A survey on cooperative communication: Review of literature.

1. A Survey on Cooperative Communication in 4G-LTE
Wireless Networks: A Review of the Literature
Aklilu Alemayehu
Jimma Institute of Technology,
Department of Communication Engineering
August, 2019
1 Abstract
Due to the limited spectrum and power, only physical layer technologies such as MIMO and
OFDM are not sufficient to deal with the ever increasing data rate demand. As a result, increasing
the spectral efficiency of the wireless network through cooperative communication where coop-
eration between source, relay and destination is required. The cooperative relaying will enhance
capacity, transmission reliability, spatial diversity gains and increased coverage. This paper does
a survey on the various cooperative relaying mechanisms that can be used for LTE(4G) cooper-
ative communication purposes. The literature discusses various cooperative relaying approaches
and comparison is done which will help identify best technique possible.
2 Introduction
The 4G (LTE Advanced) technology came up with significant data rate improvement in both downlink and
uplink transmission by incorporating several physical layer enhancements such as carrier aggregation, HetNets
and MIMO. Among these MIMO technique aims to improve data rate and quality through system diversity
gain [1]. Although transmit and receive diversity is clearly advantageous on a cellular base station, it may
not be practical for other scenarios. Specifically, due to size, cost, or hardware limitations, a wireless agent
may not be able to support multiple transmit antennas. Examples include most handsets (size) or the nodes
in a wireless sensor network (size, power). This article discuses technique called cooperative communication
which will enable 4G networks to reap the benefit of MIMO system through sharing their resources to provide
multipath propagation that will increase diversity gain.
According to the literature recent development in wireless cooperative communication system, power and
bandwidth has led to the improvement of relays technology. In relay networks source(S) and destination (D)
terminals are interconnected using some intermediate nodes which are generally called as relaying nodes. Due
to shadowing line of site between S and D can be terminated and signals might arrive the destination with
parameters like scattering reflection or any other effect which will reduce quality and performance.
Fading in wireless channel tremendously affects the performance of wireless communications. Thus it is
imperative to mitigate the impact of wireless impairment in order to improve the performance of wireless
communication. Cooperative communication has been shown as a promising approach to combat wireless
impairments by exploiting spatial diversity without the need of multiple antennas at both transmitter and
receiver [2]. According to the literature, Co-operative communications have been employed over past few years
to overcome the drawbacks of implementing MIMO systems and the schemes are used to attain the largest
diversity order.
The authors outline that several researches indicate the challenges in wireless data transmission due to
characteristics such as signal fading, bit error rate (BER), Signal-to-Noise ratio(SNR), Channel State Informa-
tion(CSI) and QoS required new developments for data transmission technique by the use of relays: Amplify and
Forward and Decode and Forward. The literature describes various relay based Cooperative Communication
techniques, existing methodologies and finally compares surveyed approaches to indicate research result.
The generalized Alamouti transmit diversity model with multiple transmission and single reception, and
STBC (Space Time block Code) codes are proposed as a system model to provide gain and diversity.
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2. 3 Cooperative Communication
Researches have shown that cooperative communication can offer significant performance enhancements in
terms of increased capacity, improved transmission reliability, spatial diversity and diversity-multiplexing trade-
off. Different from the conventional relaying, where only one node is decoding and encoding the message, and
signals from other nodes are treated as interference, cooperative communications requires the cooperation among
nodes. When the source transmits message any of the intermediate relays receive the message. According to
the authors the optimum relay that has the highest gain of the channel is selected based on part of Channel
State Information (CSI) available at the relay and source nodes. Relay is selected based on channel prediction.
The relay with the maximal predicted channel strength or the minimal predicted outage probability is selected
to forward data [2].
In the paper it is pointed out that if there are multiple relays only a single relay is activated among the
set of available. This technique is called selective relaying. Hence, the resources available were used efficiently.
Selection of single-relay in cooperative relaying based on a maximum SNR is known as opportunistic relaying
(OR). An alternate method to OR were used DSSC (distributed switch and stay combining). In DSSC, a relay
is active till it receives the signal with larger SNR than the present SNR value. CSI plays important role in
selecting best relay and it is deployed proactively or reactively. In proactively selection, the best relay is selected
before the messages transmitted to destination from source. On the other hand, reactive relay selection involved
all the relays as part of the communication. Here all the relays have to listen to the source message. Receiving
all the messages by the relays, the optimum relay has been chosen then it would be used for retransmit the
source messages to the destination. Normally, energy utilization of reactive mode is higher than proactive mode.
4 Relay Based Cooperative Communication Techniques
Comparing to other techniques, cooperative communication is superior to other in its flexibility and hardware
feasibility, according to the authors of the literature. The literature analyzes two signal processing technique:
Decode and Forward(DF) and Amplify and Forward(AF). In the literature there are three types of relaying
protocols discussed: half-duplex(HD) relaying protocols, two-path(TP) relaying protocols and two-way(TW)
relaying protocols.
According to Geng Ke a, Gao Qiang a, Fei Li b, Xiong Huagang, in decode and forward technique the
cooperation is divided in to three phases:
1. All relays will decode the received packet and check for error using CRC bits, the relays that have decode
the data successfully are the candidates to forward it to the destination.
