A Novel Approach of Rayleigh Fading Channel and MIMO Multiple Relay Assignment in OFDM-based System using amplify-and-forward (AF) Relay Strategies

A Novel Approach of Rayleigh Fading Channel and MIMO Multiple Relay Assignment in OFDM-based System using amplify-and-forward (AF) Relay Strategies

Abstract

A 2-hop wireless relay network where multiple user pairs conduct bidirectional communication via multiple relays based on orthogonal frequency division multiplexing transmission (OFDM). Channel optimization and relay assignment, including sub- carrier pairing, sub-carrier allocation as well as relay selection, for total throughput maximization is formulated for different relay strategies. A graph theoretical approach can solve the problem optimally in polynomial time by transforming it into a maximum weighted bipartite matching (MWBM) problem. Simulations studies are carried out to evaluate the network total throughput versus transmit power per node for additionally we use AF 4and the number of relay nodes for different number of subcarriers. This paper presents a new Proposed Method Rayleigh – fading channels when the channel state is assumed unknown. In the design of this classifier, new estimators for the unknown amplitude, time offset, and noise power that are blind to the modulation scheme of the received signal are proposed. It is shown that the proposed classifier performs well compared to the optimal classifier with perfect channel knowledge for an adequate estimation interval. Rayleigh fading may be considered the most efficient in a wireless communication system. By using multi relay network In its most general form, it is modelled as a multiplicative time continuous random (zero mean complex Gaussian) distortion of the transmitted signal. In order to achieve an efficient communication here, each part of the communication link must be carefully designed based on the properties of the time continuous channel.

Keywords Two-way relaying, orthogonal frequency division multiplexing (OFDM), subcarrier pairing and maximum weighted bipartite matching (MWBM).

1. Introduction

   A relay is an electrically operated switch which is used in wireless networks for various purposes like coverage extension, power saving and throughput enhancement. There are various factors which can be considered to enable high data rates in a relay based system like selecting the right number of relays and subcarriers. This coupled with Orthogonal-frequency-division multiplexing (OFDM) can be used to employ efficient relaying systems with improved efficiency. Relay strategies can be classified as amplify-and-forward (AF) and decode-and-forward (DF). In AF, the relay simply amplifies the source signal linearly whereas, in DF, the relay fully decodes, re-encodes and retransmits the source code word. Subcarrier is assigned from source to relay link and relay to destination link.

   The OFDM is employed for transmission over time dispersive channels in the two-way relay network (TWRN), where two source terminals exchange their information through a relay terminal using AF and DF relaying schemes. We propose a two-phase protocol for the channel estimation, which is compatible with the two-phase data transmission scheme. In the first phase, the two source terminals send their individual training sequences concurrently to the relay nodes. In order to avoid inter-pair interference, each user pair occupies non-overlapping subcarriers. The intra-pair interference will be treated as back-propagated self-interference and cancelled perfectly after two-way relaying. In the second phase, the signal is modified based on the relay strategy selected and broadcasted to the destinations. Compared with the existing works in our problem involves two major technical challenges. The first one is the subcarrier pairing and assignment. Though the

   optimal subcarrier pairing has been found for one-way relaying such as only heuristic subcarrier pairing methods are available for two-way relaying. In addition, the problem is more involved in the multi-user scenario since sub carriers should not only be carefully paired for each two-way link but also be assigned adaptively for different users. The second challenge lies in the fact that subcarrier pairing and relay selection are tightly coupled, i.e. different relay selections lead to different subcarrier pairing and assignment, and vice versa.

2. System Model

   An efficient method for achieving time diversity for digital signalling on the Rayleigh fading channel by using multi relays. For transmission of more than one information bit per symbol, there is one coding strategy available. That is Quadrature amplitude modulation. The focus is on Quadrature amplitude modulation (QAM) and variations of the discrete time model was found to be appropriate for use only on slowly fading channels it is employed. Perfect knowledge of the fading process in the receiver is also assumed. The reason for using such a restrictive model is that it is commonly employed, when treating coded modulation on the Rayleigh fading channel. Fig.1 to enable fair comparisons, the same model should be used.

   Figure 2.1: Block diagram of Rayleigh fading channel by using Multi Relays

   The first step towards a systematic design of good codes for the Rayleigh fading channel was taken

   by Divsalar and Simon, where an upper bound on the error probability for TCM was derived. Based on this bound, the main design parameter for coded interleaved OFDM on that channel was recognized as the effective diversity, which was found to be the minimum number of distinct channel symbols along any error event. Sometimes the effective diversity is also referred to as the minimum symbol BER of the fading channel.

   1. Rayleigh Fading Channel Communication for Relays

      Propagation of the radio waves over multiple paths between the mobile user and the base station is commonly encountered in cellular systems. The multiple paths, which result from reflections, are superimposed at the receiving unit. When the arrival times of the different rays are of the same order of magnitude as the duration of the transmitted symbols, successive symbols are smeared together. This effect is often referred to as inter symbol interference (ISI). For paths, where the time difference is comparable to the period of the carrier frequency, it is another effect to results. Superposition of many waves with different phases here gives a spatial interference pattern, with narrow holes of extremely low signal power, so called deep fades. Those deep fades are located at distances comparable to the wavelength of the carrier frequency and the signal power in a fade can be slow that communication is impossible. In a general scenario both the ISI and the interference pattern are time- varying due to motion of the transmitter/receiver are of the environment. These fluctuations are yet another problem, since they introduce a time varying distortion of the transmitted signal. The problem becomes serious when the variations are rapid compared with the signalling rate, i.e. When so called fast fading occurs. Suboptimal receivers here suffer from high error rates, which cannot even be lowered by an increase of transmitter power. This effect is often referred to as an error floor.

