Report :- MIMO features In WiMAX and LTE: An Overview


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A comparative study report on MIMO features in IEEE 802.16m and 3GPP LTE

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Report :- MIMO features In WiMAX and LTE: An Overview

  1. 1. MIMO Techniques in WiMAX and LTE: A Feature Overview 1 MIMO Techniques in WiMAX and LTE: A Feature Overview Ananthakrishnan Ramkumar (Student #4119568) & Praveen Kalyanasundaram (Student #4118863) In this report, we give an overview of the various MIMO Abstract—IEEE 802.16m and 3GPP LTE are the two techniques employed in Mobile WiMAX and 3GPP LTE, andevolving wireless standards targeting 4G wireless systems. provide a comparison.They make use of Multiple Input Multiple Output(MIMO) technologies in order to meet the requirements of4G wireless systems. A large number of MIMO techniqueshave been developed and employed in these two standardswhich greatly enhance the data rates and spectralefficiency compared to 3G. In this case study we providean overview of the MIMO techniques including Open-loop(OL), Closed-loop (CL), Single user and Multiuser MIMOin the two standards. The MIMO features of the twostandards are surveyed. I. INTRODUCTION Since the launch of 3G mobile communication services, highspeed wireless access services that provide high speed datatransmission in a mobile environment have come to be used in Figure1: Trends in Mobile Communicationsdiverse applications including email, web access etc. Figure 1shows the trends in wireless communications. It is evident that The report is divided as follows: In Section II MIMOthe next generation (4G) is expected to provide much higher technology is introduced and the techniques are rates and mobility compared to the current one. Section III and Section IV provide an overview of WiMAX Multiple-input multiple-output (MIMO) technology is and LTE technologies respectively. In section V the variousserving as a breakthrough in the design of wireless MIMO techniques used in both technologies are analyzed.communication systems. Exploiting multi-path scattering,MIMO techniques deliver significant performance II. MIMO – INTRODUCTION AND TECHNIQUESenhancements in terms of data transmission rate and MIMO is a technique to improve communicationinterference reduction. The rapidly growing demand for performance by using multiple antennas at the transmitter andbandwidth in mobile services makes it essential to the to deliver cost effective, highly performing wirelessbroadband systems with key technologies such as OFDM,advanced antenna techniques such as MIMO andbeamforming. The IEEE 802.16e (WiMAX Profile 1.0) and ThirdGeneration Partnership Project (3GPP) Evolved UniversalTerrestrial Radio Access (E-UTRA) Long Term Evolution(LTE) (Releases 8 and 9) standards have been developed andare part of the IMT-2000 third generation (3G) technologies[1]. IEEE 802.16m (WiMAX Profile 2.0) [2] and 3GPP E-UTRA LTE-Advanced (LTE-A) (Release 10) [3] are stillbeing developed primarily to meet or exceed the requirementsof the International Telecommunication Union (ITU) for IMTAdvanced fourth generation (4G) technologies. Figure2: MIMO System
  2. 2. MIMO Techniques in WiMAX and LTE: A Feature Overview 2 In MIMO systems, a transmitter sends multiple streams usingmultiple transmit antennas. The transmit streams go through achannel which consists of all M*N paths between the Ntransmit antennas and M receive antennas. The receiverdecodes the received signal vectors into the original where ‘*’ denotes complex conjugate. Here C1 and C2 withininformation. A narrowband flat fading MIMO system is the matrix represent the two complex modulation symbols thatmodeled as: are transmitted in two timeslots. There are also higher order space time codes that provide a better error-rate performance Y=Hx+n -- (1) [4]. At the receiver maximum likelihood decoding isWhere Y and x are the receive and transmit vectors, performed with only linear processing. Space time codingrespectively, and H and n are the channel matrix and the noise assumes perfect CSI at the receiver.vector, respectively. In receive diversity the independently faded signals received at the antennas are used to provide diversity gain using It has been proven that the maximum capacity achievable techniques such as Selection Diversity, Switching, Maximalwith a MIMO configuration consisting of N transmit and M Ratio Combining (MRC) ,Equal Gain Combining (EGC).receive antennas is “min (M, N)” times the capacity of thecorresponding Single Input Single Output (SISO) system. [5] B. Spatial Multiplexing: There are three main aspects in MIMO: Spatial multiplexing is a very useful transmission technique A. Diversity: in MIMO systems to increase the overall capacity of the channel at high SNR values. Here a high data rate stream is Diversity leads to improvement of link reliability by adding split into several low data rate streams where each of themore redundancy to information going through the channel. In individual streams is transmitted by a different transmitthis technique, several copies of the same signal are antenna in the