Deploying femtocells faces a major challenge of causing downlink interference to the existing macrocell. This paper proposes an intelligent spectrum sharing mechanism for a two-layer cellular network, which comprises macrocells and femtocells. Because users are scheduled to access the spectrum in time-domain according to their channel conditions, the usage of spectrum is predictable by tracing the channel conditions. Therefore, the femtocell can be managed, in time-domain, to reconfigure the antenna radiation pattern in order to suppress the interference to closely-located macrocell users who are expecting to access the spectrum. Simulation results show that user throughput at the macrocell edge can be boosted by 83%, compared to deploying femtocells with fixed omni-directional antennas.

Frequency-selective fading and inter-carrier-interference (ICI) are two fundamental issues that should be dealt with before adopting orthogonal frequency division multiplexing (OFDM). Instead of using complex designs for exploiting frequency diversity and cancelling ICI, this paper explores how spreading signal, on either frequency-domain or time-domain, can improve the performance of OFDM systems. Based on the carrier interferometry technique, information symbols are spread over all the OFDM subcarriers to pursue frequency diversity. Then a packet retransmission scheme, which can be seem as spreading signal in time-domain, is proposed to suppress ICI. Numerical simulations show that frequency diversity can be pursued by spreading information symbols in OFDM subcarrier domain and that ICI can be suppressed by spreading packets in time domain. As a result, an enhanced OFDM system with low complexity signal spreading can be delivered.

This paper explores multiuser downlink beamforming in a distributed manner, where each user locally and sequentially optimizes its own beamforming phase and amplitude according to some performance measures. For phase optimization, the complex non-convex optimization problem is reformulated into a convex one. Given the optimal phase, the amplitude is pursued through the method of Lagrange multiplier. Our numerical simulations demonstrate that the proposed algorithm matches the conventional centralized two-user zero-forcing beamforming performance in terms of bit error rate. Meanwhile, our method utilizes 50% less feedback as compared to the conventional one. As a result, a low complexity distributed multiuser beamforming is delivered.

We derive in this paper the bit error probability and cutoff rate of an Amplify-and-Forward (AF) relay system operating in Rayleigh fast fading. In our model, we consider the relay thermal noise to be non-negligible and derive a closed form expression to approximate the bit error probability. Our approximated closed form expression matches well with simulation result. Using Chernoff bound, we upper bound its pairwise error probability and we also derive the cutoff rate of the AF relay system.

In this paper, we present analytical bit error probability results for M-ary modulation concatenated with Reed Solomon (RS) codes. The analysis of bit error probability is nontrivial as the number of bits per symbol for the RS codes may not be an integer multiple of the number of bits per symbol for a modulation symbol. We propose a Markov chain technique which allows analytical evaluation of the bit error probability for such cases. The performance of RS coding with coherent biorthogonal, coherent/non-coherent orthogonal modulation over an additive white Gaussian noise (AWGN) channel is evaluated. Simulation of the bit error probability of RS code concatenated with a Nordstrom Robinson (NR) code as an inner code is performed and compared with the case of biorthogonal modulation. From the results, we notice that a stronger inner code gives better bit error probability. In addition, the throughput of the coded system with biorthogonal modulation over an AWGN channel is discussed. For a Rayleigh flat fading and block fading channel, we analyze the bit error probability of RS codes concatenated with biorthogonal modulation. From the result, we notice that a stronger outer code gives a better bit error probability for the case of Rayleigh flat fading channel.