Time Diversity Reception Research Articles

Design and Investigation of 10 Gb/s FSO Wiretap Channel Using OCDMA Time-Diversity Reception

In this paper, 10 Gb/s free space optical (FSO) wiretap channel using optical code division multiple access (OCDMA) time-diversity reception is designed and demonstrated for the first time. Reliability and security are investigated under weak and strong turbulence conditions. Two-dimensional optical encoder and decoder are constructed by wavelength selective switches and optical delay lines, and one-dimensional optical encoder and decoder are constructed by couplers and tunable optical delay lines. Experimental results show that reliability and security can be enhanced simultaneously using OCDMA time-diversity reception. At the received power of 2.58 dBm in strong turbulence, compared to non-diversity system, bit error rate (BER) of legitimate user in OCDMA time-diversity system is reduced from 3.9 × 10−7 to 6.73 × 10−8. Furthermore, the reliability improvement is more obvious when the correlation between the two signals of OCDMA time-diversity is smaller. The secrecy capacity can be increased from 0.546 bit/symbol to 0.665 bit/symbol in strong turbulence when received power is 2.28 dBm. On the other hand, the secrecy capacity of strong turbulence is about 15% higher than that of weak turbulence.

Enhancement of reliability and security in a time-diversity FSO/CDMA wiretap channel

Due to atmospheric turbulence, the reliability and security of free space optical (FSO) communication is seriously affected. In this paper, we propose a new FSO wiretap channel model based on a time-diversity scheme and optical code division multiple access (OCDMA). At the receiving end, the legitimate user employs time-diversity reception by matched optical decoders, while an eavesdropper uses a random optical decoder for eavesdropping. Using an avalanche photodiode (APD) photon counting model, bit error rate (BER) performances of legitimate and eavesdropping users are theoretically analyzed under different turbulence and diversity. Based on the binary asymmetric channel model, the physical layer security performance of the time-diversity FSO/CDMA wiretap channel is evaluated by using security capacity. With the increase of time-diversity order, the reliability and security performances of FSO/CDMA wiretap channel are improved. Simulation results show that the performances of the time-diversity FSO/CDMA wiretap channel are better than those of a non-diversity FSO/CDMA system.

BER performance of QDPSK with postdetection diversity reception in mobile radio channels

The bit-error-rate performance of Nyquist raised-cosine-filtered quaternary differential phase shift keying (RC-QDPSK) and pi /4-shift RC-QDPSK with postdetection diversity reception is theoretically analyzed for Rayleigh fading channels. A selection combiner, equal-gain combiner, and maximal-ratio combiner are considered for postdetection diversity reception. Diversity improvements on the average BER due to additive white Gaussian noise, cochannel interference, random FM noise, and delay spread of the multipath channel are calculated. Also analyzed is the combined effect of diversity reception and error control. The required E/sub b//N/sub 0/ and allowable RMS delay spread for achieving a certain BER and spectrum efficiency in cellular mobile systems are calculated when short BCH forward error correcting codes are used. The performance of automatic repeat request combined with time diversity reception is investigated. >

4.8 kbit/s data transmission over 16 kbit/s digital mobile radio channel

A channel coding is described for 4.8 kbit/s data transmission in a digital mobile radio using a 16 kbit/s radio channel. The channel coding is a combination of time diversity reception and forward error correction. Laboratory simulation and field experimental results using 1.5 GHz carrier frequency are presented.

Simulated throughput efficiency of TD-SAW ARQ system in digital mobile radio

Computer simulation results for the throughput efficiency of stop-and-wait automatic-repeat-request with time diversity reception (TD-SAW ARQ) are presented for digital mobile radio channels. It is shown that the efficiency η in the TD-SAW ARQ system does not decrease as much as in the SAW ARQ system, when the maximum Doppler frequency fD becomes large.

Level crossing rate and average fade duration for time diversity reception in Rayleigh fading conditions

Time diversity, in which the same data are transmitted several times, is attractive in digital land mobile radio and is simple to implement because only one antenna is required. Expressions for the level crossing rate (LCR) and average fade duration (AFD) are derived in this paper for a system in which each data symbol is transmitted twice (two-branch diversity). As far as LCR is concerned, the expected diversity advantages can be obtained for a data repetition period, normalised by the maximum Doppler frequency, of about 0.2. For a large data repetition period, the AFD is halved. Measured values of LCR and AFD, obtained from 900 MHz signals received at a base station site, are in good agreement with the predicted values.

Efficient ARQ with time diversity reception technique—a time diversity ARQ

A technique for increasing the efficiency of an ARQ system using time diversity reception is proposed fo mobile radio use. In this technique, the receiver demodulator output associated with each symbol is weighted and combined every time a new retransmission is made. Theoretical results show that this technique offers a considerable reduction in the average number of transmissions, in comparison with conventional ARQ systems.

An Autocorrelation Criterion for the Time-Diversity Reception of Speech Over Burst-Error Channels

This paper proposes a new approach to signal selection in time-diversity systems. Specifically, we consider the problem of digital speech transmission over a burst-error channel using two-channel time-diversity reception. Let every speech segment (of length W) be transmitted twice so that at least one of the transmissions escapes an error burst, with a certain useful probability. Let the received speech segments be Y <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> and Y <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> . We propose an autocorrelation-maximizing signal selection procedure of the following form. Select Y <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> (or Y <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ) as the “cleaner” speech segment according as C(Y <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> , W) ≧ (or <) C(Y <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , W), where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ C(Y_u,W)=\sum_{r=2}^W({\rm sgn}\ Y_{ur}\cdot{\rm sgn}\ Y_{u(r-1)})/(W-1);u=1,2.$</tex> Y <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">uτ</inf> is the speech amplitude at sample τ in Y <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</inf> , W is a computational window that is typically a few milliseconds long, and sgn Y <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">uτ</inf> is a polarity function that is assumed to have zero mean and unit variance. The use of sgn Y <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">uτ</inf> instead of Y <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">uτ</inf> leads to a simply implemented selection procedure, and computer simulations have demonstrated its practical utility. For example, in one study of three-bit dpcm coding, autocorrelation-based burst-error detection proved to be more useful than a procedure where dpcm samples were error-protected on a bit-by-bit basis, rather than in blocks.