same frequency channel. If these signals arrivetransmitted using multiple antennas into the air interface where at the receiver antenna array with sufficiently different spatialthey may experience fading independent of each other. In such signatures, the receiver can perform processing to separatescenarios there will be high probability that only some signals these streams.will undergo deep fades while others may not. This can beused to obtain diversity gain. Transmit and receive diversity are the common schemes in aMIMO system and both of these do not require channelknowledge at the transmitter. Transmit diversity refers to the use of techniques such asSpace-time coding wherein each antenna transmits adifferently encoded, fully redundant version of the same signalleading to redundancy in space and time. Figure 4: Spatial multiplexing If we consider a system with N transmit antennas and M receive antennas then the maximum spatial multiplexing order is given by the expression: Ns = min (M,N) --(2) where Ns is the number of streams which can be transmitted in parallel [5]. At the receiver the data streams can be separated by the equalizer provided each of the data stream has undergone fading independent of each other. In spatial multiplexing there is no necessity for additional bandwidth and power. Figure 3: Transmit diversity using Space-time code One such code that was designed for a two-transmit antenna C. Beamforming:system is the Alamouti code [4]. These codes are orthogonal in Beamforming is a signal processing technique that takesnature and hence it is used to provide full diversity gain. advantage of the fading channels. It primarily improves the It is described with the coding matrix received signal gain and coverage of the communication system. In this technique the transmission radiation pattern
  3. 3. MIMO Techniques in WiMAX and LTE: A Feature Overview 3from an array of antennas is focused in the direction of specific The precoding may be channel dependant or independent.user by constructively interfering in that specific direction. In With channel dependant precoding, also referred to as closed-order to achieve this, it is required to have reliable knowledge loop precoding, the precoder matrix is chosen to match theof the channel. characteristics of the MIMO channel. With channel Based on the amount of channel knowledge gained various independent precoding, also known as open loop precoding,types of beamforming can be implemented. Three different channel characteristics are not considered in the selection ofscenarios are possible namely: the precoder matrix. (i) Full CSI: Statistical Eigen vector beamforming is a E. Open Loop and Closed Loop Transmissionreliable technique. (ii) Limited CSI: Grassmannian beamforming is used. In Open Loop (OL) transmission technique, the transmitter (iii) No CSI: Blind beamfomring technique is used where has limited or no knowledge of the channel. To obtainthe CSI is blindly estimated from the received signal statistics. knowledge of the channel, an open-loop transmission scheme uses the idea of the channel reciprocity available in TDD because both downlink and uplink are using the same frequency channel. That is why FDD is not used in an open- loop transmission since downlink and uplink channels do not use the same frequency. Each of these channels is totally different and hence not reciprocal. OL transmission is suitable for high mobility scenario, where channel varies rapidly and feedback from receiver is not very useful. Some of the OL- MIMO techniques include Spatial Multiplexing and Space- Time codes which were discussed earlier in this report. Closed loop MIMO (CL-MIMO) system on the other hand uses feedback from the receiver to obtain Channel State Information (CSI) and hence uses it for increasing throughput or coverage. The major challenge in CL-MIMO is efficiently Figure 5: Beamforming obtaining the CSI which is then used to construct the beamforming or precoding matrix. In MIMO systems multilayer beamforming is also supportedwith the help of precoding which is explained in the later F. Single User vs Multi user MIMOsection. In such a case the full channel matrix (CSI) must be In SU-MIMO transmissions, time-frequency resources areknown to the system. By applying Singular Value dedicated to a single terminal/user with the aim of achievingDecompostion (SVD) the channel matrix is diagonalized as peak user spectral efficiency. In MU-MIMO time-frequencyshown below: resources are shared by multiple users. Multi User MIMO combines the high capacity achievable with MIMO with the benefits of Space division Multiple Access (SDMA). Where σM is the mth non-negative singular value from a setof σ1 ≥ σ2 ≥ …. ≥ σM, U and V are the corresponding singularvector unitary matrices. The two unitary matrices are removedthrough pre- and post-multiplication at the transmitter andreceiver, respectively. Figure 6: Single user vs Multi-user MIMO Once SVD is applied, one data stream per singular valuecan be transmitted with appropriate power in a defined G. MIMO Receiver Designdirection without creating any interference [7]. A number of MIMO receiver algorithms are used, depending on the receiver complexity: Zero-Forcing (ZF), Minimum D. Precoding Mean Square Error (MMSE) and MIMO Maximum Precoding is generalized beamforming which permits to Likelihood Detector (MLD).maximize the received signal level. It improves the capacity of The receiver receives signals transmitted by more than onethe system and also limits the transmit power. transmit antenna. To separate the mixed data streams at the Precoding is done to exploit beamforming and for spatial receiver, the received mixed-signal is multiplied by the inversemultiplexing. The process of precoding creates some of the MIMO channel matrix. This is called a ZF receiver.redundancy into the data sequence before the transmission. For an output given by
