1) The document presents an overview of relay technologies used in IEEE 802.16j and 3GPP LTE-Advanced standards. 2) It discusses different relay types (Type-I non-transparent and Type-II transparent), transmission schemes (Amplify and Forward, Selective Decode and Forward, Demodulation and Forward), and relay path selection methods (Centralized and Distributed pairing schemes). 3) MATLAB simulations show that using a simple relay transmission method can significantly reduce the required transmission power level compared to direct transmission, especially when the mobile station is moving away from the base station.

1. Relay Technologies for WiMax and LTE-Advanced Mobile Systems
Devdatta Ambre
Abstract-- IEEE 802.16j and 3GPP LTE- rate wireless access to far-reached place of
Advanced standards are the next the coverage area.
generation wireless communications
To enable rapid and cost effective
system that provides considerable
deployment of the network infrastructure the
increase in data throughput than the
IEEE has made amendments in 802.16j
previous 3G communication system. To
framework, which focuses [3] on
achieve these throughput requirements
enhancements to OFDMA physical layer
and provide better quality of service,
and MAC layer to enable operation of a
these standards have opted for relay
Mobile multi-hop relay system (MMR)
technology for signal transmission over
using relay stations. MMR would allow base
the conventional direct transmission. This
stations without an E-1 or T-1 backhaul
paper presents an overview of the relay
connections (i.e. RS) to communicate with
technologies used in the two standards.
base stations that have link connections with
By Matlab simulation, the reduction in
some portion of the air link bandwidth. The
transmission power level using Simple
relay stations (RS) would help to forward
Relay is shown, comparing to the
user information from neighboring mobile
conventional Direct transmission method.
station (MS) to a local e-node-B (eNB- base
I. Introduction stations with integrated Radio network
controllers)/ base stations (BS). The MMR
The IEEE 802.16j (WiMax), IEEE 802.16m can effectively extend the signal and service
and 3GPP LTE advanced are the next coverage of a BS and enhance the overall
generation (4G) mobile communication throughput performance of a wireless
systems that meet the requirements of communication system.
International Telecommunication Union
(ITU), for the 4G systems. According [1] to 1. Relay Types and benefits
the ITU’s requirements, the 4G systems
should support peak data rates of 100 Mbps The Relay technologies have been used in
and 1 Gbps, respectively, in high speed earlier wireless carrier systems e.g.
mobility environments (up to 350 kmph) and repeaters. Relay technique was used to
stationary or pedestrian environments (up to increase the coverage to a potential coverage
10 kmph). In order to meet the requirements hole due to urban clutter. Relays were used
on higher wireless access data rate and to better coverage inside a building by
better quality of service (QOS), the LTE and providing In-build solutions with installing
WiMax operators would need to increase the low capacity BS for the building connected
density of Base Stations to provide high data to Master BS. But in WiMax and 3GPP
standards, to meet the data rate requirements

2. the relay technique is even used inside the the RS is used to relay the traffic signal
BS coverage area. This method helps in between BS and MS. Such a mode helps in
increasing the throughput along with improving the throughput within the
reducing the required transmission power coverage cell, compared to the case without
level for the signal to reach from BS to MS, RS [1].
which we have shown in Matlab simulation.
Along the lines, that relays are deployed to For the different 802.16j and 3GPP
provide coverage inside the BS coverage standards the types are given different
area and to extend the BS coverage beyond notations of Transparent/non-transparent
its coverage borders, relays are distinguished and I & II, respectively
in two types 1) Type-I (or non-transparency 2. Transmission Schemes
, NT-RS) RS and Type-II (or transparency,
T-RS) RS which are shown in figure below. The two standards, have mainly proposed
three transmission schemes [1] for the
processing of the signals at the RS, while
also trying to avoid the processing delay.
Amplify and Forward (AF) – This scheme
is known as simple relay and is mostly used
to increase the coverage area. In AF scheme,
the RS amplifies the received signal from
BS and forwards it to MS. It has a very short
processing delay.
[1] Network scenario for Type-I and Type-II
Selective Decode and Forward (DCF) – In
Fig: I this scheme, [1] the RS decodes (channel
decoding) the received signal from the BS.
Type-I RS (or NT-RS for 802.16j
The RS checks, whether the decoded data is
standard) provide coverage to MS i.e.
correct using cyclic redundancy check, and
beyond or at the edge of BS coverage area.
if correct performs channel coding and
It contributes to the overall system capacity
forwards the new signal to MS. DCF
and enables communication ser-vices and
effectively avoids error propagation, but has
better data throughput to a MS at the edge of
a long processing delay.
a cell. The BS and MS in Type-I relay have
no direct connection and the transfer of Demodulation and Forward (DF): The RS
preamble and other control information are demodulates the receiving signal from the
sent through RS along with traffic signals. BS and modulates and forwards the signal to
MS. It has simple operation and low
In Type-II (or T-RS), the RS is placed
processing delay, but is unable to avoid error
within the particular BS cell coverage area.
propagation.
In this relay mode [3] the base stations
control information can reach the MS but

