AN OVERVIEW OF EMBEDDED WIRELESS NETWORK SYSTEM EMPLOYING COOPERATIVE WIRELESS COMMUNICATION SYSTEM Aznor Hanah*, Abid Yahya, Ahmad R.B, Farid Ghani, Norlydiana, Hasneeza Liza School of Computer and Communication Engineering Universiti Malaysia Perlis (UniMAP) 02000 Kuala Perlis, Perlis, MALAYSIA *
[email protected]
Abstract This paper presents an overview as well as proposed model of cooperative communications in detailed. The concept of relay channel being introduced to the current experimentation for cooperative communication has been presented. There are many researches done on cooperative protocols and cooperative relay to exploit the degree and information which would enhance the performance of cooperative communication. Modulation techniques are also incorporating better operation in achieveing full diversity gain in the number of cooperating terminals. In a cooperative diversity system that uses randomized space-time block coding, there are relationship between node density and range improvement with the existence of timing error based on Gaussian approximation. To guarantee full diversity for any number of mobile relays with minimum delay, cooperative balances space time block coding scheme has been proposed. OFDM in cooperative communications can be used for a relay management strategy to simplify the cooperation in wireless network. Since OFDM distributes the data over a large number of carriers that are spaced apart at precise frequencies. In terms of diversity scheme, performance analysis has been carried out and the range improvement as the cooperative communication is implemented. A summary of projects on cooperative experimental has been presented in this paper. An embedded system is being proposed which would need individuals to share data freely and having an intelligent transportation system network.
Keywords: Cooperative communications, cooperative protocols,embedded system, OFDM,.
1
INTRODUCTION
Cooperative communication [1,2] is introduced as an important concept to enhance reliability and throughput in wireless systems. Cooperative communications are related to the wireless network where that can improve their efficient quality by cooperation. The effective quality can be measured at the physical layer by using bit error rates, block error rates or using outages probability. In a wireless network, signal fading arising from multipath propagation is a particularly severe form of interference that can be mitigated through the use of diversity. Fading means the variation of the signal strength or the signal power, which means that the signal strength can be varied; it can be increased or decreased. Problems occur when the signal strength decreases and this problem need to be solved by many ways such as frequency diversity, antenna diversity or polarization diversity. If the signal strength is increased means that the signal power is also increased, but of course no more than the original transmitted power. It just increased to be more than the expected received level. In a diversity system the fading process in each diversity path differs from the other paths. Hence, if the probability of a path being faded is p, the probability of all paths being totally faded will be p to power L , where L is the number of diversity paths. Diversity is a method for directly combating the effects of fading. Some well-known forms of diversity are spatial diversity, temporal diversity, and frequency diversity [3]. Spatial diversity is obtained when signals transmitted from antennas separated far enough to experience independent fading channels. In wireless networks, signal fading arising from multipath propagation is a particularly severe form of interference that can be decreased through the use of diversity. If multiple independent channels is used on which the same information is transmitted, then it is creating additional spatial degree of freedom that improves the signal strength and will have good coverage. With diversity, reliability of the signal can be improved.
2
RELAY CHANNEL
The beginning of cooperative communication is introduced by Cover and El Gamal [3] work that explained on relay channel. The relay channel model that showed in Figure 1 shows that transmitter A sends a noisy signal X. The attenuation signal is received by the receiver C and a relay B. The relay then transmits the overheard signal X1 to the same destination. Authors then calculate the information theoretic capacity of this channel and found that it is bounded by the minimum rates of transmission. Authors then conclude that the overall capacity is better compared to individual capacity between A and C.
