Digital modem techniques for emerging digital cellular telecommunications-mobile radio system applications are described and analyzed. In particular, theoretical performance, experimental results, principles of operation. and various architectures of pi /4-QPSK ( pi /4-shifted coherent or differential QPSK) modems for second-generation US digital cellular radio system applications are presented. The spectral/power efficiency and performance of the pi /4-QPSK modems (emerging American and Japanese digital cellular standards) are studied and briefly compared to GMSK (Gaussian minimum-shift keying) modems (proposed for European DECT and GSM cellular standards). Improved filtering strategies and digital pilot-aided (digital channel sounding) techniques are also considered for pi /4-QPSK and other digital modems. More spectrally efficient modem trends for future cellular mobile (land mobile) and satellite communication system applications are also highlighted. >

Modems for emerging digital cellular-mobile radio system

A brief review of the principles and properties of F-QPSK (quadrature phase shift keying) modulation is given. Its spectral efficiency is compared with that of Gaussian minimum-shift keying (GMSK) using adjacent channel interference (ACI) as a parameter. It is established that in a noncellular environment, hardlimited F-QPSK has a spectral efficiency of 1.42 b/s/Hz, which is up to 51% more spectrally efficient than GMSK BT=0.5 for ACI=20 dB. F-QPSK's BER performance in AWGN and Rayleigh fading channels is shown to be superior to that of GMSK. The spectral efficiency of the modulations in cellular/microcellular environments where frequency is reused in geographically separate cells to achieve higher capacity is compared. It is shown that the application of F-QPSK in such environment would leads to a 95% increase of system capacity compared to GMSK. It is concluded that the power and spectrally efficient F-QPSK make it an excellent candidate for future high-capacity personal communication system (PCS) networks. >

F-QPSK-a superior modulation technique for mobile and personal communications

Pilot-symbol aided coherent M-ary PSK modems in digital cellular mobile radio systems are analyzed theoretically. The error-floors caused by the Doppler spread in a fast fading channel are removed in both flat and selective fading channels. However, the error-floors caused by the delay spread are lower-bounded by those that exist in the ideal coherent detection. The systems are modeled as frequency-selective fast Rayleigh fading channels, corrupted by co-channel interference (CCI) and additive white Gaussian noise (AWGN). In the proposed scheme, pilot symbols are inserted periodically to monitor the channel characteristics. The fading processes experienced by the pilot symbols are used to estimate those suffered by the data symbols using interpolation or filtering. The estimated fade characteristics are used to compensate the random phase variation caused by the Doppler spread, so that the signals can be demodulated coherently. The theoretical performances of the fade compensated coherent modems are evaluated. The results show that the fade compensated coherent demodulation with the least redundancy achieves the same performance as the ideal differential detection in a fading channel. The performance approaches that of the ideal coherent demodulation as more redundancy is allowed. The pilot-symbol-insertion (PSI) scheme is also applicable to M-ary QAM modems and Rician channels. The residual frequency offset can also be compensated by the PSI technique. >

Pilot-symbol aided coherent M-ary PSK in frequency-selective fast Rayleigh fading channels

A digital radio communication system employs an encoder for creating a set of encoded bits for each set of input bits. A mapper in the transmitter employs a modulation scheme which is constrained in the maximum phase angle difference between subsequently transmitted symbols. A constellation having a number of redundant symbols within the constrained maximum phase angle is chosen. An iterative search is performed to assign each of these symbols to a set of encoded bits such that the resultant coded scheme provides optimal bit error rate (BER) performance for the channel type. Configuring the encoder and mapper in this manner reduces the transmitted power envelope variation of the transmitted signal, thereby reducing the linearity requirements on a transmitter amplifier. Since transmitted power envelope variation is reduced, battery life of a mobile unit is extended. This manner of configuration also retains significant separation between transmitted sequences, resulting in substantial resistance to interference.

Trellis coded modulation scheme with low envelope variation for mobile radio by constraining a maximum modulus of a differential phase angle

Since the mid 1980s, research and development in satellite modem techniques has been very active and has yielded promising results. In the article the author presents a summary of the current status of work in this area, and discusses the directions this work is taking. >

Modem techniques in satellite communications

The use of pi /4 quadrature phase-shift keying(QPSK) modems for satellite sound broadcast systems (SSBS) broadcasting to mobile or portable receivers is proposed. Three different differential detectors (including the FM-discriminator followed by integrate-sample-and-dump filter) and a novel coherent detector are discussed. The degradation caused by the frequency offset between the local oscillator and the unmodulated carrier in the baseband differential detector is studied. The performance of both coherently and differentially detected pi /4-QPSK in a Gaussian channel is also studied. It is shown that with a frequency offset of more than 3% of the symbol rate, the performance degradation is more than 1 dB at 10/sup -4/. The out-of-band power of the nonlinearly amplified bandlimited pi /4-QPSK signals is reduced from -13 dB to -37 dB if a 2-dB output back-off amplifier is used instead of a hardlimiter. The performance of the pi /4-QPSK is equivalent to that of QPSK, although the pi /4-QPSK has the advantage of less spectrum restoration after nonlinear amplification. The coherent demodulator and differential decoder avoid the three-level detection and achieve the same bit-error-rate (BER) performance as DEQPSK with a simple circuit. >

C.-L. Liu

Papers

pi /4-QPSK modems for satellite sound/data broadcast systems