3G Evolution Outline Chapter: 5 p 5 • Why wider-band single carrier transmission? • Equalization against radio- channel frequency selectivity – Time domain linear equalization – Frequency domain equalization Wider-band single-carrier Wider band single carrier – Other equalizer strategies transmission • Uplink FDMA with flexible bandwidth assignment • DFT- spread OFDM Payam Amani – Basic principles – DFTS-OFDM receiver DFTS OFDM i Payam.Amani@eit.lth.se P A i@ it lth – User multiplexing with DFTS-OFDM Department of Electrical and Information Technology – Distributed DFTS-OFDM 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 1 1 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 2 2 Equalization against radio- channel frequency Why wider-band single carrier transmission selectivity selectivity • OFDM • Equalization: main method to handle signal corruption due to radio channel frequency selectivity channel frequency selectivity Advantages: – Provides overal very high transmission bandwidth. • Time domain linear equalization q – Robust to signal corruption due to radio channel frequency selectivity. g p q y y – Rake receiver in DS-CDMA Drawbacks: • Channel mached filtering with filter response as the complex conjugate of the – Large variations in the instantaneous power of transmitted signal. time reversed channel impulse response. p p • Reduced power amplifier efficiency • Also called Maximum Ratio Combining (MRC) • High power amplifier cost • Critical for uplink τ = h − τ w ( ) * ( ) – Some methods to reduce this power variations discussed in chapter 4. Some methods to reduce this power variations discussed in chapter 4 • Limitations on the amount of reduction in these variations. • Maximizes post filter signal to noise ratio • Significant computational complexity and/or reduced link performance. • No compensation for radio channel frequency selectivity • wider-band single carrier transmission as an alternative for multicarrier n ( t ) Channel Model transmission especially for Uplink. Receiver ˆ t Transmitter s ( t ) s ( ) r ( t ) ( τ ( τ w ) h ) 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 3 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 4
Time domain linear equalization MMSE Equalization – Zero Forcing (ZF) equalizer: • Time domain linear equalization – Select the receiver filter to fully compensate for the radio channel frequency – MMSE Equalization selectivity. • A trade-off between signal corruption due to radio channel frequency selectivity τ ⊗ τ = h ( ) w ( ) 1 and noise/ interference. { { } } ε = − 2 E s ˆ ( ( t ) ) s ( ( t ) ) – Denotes linear convolution. Denotes linear convolution ⊗ • Select a filter to minimize the mean squared error between the equalizer output – Suppression of any signal corruption caused by radio channel frequency and the transmitted signal. selectivity. selectivity – Large or potentially very large increase in noise level after equalization. – Linear equalization implemented as time discrete FIR filter. q p – Overal degradation of the link performance. – Especially for channel with large variations in frequency response. 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 5 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 6 Time domain linear equalization – Complexity of such time discrete equalizer grows relatively rapidly equalizer grows relatively rapidly with bandwidth of the sigal to be – More wideband signal leads to a nT equalized: s correspondingly higher sampling r ( t ) nT s ˆ t ( ( ) ) rate for the received signal. rate for the recei ed signal s w w r ( t ) s ˆ t ( ) Filter processing shall run with w higher sampling rates. – More wide band signal is subject to relatively more radio channel t l ti l di h l } } selectivity r r n n – High complexity in the w w w w w w 0 − 1 L 1 − – Requires the equalizer to have a 0 equalization and also calculating 1 L 1 larger span to be able to g p the inverse of channel output p s s ˆ compensate it. autocorrelation matrix. n s ˆ 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 7 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 8
Frequency domain equalization Frequency domain equalization • Reduce complexity of equalization. • Time domain filtering implements a time discrete implements a time discrete • Carried out block wise in block size linear convolution. nT nT N. ˆ ˆ s s r ( t ) s r ˆ r ( t ) s r ˆ R S R S n n n n k k k k • Frequency domain filtering • Equalization as frequency domain corresponds to circular filtering. { { convolution in the time domain convolution in the time domain. • Block size N preferably selected as for some integer n to = N 2 n • First L-1 samples at the output w w allow for computational efficient p 0 0 of the frequency domain radix-2 FFT/IFFT implementation of DFT/IDFT. equalizer will not be identical to ˆ ˆ s s R R 0 0 0 0 the corresponding output of the p g p w w − − N 1 N 1 • For channels with extensive time domain equalizer. frequency selectivity frequency R R ˆ ˆ s s − − N 1 N 1 domain equalization is less − − N 1 N 1 complex. complex 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 9 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 10 Frequency domain equalization Frequency domain equalization • Cyclic prefix � channel will seem to the receiver as circular convolution • Overlap of at least L-1 samples. O l f t l t L 1 l over a receiver processing block of size N. • Discard first L-1 samples at Discard first L 1 samples at • Frequency domain taps can be output of the frequency domain x ( t ) calculated directly. equalizer as they are also ≤ L − 1 provided as the last part of the provided as the last part of the • Estimate of MMSE equalizer in previously received / equalized frequency domain. block. • Overhead in power and bandwidth. • Computational overhead or Transmitter + + * H • Less overhead � increase block higher receiver complexity. higher receiver complexity. = Single carrier signal D/A W W K CP Insertion x ( t ) generation generation Conversion Conversion size : channel shall be constant in 2 2 + K + + + H N duration of a block size ; upper limit K 0 N Samples N+Ncp Samples for the block size • Cyclic prefix insertion. 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 11 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 12
Uplink FDMA with flexible bandwidth Other equalizer strategies assignment assignment • Share uplink radio resource using plink intra-cell multiple access scheme. • D Decision feedback equalization (DFE) i i f db k li ti (DFE) Frequency cy – Previously detected symbols are fed back and used to cancel the • High rate packet data transmission � Assign the entire system bandwidth to a contribution of the corresponding transmitted symbols to the overal signal terminal. terminal corruption. Used in combination with time domain linear filtering. Also used in combination with frequency domain linear equalization. • Burstiness of most packet data Frequency services � in most cases mobile terminals do not have anything to send • Minimum Likelihood (ML) Detection or Maximum Likelihood Sequence in uplink. TDMA required. Estimator (MLSE) – Uses the entire received signal to decide on the most likely transmitted Uses the entire received signal to decide on the most likely transmitted • • Just TDMA is not bandwidth efficient Just TDMA is not bandwidth efficient. sequence, taking into account the impact of time dispersion on the signal. • Uplink is power limited. – Viterbi Algorithm – Used widely in 2G but too complex for 3G evolution (much wider • Allocating the entire system bandwidth to transmission bandwidth, much more channel frequency selectivity, much one terminal is inefficient in terms of higher sampling rate ) bandwidth utilization. 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 13 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 14 DFT Spread OFDM Basic principles • Small variations in the instantaneous power of the transmitted signal. • Small variations in the instantaneous power of the transmitted signal. • Possibility of low complexity and high quality equalization in frequency domain. • PAR distribution is independent of modulation in OFDM. • Cubic metric: a measure of the additional back off required for a certain Cubic metric: a measure of the additional back off required for a certain • Possibility for FDMA with flexible bandwidth assignment. signal wave form relative to the back off needed for some refrence wave form. • Uplink transmission scheme for LTE Uplink transmission scheme for LTE. Transmitter 0 0 a , a ,..., a Size -M OFDM D/A − 0 1 M 1 CP x ( t ) DFT (IDFT) Conversion M 0 N N 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 15 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 16
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