Figure 1 : Relay channel
3
COOPERATIVE PROTOCOLS
Wireless communication has been flourishing and increasing smoothly during the previous year. The transmitted signal usually use diversity schemes to enhance the performance. Transmit diversity commonly use more than one antenna at the transmitter but the problem still occur when the wireless device are limited by the size or hardware complexity to each antenna. A new class scheme has been proposed call cooperative wireless communication by Aria and Ahmadreza [4]. This scheme overcomes the problem by enabling an antenna mobile to share their antenna and produces a virtual multiple antenna transmitter. Three methods have been discussed are Amplify and Forward, Decode and Forward and Coded cooperation. Comparing to these three cooperative transmission schemes, it shows that amplify and forward and decode and forward are not suitable at low signal to noise ratio (SNR). This is because their signalling is proportional repetition coding and not efficient at low SNR. Compared to amplify and forward and decode and forward, coded cooperation has smooth degradation and performs better at all SNR. Evolving of the wireless transmission, Andreas et al. [5] have compared amplify and forward with decode and forward transmission protocol. The diversity is considered by building an ad-hoc network using third station as a relay. The different combining methods must taking into account to compare which method has the greater performance. Using several combining methods such as Equal Ratio Combiner (ERC), Fixed Ratio Combiner (FRC) and Enhanced Signal-to-Noise Ratio Combining (ESNRC) gives AAF protocol as the better performance rather than DAF. The best performance has been performed when the relay is at same distance from the sender and the receiver or a bit closer to the former. A class of slotted amplify and forward (SAF) being studied by Sheng and Jean [6] to overcome the problem regarding to the previous article that proposed amplify and forward (AF) and decode and forward (DF) schemes produce results that it do not improve with increasing number of relays in terms of diversity multiplexing tradeoffs (DMT) when multiplexing gains, r higher than 0.5. The authors construct upper bound on DMT and produced result that asymptotically reach the multiple input single output (MISO) bound. Investigation of a sequential SAF scheme shows that the schemes achieves the proposed DMT upper bound which tends to the transmit diversity when the number of slots, M reach infinity and gouge the diversity gain in high multiplexing gain regime (r > 0.5). From the observations, it shows that outage and error rate performance expose a significant gain of the proposed scheme compared to previously proposed schemes.
Amplify and forward protocol in wireless communication continually being analyzed by Patrick et al. [7] as shown in Figure 2. Full cooperative communication system has been achieved where it operates in real time over real wireless channel. The results while employing amplify and forward show with a significant improvement in bit error rate (BER) under realistic wireless condition. The authors also focused on OFDM-based physical layer which used a distributed version of Alamouti block code which it uses relay where it sends one branch of Alamouti encoded symbol. This protocol is not influenced by carrier frequency offset at the relay and conventional Alamouti receiver can be used without change for the distributed relay system.
Figure 2 : Nodes in basic amplify and forward system
4
COOPERATIVE RELAY
One of the communication techniques in wireless area is cooperative, relaying where the nearest nodes support communication pairs to reduce the unwanted effects of multipath fading. The outcomes performance mightily depends on the selected relay. Even though it is providing the good outcomes, overall network performance probably reduced because of the increasing level of interference. Two basic schemes for relay network that are decode and forward and compressed forward have been studied by Gerhard et al. [8] to exploit terminal cooperation. In this scheme the transmitters cooperate and each receiver uses some or all of its previous channel output blocks to decode. Through the compression, the relays take advantage of the statistical dependence between their channel outputs and the receiver’s channel output. This strategy can be used to approach capacity if the terminals form two closely spaced cluster. The analysis shows that decode and forward have achieved the ergodic capacity with phase fading if the relays are in a region near the source terminal. Xiaoyan et al. [9] have studied about two sources that share a single relay to produce a simple cooperative diversity scheme as shown in Figure 3. This shared-relay scenario use cooperative coding scheme that only require channel coding at sources and network coding at the relay and hence it reduces system complexity for the base station. The message passes at the destination decoder where it obtains benefit from spatial diversity over several transmission frames. The combination of the channel coding at the sources and network coding at the relay has produce an efficiently diversity gains. They conclude that the proposed scheme give better competitive schemes based on consecutive relaying. Nikolaj et al. [10] have discussed a method that is relay selection mechanism to exploit the degree and information prepared for the potential relay nodes. Performance of several nominations and choosing method in terms of probability of victorious relay selection and the amounts of spatial resources used by cooperative relay has been discussed. A high success probability for relay selection can be achieved with decreasing the amount of additional block nodes. This happened when we take into account that the proposed method is not maximum in terms of spatial usefulness.
Figure 3 : Four node communications network. Sources Sa and Sb share a relay, R to transmit signal to destination, D.