3. The DCF scheme [1] is identified to achieve from all RS’s and MS’s units within the cell.
a better throughput improvement, compared The channel and location information is
to AF and DF. periodically updated and reported to the BS.
Using the information, the BS generates a
3. Cooperative Relaying matrix C with i and j rows corresponding to
Diversity [4] techniques are used to achieve MS ID’s and RS ID’s. The matrix elements
signal improvement by using multiple paths represents the achievable data rate when the
between transmitter and receiver. IEEE ith MS is served/paired with jth RS. If the
802.16j [2] has introduced an optional MS is not served by the RS the corresp-
feature of cooperative diversity, to use the onding row and column are set to zero,
multiple RS antennas and BS available otherwise, Ci,j is calculated between the
within the cell coverage area. The 802.16 instantaneous channel conditions. The
standard provides three mechanisms for Centralized pairing scheme is developed for
cooperative diversity. a) Cooperative multiple RS and single MS scenario and
Source Diversity using antennas distributed vice-versa. In this scenario, once an RS
among RS and BS to transmit identical selects an MS unit, it cannot serve any other
signal simultaneously in time and frequency. MS. The BS sets all corresponding rows to
b) Cooperative transmit diversity using zero. This avoids the MS to attempt to
pre-defined space time codes distributed connect an already serving RS. The values
among RS’s and BS and c) Cooperative of matrix C are constantly updated to check
hybrid diversity, which is a combination of for non-serving RS. The overall throughput
the above two mechanisms. for the served MS units is calculated by
adding together all serving elements in
4. Relay Path Selection matrix C. The Centralized pairing scheme is
mainly used for two hop relays and in type-
In a network of multiple RS and multiple
II relay mode, due to its periodic
MS units in each cell, the important aspect is
information exchange.
to select appropriate RS to transmit signals
in able to achieve better throughput along 4.2 Distributed Pairing Scheme
with low processing delay. The pairing
scheme also serves the purpose of the RS In a Distributed pairing, the RS selects its
routing selection method in more than two own MS units and it’s serving RS in more
hop routing. Hence, the two standards have than two hop relay system. It gathers local
provided two types of pairing schemes for channel information from neighboring MS’s
Relay selection. The selection is done by & RS and from serving BS. Each RS’s of a
using channel and location information. particular serving BS uses a common
communication channel.
4.1 Centralized Pairing Scheme
In a Centralized pairing scheme [1], the BS
serves as a central node to collect the
required channel and location information

4. since smaller hops are favorable as it
increases delay.
Path cost = (Link Throughput/Hop count)
[5]
The best path is selected i.e. the path
maximum value among the calculated costs,
since it provides the higher link throughput.
Both Centralized pairing scheme and
distributed pairing schemes are able to
increase the probability of network
[5] Relay network entry scenario connection, hence increases the traffic and
are also able to provide maximum
Fig: II throughput.
As distributed pairing schemes are used in II. Simulation Results
more than two hop relay connections, it also
serves the purpose of path selection. The A Matlab simulation was performed
channel information is sent through UCD to demonstrate the reduction in transmission
messages. The UCD [6] message is an power level in simple two-hop relay
uplink channel descriptor which is compared to conventional line of sight
broadcasted by the BS at a periodic interval transmission. The Matlab codes for two
in order to provide burst profiles (physical cases are provided in Appendix A.
parameter sets) that can be used by uplink
Case 1: Calculation of transmission power
physical channels. For RS routing selection
level over a fixed distance between BS and
purpose, the UCD messages contain link
MS, while RS is placed at variable distance.
available bandwidth, SNR and Hop count.
The latency or delay in routing depends on
these parameters. Each RS sets its own path
metric table using the parameters in UCD
message.
Figure II demonstrates how the relay station
selects its routing path in distributed pairing
scheme. The RS sets the metric path table
using the information of neighboring RS
sent by the serving BS of the respective
neighboring RS. Using the path metric table,
the RS calculates the cost of each route
dividing the link throughput by hop count,
Fig: III

5. The calculations were performed for The result in fig IV shows that there is
receiving power level of -30 dBm with considerable reduction in total transmitted
pathloss exponent 4. The values were taken power level, when simple relay is used,
over a distance of 0 to 2000 meters. when the mobile is in motion moving away
from BS.
The result in figure III shows that using a
simple Relay reduces the transmission III. Conclusion and Summary
power level between BS and MS. The
minimum power level in this ideal case is The Relay Technology to be used in the
obtained, when the RS is placed at the WiMax and LTE networks provides better
halfway distance between BS and MS. performance compared to conventional
transmission methods, in terms of increase
in coverage capacity, achievable peak data
rate and reduction in power level. Thus, the
Case 2: Based on the study in [6], to cost effective Relay technique is a better
demonstrate the reduction in transmission alternative for signal transmission to meet
power level by using the simple relay the requirements of high data rate and QOS
compared to direct line of sight in advanced mobile systems.
transmission, when the MS is in motion
away from BS. For future we would like to demonstrate the
power consumption level in DCF and to
simulate effects of noise in our current
simulation results. The current results are
based on ideal noise free environments.
Fig: IV
The calculations with the same parameters
as case 1 to receive power level of -30 dBm
at MS. The RS was placed at 400 meters
from BS. The location of MS is changed
further away from BS, for each calculation.

