Network coding on cooperative relay networks

3 1.3 Cooperative Diversity Relaying Networks using network coding.. 12 3.4 Outage Probability of Single Relay Networks using Network coding.. 12 3.5 Outage Probability of Multiple-Relay

Viet Nam National University, Ha Noi University of Engineering and Technology

Lam Sinh Cong

Network Coding

On Cooperative Relay Networks

Branch: Electronics and Telecommunications Technology

Major: Electronics Engineering

Code: 60 52 70

Master Thesis Summary

Ha Noi-2012

1.1 Introduction to cooperative relay networks 1

1.1.1 The relay protocols 2

1.1.2 Advantages of Cooperative Diversity Relaying Networks 2

1.2 Introduction to Network Coding 2

1.2.1 Non-Binary and Binary Network Coding 3

1.2.2 Advantages of Network Coding 3

1.2.3 Weaknesses of Network Coding 3

1.3 Cooperative Diversity Relaying Networks using network coding 3

2 System models 5 2.1 Traditional Relay Multiple-Wireless Networks 5

2.2 Single Relay Networks using Network Coding 6

2.3 Multiple-Relay Networks using Network Coding 8

3 Outage Probability Calculations 9 3.1 Mutual Information 9

3.2 Outage Probability Definition 9

3.3 Outage Probability of Multiple-Relay Networks 11

3.3.1 Traditional Decode-and-Forward relaying 11

3.3.2 Selection Decode-and-Forward relaying 12

3.4 Outage Probability of Single Relay Networks using Network coding 12

3.5 Outage Probability of Multiple-Relay Networks using Network Coding 16

Bibliography 19

List of Figures

2.1 A traditional single relay network 6

2.2 A traditional multiple-relay network 6

2.3 Network coding in single relay network 7

2.4 Multiple-relay network using network coding 8

3.1 The direct link between the input and the output 10

3.2 Outage probability of a direct link 11

3.3 Outage Probability of fixed and selection DF relay 13

3.4 The degraded system model of a single relay network based on NC 13

3.5 The degraded system model of a single relay network based on NC 14

3.6 Outage probability of the single relay network with and without network coding 15 3.7 Link s1r1 is in outage 17

3.8 Outage probability of relay networks with different scenarios 18

to use Selection Decode-and-Forward instead of Traditional Decode-and-Forward protocol at therelay We also use the instantaneous channel gains to calculate the outage probability of the proposalsystem model.

The rest of the thesis is organized as follows In Chapter II, the system model of a multiple-relaynetwork is described The outage probability is calculated in Chapter III Finally, the conclusionsand the future works are drawn in Section IV

Chapter 1

Introduction

1.1 Introduction to cooperative relay networks

In recent years, MIMO (multi-input multi-output) technology based on spatial diversity andspatial diversity has attracted attention in wireless communication because it greatly improves thereliability, the throughput and the transmission rate without additional bandwidth nor requiringhigher transmitter power However, this technique requires both the transmitter and the receiver

to have multi-antennas, and all channels must be independent In practice, users do not oftenachieve full-rank MIMO because they either do not have multiple-antennas installed on a small-sizedevices, or the propagation environment cannot support MIMO, for example, there is not enoughscattering Even if the users have enough antennas, full-rank MIMO is not guaranteed because thelinks between several antenna elements are often correlated

To overcome the limitations in diversity gain MIMO, a new communication paradigm which uses

an intermediate node to generate independent channel between the user and the base station wasintroduced The intermediate node often called relay node receives the signal transmitted from theuser and forward it to the base station And this paradigm is called Cooperative Diversity RelayingNetwork

1.1.1 The relay protocols

A key aspect of the cooperative communication process is the processing of the signal receivedfrom the source node carried out by the relay These different processing schemes depend on theprotocols of the relays which can be generally categorized into fixed relaying schemes, selectionrelaying protocol (adaptive relaying schemes) and incremental relaying protocol

Cooperative Diversity Relaying refers to devices communicating with one another with the help

of relays in order to increase the performance of the network [3] Thereby, the relay channel can beconsidered as an auxiliary channel to the direct channel between the source and destination

In Cooperative Diversity Relaying, the user can guarantee the maximum diversity which is equalthe number of the relays plus the direct link, i.e being the minimum cut at each source It meansthat the limitation of MIMO technique has been overcome

However, in cooperative relay network, we are able to use one or more relays, but in one timeslot,the relay only transmits the signal of one source