2. Some of the decodable relays (if no error) are selected to forward the data based on CSI.
3. The selected relays forward the data to the destination. The destination combines the signals from the
relays using Maximal Ratio Combiner (MRC) and decodes it.
In the literature, under this technique in the indirect connection between source and destination node, the relay
receives message at tth
time slot and transmits at (t + 1)th
time slot. From the literature:
At the relay, y[t] = h[t]x[t] + n[t]
At the destination, y[t + 1] = h[t + 1]x[t + 1] + n[t + 1]
With the amplify-and-forward (AF) scheme, the relay will amplify its received signal in the first stage and
then forward it to the destination in the second stage [3]. From the literature:
At relay, y[t] = x[t]h[t] + n[t]
At destination, y[t + 1] = h[t + 1]g[t]h[t]x[t] + h[t + 1]g[t]n[t] + n[t + 1]
The above equation indicates that relays can serve only a single source in a given time or frequency slot,
and therefore the available resources are not shared efficiently by the sources.
5 Existing Cooperative Relaying Methodologies discussed in the lit-
erature:
1. DF Co-operative relaying scheme: decode the received signal at the relay, re-encode it, and then retransmit
it to the receiver. According to the literature this technique is used for reduced BER and improved
coverage.
2. DNF-TPSR Scheme (De Noise and Forward-Two Path Successive Relaying): used to improve the spectral
efficiency loss due to HD transmission. It uses indirect transmission between source and destination.
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3. 3. STBC-NOMA (Space Time Block coding Non-Orthogonal Multiple Access): The transmission reliability
and system capacity may increase by deploying multiple antennas at relaying nodes. This technique
requires some complex operation due to multiple antennas at the receiver.
4. DF-TWFD Scheme: To achieve more efficiency in spectral usage, two way (TW) relay had used in full
duplex mode.
5. HDAF-NOMA Scheme: is a technique with multiple relay to decode-amplify-forward scheme. The HDAF-
NOMA relaying protocol was used to enhance the entire network performance.
6. Joint antenna and Relay Selection: single antenna multi relay cooperative diversity scheme consist of more
number of antennas at the source end (S) and the destination end (D). The Proposed antenna strategy
and relay selection methods is same as path and antenna selection technique but it is limited to single
relay cooperative MIMO system. The selected source antenna transmits to all available relays and selected
antenna will receive at the destination.
7. Selective Decode-and-Forward Relaying (SD-FR): SD-FR networks considered different modulation levels
at transmitting nodes. It has shown that, performance of the proposed method had been improved in
terms of average error probability and spectral efficiency when compared to the conventional modulation
methods by modulation level selection (Adaptive) and power allocation.
8. Threshold based Selective Weighted Least Square (TSWLS) method: For combining a signal in LTE-A
networks, TSWLS method is used also for the purpose of improving the signal strength of received signal
at the user equipment and improve system capacity.
9. Resource Allocation: This technique considers the problem of power allocation to improve the throughput.
6 Discussion and Conclusion
The very high data rates targeted by 4G(LTE-Advanced) requires a denser infrastructure to reduce transmitter
to receiver distance thereby increasing the data rate. In order to achieve this in addition to coordinated
multipoint transmission, relaying is another solution. The literature discusses various cooperative relaying
mechanisms with their respective signal processing techniques used and compares their performance against
direct communication.
The survey has addressed all the recent cooperative relaying techniques developed to overcome the issues
with the old techniques such as spectral inefficiency due to one-way relaying. The literature focused on AF
and DF signal processing techniques in the relays but other researches include third technique with compress
and forward technique. Much emphasis is given to relays with DF signal processing (layer 2 and layer 3) since
it is best suited to deal with interference and noise due to its error detection capability. Relaying techniques
equipped with concepts of NOMA, SD-FR, TSWLS are also discussed.
The signals are broadcast in free space in wireless communication, so signals are normally accessible to anyone
who has device capable of getting them which will cause security concern. This literature can be expanded to
include the security issues related to each relaying technique.
Currently, several researches, such as adaptive modulation, best relay selection or combination of the two,
are being considered on improving spectral efficiency loss of conventional cooperative relaying [4]. I believe
bidirectional/full duplex cooperative relaying with improved spectral and energy efficiency will be a challenge
in future research of cooperative communication, especially in 5G.
7 Literatures Cited
References
[1] “LTE-Advanced, An Evolution Built for the Long-haul” Qualcomm Incorporated, 5775 Morehouse Drive,San
Diego, CA 92121 U.S.A., c 2013
[2] ”Relay selection in cooperative communication systems over continuous time-varying fading channel”, Geng
Ke a, Gao Qiang, Fei Li, Xiong Huagang
[3] ”Wireless Communications: Principles, Theory and Methodology”, First Edition. Keith Q.T. Zhang. c
2016 John Wiley ‘I&’ Sons, Ltd. Published 2016 by John Wiley ‘I&’ Sons, Ltd.
[4] ”On the Spectral Efficiency of Selective, Decode-and-Forward Relaying”, Hamza Umit Sokun and Halim
Yanikomeroglu
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