  4. 4. MIMO Techniques in WiMAX and LTE: A Feature Overview 4 receivers, conversely, can both suffer sharp performance losses when MIMO channels are correlated. Another advantage is diversity gain. The MLD receiver not only can separate the transmit data streams but also can achieve the receive diversity gain for multiple receive The data obtained at the receiver end can be expressed as antennas. Further, the receiver can support a very high mobility environment. But MLD receiver has higher Ŝ = H #X -- (3) complexity load compared to MMSE receiver. Where H# = (H*H)-1 H* Each MIMO technique has advantages and disadvantages. Here ‘H’ is the channel matrix and ‘X’ is the output at the When designing a wireless system, appropriate MIMOreceiver. It is a simple MIMO receiver that suffers technique is chosen by considering the service type, channelperformance loss at higher noise and interference levels at the condition and complexity.receivers. In a MMSE receiver on the other hand, in order to minimize III. WIMAXperformance loss, the inversion of the MIMO channel matrixoperation is adjusted according to the interference or noise A. Introductionlevel given as: WiMAX which stands for Worldwide Interoperability for Ŝ = H*(HH* + Rn)-1x -- (4) Microwave Access is the trade name for the IEEE 802.16 international standards. It is a rapidly growing broadbandwhere Rn is the noise or interference covariance wireless access technology that replaces the current existing systems such as Wi-Fi and 3G. Improved reception of spatial multiplexing MIMO IEEE 802.16-2005 or IEEE 802.16m known as Mobiletransmission requires an exhaustive search of MIMO signal WiMAX is an extension of IEEE 802.16-2004 or IEEEconstellation combinations. The MIMO transmission process 802.16e (fixed WiMAX). Mobile WiMAX introduces newis emulated at the receiver in such a way that a specific features to support enhanced Quality of Service to providecomplex modulation constellation is generated for each high mobility at very high data rates.transmit antenna stream. The constellations are then applied tothe MIMO channel input. At the MIMO channel output, the B. Featurescorresponding MIMO reception signal is generated for each a) Access Technology: OFDMA with Cyclic Prefix inreceive antenna. At this point, the Euclidean distance iscomputed for the emulated MIMO output signal against the both UL and DLreceived signal. In this way, the different modulation b) TDD and FDD as the duplexing modesconstellations at MIMO channel input constitute different c) Adaptive modulation and coding (AMC)hypothesis tests. - QPSK, 16QAM, and 64QAM The minimum Euclidian distance associated with the - Forward Error Correction (FEC)constellation combination hypothesis provides the most likely d) MIMO Matrix A (Space Time Block Coding) anddecoding. This is the principle behind MLD receiver. Optimal Matrix B (Spatial Multiplexing) supportreceiver performance, however, comes at the cost of receiver e) Open Loop and Closed Loop configurations includingcomplexity. Transmit Diversity, Spatial Multiplexing and Beam The advanced receiver can be further extended by aSuccessive Interference Cancellation (SIC) receiver forming techniques.architecture. The idea behind the SIC receiver is that the signal C. Requirementsquality of the multiple transmit data streams is not the same,because of the fading variations of the MIMO channel. We a) High peak data rates: max 74Mbps in 20MHz widecan successfully demodulate the first data stream, re- spectrummodulate it and then subtract the first data stream from b) Carrier Frequency: Unlicensed band-2.5 and 3.5the receive input mixture, absent interference from the GHz, Licensed band under 6GHzfirst data stream. We can then demodulate the second data c) Operating bandwidth: 1.25-20MHzstream successfully with the simplest maximum ratio d) Mobility support – with appropriate pilot designcombining (MRC) receiver. and H-ARQ Here we give a comparison of the various receivers that canbe used for MIMO. MLD-based MIMO receivers have many e) Flexible Frequency Reuseadvantages compared with ZF and MMSE receivers. The f) Flexible bandwidth allocationMLD receivers performance advantages are significant when g) Quality of Service (QoS) support - allowing videothe MIMO channels are correlated. The ZF and MMSE calls, mobile entertainment, multimedia chat and high speed internet access.
