Performance Evaluation of Encoded Opportunistic Transmission Schemes

Abstract

The performance of encoded opportunistic transmission schemes in wireless channels affected by Rayleigh fading and additive white Gaussian noise (AWGN) is analyzed. In opportunistic transmission, the information is transmitted only when the fading amplitude is above a threshold. For this, the receiver with the knowledge of the channel state information notifies the instants the transmitter should transmit. Opportunistic systems with convolutional error correcting codes or with trellis coded modulation are analyzed in terms of closed-form bit error rate (BER) expressions. Nevertheless, the approach presented can be employed with any kind of error correcting codes. Hence, the performance of turbo codes is also presented in the simulations. Monte Carlo simulations verify the accuracy of the derived expressions and provide insights on the system performance. Performance results show that uncoded and encoded opportunistic systems are superior to uncoded and encoded ordinary systems (non-opportunistic), respectively. In particular, the BER curves of the opportunistic system decay exponentially when the signal-to-noise ratio (SNR) increases. On the other hand, BER curves for ordinary transmission decay linearly, where the slope is proportional to the diversity that depends on the error correcting code. Thus, opportunistic systems require less SNR to guarantee the same BER of ordinary transmission. The BER gain increases as the SNR increases. It is also observed that uncoded opportunistic systems are even superior to encoded ordinary ones. The results are validated guaranteeing the same spectral efficiency for all the scenarios. Finally, due to the exponential decay of the BER curves, coding gain expressions, used in ordinary systems over AWGN, can be used as approximations for opportunistic transmission in fading channels. INDEX TERMS Wireless communication, opportunistic transmission, error correction codes, bit error rate, Rayleigh channels.

On the Bit Error Probability and the Spectral Efficiency of Opportunistic Wireless Transmissions in Rician Fading Channels

“…At this point, it is important to remember that the mean SE is proportional to q (refer to (43)). As a consequence, the cell area can be reduced, 7 or, a lower transmission probability can be used at the cell edge. We also observe that as q is reduced to 1/2 or 1/3, the SE decreases, but coverage is guaranteed throughout the cell.…”

“…that is defined as the total power received in all paths, i.e., (7) is the mean squared fading amplitude. In the receiver, after the coherent demodulation stage and the correlator (matched filter), the received sample during a symbol interval, assuming flat fading, can be written as…”

“…Hence, widely used techniques to counteract the undesirable fading effects are time, frequency or spatial diversity [4]- [6]. Other techniques such as error correction codes also reduce the BEP and even make it possible to obtain diversity [7]. Frequency or time diversity techniques present loss of spectral efficiency (SE) in exchange for performance improvement.…”

“…Besides, the loss of SE is reduced once the use of several receiving antennas reduces the probability that the fading is below the established threshold and therefore, the non-transmission probability is very small. Besides, in [7], the performance of OpT is evaluated in encoded systems considering trellis-coded-modulation (TCM) and turbo codes are evaluated. Upper-bound expressions to evaluate the ABEP are obtained considering a Rayleigh fading channel.…”