The Low Bandwidth Utilization Of Relay Selection

Published Date: 02 Nov 2017

INTRODUCTION

Communication through a wireless channel is a challenging task because the medium introduces much impairment to the signals. The transmitted signals are affected by effects such as noise, attenuation, distortion and interference[1].Cooperative relay communication has received considerable attention because it can extend coverage and mitigate channel impairments [2].

Transmit diversity requires more no of antennas at the transmitter. The main limitation of wireless devices is hardware complexity so it is limited to one antenna at the receiver side [3]. Some recent works have put forwarded new class of methods called cooperative communication that enables single antenna nodes in an environment where there are many users to share their antennas and a virtual multiple-antenna transmitter is formed that allows them to achieve more transmit diversity. Transmit diversity may be advantageous on a cellular base station, it may not be useful for other process. Due to size, cost, or hardware limitations, many wireless agents may not be able to support many transmit antennas at a time. The main idea of cooperation is more suitable to ad-hoc wireless networks and wireless sensor networks than cellular networks [4].

In cooperative wireless communication, we are concerned with wireless network, of the ad-hoc or cellular variety, where the wireless agents, the users, may increase their effective quality of service via cooperation of the users [5]. In cooperative communication system model, each user which is wireless is assumed to some transmit data and act as a cooperative node for another users [6].

Cooperative relay communication uses two major protocols: amplify-and-forward (AF) protocol where each relay forwards the amplified version of an incoming and decode-and-forward (DF) protocol where each relay forwards the detected version of an incoming signal [7]. Many previous works in the area have focused on repetition based orthogonal transmission in cooperative relay networks. In such transmission, different relays transmit in different time slots, and thus, synchronization across multiple relays is not a critical issue. On the other hand, many researchers have studied non repetition-based transmission in cooperative relay networks. In particular, relay selection (RS) has been considered as one of the most promising strategies because it substantially improves system performance with low feedback signalling overhead. In particular, compared to distributed space-time coding, RS performs better; and compared to distributed beam forming, RS requires much less feedback signalling overhead. RS exploits time-varying channel fluctuations by selecting a single best user associated with the best channel gain at a given instant [8].

In cooperative communications, independent paths between the user and the base station are generated via the introduction of a relay channel. The relay channel is an auxiliary channel to the direct channel between the source and destination [9]. A key aspect of the cooperative communication process is the processing of the signal received from the source node done by the relay. These different processing schemes result in different cooperative communications protocol. Cooperative communications protocols can be generally categorized into fixed relaying schemes and adaptive relaying schemes. In fixed relaying, the channel resources are divided between the source and the relay in a fixed (deterministic) manner. The processing at the relay differs according to the employed protocol. In a fixed amplify-and-forward (AF) relaying protocol, the relay simply scales the received version and transmits an amplified version of it to the destination. Another possibility of processing at the relay node is for the relay to decode the received signal, re-encode it and then retransmit it to the receiver. This kind of relaying is termed a fixed decode-and-forward (DF) relaying protocol [10].

Fixed relaying has the advantage of easy implementation, but the disadvantage of low bandwidth efficiency. This is because almost half of the channel resources are allocated to the relay for transmission, which reduces the overall rate. This is true especially when the source–destination channel is not very bad, because in such a scenario a high percentage of the packets transmitted by the source to the destination could be received correctly by the destination and the relay’s transmissions would be wasted. Adaptive relaying techniques, comprising selective and incremental relaying, try to overcome this problem.

Lingyang Song [11] proposed a simple sub-optimal Min-Max criterion for relay selection, where a relay will be selected on the priority that which minimizes the maximum SER from the source nodes. Ioannis Krikidis [12] proposed conventional max−min selection scheme suitable for different system setups. Seunghoon Nam [13] proposed a method to select a set of cooperating relays to minimize the total transmission time of a fixed amount of data. M. D. Selvaraj [14] proved that the use of conventional maximal-ratio combining technique is not suitable for a cooperative diversity network. Hence, proposed a scaled selection combining scheme, where the relay-to-destination links is scaled by a fixed positive scale factor to incorporate the effect of the source-to-relay links.

In our work, we consider a five-hop relay system performing relay selection in amplify and forward relaying protocol and selection combining at the destination terminal.

