LTE Course

OFDMAND OFDMA TECHNOLOGIES OUTLINE NEED FOR MULTI-CARRIER OFDM ENTERS INTO THE PICTURE FFT / IFFT GUARD TIME INSERTION OFDM DRAWBACKS CHANNEL ESTIMATION OFDM BLOCK DIAGRAM SIMULATION RESULTS NEED FOR MULTI-CARRIERTime Domain AnalysisNEED FOR MULTI-CARRIER cont.Pulse completely distorted. ISI is significant in this case.Pulse extended but the extension are much smaller than T the output behaves like thetransmitted rectangular pulse.NEED FOR MULTI-CARRIER cont.Frequency Domain AnalysisNEED FOR MULTI-CARRIER cont. Conclusion Wide pulses is needed for simple equalization, But Narrow pulses is needed for high data rate SolutionMultiplexing NEED FOR MULTI-CARRIER cont.NEED FOR MULTI-CARRIER cont.ProblemSolution OrthogonalityNEED FOR MULTI-CARRIER cont.NEED FOR MULTI-CARRIER cont.OFDM ENTERS INTO THE PICTUREInterference OrthogonalityB.W efficiency Min SeparationOFDM ENTERS INTO THE PICTURE cont. Min Separation Problem Difficult Implementation with traditional oscillators Solution DFT But DFT needs high processing Solution Easy implementation using FFT/IFFT FFT / IFFTIFFTDAC Channel ADCFFT FFT/IFFTGUARD TIME INSERTIONX1X2.Xn. Y1Y2.Yn. Channel Filteringhv.h1h0 GUARD TIME INSERTION cont. X1X2.Xn. Y1Y2Yv+1YnYv+2.Yv X1X2.Xn. .ProblemISI occurshv.h1h0 GUARD TIME INSERTION cont. X1X2.Xn Xn-v+1 Xn. Y1Y2Yv+1YnYv+2.Yv X1Xn. .Solution Cyclic Prefix No ISI Circular Convolution achieved.hv.h1h0 Cyclic prefix The CP allows the receiver to absorb much more efficiently the delay spread due to the multipath and to maintain frequency Orthogonality. The CP that occupies a duration called the Guard Time (GT), often denoted TG, is a temporal redundancy that must be taken into account in data rate computations. OFDM DRAWBACKS cont. Peak to Average Power Ratio (PAPR) OFDM DRAWBACKS cont. Sensitivity to frequency offset Pilot Signal Extraction Lowpass FIR Filter PilotSignal Estimation CHANNEL ESTIMATION Pilot Based Channel Estimation Received Signalafter FFTEstimated Channel ResponseKnown PilotsCHANNEL ESTIMATION cont. Frequency( sub carriers) Data symbolsPilot symbolsTime (OFDM Symbols) Time (OFDM Symbols)Frequency( sub carriers) Pilot Arrangement Types Block Pilot PatternsComb Pilot Patterns High channel frequency selectivityrapid changing channelsOFDMA OFDMA is a multiple access method based on OFDM signaling that allows simultaneous transmissions to and from several users along with the other advantages of OFDM. OFDM versus OFDMA IEEE802.16d IEEE802.16e Fixed WiMAX,256-OFDMMobile WiMAXDIVERSITY AND MIMO PRINCIPLESWhat is diversity?Is a technique that combats the fading by ensuring that there will be many copies of the transmitted signal effected with different fading over time, frequency or space. Diversity typesTime diversityFrequency diversitySpatial diversity1- Time diversity: We averaging the fading of the channel over time by using :1-The channel coding and interleaving.2-Or sending the data at different times.to explain this we will see an example: 1-time diversity:

No interleaving x1 x2 x3 x4y1 y2 y3 y4z1 z2 z3 z4 h1 h2 h3 h4 interleaving x1 y1 z1 h1 x2 y2 z2 h2x3 y3 z3 h3x4 y4 z4 h4 So we can see that only the 3rd symbol from each codeword lost and we can recover them from the rest symbols in each codeword. |H(t)|t2- frequency diversity:This type of diversity used for the frequency selective channels as we will averaging the fading over the frequency by using:1-Multi-carrier technique like OFDM.2-FHSS (frequency hope spread spectrum).3-DSSS (direct sequence spread spectrum). 