Long Term Evolution Technology training (Session 3) 1 Outline LTE TDD frame structure Downlink reference sequences Uplink transmission 2 Allocatable resources LTE radio resource = time-frequency chunk + MCS Resource Block (RB) = 12 carriers in one TS (12*15KHz x 0.5ms) 3
Time domain 1 frame = 10 sub-frames 1 subframe = 2 slots 1 slot = 7 (or 6) OFDM symbols Frequency domain 1 OFDM carrier = 15KHz Note: In LTE resource management is along three dimensions: Time, Frequency, Code Bandwidth flexibility LTE supports deployment from 6RBs to 110 RBs in 1 RB increments 6RBs = 6 x 12 x 15KHz = 1080KHz -> 1.4MHz (with guard band) 110RBs = 110 X 12 X 15KHz = 19800KHz -> 20MHz (with guard band) Typical deployment channel bandwidths: 1.4, 3, 5, 10, 15, 20 MHz Straight forward to support other channel bandwidths (due to OFDM)
UE needs to support up to the largest bandwidth (i.e. 20MHz) 4 LTE duplexing LTE may be: Time Division Duplexed (TDD) Frequency Division Duplex (FDD) Half Duplex Frequency Division Duplexing HD-FDD LTE supports different channel bandwidths
LTE supports frequencies from 450MHz to 3.8GHz LTE-A supports carrier aggregation 5 1.4, 3, 5, 10, 15 or 20 MHz in a signal channel Bandwidth flexibility facilitates deployment across wide spectrum range Multiple channels may be aggregated in data delivery Duplexing schemes in LTE Use of TDD
Unpaired spectrum allocation Imbalance between DL and UL traffic 3GPP TDD bands 6 KSA TDD bands Reported by Ericsson (annual report 2009) Time domain structure Two time domain structures Type 1: used for FDD transmission (may be full duplex or half duplex) Type 2: used for TDD transmission Both Type 1 and Type 2 are based on 10ms radio frame
Radio frame : Type 1 Radio frame : Type 2 7 TDD frame configurations Different configurations allow balancing between DL and UL capacity Allocation is semi-static Adjacent cells have same allocation Transition DL->UL happens in the second
subframe of each halfframe Note 1: By convention special sub-frame belongs to DL Note 2: TDD frame structure allows co-existence between LTE TDD and TD-SCDMA 8 Example DL throughput calculation in TDD Estimate PHY throughput range of DL in a TDD system using 15MHz of spectrum, TDD configuration 4:1 SISO antenna system and normal CP 15MHz = 75 resource blocks = 75 * 12 = 900 sub-carriers In 4:1 TDD there are 6 downlink sub-frames per frame For normal CP there are 14 OFDM symbols perf subframe Number of resource elements in a frame: 6*14*900 = 75,600 Lower bound: All resource elements are carrying QPSK (2 bits/symbol)
Date rate: 75,600*2 bits / 10ms = 15.12 Mbps Upper bound: All resource elements are carrying 64QAM (6 bits/symbol) Date rate: 75,600*6 bits / 10ms = 45.36 Mbps Note: Both lower and upper bounds may be slightly higher is special sub-frame is used for data transmission 9 Review questions 10
What is the smallest bandwidth that may be allocated on DL? How many different MCS schemes may be used on the DL? Can LTE be deployed in 1MHz of spectrum? Can LTE TDD be deployed in 10MHz of spectrum? How many different TDD configurations are standardized? What is the reason for using TDD? Consider 10MHz TDD deployment in 7:3 frame configuration. Estimate aggregate DL throughput range for SISO configuration and extended CP. Channel state information LTE implements radio resource management based on UE feedback UE measures DL channel properties using reference signals Sends information back to eNode B The information is referred to Channel State Information (CSI) CSI contains variety of information (Power, CINR, Rank, PMI, ) eNode B sends a reference
sequence Mobile estimates the channel by listening to the reference sequence 11 Process of channel estimation eNode B sends a known reference sequence Channel distorts the sequence The mobile compares known sequence against the distorted version it receives Based on the difference it calculates and sends back eh CSI SISO / MIMO eNodeB mandatory support for 4 antennas (Ntx DL / Nrx UL = 4)