  5. 5. MIMO Techniques in WiMAX and LTE: A Feature Overview 5 h) Integrated Security for voice and data transmission OFDM technology has been incorporated into LTE because using Advanced Encryption Standard (AES) it enables high data bandwidths to be transmitted efficiently while still providing a high degree of resilience to reflections and interference. The access schemes differ between the uplink and downlink: OFDMA is used in the downlink; while SC- FDMA is used in the uplink. SC-FDMA is used in view of the fact that its peak to average power ratio is small and the more constant power enables high RF power amplifier efficiency in the mobile handsets - an important factor for battery power equipment. MIMO technologies have been widely used to improve downlink peak rate, cell coverage, as well as average cell throughput. LTE-Advanced has recently started in 3GPP wherein the existing SU-MIMO technologies are extended to support configuration with up to eight transmit antennas in the downlink, and up to four transmit antennas in the uplink.Figure 7: WiMAX multi-antenna implementation C. LTE Requirements a) Higher performance IV. LONG TERM EVOLUTION - 100 Mbit/s peak downlink, 50 Mbit/s peak uplink - 1G for LTE Advanced A. Introduction - Better cell edge performance Long Term Evolution (LTE) is an upcoming technology - Reduced latency (to 10 ms) for better usertargeting 4G wireless systems. The objective of LTE is to experienceprovide technical benefits to cellular technologies in terms of - Scalable bandwidth: 1.25-20 MHzbetter spectral efficiency (i.e. higher data rates with available b) Backwards compatible:bandwidth) and cell coverage as compared to 3G. - Works with GSM/EDGE/UMTS systems 3rd Generation Partnership Project (3GPP) Evolved - Utilizes existing 2G and 3G spectrum and newUniversal Terrestrial Radio Access (E-UTRA) Long Term spectrumEvolution (LTE releases 8 and 9) standard has been developed - Supports hand-over and roaming to existing mobileas part of the IMT 2000 third generation technologies (3G). networksLTE-A (Release 10) is still being developed primarily to meet c) Wide applicationor exceed the requirements of the International - Mobility up to 350kmphTelecommunications Union (ITU) for IMT fourth generation - Large range of terminals (phones and PCs to(4G). LTE is used to denote 3GPP releases 8 and 9, LTE-A to cameras)denote 3GPP release 10 and E-UTRA for releases 8 to 10. B. Featuresa) Multiple access schemes: - DL: OFDMA with Cyclic Prefix (CP) - UL: Single Carrier FDMA (SC-FDMA) with Cyclic Prefix (CP)b) Adaptive modulation and coding Table 1: Wimax and LTE features summary - DL/UL modulations: QPSK, 16QAM, and 64QAM - Convolutional code and Rel-6 turbo code Aspect 3GPP-LTE Mobilec) Advanced MIMO spatial multiplexing techniques WiMAX - (2 or 4)x(2 or 4) downlink and uplink supported. 802.16m - Multi-user MIMO also supported. Legacy GSM/GPRS/EDGE IEEE 802.16d) Support for both Frequency Division Duplexing (FDD) and Time division Duplexing (TD) /UMTS/HSPA a through ee) Hybrid-ARQ, mobility support, rate control, security. Access Technology The main technologies used in LTE are: Orthogonal DL OFDMA OFDMAFrequency Division Multiple Access (OFDMA), Single UL SC-FDMA OFDMACarries Frequency Division Multiple Access (SC-FDMA) and Radio Access TDD and FDD TDD and FDDMultiple Input Multiple Output (MIMO). Mode
  6. 6. MIMO Techniques in WiMAX and LTE: A Feature Overview 6 Frequency Existing 2-11 GHz Band (800,900,1800,1900 MHz) and new bands (Range 800 MHz to 2.62 GHz) Channel Scalable from 1.25 Scalable from Bandwidth to 20 MHz with 1.25 to 20 MHz system profiles with system Figure 8: OFDMA-uplink in WiMAX and SC-FDMA-uplink in 1.25,1.4,2.5,3.5,10, profiles 1.25,2.5, LTE (Note: Different colors indicate different users) 15 and 20 MHz 5, 10and 20 MHz B. SU-MIMO: Spatial Multiplexing Antenna MIMO MIMO Scheme The major constraints in implementing the spatial DL 2x2,4x2,4x4 2x2,4x2,4x4 multiplexing MIMO technology in 802.16m and LTE are the UL 1x2,1x4, 2x2 1x2,1x4,2x2 , cost of multiple antennas, the size limitation of multiple antennas for handheld devices, backward compatibility Number of 2 1 constraints and receiver complexity. Reception of the spatial code-words multiplexing MIMO transmissions is optimized by selecting Mobility: the best receiver operation, based on the MIMO channel Speed Up to 350 Km/h Up to 120 Km/h