5
MODULATION TECHNIQUES FOR COMMUNICATION
Ahmed et al. [11] have developed a class of cooperative diversity protocol for multinode wireless network employing decode- and-forward relaying. This protocol provides a scheme that each relay can combine the signal arrived from an arbitrary but fixed number of previous relays along with that received from the transmitter. They derive the expression for the SER of a general cooperation scheme for both MPSK and MQAM modulation. They also provide approximations for the SER which are shown to be tight at high enough SNR. The class of cooperative protocols shares the same asymptotic performance at high enough SNR and does not depend on the number of previous nodes involving in coherent detection, hence, the asymptotic performance of a simple cooperative scenario in which each relay combines the signals from the source and the previous relay is exactly the same as that for a much more complicated scenario in which each relay combines the signals from the source and all the previous relays. This analysis also exposed that the proposed protocols achieve full diversity gain in the number of cooperating terminals. Other than that, they formulate the optimal power-allocation problem, and show that the maximum power allocated at the nodes for an arbitrary network follow a certain ordering. They find that the maximum power-allocation scheme does not depend on the quality of the direct link between the source and the destination. Furthermore, they provide closed-form solutions for the optimal power allocation for some network topologies of practical interest, and conclude that through numerical examples that their theoretical results match with the simulation results.
6
SPACE TIME CODING SCHEME
Transmission of cooperative communication is from a cluster of node to a receiver with using randomized orthogonal space-time block coding protocol. This protocol allows every node to transmit one column of a given code matrix. This system is widely used in sensor network applications. Ramesh et al. [12] have verified the relationship between node density and range improvement with the existence of timing error based on Gaussian approximation for a cooperative diversity system that uses randomized space-time block coding. The range improvements are limited as the timing errors are limited to the fraction of the symbol duration. The range improvement stopped with the increasing number of nodes. The scaling of range as the number of nodes increase with timing error is not linear. The range approaches a limit beyond which range improvement is not significant. To guarantee full diversity for any number of mobile relays with minimum delay, cooperative balances space time block coding (CBSTBC) scheme has been proposed by Ali and Mehmet [13]. This method improves the error performances of the wireless system where each user has their own antenna. All transmitted symbols are individually decode in the relays and in the destination for orthogonal space time block codes and resulting decoding complexity at the destination increase linearly. The relay selection method does not increase the diversity even though the number of relays is higher because of the lack of source-relay channels information and opposite effect to the proposed scheme. In contrast to the relay selection, CBSTBC shows the source and relays share the maximum transmit power together in cooperative regime.
7
ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (OFDM)
The uses of Orthogonal Frequency Division Multiplexing (OFDM) have been reviewed by Bo Gui et al. [14] where it uses for a relay management strategy to simplify the cooperation in wireless network. They consider different relay and sub channel assignment and combining methods with different levels of CSI at the potential relay nodes to improve the end-to-end performance. Some simulation has been done to illustrate the potential space and frequency diversity and this simulation is done with focusing on block error rate. This analysis also includes the restrictions when sub channels can’t operate for retransmission. The significant performance gains still can be obtained even with little channels state information (CSI). The main purpose of OFDM is it distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own. Hence, the signal bandwidth is less than the channel coherence bandwidth. This aspect will effectively sweep out the ISI. Leng Zhi et al. [15] have combined cooperative diversity with OFDM by taking advantages of cooperative communication. Through this analysis technology of OFDM, amplify and forward and decode and forward protocols, two combining methods that are equal ratio combining and signal to noise
combining are performed. The authors then identified the performance of the cooperative diversity OFDM system with relay nodes placed at different locations.
8
AMPLIFY AND FORWARD COOPERATIVE OFDM
There are need to improve cooperation communication for better diversity reception. Aznor Hanah et all [16] has done performance analysis of cooperation wireless communication system with diversity scheme. They also investigate the range improvement as the cooperative communication is implemented. Analyses are conducted bases on optimization and characteristic for two types of diversity which are space and time. The model of the cooperative communication has been design in that includes source, relay and destination nodes as illustrated in Figure 4. Amplify and forward has been chose since it provide complexity of the system and low cost implementation.
Figure 4: Model for the wireless relay channel It has been demonstrate performance enhancement for the system that have diversity is accomplished when the communication error probability is decreased at a high channel signal-to-noise ratio (SNR) compared to any system that have less or no diversity. The presented results of the OFDM system with the presence of amplify and forward as the relay protocol shows the better performance of Bit Error Rate compared to conventional OFDM system. The evaluation shows clear benefits to implement amplify and forward relay rather than direct transmission, leading a significant BER improvement under wireless condition.
