Ahmed Kamel Sadek

Cooperative Communication Protocols in Wireless Networks: Performance Analysis and Optimum Power Allocation

In this paper, symbol-error-rate (SER) performance analysis and optimum power allocation are provided for uncoded cooperative communications in wireless networks with either decode-and-forward (DF) or amplify-and-forward (AF) cooperation protocol, in which source and relay send information to destination through orthogonal channels. In case of the DF cooperation systems, closed-form SER formulation is provided for uncoded cooperation systems with PSK and QAM signals. Moreover, an SER upper bound as well as an approximation are established to show the asymptotic performance of the DF cooperation systems, where the SER approximation is asymptotically tight at high signal-to-noise ratio (SNR). Based on the asymptotically tight SER approximation, an optimum power allocation is determined for the DF cooperation systems. In case of the AF cooperation systems, we obtain at first a simple closed-form moment generating function (MGF) expression for the harmonic mean to avoid the hypergeometric functions as commonly used in the literature. By taking advantage of the simple MGF expression, we obtain a closed-form SER performance analysis for the AF cooperation systems with PSK and QAM signals. Moreover, an SER approximation is also established which is asymptotically tight at high SNR. Based on the asymptotically tight SER approximation, an optimum power allocation is determined for the AF cooperation systems. In both the DF and AF cooperation systems, it turns out that an equal power strategy is good, but in general not optimum in cooperative communications. The optimum power allocation depends on the channel link quality. An interesting result is that in case that all channel links are available, the optimum power allocation does not depend on the direct link between source and destination, it depends only on the channel links related to the relay. Finally, we compare the performance of the cooperation systems with either DF or AF protocol. It is shown that the performance of a systems with the DF cooperation protocol is better than that with the AF protocol. However, the performance gain varies with different modulation types and channel conditions, and the gain is limited. For example, in case of BPSK modulation, the performance gain cannot be larger than 2.4 dB; and for QPSK modulation, it cannot be larger than 1.2 dB. Extensive simulation results are provided to validate the theoretical analysis.

Cognitive multiple access via cooperation: Protocol design and performance analysis

In this paper, a novel cognitive multiple-access strategy in the presence of a cooperating relay is proposed. Exploiting an important phenomenon in wireless networks, source burstiness, the cognitive relay utilizes the periods of silence of the terminals to enable cooperation. Therefore, no extra channel resources are allocated for cooperation and the system encounters no bandwidth losses. Two protocols are developed to implement the proposed multiple-access strategy. The maximum stable throughput region and the delay performance of the proposed protocols are characterized. The results reveal that the proposed protocols provide significant performance gains over conventional relaying strategies such as selection and incremental relaying, specially at high spectral efficiency regimes. The rationale is that the lossless bandwidth property of the proposed protocols results in a graceful degradation in the maximum stable throughput with increasing the required rate of communication. On the other hand, conventional relaying strategies suffer from catastrophic performance degradation because of their inherent bandwidth inefficiency that results from allocating specific channel resources for cooperation at the relay. The analysis reveals that the throughput region of the proposed strategy is a subset of its maximum stable throughput region, which is different from random access, where both regions are conjectured to be identical.

SER performance analysis and optimum power allocation for decode-and-forward cooperation protocol in wireless networks

We derive a closed-form symbol-error-rate (SER) formulation for the cooperation system with PSK and QAM signals. Moreover, two SER upper bounds are established to show the asymptotic performance of the cooperation protocol, in which one of them is tight at a high signal-to-noise ratio. Based on the SER performance analysis, we also determine the optimum power allocation for the cooperation systems. It turns out that an equal power strategy is, in general, not optimum in cooperation communications, and the optimum power allocation depends on the channel link quality. An interesting result is that, in the case that all channel links are available, the optimum power allocation does not depend on the direct link between source and destination, it depends only on the channel links related to the relay. Extensive simulations are performed to validate the theoretical results.

Outage analysis and optimal power allocation for multinode relay networks

In this letter, a novel approach for outage probability analysis of the multinode amplify-and-forward relay network is provided. It is shown that the harmonic mean of two exponential random variables can be approximated, at high signal-to-noise ratio (SNR), to be an exponential random variable. The single relay case considered before is a special case of our analysis. Based on that approximation, an outage probability bound is derived which proves to be tight at high SNR. Based on the derived outage probability bound, optimal power allocation is studied. Simulation results show a performance improvement, in terms of symbol error rate, of the optimal power allocation compared to the equal power-allocation scheme

Technical challenges for cognitive radio in the TV white space spectrum

The FCC recently issued the regulatory rules for cognitive radio use of the TV white space spectrum. These new rules provide an opportunity but they also introduce a number of technical challenges. The challenges require development of cognitive radio technologies like spectrum sensing as well as new wireless PHY and MAC layer designs. These challenges include spectrum sensing of both TV signals and wireless microphone signals, frequency agile operation, geo-location, stringent spectral mask requirements, and of course the ability to provide reliable service in unlicensed and dynamically changing spectrum. After describing these various challenges we will describe some of the possible methods for meeting these challenges.