condition and the modulation type, to reduce the average Handover Inter-cell soft Optimized hard processing power required. handovers handovers 802.16m uses Vertical Encoding (VE)/Single Codeword DL Spectral 1.57 1.59 (SCW) transmission for both uplink and downlink. The reason Efficiency bps/Hz/Sector bps/Hz/Sector for this choice is that advanced receivers would be better (2x2) MIMO2 (2x2) MIMO implemented with an optimal Maximum Likelihood Detector UL Spectral 0.64 0.99 (MLD).The advantage of VE is that the implementation of Efficiency bps/Hz/Sector bps/Hz/Sector HARQ process is simple and it requires only a single report of (1x2) SIMO2 (1x2) SIMO channel quality indicator (CQI) for all multiplexed layers. On the other hand, LTE uses Multiple Codeword (MCW) transmission on the downlink. The reason for this choice is V. SURVEY OF MIMO TECHNIQUES because of lower complexity and better performance of MMSE-SIC receivers for MCW transmission in LTE. A. Uplink and Downlink Modeling of the effective SNR for each codeword is much WiMAX uses OFDMA in both uplink and downlink whereas more difficult in MLD than MMSE-SIC. The disadvantage ofLTE uses SC-FDMA in uplink and OFDMA in downlink. using MCW is that it requires one CQI report and one HARQ Despite its many advantages, OFDMA has the disadvantage process for each FEC codeword. Each HARQ process requiresof high frequency (esp. Doppler spread) sensitivity and high an ACK/NAK feedback signaling on uplink.peak-to-average power ratio (PAPR). PAPR occurs due to C. Reference Signal (RS)/Pilotrandom constructive addition of sub-carriers and results in Reference Signal (RS) also known as the pilot signal is usedspectral spreading of the signal leading to adjacent channel for measuring the spatial channel and help in coherentinterference. It is a problem that can be overcome with high demodulation at the terminal. They perform the operation ofcompression point power amplifiers and amplifier linearization supervision, control, equalization, synchronization or referencetechniques. purposes within a transmission system. It is possible to make While these methods can be used on the base station, they an estimate of the channel response at various frequencies bybecome expensive on the User Equipment (UE). Hence, LTE comparison with the known reference pilot subcarrier.uses Single Carrier FDMA (SC-FDMA) with cyclic prefix on The reference signals can be classified into Commonthe uplink which reduces PAPR as there is only a single carrier Reference Signal (CRS) and Dedicated Reference Signalas opposed to N carriers. (DRS). The cell common reference signal is a reference signal used by all UEs within a cell. The DRS or UE specific reference signal is a reference signal used by an UE within the cell or used by a UE group. The RS can be further classified as precoded or non-precoded. If the pilot/RS is also multiplied by the precoding matrix before transmission then it is called precoded pilot. Precoded pilots offer lesser overhead compared to non- precoded pilots. This is because in case of non-precoded pilots
  7. 7. MIMO Techniques in WiMAX and LTE: A Feature Overview 7the RS has to be transmitted by each of the transmit antennas, D. Multi-User MIMOwhereas in precoded pilots the number of RS to be transmitted MU-MIMO allocates multiple users in one time-frequencyis given by the number of spatial streams which is bounded by resource to exploit multi-user diversity in the spatial domain,m= min (M, N), where N and M are the number of transmit which leads in significant gains over SU-MIMO.and receive antennas. Moreover in non-precoded RS, the In the uplink scenario, users transmit to the base station overspatial precoder chosen from the codebook has to be indicated the same channel i.e multiple access. The challenge here is forto the terminal in each transmission assignment which adds to the base station to separate the signals transmitted by the usersthe overhead. But the advantage of using non-precoded pilots using Multi-User Detection (MUD) or other techniques. In theis that it enables finer channel estimation in the frequency downlink, the base station transmits simultaneously to a groupdomain. of users i.e broadcast. Here the challenge is to overcome the The RS signal used in the uplink for both 802.16m and E- inter-user interference to detect the signals.UTRA are similar. Non-precoded DRS is used in bothstandards for channel adaptation and beam selection in theuplink. Moreover, precoded DRS is used for coherentdemodulation in the uplink. However different designs have been adopted for downlinkpilots in the two technologies. 