Relay Technologies for WiMax and LTE-Advanced Mobile Systems Devdatta AmbreAbstract-- IEEE 802.16j and 3GPP LTE- rate wireless access to far-reached place ofAdvanced standards are the next the coverage area.generation wireless communications To enable rapid and cost effectivesystem that provides considerable deployment of the network infrastructure theincrease in data throughput than the IEEE has made amendments in 802.16jprevious 3G communication system. To framework, which focuses  onachieve these throughput requirements enhancements to OFDMA physical layerand provide better quality of service, and MAC layer to enable operation of athese standards have opted for relay Mobile multi-hop relay system (MMR)technology for signal transmission over using relay stations. MMR would allow basethe conventional direct transmission. This stations without an E-1 or T-1 backhaulpaper presents an overview of the relay connections (i.e. RS) to communicate withtechnologies used in the two standards. base stations that have link connections withBy Matlab simulation, the reduction in some portion of the air link bandwidth. Thetransmission power level using Simple relay stations (RS) would help to forwardRelay is shown, comparing to the user information from neighboring mobileconventional Direct transmission method. station (MS) to a local e-node-B (eNB- baseI. Introduction stations with integrated Radio network controllers)/ base stations (BS). The MMRThe IEEE 802.16j (WiMax), IEEE 802.16m can effectively extend the signal and serviceand 3GPP LTE advanced are the next coverage of a BS and enhance the overallgeneration (4G) mobile communication throughput performance of a wirelesssystems that meet the requirements of communication system.International Telecommunication Union(ITU), for the 4G systems. According  to 1. Relay Types and benefitsthe ITU’s requirements, the 4G systemsshould support peak data rates of 100 Mbps The Relay technologies have been used inand 1 Gbps, respectively, in high speed earlier wireless carrier systems e.g.mobility environments (up to 350 kmph) and repeaters. Relay technique was used tostationary or pedestrian environments (up to increase the coverage to a potential coverage10 kmph). In order to meet the requirements hole due to urban clutter. Relays were usedon higher wireless access data rate and to better coverage inside a building bybetter quality of service (QOS), the LTE and providing In-build solutions with installingWiMax operators would need to increase the low capacity BS for the building connecteddensity of Base Stations to provide high data to Master BS. But in WiMax and 3GPP standards, to meet the data rate requirements
the relay technique is even used inside the the RS is used to relay the traffic signalBS coverage area. This method helps in between BS and MS. Such a mode helps inincreasing the throughput along with improving the throughput within thereducing the required transmission power coverage cell, compared to the case withoutlevel for the signal to reach from BS to MS, RS .which we have shown in Matlab simulation.Along the lines, that relays are deployed to For the different 802.16j and 3GPPprovide coverage inside the BS coverage standards the types are given differentarea and to extend the BS coverage beyond notations of Transparent/non-transparentits coverage borders, relays are distinguished and I & II, respectivelyin 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  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. Network scenario for Type-I and Type-II Selective Decode and Forward (DCF) – In Fig: I this scheme,  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 isstandard) provide coverage to MS i.e. correct using cyclic redundancy check, andbeyond or at the edge of BS coverage area. if correct performs channel coding andIt contributes to the overall system capacity forwards the new signal to MS. DCFand enables communication ser-vices and effectively avoids error propagation, but hasbetter data throughput to a MS at the edge of a long processing delay.a cell. The BS and MS in Type-I relay haveno direct connection and the transfer of Demodulation and Forward (DF): The RSpreamble and other control information are demodulates the receiving signal from thesent through RS along with traffic signals. BS and modulates and forwards the signal to MS. It has simple operation and lowIn Type-II (or T-RS), the RS is placed processing delay, but is unable to avoid errorwithin the particular BS cell coverage area. propagation.In this relay mode  the base stationscontrol information can reach the MS but
The DCF scheme  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 isto 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 toDiversity  techniques are used to achieve MS ID’s and RS ID’s. The matrix elementssignal improvement by using multiple paths represents the achievable data rate when thebetween transmitter and receiver. IEEE ith MS is served/paired with jth RS. If the802.16j  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 thewithin the cell coverage area. The 802.16 instantaneous channel conditions. Thestandard provides three mechanisms for Centralized pairing scheme is developed forcooperative diversity. a) Cooperative multiple RS and single MS scenario andSource Diversity using antennas distributed vice-versa. In this scenario, once an RSamong RS and BS to transmit identical selects an MS unit, it cannot serve any othersignal simultaneously in time and frequency. MS. The BS sets all corresponding rows tob) Cooperative transmit diversity using zero. This avoids the MS to attempt topre-defined space time codes distributed connect an already serving RS. The valuesamong RS’s and BS and c) Cooperative of matrix C are constantly updated to checkhybrid diversity, which is a combination of for non-serving RS. The overall throughputthe 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 periodicMS units in each cell, the important aspect is information exchange.to select appropriate RS to transmit signalsin able to achieve better throughput along 4.2 Distributed Pairing Schemewith low processing delay. The pairingscheme also serves the purpose of the RS In a Distributed pairing, the RS selects itsrouting selection method in more than two own MS units and it’s serving RS in morehop routing. Hence, the two standards have than two hop relay system. It gathers localprovided two types of pairing schemes for channel information from neighboring MS’sRelay selection. The selection is done by & RS and from serving BS. Each RS’s of ausing channel and location information. particular serving BS uses a common communication channel. 