2- frequency diversity:We can see that each sub-band will effecting with different fading over the frequency.3-spatial diversity:we will have many copies of the transmitted signal effects with different fading over the space .we use multi-antenna systems at the transmitter or the receiver or at both of them. Spatial diversityMISO SIMO MIMOMIMO-MUReceive diversity:1-The receiver will has many antennas .2-Each one has signal effecting with different fading.3-number of different paths =Mr.Diversity order=MrMIMO:In this type we use multi antennas at both the transmitter and receiver as shown.Diversity order=Mt x MrNotes:The higher diversity order we have the better we combat the fadingNotes:1-The diversity reduces the BER of the communication system.2-Diversity orderBER .Notes:The distance between the antennas must be larger than the coherent distance to ensure that data streams are not correlated .Question?How the receiver get the signal from the many copies reached ?Answer Diversity combining techniquesSelective combining SCMaximal ratio combining MRCEqual gain combining EGCDiversity combining technique1-Combines the independent fading paths signals to obtain a signal that passed through a standard demodulator.2-The techniques can be applied to any type of diversity.3-combining techniques are linear as the output of is a weighted sum of the different fading signals of branches.4-It needs co-phasing.Diversity combining techniqueThe signal output from the combiner is the transmitted signal s(t) multiplied by a random complex amplitude term Random SNR from the combinerFading of the pathType of techniqueDiversity orderDiversity combining techniqueTypes of combining techniques Selection combining Threshold combining Maximal ratio combining Equal gain combining selection combining technique1-the combiner outputs the signal on the branch with thehighest SNR.2-no need here for the co-phasing.0 0 0 1Threshold combining techniqueAs in SC since only one branch output is used at a time and outputting the first signal with SNR above a given thresholdso that co-phasing is not required.Special case at diversity order =2 (SSC)Does not take the largest SNR so that its performance less than the SC technique.Maximal ratiocombiningIn maximal ratio combining (MRC) the output is a weighted sum of all branches due to its SNRh1* h2* h3* hi*Equal gain combining techniqueA simpler technique is equal-gain combining, which co-phases the signals on each branch and then combines them with equal weightingMIMO Traditional diversity is based on multiple receiver antennas Multiple-In Multiple-Out (MIMO) is based on both transmit and receive diversity Also known as Space Time Coding (STC) With Mt transmission antennas and Mr receiver antennas we have Mt Mr branches Tx and Rx processing is performed over space (antennas) and time (successive symbols)47MIMO or STC In Mobile communication systems it may be difficult to put many antennas in the mobile unit Diversity in the downlink (from base station to mobile station) can be achieved by Multiple-In Single-Out (MISO) (i.e., Mr=1) In the uplink (from mobile station to base station) diversity is achieved my conventional diversity (SIMO) Hence, all diversity cost is moved to the base station All 3G and 4G mobile communication system employ MIMO in their standard 48Type of MIMO Two major types of space time coding Space time block coding (STBC) Space time trellis coding (STTC) STBC is simpler by STTC can provide better performance STBC is used in mobile communications. STTC is not used in any systems yet We will talk only about STBC 49Space Time Block Codes There are few major types Transmit diversity: main goal is diversity gain Spatial multiplexing: main goal is increase data rate Eigen steering: main goal is both. Requires knowledge of the channel at the transmitter side Mix of the above: Lots of research Transmit diversity, spatial multiplexing and simplified version of Eigen steering are used in 3G and 4G standards While in 3G standards MIMO was an enhancement, in 4G MIMO is a main part 50Transmit Diversity Take Mt=2 and Mr=1 Two symbols so and s1 are transmitted over two transmission periods No change in data rate (denoted as rate 1 STBC) Channel is known at receiver only 51Transmit Diversity Transmission matrix: Transmission matrix columns are orthogonal to guarantee simple linear processing at the receiver Other transmission matrices are defined in literature Received signal is: Performance is