802.16m uses both non-precoded common pilots and precoded dedicated pilots forchannel measurements and coherent demodulation, supportingup to eight transmit antennas. On the other hand, non-precodedCRS’s supporting up to four transmit antennas have beendefined for LTE release 8. LTE release 9 and LTE-A havechosen DL precoded DRS (UE-Specific RS).Also, eightantenna port transmissions supporting up to eight layers(spatial) has been proposed for LTE-A. Figure 10: MU-MIMO Downlink There are two schemes in MU-MIMO: Linear and Non- linear. In the linear case, the data symbols are precoded with the pseudo inverse of the channel, so at the receiver the interference due to other users is cancelled. Zero-forcing (ZF) MU-MIMO technique is one linear MU-MIMO technique. [6] Non-linear MU-MIMO uses Dirty Paper Coding, in which precoding is done, given the interference is known at the transmitter. It was shown in [8] that the capacity of a channel where the transmitter knows the interfering signal, is the same as if there were no interference. Although non-linear MU-MIMO with dirty paper coding theoretically offers the best performance, the practical implementation is difficult, and hence linear MU-MIMO has been adopted by both standards for its simplicity. Figure 11: Inverse Channel multiplication in ZF MU-MIMO In 802.16m and LTE downlink scenario, a scheduler is Figure 9: Reference signal transmission by two transmit used, which selects several users with good spatial separationantennas and performs pseudo inversion of the combined channel matrix to obtain the precoding matrix. The CQI reported by each user is then adjusted at the base station to fit the channel quality
  8. 8. MIMO Techniques in WiMAX and LTE: A Feature Overview 8after precoding. 802.16m also supports OL MU-MIMO. Herea unitary precoding matrix is preset for each frequency domainresource. For uplink MU-MIMO, both WiMAX and LTE allowmultiple users to transmit simultaneously in the same uplinkresource. The base station (ABS/eNB) distinguishes thesignals from different user terminals through the pilots/RSsallocated to each terminal and separates them using anadvanced receiver which is MLD in case of 802.16 andMMSE in LTE. Figure 12: Layer permutation with four codewords E. Open Loop MIMO In LTE, since non-precoded CRS is used, the predefined(i) Space Time/Frequency Code: precoders can be changed within the subcarriers of a resource This is an Open loop technique wherein transmit diversity block (RB) so that beam diversity gains are fully used. Layertechnique provides spatial diversity gain. Both WiMAX and permutation is performed along with precoder cycling in E-LTE-A have adopted the frequency domain version of the UTRA to further increase diversity gain from virtual antennasAlamouti Code [4] as the basic transmit diversity MIMO with MCW transmission. This combination of precodertechnique, where coding is performed to pairs of adjacent cycling and layer permutation is called large-delay CDD andsubcarriers rather than two adjacent time slots. The main has been adopted as OL-SM technique in LTE.reason for this is to sustain the orthogonality of the code underhigh mobility of the terminals. Hence SFBC outperforms (iii) SFBC and FSTDSTBC in high speed scenarios. In MIMO modes where more Space Frequency Block Coding (SFBC) is used along withthan two transmit antennas are required, the application of Frequency Switched Transmit Diversity (FSTD).In SFBC theprecoders become necessary. This technique that is adopted in codeword symbols are mapped across frequency. The FSTDboth the standards makes effective use of all the spatial mainly cycles the transmissions over pairs of transmit antennasdegrees of freedom over a set of subcarriers by limiting the across subcarriers within a Resource Unit (RU).transmission of SFBC to a pair of subcarrier. As a result itimproves the robustness against spatial correlations in thechannel.