Visible light communication (VLC) is a promising technology for next-generation indoor wireless broadband communication systems. Physical layer security (PLS) is one proposed method to ensure data security for VLC in public areas. However, most PLS studies in the literature neglect the reflections in the VLC channel and do not consider inter-symbol interference (ISI). This paper investigates the ISI effects on the secrecy rate in multiple-input single-output VLC systems where the reflections are not neglected for the cases of perfect and imperfect knowledge of the eavesdropper’s channel state information. We aim to compensate for the ISI effects by designing zero-forcing (ZF) and artificial noise (AN) beamformers. In addition, we formulate optimization problems to maximize the secrecy rate resulting in non-convex and NP-hard problems. Thus, we utilize the particle swarm optimization algorithm for solutions and also convert non-convex optimization problems into easily solvable linear problems. Furthermore, robust ZF and AN beamformers for the worst-case scenario are considered for the cases of ISI and no ISI. The results reveal that ISI severely degrades the secrecy rate; however, well-designed beamformers can reverse the ISI effects.

As an attempt to improve throughput of WiMAX system, the oversampled WiMAX receiver is presented in this paper. Oversampling samples each transmitted symbol more than one time. Thus, several signal sequences which contain different information about channel may be obtained at the receiver. This study presents throughput of the oversampled WiMAX system that obtained by simulation over a realistic channel model and by experimental tests. Since the performance of oversampled system is very much depends on channel conditions, realistic channel model and experimental study are essential to exhibit the realistic performance benchmarks. Simulation and experimental results clearly show that oversampled WiMAX receiver significantly outperforms the spectrum efficiency of the conventional system.

Outage capacity is one of the widely accepted metrics to predict the maximum reliable information rate over fading channel with channel side information (CSI) at the receiver only. This metric is calculated with the assumption that since CSI is not available at the transmitter side, transmission strategies such as rate adjustment cannot be employed to match channel varying to improve the system spectral efficiency. But it is possible to adjust transmission rate at the receiver side instead of adjusting at the transmitter side. In this case, is the widely accepted concept on outage capacity still true? To answer this question, we evaluate spectrum efficiency of two different systems that use rateless codes for several modulation schemes over Rayleigh flat fading channel by theoretical analysis and simulation in the paper. It is shown that outage capacity cannot be well used to predict maximum reliable information rate for fading channels in middle regime of signal to noise ratio.

Adaptive modulation and coding (AMC) and incremental redundancy (IR) are two popular rate adaptive systems to improve system capacity, peak data rate and reliable coverage of transmission in wireless mobile communications suffering from time-varying channel conditions. Adaptive demodulation (ADM) using rateless codes has been considered as an alternative solution for these rate adaptive systems in order to avoid their resource consumptions. In ADM system, data stream is modulated with a fixed modulation scheme at the transmitter but it is demodulated at a non-fixed rate at the receiver. If the signal to noise ratio (SNR) of the communication channel is not high enough to demodulate the message to reach predefined bit error rate (BER), demodulation level is then decreased by treating some of the bits in each symbol as erasures and being discarded. In this paper, we shall propose a log-likelihood ratio (LLR) based ADM algorithm to select the bits to demodulate. In our algorithm we shall select the bits to demodulate in whole information packet by comparing their probability to be correct instead of using modified constellation diagram to select bits per symbol. Therefore, the probability of discarding incorrect bits will increase, resulting in BER performance and transmission rate improvement. In addition, our developed algorithm can be applied easily to any modulation scheme and the corresponding demodulation rate can be selected as any rational rate value which relaxes the limitation of ADM systems. Various simulations show that our proposed LLR based ADM technique significantly outperforms the conventional ADM algorithm in term of BER performance.

This paper presents an experimental evaluation of bit-error-rate (BER) performances for adaptive training algorithms and obtaining matched filter bound in real-time WiMax (3.5 GHz) radio channels. Without involving with any modified or improved versions, two conventional adaptive equalizer training methods, the least mean squares (LMS) and recursive least squares (RLS), are implemented in a C code running over sampled received sequence and their performances are compared in different modulation techniques before and after coding. In experimental trails the obtained channel impulse responses were generally non-minimum phase and the obtained BER performance of receiver using LMS training has been quite close to whom using RLS particularly after coding, in real-time channel conditions.

Field trails of orthogonal frequency division modulation (OFDM) and Single Carrier (SC) transmissions are presented in this paper which puts forward experimentally obtained BER performances ahead of simulation works submitted in the literature. Instead of giving raw error performance results, experimental studies are also extended to calculate matched filter bound for both OFDM and SC systems, which matches the theoretical limit of real-time wireless medium.