This work is organized as follows: section II presents the system model, section III presents the relay selection for amplify and forward relay networks, section IV presents the simulation results, section V presents the discussion and section VI presents the conclusion and future work.

system description

We consider a five-hop wireless system with relay1 (R1), relay2 (R2), relay3 (R3), relay4 (R4), source(S) and destination (D) terminals as shown in Fig 2.1.There are mainly two ways for transmission of information from source to destination. The first way of transmission is through direct link from source to destination and the second way is through relay link. The source transmit the information to the relay node, after processing the information, i.e., amplifying the signal by the amplify and forward protocol used to improve the gain. The information from the direct link and from the relay link is received by the destination. Finally the destination

exploits the well known selection combining technique to combine the received signal from the two ways of transmission. During the broadcast phase the signal received at the destination by the direct link is given by

By the relay link, the received information by the destination is given by

The gain amplification factor at the relay is given by

By the use of selection combining, the instantaneous SNR of Amplify and forward protocol is given by

D

S

R3

R2

R4

R1

Fig 2.1 Five-hop relaying network

The conventional relay selection policy can be expressed as

The conventional relay selection criterion ensures that the relay with the "best" end-to-end path between source and destination is used and provides diversity gain on the order of the number of relays. However, this selection criterion has been designed for environments without interference and thus does not take into account the effects of interference. In the work presented here, the relay selection criteria investigated can be regarded as an extension of this basic selection scheme.

RELAY SELECTION FOR AMPLIFY AND FORWARD RELAY NETWORKS

In this section, we propose three criterion for relay selection in amplify and forward relay networks to enhance the throughput of the network and also analyzed the effective utilization of channel bandwidth. Relay selection criteria are suitable for different system configurations and constraints.

Algorithm1: Max-Min relay selection

1: number of relays = n

2: for k= 1: n

3: generate the source – relay channel matrix, where the

channel is varied for each bit.

4: temp (k) = max (power (k))

5: sort the values in ascending order.

6: find the best relay by calculating the minimum index

value from the temporary calculated value.

Algorithm 2: Balanced Max-Min relay selection

1: generate the source – relay channel matrix

2: temp (k) =

where P is the path from source to relay, relay to destination, source to destination.

3: find the best relay by calculating the maximum index values from the temporary calculated value.

Algorithm3: link quality relay selection

1: generate the source – relay channel matrix

2: temp (k) =

3: X (k) = min (temp (k))

3:find the best relay by calculating the maximum index values from the temporary calculated value and make the channel coefficient biggest value.

SIMULATION RESULTS

Simulations are done using MATLAB and the graphs are analyzed. Fig 3.1 to 3.3 shows the relay selection based on power and channel coefficients.

Fig 3.1 Max-Min relay selection

Max-min relay selection is appropriate for interference limited environment with many relays.

Fig 3.2 Balanced max-min relay selection

Fig 3.3 Based on channel coeffients

The following graphs shows the effective utilization of channel bandwidth .The bandwidth of the channel is chosen as eight and the total number of users as fifteen. The following figure shows the SNR of each user.

Fig 3.4 Users SNR Rates

In selection combining, those users having SNR greater than a threshold value are selected .In Fig 3.5 , the plot of acceptable users are shown.

Fig 3.5 Acceptable Users SNR

Fig 3.6 Acceptable Users

Fig 3.6 shows the indexes of acceptable users having SNR value greater than the threshold value. As the channel bandwidth is not completely occupied, the users having SNR less than the threshold are sorted in descending order and the portions which are left unoccupied are filled with the highest unacceptable user SNR. By doing this the channel bandwidth is effectively utilized. The following figure shows the unacceptable users SNR.

Fig 3.7 Unacceptable Users SNR

Fig 3.8 Unacceptable Users

DISCUSSION

Relay selection is studied in Fig 3.1 to Fig 3.3. The max−min criterion is an efficient selection metric for AF techniques. It is the optimal solution for DF protocols and efficiently approximates the performance of the optimal AF selection scheme, which is based on instantaneous AF statistics. The max−min criterion can be adopted as a general selection technique for AF techniques. The max−min criterion has some interesting computational properties, which can simplify the implementation complexity. 1) The selection metric does not involve computational operations (multiplications, additions, and divisions) and thus corresponds to a low complexity hardware core. If the complexities overhead for the computation of the instantaneous AF statistic is equal to 2 multiplications (1 multiplication and 1 division) the total complexity is equal to 2K, where K is the number of relays. 2) It can easily be extended for interference environments without complicated modifications and is amenable to a distributed implementation.

In selection combining, the channel bandwidth is effectively utilised. Fig 3.4 to Fig 3.8 shows the graphs of selection combining.

CONCLUSION AND FUTURE WORK

The performance of the cooperative network heavily depends on relaying strategies. Relay Selection in AF protocol for a five-hop network with selection combining technique at the receiver end is analyzed. By performing this, path loss can be reduced. In MRC based cooperative relay systems, the relay will forward the incoming signal only if it correctly detects that signal and will not perform well when the channel condition between the source and the relay is very bad. So the relay selection is done with selection combining technique. Relay Selection is the optimal solution for DF protocols and efficiently approximates the performance of the optimal AF selection scheme, which is based on instantaneous AF statistics.