(ii) Precoder Cycling Random beamforming is a method to increase channel - (a)selectivity by changing beams within allocated time/frequencyresources. Precoder cycling is a random beamformingtechnique. Here a predefined set of precoders are chosen from apredefined codebook and are cyclically allocated to a group ofadjacent subcarriers. The Chordal distance (separationbetween beams) property of the set of precoders must begood in order to increase the order of the diversity. - (b) In both 802.16m and LTE standards, precoder cycling isemployed to achieve beamforming. This technique in 802.16m Figure 13: (a) SFBC with two transmit antennas on downlinkprovides both beam diversity gain and beam selection gain. (b) SFBC+FSTD with four transmit antennas on downlinkBeam diversity gain is achieved by distributing the resourceswithin a wide frequency band. Here the predefined precoders In case of two antennas only SFBC is used whereas in casesform different beams in each localized frequency band. At the where four transmit antennas are used, a combination of SFBCreceiver, all the resources that are added up will benefit from and FSTD are employed. Figure 12 shows how SFBC andbeam diversity gain. FSTD is implemented, where in SFBC is limited to On the other hand beam selection gain is obtained based on transmission over a pair of subcarriers. By using differentthe CQI feedback, by allocating a localized resource to a antennas to transmit over the two subcarriers, spatial diversityterminal for its preferred sub-bands. According to the sub- is obtained. This provides robustness against spatialbands the precoders are cyclically changed and thus correlations in the channel.opportunistic beamforming gain can be achieved by allocating Both 802.16m and LTE make use of precoders in order tothe preferred sub-bands as reported by the terminal. achieve spatial diversity gain. However there is a slight variation in the use of precoders in both these standards. The variation lie in the design of DL demodulation pilots. In IEEE 802.16m, SFBC with precoder cycling is employed with
  9. 9. MIMO Techniques in WiMAX and LTE: A Feature Overview 9precoded pilots. These precoded pilots reduce the overhead The precoding operation for the closed-loop spatialwhen compared to non-precoded pilots. The precoder cycling multiplexing is defined bycreates a fixed set of two virtual antennas across all subcarrierswithin a RU and uses various precoded weights to change y = Wx - (5)these virtual antennas. Where y = [y0. . . yN-1]T, yn denotes the complex symbol On the other hand LTE makes use of SFBC with FSTD and transmitted on the nth antenna,uses non-precoded CRS. Although non-precoded CRS leads to x = [x0. . . xM-1]T, xm denotes the modulation symbolhigher overhead, it provides a wider range of interpolation in transmitted on the mth layer, and W denotes the N × Mthe frequency domain for finer channel estimation. precoding matrix. F. Closed Loop MIMO For transmission on two antennas, the precoding matrix W is Feedback is required when channel reciprocity is unavailable selected from Table 2, where each column vector is in the(e.g., in frequency-division duplex systems). The major form of [ 1 e^ j(θ+kπ)]T multiplied by a scaling factor [9].challenge lies in how to report the preferred beamforming Based on the codebook index the precoder is chosen.matrix, which is used for the transmitter to compute the actual The factors to be considered for the base codebook designprecoder over a limited feedback. For overhead reduction, the are performance gain, overhead, robustness and complexitywhole beamforming matrix is quantized by a matrix or vector First, 802.16m defines 3-bit for 2-transmit antennas (2-Tx) ascodebook. The index of the selected quantization codeword is well as 4-bit and 6-bit feedbacks for 4-transmit antennas (4-fed back. An L bit codebook consists of 2L codewords, where Tx), while LTE defines 2-bit and 4-bit feedbacks for 2-Tx andL is the required number of bits for indexing each codeword. 4-Tx, respectively. Besides the preferred beamforming matrix, In the closed-loop spatial multiplexing mode, the base station an indication of the preferred number of spatial streams is also(also known as eNodeB) applies the spatial domain precoding defined. More the number of bits in codebook index, more theon the transmitted signal taking into account the precoding codewords. This gives wider range for choosing the bestmatrix indicator (PMI) reported by the User Equipment(UE) precoder at the cost of signaling that the transmitted signal matches with the spatial channelexperienced by the UE. To support the closed-loop spatial The high rank codewords with more columns include the lowmultiplexing in the downlink, the UE needs to feedback the rank codewords with a few columns as subset. This reduces therank indicator (RI), the PMI, and the channel quality indicator complexity of searching for the best number of spatial streams.