1.2 Introduction to Network Coding

As discussed in the previous section, in a typical network, information is transmitted from thesource node to each destination node through a chain of intermediate nodes by a method known

as store-and-forward In this method, the intermediate node only processes and transmits a uniquesignal at one time without overlapping, thus slow down the through In order to increase thethroughput of the network, network coding technique was introduced in [5] and then further devel-oped in [6], as a new paradigm which exploits the characteristics of the broadcast communicationchannel to combine several input signals into one output signal at the intermediate node

1.2.1 Non-Binary and Binary Network Coding

In binary network coding, the intermediate node uses XOR operator to consolidate the receivedmessages transmitted form sources

In non-binary network coding, each intermediate node uses a linear equation to combine theinputs and the destination uses the system of linear equation to decode the received messages

Increasing throughput achieved by increasing the efficiency of packet transmission is the mostwell-know benefit of network coding

The main issue of using network coding is that if a transmission error occurs, it could affectthe detecting and coding at the intermediate node, and the destination node could receive uselessinformation

Besides, synchronization and transmission delay among the incoming data streams at the input

of the intermediate node or destination node are also significant issues that need to be consideredwhen network coding is applied The transmitted data can not be recovered until all the neces-sary information is received These are not big problems for non-real time services (e.g data andvoice transmission), but they are should be considered carefully for real time services (e.g videotransmission, )

1.3 Cooperative Diversity Relaying Networks using

net-work coding

The most common example of NC-based network model is two-source one-relay topology, asshown in Figure 2.3 In this topology, two sources transmit their signals to the relay and thedestination using broadcast technique Then, the relay combines its received signals into a uniquesignal and sends it to destination The traditional Decode-and-Forward (DF) protocol is often used

at the relay which decodes the messages from its input nodes before sending them to its outputnodes Often, the links between the sources and relay are assumed to be error-free so that the relaydecodes the received messages successfully [3, 11–13] In [14], taking into account of link errors, therelay is assumed to perform DF without error checking and the network codes are designed for errorcorrection.

In this thesis, instead of using DF relaying as in [14], we propose to use selection DF relaying

at the relay The selection DF relaying protocol is designed to overcome the shortcomings of

DF relaying when the measured SNR at the relay falls below a threshold such that the relaybecomes unable to decode the messages, the source simply continues its direct transmission to thedestination using repetition coding [15] In addition, we use Maximum Ratio Combining (MRC)

at the destination Finally, we analyze the performance of the proposed scheme in terms of outageprobability by using the instantaneous channel gains The analysis is based on a newly developedmethod for exact calculation of the outage probability [16]

Chapter 2

System models

2.1 Traditional Relay Multiple-Wireless Networks

In this section, we will discuss about end-to-end signal of the selection Decode-and-Forwardrelay Relaying is assumed to operate in the time division mode having two phases (two time slots):the relay-receive phase and the relay-transmit phase

The total received signal at the destination is given by equation (2.1) and (2.2)

In order to increase the network’s throughput by reducing the number of timeslots, we increasethe number of relay Figure 2.2 shows a relay network using two relays (R1, R2) with selection-DFprotocol relaying information for two sources (S1, S2) to the destination [18] It is clear that itrequires at least 3 time slots in order to complete a transmission process

( x

2

) 3

( x

2

) 4

Figure 2.2: A traditional multiple-relay network

In this thesis, we review the calculation of the cumulative distribution function (cdf) of taneous channel gains of various wireless links in a diversity relay network, which was published bythe author of this thesis and his co-author in [16, 19]

instan-2.2 Single Relay Networks using Network Coding

Figure 2.3 shows a single relay network with two sources using network coding [20]

• In timeslot 1:

– S1 sends its signal x1 to both relay and destination by using broadcast mode

1

rdh

d s

Figure 2.3: Network coding in single relay network

– The relay can or cannot decode x1

• In timeslot 2

– If R can not decode x1, S1 repeats sending x1 to D, thus D receives x1 on 2hs1d

– If R can decode x1, S1 will do nothing in timeslot 2 and R store x1 in it and waits for x2

In the meantime

– S2 transmits its signal x2 to relay and destination using broadcast mode

– R can or cannot decode x2

– If R cannot decode x2, S2 repeats sending x2 to D, thus D receives x2 on 2hs2d

• In timeslot 3

– If the relay is unable to decode neither x1 nor x2, it must be silent

– If the relay only decodes xi, it will forward xi to destination

– If the relay decodes x1 and x2, successfully, it combines x1and x2by using XOR operatorbefore sending it to the destination

When all source-relay links are perfect, the destination decodes the received messages by usingalgorithm:

x1⊕ x1⊕ x2 = x2 or x2⊕ x1⊕ x2 = x1

Because of using less time slot than system model depicted in Figure 2.1, it indicate that systemmodel can improve the throughput of the networks Comparing with the system model depicted infigure 2.2, it cannot save any time slot, but we can save one relay.