On the energy efficiency of cooperative communications in wireless sensor networks

Cooperative communications represent a potential candidate to combat the effects of channel fading and to increase the transmit energy efficiency in wireless sensor networks with the downside being the increased complexity. In sensor networks the power consumed in the receiving and processing circuitry can constitute a significant portion of the total consumed power. By taking into consideration such overhead, an analytical framework for studying the energy efficiency trade-off of cooperation in sensor networks is presented. This trade-off is shown to depend on several parameters such as the receive and processing power, the required quality-of-service, the power amplifier loss, and several other factors. The analytical and numerical results reveal that for small distance separation between the source and destination, direct transmission is more energy efficient than relaying. The results also reveal that equal power allocation performs as well as optimal power allocation for some scenarios. The effects of the relay location and the number of employed relays on energy efficiency are also investigated in this work. Moreover, there are experimental results conducted to verify the channel model assumed in the article.

Cooperative communications with partial channel state information: When to cooperate?

In this paper we propose a new cooperative protocol, which takes into consideration the partial channel state information (CSI) available at the source. With such protocol a significant improvement in the transmission rate can be achieved in decode-and-forward cooperative transmission, while guaranteeing full diversity order. We derive closed-form expressions for the transmission rate and the symbol error rate (SER) for the M-PSK and the M-QAM signalling. Moreover, we consider two optimization metrics in the protocol design to enhance the system performance; the first is based on minimizing the SER only, while the second is based on minimizing a joint function of both the SER and the transmission rate. Finally, the obtained analytical results are verified through computer simulations

A TV white space spectrum sensing prototype

The paper provides an overview of a real-time spectrum sensing prototype operating in the TV white space. There has been considerable research on the feasibility of spectrum sensing in the TV white space. This is a challenge due to the very weak signal power levels specified by the FCC. In the TV white space there are a number of incumbent systems that must be protected from interference by a TV white space device. One method of protection is spectrum sensing, which can be used in locations where geo-location is problematic. This paper describes a real-time prototype which performs sensing for ATSC, NTSC and wireless microphones in the UHF TV band. An overview of the sensing techniques are provides as well as a description of how the sensing techniques are realized in the prototype.

Cooperative communications protocol for multiuser OFDM networks

This paper proposes a relay-based scheme where a fixed number of relays are utilized to design an OFDM cooperative protocol. By exploiting limited feedback from the destination terminal (central node) such that each relay is able to help forward information of multiple sources in one OFDM symbol, the proposed cooperative protocol not only achieves full diversity but also efficiently utilizes available bandwidth. A practical relay assignment scheme is also proposed to specify the pairing of sources and relays in cooperative networks. The outage probability analysis of the proposed cooperative protocol is provided, and the analytical results are validated via simulations. Moreover, a closed-form lower bound on the outage probability of any relay-assignment schemes is established to provide a performance benchmark of the proposed cooperative protocol. Based on the outage probability analysis, the optimum relay location for the proposed relay-assignment scheme is determined. Both theoretical and simulation results show that in WLAN scenario the proposed cooperative protocol can achieve significant coverage extension compared to the direct transmission.

An efficient cooperation protocol to extend coverage area in cellular networks

In this paper, we propose a bandwidth efficient cooperation protocol to extend coverage area in cellular networks. An important question, however, in implementing any cooperation protocol is how to assign cooperating partners. We introduce a BS-controlled relay-assignment algorithm to assign cooperating users. First, we derive a lower bound on the outage performance of any practical relay assignment protocol. The lower bound can be achieved by a hypothetical Genie-aided protocol that places a relay at the optimal theoretical location for every source in the network. Then we describe and analyze the performance of the base-station/access-point (BS/AP) controlled protocol. The BS/AP-controlled protocol can be implemented at the BS or the AP, and it tries to emulate the Genie-aided protocol. Simulation results for rural cellular systems are provided. By utilizing the proposed protocol, simulation results indicate a significant gain in coverage area over the direct transmission scheme under fairly the same bandwidth efficiency and fixed average transmitted power. For the rural cellular system case, coverage extension of about 250% can be achieved by the BS/AP-controlled protocol