(CQI) in the uplink. The RI indicates the number of spatial Also, in each LTE codeword and most of 802.16m codewordslayers that can be supported by the current channel equal power is given to all antennas. This reduces theexperienced at the UE. complexity of power amplifier. The base station decides the transmission rank, M, takinginto account the RI reported by the UE as well as other factors Table 2: Precoding codebook for transmission on twosuch as traffic pattern, available transmission power, etc. The antennas.CQI feedback indicates a combination of modulation schemeand channel coding rate that the eNodeB should use to ensurethat the block error probability experienced at the UE will notexceed 10%.[9] Since the optimal codebook varies with the deployment scenario, adaptive codebook is defined in 802.16m. The adaptive codebook changes its codeword distribution according to long-term channel statistics. By doing this, codewords are transmitted in the ideal beamforming directions. Also, for overhead reduction, 802.16m has adopted differential feedback, where the correlation between consecutive beamforming reports is exploited. Each feedbackFigure 14: Closed loop spatial multiplexing with N antennas specifies only the incremental change between the current andand M layers previous matrices. But the down side of this is the error propagation effect.
  10. 10. MIMO Techniques in WiMAX and LTE: A Feature Overview 10 VI. SUMMARY OF FEATURESFeature LTE WiMAX Explanation APPENDIXCapacity +++ +++ Use of MIMO A. List of Terminologies: TechnologySpectral Use of Spatial AES Advanced Encryption StandardEfficiency Multiplexing, AMC Adaptive Modulation CodingUplink + ++ Beamforming. ARQ Automatic Repeat RequestDownlink +++ +++ OFDMA in downlink CP Cyclic Prefix in both. But LTE uses CQI Channel Quality Indicator SC-FDMA in uplink. CSI Channel State InformationMobility +++ ++ Use of Open-Loop DL Downlink techniques E-UTRA Evolved-UMTS Terrestrial Radio AccessReceiver ++ + MLD (higher HARQ Hybrid Automatic Repeat RequestComplexity computational MLD Maximum Likelihood Detector complexity) used in MMSE Minimum Mean Squared Error WiMax. MMSE in SM Spatial Multiplexing LTE. STBC Space Time Block CodePilot + ++ Precoded Pilots in STC Space Time CodingOverhead WiMAX and non- UL Uplink precoded RS used in ZF Zero Forcing Detector LTE CRS Common Reference SignalFeedback ++ +++ Differential feedback DRS Dedicated Reference Signal in 802.16m leads to lesser overhead compared to LTE B. IEEE802.16m and 3GPP-LTE TerminologiesPower ++ ++ Adaptive powerConsumption control.Note: + indicates a positive feature. More the + better it is. VII. CONCLUSION In this report we have provided an overview of the variousMIMO techniques that are implemented on the two standardsnamely IEEE 802.16m Mobile WiMAX and 3GPP LTE.Various MIMO schemes such as Open Loop, Closed Loop,Single and Multi User adopted in these two technologies havebeen studied and analyzed. From the case study we conclude that IEEE 802.16m(Mobile WiMAX) and 3GPP LTE are both capabletechnologies designed to meet the requirements of the nextgeneration (4G) mobile wireless communication system interms of data rates, mobility and spectral efficiency. Both aretechnically similar when it comes to employing MIMOtechniques. Both are based on the same fundamental elements,namely OFDMA modulation, use of smart antenna techniques,and flat all-IP networks. Although there are minor differencesin choices and approaches in the two standards, each havingsubtle technical advantages and disadvantages i.e. tradeoffs,performance wise they are both equally competent. However from a market perspective, the two technologies REFERENCESdiffer in terms of legacy and time to market. WiMAX has edge Primary Reference:over LTE due to its early deployments. LTE was designed Qinghua Li; Guangjie Li; Wookbong Lee; Moon-il Lee;with mobility in mind from start, while 802.16m evolved from Mazzarese, D.; Clerckx, B.; Zexian Li; , "MIMO techniques instandards based on fixed wireless networking.
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