4.1 Centralized Pairing SchemeIn a Centralized pairing scheme , the BSserves as a central node to collect therequired channel and location information
since smaller hops are favorable as it increases delay. Path cost = (Link Throughput/Hop count)  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 Relay network entry scenario connection, hence increases the traffic and are also able to provide maximumFig: II throughput.As distributed pairing schemes are used in II. Simulation Resultsmore than two hop relay connections, it alsoserves the purpose of path selection. The A Matlab simulation was performedchannel information is sent through UCD to demonstrate the reduction in transmissionmessages. The UCD  message is an power level in simple two-hop relayuplink channel descriptor which is compared to conventional line of sightbroadcasted by the BS at a periodic interval transmission. The Matlab codes for twoin 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 powerphysical channels. For RS routing selection level over a fixed distance between BS andpurpose, 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 onthese parameters. Each RS sets its own pathmetric table using the parameters in UCDmessage.Figure II demonstrates how the relay stationselects its routing path in distributed pairingscheme. The RS sets the metric path tableusing the information of neighboring RSsent by the serving BS of the respectiveneighboring RS. Using the path metric table,the RS calculates the cost of each routedividing the link throughput by hop count, Fig: III
The calculations were performed for The result in fig IV shows that there isreceiving power level of -30 dBm with considerable reduction in total transmittedpathloss 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 asimple Relay reduces the transmission III. Conclusion and Summarypower level between BS and MS. Theminimum power level in this ideal case is The Relay Technology to be used in theobtained, when the RS is placed at the WiMax and LTE networks provides betterhalfway 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, theCase 2: Based on the study in , to cost effective Relay technique is a betterdemonstrate the reduction in transmission alternative for signal transmission to meetpower level by using the simple relay the requirements of high data rate and QOScompared to direct line of sight in advanced mobile systems.transmission, when the MS is in motionaway 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: IVThe calculations with the same parametersas case 1 to receive power level of -30 dBmat MS. The RS was placed at 400 metersfrom BS. The location of MS is changedfurther away from BS, for each calculation.
References  Sojeong Ann, “A Path selection method in IEEE 802.16j Mobile Multi-hop relay Yang Yang, “Relay Technologies for Networks”, Computer Society, IEEE, 2008WiMax and LTE-Advanced MobileSystems”, Communications Magazine,  Jee Young Song, “Power ConsumptionIEEE, October 2009 Reduction by Multi-hop Transmission in Cellular Networks”, IEEE, 2004 Steven W. Peters, “The future ofWiMax: Multihop Relaying in IEEE802.16j” Communications Magazine, IEEE,January 2009 Mustafa Ergen, “Mobile Broadband:Including WiMax and LTE”, Springerpublications Vijay Garg, “Wireless Communicationsand Networking”, Morgan KaufmannPublishers
Appendix A: MATLAB CodeSimulation 1: To demonstrate the effects of using relay transmission method over direct line ofsight method on the transmission power required to provide a receiving power level of -30 dBmat the mobile station.function powerclose all;% Part 1: No-Relay Transmitted powerPr = 10^-3; % Rx. power requirement of the M.Sdt = 2000; % Fixed distance from B.S to M.Sy=4; % Pathloss exponent in urban areasPtbm = Pr*(dt^y); % Calculate power transmitted in No relay systemPtbmdB = pow2dB(Ptbm) % Convert Milli Watts into dBnorelay=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 & MSfor 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); endend% Part 3: Plot the graphfigure,hold on % To plot the graph at the same plotxlabel(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 Relayp2=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;
Simulation 2: To observe the trend of transmission power level, due to change in location ofMS, between direct line of sight and Simple relay method.function transmissionclose all;% Part 1: No-Relay Transmitted powerPr = 10^-3; % Rx. power requirement in milli watts of the M.Sdt = 500:100:2000; % Range of distance between B.S to M.Sy=4; % Pathloss exponent in urban areasfor count1=1:16Ptd(count1) = Pr*(dt(count1)^y); % Calculate power transmitted inNo relay systemPtddB(count1) = pow2dB(Ptd(count1)) % Convert Milli Watts into dBmend% Part 2: Two Hop transmissiondr = 400 % Fixed distance at which RS isplaced between BS & MSfor count1=1:16 % To calculate multiple values ofTx. 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 dBmend%Part 3:Plotting the graphfigure,hold on % To plot the graph at the sameplotxlabel(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 relaysystemplot(dt,PtmdB,ko-) % To plot the result of SimpleRelaylegend(No Relay, Simple Relay);hold off;