintained in microcellular PCNs. Wehn using diversity, our system has similar robustness to the 11.4 kbps quadruple-complexity, lower speech-quality code excited linear predictive speech codec/convolutional coding/minimum shift keying (CELP/CC/MSK) benchmarker utilised. Yes, 16-QAM does provide an attractive alternative to half-rate speech coding
Bandwidth Efficient QAM Schemes for Rayleigh Fading Channels
Full text linkTechniques to improve the BER performance of 16-level QAM transmission over Rayleigh fading channels for PCM systems are presented, simulations were carried out with a carrier frequency of 1.9GHz, data rate of 64 kbit/s and mobile speed of 30mph. The residual BER was reduced by over an order of magnitude, by introducing circular constellation coupled differential amplitude and phase encoding, compared with a square QAM constellation. When an oversampling and interpolation technique was combined with circular constellation with differential encoding another order of magnitude reduction in residual BER was obtained. By expanding the number of QAM levels to 64, and on using the two extra bits gained for block coding, the BER was reduced 10-6 for channel SNRs in excess of 35 dB. Decreasing the data throughput to 48 kbit/s using 3/4 rate RS(60, 44, 12) code, 16 level QAM, interleaved over 40ms, transmitted over a Rayleigh fading channel yielded a BER of 10-6 for channel SNRs above 25 dB
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Performance of PRMA Schemes via Fading Channels
No full textA packet reservation multiple access (PRMA) assisted microcellular cordless telecommunications (CT) system is investigated for office type environments. The objective speech quality is found as a function of channel signal to noise ratio (SNR) for transmission over narrowband Rayleigh fading channels, parameterised with the number of PRMA users. A moderate complexity 16-ary CT scheme constituted by a 4 bit/symbol 32kbps adaptive differential pulse code modulation (ADPCM) speech codec, Reed-Solomon forward error correction (FEC) codec and diversity-assisted 16-level star quadrature amplitude modulation (16-StQAM) modem is proposed. The 264kBd 20-slot PRMA scheme supports 36 users while maintaining virtually imperceptible speech degradation for channel SNRs in excess of about 25dB and for mobile speeds above 2mph
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