same as MRC with M=2 However, if Tx Power is the same, then transmit diversity (2x1) is 3 dB worse than (1x2) 521* *1 1 1 1o o o oos s r g nRs s r g n ((((= = + (((( 11* *1 1oAnt Anto oos s TimeSs s Time (=( Transmit Diversity Take Mt=2 and Mr=2 Performance is the same as MRC with M=4 However, if Tx Power is the same, then transmit diversity (2x2) is 3 dB worse than (1x4) 53Performance MRRC=Maximal Ratio Receiver Combining Note 3 dB difference in favor of Rx MRC diversity Reference: S. Alamouti, a simple transmit diversity technique for wireless communications,IEEE JSAC, October 98 54Order 2Order 4No diversitySpatial Multiplexing Purpose is to increase data rate (2x2 gives twice data rate) The 4 gains must be known at receiver Simplest way zero forcing algorithm: 551 1 o o or s g s g = +1 2 1 3 or s g s g = +12 3 1 1o o oGg g r sg g r s (((=((( 11 1o o H Hs rG G Gs r (( ( = (( Spatial Multiplexing Optimum method: Maximum Likelihood Try all combinations of s1 and s2

Find the combination that minimizes the squared error: Complexity increases with high order modulation 562 22 21 1 1 1 2 1 3 o o o o oe e r s g s g r s g s g + = + 1 1 o o or s g s g = +1 2 1 3 or s g s g = +Performance Equal rate comparison Reference: David Gesbert, Mansoor Shafi, Da-shan Shiu, Peter J. Smith, and Ayman Naguib, From theory to practice: an overview of MIMO spacetime coded wireless systems, IEEE JSAC, April 2003 57 Zero forcing ML Alamouti Eigenvalue Steering Assume a MIMO system 58Eigenvalue Steering Example with Mt = 2 and Mr=4 Any matrix H can be represented using Singular Value Decomposition as U is Mr by Mr and V is Mt by Mt unitary matrices E is Mr by Mt diagonal matrix, elements i 59|| | || |y H x n = + 1 11 12 12 21 22 1 23 31 32 2 34 41 42 4Hy h h ny h h x ny h h x ny h h n ((( ((( ( (((= + ( ((( ((( HH U V = EEigenvalue Steering Using transmit pre-coding and receiver shaping60( )( )( )HH HH HH H Hy U H x nU U V x nU U V Vx nU U V Vx U nx n= += E += E += E += E +Eigenvalue Steering This way we created r paths between the Tx and specific Rx without any cross interference The channel (i.e., Channel State Information) must be known to both transmitter and receiver The value of r = rank of matrix H,r smin(Mt, Mr) Not all r paths have good SNR Data rate can increase by factor r See Appendix C for Singular Value Decomposition See Matlab function [U,S,V] = svd(X) 61Example Reference: Sanjiv Nanda, Rod Walton, John Ketchum, Mark Wallace, and Steven Howard, A high-performance MIMO OFDM wireless LAN, IEEE Communication Magazine, February 2005

62INTRODUCTION TO LTE AND ITS UNIQUETECHNOLOGIES. What is LTE?? The 3GPP LTE is acronym for long term evolution of UMTS . In order to ensure the competitiveness of UMTS for the next 10 years and beyond, concepts for UMTS Long Term Evolution (LTE) have been introduced in 3GPP release 8. LTE is also referred to as EUTRA (Evolved UMTS Terrestrial Radio Access) or E-UTRAN (Evolved UMTS Terrestrial Radio Access Network) What is LTE(cont.)? The architecture that will result from this work is called EPS (Evolved Packet System) and comprehends E-UTRAN (Evolved UTRAN) on the access side and EPC (Evolved Packet Core) on the core side. Can be considered thereal 3.9G & invited to join the 4G family. Also considered a competitivesystem to mobile WiMAX as we will show What is LTE (cont.)?LTE DESIGN TARGETS- (a) capabilities: Scalable BW: 1.25, 2.5, 5.0, 10.0 and 20.0 MHz. Peak data rate: Downlink (2 Ch MIMO) peak rate of 100 Mbps in 20 MHz channel Uplink (single Ch Tx) peak rate of 50 Mbps in 20 MHz channel Supported antenna configurations: Downlink: 4x4,4x2, 2x2, 1x2, 1x1 Uplink: 1x2, 1x1 Duplexing modes: FDD and TDD Number of active mobile terminals: LTE should support at least 200 mobile terminals in the active state when operating in 5 MHz. In wider allocations than 5 MHz, at least 400 terminals should be supported Spectrum efficiency Downlink: 3 to 4 x HSDPA Rel. 65bits/s/Hz Uplink: 2 to 3 x HSUPA Rel. 62.5bits/s/hz Latency C-plane: