SUMMARYThe transmission quality of mobile radio communications is strongly affected by fading with rapid fluctuations in the envelope and phase of the received signal, in addition to the effect of thermal noise. This paper considers a multiple access system (DS-CDMA system) using the spread spectrum technique with direct spread, and presents a method of deriving the approximate average bit error rate in a frequency-selective fading environment (the n-ray Rayleigh fading model), with the maximum Doppler frequency, the delay time of the delayed wave, and the number of stations simultaneously utilizing the channel as parameters. The two-wave model and the IMT2000 model are considered as examples, and the validity of the proposed method is demonstrated by the fact that the results obtained by the approximate formula agree well with the simulation results even if the parameters are varied.
Abstmct-A PSK group modem which modulates and demodulates multiple PSK signals en bloc will play an important role in realizing a cost-effective satellite communications system based on the narrowband multicarrier PSK/FDMA technique. This paper briefly discusses its possible applications, and presents a developed PSK group modem including its algorithm, hardware design, and implementation. In the algorith'm, filtering of respective PSK signals is effectively carried out by digital signal processing techniques applied to the digital transmultiplexer. Furthermore, in order to compensate for the ,difference between the operation timing of the PSK group modem and the symbol timing of each input/output data signal, digital signal processings based on a table-look-up method using ROM and a rate-conversion filter with adaptive weighting are proposed for the modulator and demodulator, respectively. An experimental 4-phase PSK group modem has been developed, and experiments have been conducted under various conditions. The measured performance of the developed hardware is also demonstrated.
Orthogonal frequency division multiplexing (OFDM) systems can avoid the eãect of multipath fading by absorbing delayed waves into guard interval. But as the eãect of delayed waves over guard interval becomes stronger, it causes the bit error rate (BER) performance in OFDM systems to be more degraded. This paper proposes the approximate derivation method of the BER in DQPSK/OFDM systems over frequency selective fading channels. In this study, we will derive the approximate equation of the BER with diãerential coding/diãerential detection in the time domain. We conårm that the BER derived from the proposed approximate equation coincides with that by computer simulation even when such parameters as the Doppler frequency shift, the delay spread, the Rician factor and so on are varied. I. INTRODUCTIONOrthogonal frequency division multiplexing (OFDM) systems have attracted considerable attention in the recent past because of its robustness against multipath fading (frequency-selective fading) and high spectrum eé-ciency. The futures have warranted the designation of OFDM as a standard transmission technique for various services, for example, forth generation mobile communications systems, digital terrestrial TV broadcasting and high speed wireless LAN.Besides the above appealing properties, however, OFDM systems suãer severe performance degradation from the impacts of Doppler frequency shift and delayed waves caused by multipath transmission because of the co-channel interference and the inter-carrier interference (ICI) in OFDM systems [1]- [3].We have already derived the approximate equation of the bit error rate (BER) in DQPSK/OFDM systems over frequency non-selective Nakagami-Rice and Rayleigh fading channels in [4]. We evaluated the performance degradation caused by the impact of Doppler frequency shift f D T s , where f D is the maximum Doppler frequency and T s is a symbol duration. OFDM systems can avoid the interference caused by multipath fading by absorbing delayed
This paper proposes an inter-symbol interference (ISI) suppression scheme using the periodic signal waveform for fixed-rate coded OFDM systems. In this scheme, the periodic signal waveform composed of the even-numbered sub-carrier group is used for extending the guard interval, and the odd-numbered sub-carrier group is also enable to be available by means of frequency conversion in order to realize the sub-carrier group selection. This paper mainly focuses on the combining effect of the sub-carrier group selection and the channel coding scheme in the inter-symbol interference (ISI) condition. Computer simulation results show that the proposed scheme has the great potential to create the combined frequency diversity effect without the error floor even in the severe ISI condition.
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