2.3 Multiple-Relay Networks using Network Coding

Figure 2.4 shows a multiple-relay network using coding in the cooperative relays, it is considered

as a technique to improve the robustness The system model under analysis is given by the access relay channel, where two source nodes, S1 and S2, communicate with destination with thehelp of two relays R1 and R2 The notation used for this system and their operation are the same

1

d sh

2 2

Figure 2.4: Multiple-relay network using network coding

in Session 2.2 for relay R1 and R2

At destination, we use MRC to combine the signals from R1 and R2 in to a better signal ofhigher SNR I hope that this may improve the robustness of the network Note that, if both relays

R1 and R2 are not able to decode the messages of source Si, Si repeats its transmission to the

destination by using repetition code

In chapter 3, we will show that by using MRC, the performance of the network is increased

3.2 Outage Probability Definition

In this section, we define the outage probability of direct transmission between two nodes Thereceived signal at the destination is given by

y[n] = x[n]h[n] + w[n]

Figure 3.1: The direct link between the input and the output

Where x[n], h[n] and w[n] are transmitted signal, channel gain, and Addition White Gaussian Noise(AWGN), respectively We assume that h is independent and identically distributed Thus, themaximum average information between the input and the output is given by using (3.2) with

M = 0, A = h, SN R = P/No:

I = log(1 + SN R|h|2)

in which SN R is the received signal-noise ratio at the destination

The outage event of the information rate for a given threshold Rth is defined as: I ≤ Rth andequivalently [15]

|h|2 < 2

R th− 1

µth is called the channel power threshold

Then the outage probability is expressed as:

Figure 3.2: Outage probability of a direct link

3.3 Outage Probability of Multiple-Relay Networks

3.3.1 Traditional Decode-and-Forward relaying

The maximum average mutual information between the input and the two outputs is expressed

3.3.2 Selection Decode-and-Forward relaying

The information rate of a selection DF relay network in this case can be expressed as below [22]:

So, its outage probability under Rayleigh fading condition is [16]

PSDFout = Fsd(µth

2 )Fsr(µth) +

1 − Fsr(µth)

µsd− µrd {µsdFsd(µth) − µrdFrd(µth)} (3.13)

in which µth is defined as in equation (3.3)

3.4 Outage Probability of Single Relay Networks using

Network coding

We analyze all events which cause system outage

Figure 3.3: Outage Probability of fixed and selection DF relay

• Link s1r is in outage, then the source s1 repeats transmitting its signal to D The system

model in Figure 2.3 is degraded to the one which is depicted in Figure 3.4

h

2

r s

Therefore, the outage probability of this degraded model is given by:

p1(µth) =P (|hs1r|2< µth)P (2|hs1d|2 < µth) (3.14)

• Link s1r and s2r are free of errors It means that the relay decodes fully the sources’ messages,

and then combine them into a unique signal before sending it to the destination The system

is in outage if both link s1d and s2d are in failure The outage probability in this case isexpressed as follows:

p2(µth) =P (|hs1r|2 > µth)P (|hs1d|2< µth)

P (|hs2r|2 > µth)P (|hs2d|2 < µth) + P (|hs2d|2 > µth)P (|hrd|2 < µth) (3.15)

• Link s1r is free of error, link s2r is in outage, then the source s2repeats transmitting its signal

to D In this case, the relay only sends the signal of the source s1 The system model in thiscase is as shown in 3.5

h

2

r s

Figure 3.5: The degraded system model of a single relay network based on NC

Therefore, the outage probability in this case is given by:

p3(µth) =P (|hs 1 r|2> µth)P (|hrd|2+ |Ps1d|2 < µth)P (|hs 2 r|2 < µth) (3.16)

Finally, the outage probability of the system is PSDFout (µth) is calculated as follow:

Single Relay Network using Network Coding

Figure 3.6: Outage probability of the single relay network with and without network coding

3.5 Outage Probability of Multiple-Relay Networks

us-ing Network Codus-ing

Because |hr 1 d|2 and |hr 2 d|2 are exponentially distributed, the probability density function andcumulative distribution function of |hrd|2 respectively are:

µr1d− µr2d

e

The outage probability of the source S1 is obtain by calculating the probability of all events in

the source-to-relay links which make the destination unable to decode the x1 messages from S1

• All source-relay links are free of error

The probability of this event is given by:

Then,

P1co(µth) = Fs1d(µth)(FRD(µth) + (1 − FRD(µth))Fs2d(µth)) (3.21)

In which, FRD is calculated by using equation 3.19

• Link s1r1 is in outage and others are free of errors The relay R1 only decode fully the message

of S2