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Nowadays, low reuse factors are used in cellular systems because of high traffic demand, despite it produces high co-cell interference (CCI) levels. Consequently, soft-frequency-reuse (SFR) and sectorization are used to improve the spectral efficiency and to mitigate CCI. In addition, diversity techniques are necessary for a good system performance. Motivated by this scenario, for the uplink of orthogonal-frequency-division multiple access (OFDMA) systems, the bit error rate (BER) and the cellular spectral efficiency using multilevel-quadrature-amplitude-modulation (M-QAM) and maximalratio-combining (MRC) in Rician fading channels are analyzed, where diversity branches have different Rician K-factors (unbalanced diversity). SFR is used assuming non-ideal sectorized cells due to the irregular radiation pattern of base station antennas. An exact integral-form expression and a closed-form upper-bound to evaluate the BER are obtained. In addition, an algorithm, and an expression to calculate the cellular spectral efficiency are presented considering that a target BER must be guaranteed for all users in the cell. From the analysis, it is determined that the BER can be reduced and the spectral efficiency can be improved if some system operating parameters are selected in an adequate manner. Thus, it was noticed that the number of diversity branches, the sum of the K factors of these branches, and the antenna type, are decisive to guarantee the target BER and to maximize the cellular spectral efficiency.

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.

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