      The received signal in fading is often modelled by a Rayleigh distributed envelope. Hence the name Rayleigh fadng. It arises not only in cellular systems but also in terrestrial long-distance radio communication and in under water acoustic communications. The radio spectrum available for wireless services is extremely scarce, while demand for this service is growing at a rapid pace. Spectral efficiency is therefore of primary concern in the design of future wireless data communications systems. High overall spectral efficiency of a wireless cellular system may be achieved at several levels of the system design.

      At the radio coverage planning level by minimizing cell area and the co channel reuse distance. At the network/system level by using sophisticated channel allocation schemes that maximize the overall carried

      over sub carrier pair (n, n) with the assistance of relay can be expressed as:

      n n'

      '

      '

      traffic by using multi relays.

      Rn,n

      1 C

      k1r rk2

      k ,r 2 1 n'

      n n

      1. Amplify-and-forward (AF) Relaying

         1 n

         rk2

         n'

         k1r

         k2r

         • C k2r rk1

      In AF relaying, when a relay node overhears the

      2 1 n' n n

      transmission from a source, it amplifies the signal and forwards it to the destination. The single link cooperative capacity, between a source s and a

      rk1

      '

      '

      For AF

      k1r k2r

      destination deserved by are lay node r with AF

      Rn,n minC n

      ,C n'

      minC n'

      ,C n

      capabilities, can be written as follows:

      k ,r

      k1r

      rk2

      k2r

      rk1

      Where, W is the channel bandwidth and SNRsd, SNRsr, SNRrd are the signal to noise ratio at the destination and relay nodes. The multiplicative term1/2 is from the fact that the cooperative communication is done over two time slots.

      When relay aided cooperative communications is not considered the source can directly communicate with

      the destination over both the time slots an the achievable capacity can be written as Shannon Capacity:

      In AF relay mode a relay only retransmits a scaled version of their received signals from the source node according to their power constraint. Therefore, AF relay employed in this thesis is a reasonable strategy when relay nodes have limited power. Moreover, the complexity pertained to AF relaying is much simpler, since it does not require any signal processing at the relay node for decoding and encoding process. Although we have only presented our work considering AF relaying, the techniques proposed in the paper can also be used for DF relaying.

3. Problem Formulation

   Let N= {1 , 2 , ,} denote the set of subcarriers,

   = {1 , 2 , ,} denote the set of relays, and K= {1

   , 2 , , k} denote the set of user pairs. Denote K1 and K2 as the two sources in the K-th user pair, respectively. The achievable sum-rate of user pair K

4. Simulation Results

   A two-dimensional plane of node locations where source nodes and relay nodes are distributed uniformly has been considered. We adopt the path loss model. All the sources have same maximum power constraints; hence, all the relays have same maximum power constraints.

   Table1. Comparison between flat fading and Rayleigh fading

   The figure 4.1 shows the Performance Comparison for AF and DF Relay using Rayleigh Fading Channel Strategies Simulation results for the proposed Relay Assignment in OFDM-Based Multi-Relay are presented in Figure.4.1. As shown, the classifier first estimates amplitude, time constant and noise using correct classification.

   The figure.4.1 Represents the performance of the proposed classifier for Rayleigh fading environment. The effect of reliability of the estimates improve, be it through values of Nc and SNR, the performance of the classifier improves, as expected. It can be seen that the classifiers performance. the throughput performance for two different types of relay strategy of which one is amplify-and forward and the other is decode-and forward relay strategy. It has been inferred that the data rate has been increased when decode-and forward relay strategy is used compared to that of amplify-and forward strategy in case of both fixed and variable subcarrier pairing

   Figure4.3: B mismatch for Rayleigh fading channel (Nc= 1000)

   The figure 4.3 Represents the performance of the proposed classifier for Rayleigh fading environment maximum weighted bipartite matching (MWBM).

5. Conclusion

Figure 4.1: Average probability of correct classification by using Rayleigh fading channel (Nc= 1000)

Figure 4.2: Comparison of Average MSE & SNR for Rayleigh Fading Channel

The comparison of Average MSE & SNR for Rayleigh Fading channel. The power is 20 db or 15db

The joint optimization of subcarrier-pairing based subcarrier assignment and relay selection for multi-pair two-way relay networks by using Rayleigh Fading was investigated. The problem was formulated as a combinatorial optimization problem solved by bipartite matching approach and the unknown amplitude, time offset, and noise power that are blind to the received signal. The proposed method Relay fading channels when the channel state is assumed unknown. The work assumed the AF based Relay strategies. The fading environment and path loss model also can be changed and the results can be compared. The similar problem based on more advanced regenerative two-way relay strategies can be considered in the future work.

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