9
EXPERIMENTATION OF COOPERATIVE COMMUNICATION
Modeling and simulation can become unmanageable as the number of nodes within a network grows, making experimentation “one of the de facto approaches for benchmarking” [17]. Complex interactions between layers will also be impossible to completely predict, making full implementation necessary to identify conflicts that may degrade performance.Currently experimentation efforts can be separated into two groups on the architecture [18]: a) Legacy-based Existing platforms are used for implementation and the benefits using this architecture are promoting quicker adoption and facilitating direct comparison with the existing schemes. The withdraw on this architecture is to allow full cooperation are often not possible. b) Clean-slate The system needs to be design from scratch and it will allow full cooperation. This can be costly and time consuming project, Software-Defined Radio (SDR), have become popular for cleanslate implementation and experimental research. Seven projects of cooperative experimental have been done by several groups. The summary of the project is in Table 1. Wide experimentation in terms of number of topologies and nodes used has been done with the implementation of CoopMAC. The benefits of cooperation at the MAC layer has exposed and some approaches needed for intergrating cooperation into existing standards. Others experimentation stress on PHY cooperation in simple setups with no more four nodes, limited induced fading and little MAC integration. This is because much focus is on developing frameworks and testbeds which to experiment. A number of important basic theoretical predictions and demonstrating that currently available hardware and software solutions are confirmed with those experimentations.
Table 1: Summary of cooperative experimental projects Project Rice CoopMac WiFi CoopMac WARP Polytechnic PHY KTH ETH Notre Dame
Architecture clean-slate (FPGA) Legacy clean-slate (FPGA) clean-slate (FPGA) clean-slate (DSP) RF front-end clean-slate (GPP)
Layers PHY MAC MAC PHY PHY N/A PHY
Nodes 3 10-20 3 3 4 10 3
Topology line random line line random random triangle
Band Data 2.4 GHz 2.4 GHz 2.4 GHz 2.4 GHz 1.77 GHz 5.25 GHz 400 MHz
Rate 15 Mbps 11 Mbps 24 Mbps 10 Mbps 19.2 kbps N/A 50 kbps
Fading no no no no no yes yes
10 PROPOSED WORK i. ii. iii. iv.
Based on the review of cooperative communication, following objectives can be outlined: To develop ad-hoc networks using cooperative communication for better diversity reception. To investigate the range improvement as the number of Cooperating nodes increases. To measure the capabilities of wireless communication between moving objects. To transmit emergency medical information.
Generally in ad-hoc networks transmit diversity usually needs more than one antenna at the transmitter. On the other hand, many wireless systems are bound to size or hardware complexity to one antenna. In this work cooperative communication will be employed rather than conventional communication schemes. In cooperative communication single antenna mobiles in a multi-user environment share their antennas and produce a virtual multiple-antenna transmitter that permits them to accomplish transmit diversity. In cooperative wireless communication, a wireless network, of the cellular or ad hoc variety, enhances their effective quality of service via cooperation. Traffic jams on highways due to accidents or road works are a common occurrence in our lives. There is a way to prepare of an impending gridlock. Using a kind of mobile ad-hoc network, where, the mobile nodes are the vehicles equipped with a wireless communication device, information regarding accidents or road closures due to road works could be relayed to other drivers. For the next generation of intelligent transportation systems, among the visions for this system include a decentralized, autonomous communications network between vehicles that leverages the unlicensed spectrum provided by the government and an off-the-shelf hardware that has been developed to exploit this spectrum. The embedded system that is being proposed would need individuals to share data freely and allow each traveller to buy as much or as little a device as they need.
11 CONCLUSION Nowadays cooperative communication has been analyzed with much cooperative protocols. Even though the results obtained have shows the performance but many issue remain to be addressed and many directions for future research exist. There are many researches done on cooperative protocols and cooperative relay which would enhance the performance of cooperative communication. Modulation techniques also incorporating better operation in achieveing full diversity gain in the number of cooperating terminals. For randomized space-time block coding in cooperative diversity system, there are relationship between node density and range improvement with the existence of timing error based on Gaussian approximation. To achieve full diversity for any number of mobile relays with minimum delay, cooperative balances space time block coding scheme has been proposed. OFDM in cooperative communications can be used for a relay management strategy to simplify the cooperation in wireless network. Since OFDM distributes the data over a large number of carriers that are spaced apart at precise frequencies. An embedded system is being proposed which would need individuals to share data freely and having an intelligent transportation system network.
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