6. References [5] Sojeong Ann, “A Path selection method
in IEEE 802.16j Mobile Multi-hop relay
[1] Yang Yang, “Relay Technologies for Networks”, Computer Society, IEEE, 2008
WiMax and LTE-Advanced Mobile
Systems”, Communications Magazine, [6] Jee Young Song, “Power Consumption
IEEE, October 2009 Reduction by Multi-hop Transmission in
Cellular Networks”, IEEE, 2004
[2] Steven W. Peters, “The future of
WiMax: Multihop Relaying in IEEE
802.16j” Communications Magazine, IEEE,
January 2009
[3] Mustafa Ergen, “Mobile Broadband:
Including WiMax and LTE”, Springer
publications
[4] Vijay Garg, “Wireless Communications
and Networking”, Morgan Kaufmann
Publishers

7. Appendix A: MATLAB Code
Simulation 1: To demonstrate the effects of using relay transmission method over direct line of
sight method on the transmission power required to provide a receiving power level of -30 dBm
at the mobile station.
function power
close all;
% Part 1: No-Relay Transmitted power
Pr = 10^-3; % Rx. power requirement of the M.S
dt = 2000; % Fixed distance from B.S to M.S
y=4; % Pathloss exponent in urban areas
Ptbm = Pr*(dt^y); % Calculate power transmitted in No relay system
PtbmdB = pow2dB(Ptbm) % Convert Milli Watts into dB
norelay=PtbmdB;
contnplot=0:1:2000; % For continuous graph
% Part 2: Relay Transmitted Power (Two-hop transmission)
dr = 0:50:2000 % distance at which RS is placed between BS & MS
for count1=1:41 % To calculate multiple values of Tx. Power
Ptb(count1)= (Pr*((dr(count1))^y)); % Calc. Tx Pwr from BS to RS
Ptr(count1)= (Pr*((dt-dr(count1))^y)); % Calc. Tx Pwr from RS to MS
Ptm(count1)=Ptb(count1)+ Ptr(count1); % Calc. total Tx Pwr
PtmdB(count1)= pow2dB(Ptm(count1)) % Convert Milli Watts into dB
Ptmin=min(PtmdB) % Find the minimum Tx. pwr
% find the distance at which minimum Tx. Pwr is achieved
if PtmdB(count1)== Ptmin
dmin = dr(count1);
end
end
% Part 3: Plot the graph
figure,
hold on % To plot the graph at the same plot
xlabel('Distance');
ylabel('Tx Power in dBm');
title('Tx. Power v/s distance')
xlim([0 2000]);
ylim([80 110]);
p1=plot(dr,PtmdB,'ro-'); % Plot the graph for Tx. Pwr in No Relay
p2=plot(contnplot,norelay); % Plot the graph for Tx. Pwr in Relay Tx.
p3=plot(dmin,Ptmin,'*'); % To plot the Min. Tx. Pwr in Relay Tx.
legend('Simple Relay', 'No Relay' , 'Optimum Power level');
hold off;

8. Simulation 2: To observe the trend of transmission power level, due to change in location of
MS, between direct line of sight and Simple relay method.
function transmission
close all;
% Part 1: No-Relay Transmitted power
Pr = 10^-3; % Rx. power requirement in milli watts of the M.S
dt = 500:100:2000; % Range of distance between B.S to M.S
y=4; % Pathloss exponent in urban areas
for count1=1:16
Ptd(count1) = Pr*(dt(count1)^y); % Calculate power transmitted in
No relay system
PtddB(count1) = pow2dB(Ptd(count1)) % Convert Milli Watts into dBm
end
% Part 2: Two Hop transmission
dr = 400 % Fixed distance at which RS is
placed between BS & MS
for count1=1:16 % To calculate multiple values of
Tx. Power
Ptb(count1)= (Pr*((dr^y))); % Calc. Tx Pwr from BS to RS
Ptr(count1)= (Pr*((dt(count1)-dr)^y)); % Calc. Tx Pwr from RS to MS
Ptm(count1)=Ptb(count1)+ Ptr(count1); % Calc. total Tx Pwr
PtmdB(count1)= pow2dB(Ptm(count1)) % Convert Milli Watts into dBm
end
%Part 3:Plotting the graph
figure,
hold on % To plot the graph at the same
plot
xlabel('Location of Mobile station');
ylabel('Tx Power in dBm');
title('Tx. Power v/s distance')
xlim([0 2200]);
ylim([60 130]);
plot(dt,PtddB,'ro-'); % To plot the result of No relay
system
plot(dt,PtmdB,'ko-') % To plot the result of Simple
Relay
legend('No Relay', 'Simple Relay');
hold off;