UE mandatory support for 2 antennas (Ntx UL /Nrx UL = 2) UE - optional support for 4 antennas. When multiple antennas are used multiple wireless channels are created CSI needs to be estimated for each of the channels Separate reference (i.e. training) sequence is required for each channel Note: under favorable propagation condition MIMO increases throughput by a factor min(Ntx, Nrx) 12
Different TX/RX configurations supported in LTE In LTE terminology antenna = port Downlink reference signals For coherent demodulation terminal needs channel estimate for each subcarrier Reference signals used for channel estimation There are three type of reference signals 1. Cell specific DL reference signals Every DL subframe Across entire DL bandwidth 2. UE specific DL reference signals
Sent only on DL shared channel Intended for individual UEs 3. MBSFN reference signals Support multicast/broadcast Note: Reference signals are staggered in time and frequency. This allows UE to perform 2-D complex interpolation of channel timefrequency response 13 Cell specific reference signals Two port TX DL transmission may use up to four antennas Each antenna port has its own pattern of reference signals Reference signals are transmitted at higher power in multiantenna case Reference signals introduce overhead
4.8% for 1 antenna port 9.5% for 2 antenna ports 14.3 % for 4 antenna ports Four port TX Reference symbols vary from position to position and from cell to cell cell specific 2 dimensional sequence Period of the sequence is one frame One port TX 14
Cell specific reference signals (2) There are 504 different Reference Sequences (RS) They are linked to PHY-layer cell identities The sequence may be shifted in frequency domain 6 possible shifts Each shift is associated with 84 different cell identities (6 x 84 = 504) Shifts are introduced to avoid collision between RS of adjacent cells In case of multiple antenna ports only three shifts are useful For a given PHY Cell ID - sequence is the same regardless of the bandwidth used UE can demodulate middle RBs in the same way for all channel bandwidths Shifts for single port transmission 15
UE Specific RS UE specific RS used for beam forming Provided in addition to cell specific RS Sent over resource block allocated for DL-SCH (applicable only for data transmission) Note: additional reference signals increase overhead. One of the most beneficial use of beam forming is at the cell edge improves SNR 16 Review questions
17 What is MIMO transmission? How many antenna ports are supported by eNode B? If the transmission is in 4 by 2 MIMO mode, what is the maximum increase in data rate? What are three types of reference sequences? Consider 10MHz TDD deployment in 7:3 frame configuration. Estimate aggregate DL throughput range for 2 by 2 MIMO configuration and extended CP. Part 4 LTE UPLINK TRANSMISSION Peak to Average Power OFDM simple addition of multiple
independently modulated carriers OFDM vary high Peak to Average Power Ratio (PAPR) To make sure that TX amplifier is working in linear region OFDM waveform requires large back off Large back off = poor power efficiency To improve PAPR UL uses DFT spread OFDM Note: If uncompensated, PAPR for OFDM is 10log10(Nsc), where Nsc = number of subcarriers 19 BW (MHz) 1.4 5 10 15
Nsc 72 300 600 900 10log10(Nsc) 18.6 24.8 27.8 29.5 Note: very high PAPRs occur with very low probability DFTS-OFDM
DFTS-OFDM = DFT Spread OFDM Also known as s Single Carrier FDMA (SC-FDMA) Used on RL of LTE Advantages: Lower PAPR than OFDM (4dB for QPSK and 2dB for 16-QAM) Orthogonality between the users in the same cell Low complexity TX/RX due to DFT/FFT Disadvantage: Needs an equalizer at the Node B RX
Need for some synchronization in time domain Outline of the DFTS-OFDM 20 Note: In DFTS-OFDM, M < N DFTS-OFDM TX/RX chain Note: the TX/RX of DFTS-OFDM is almost the same as OFDM. The DFT precoding / decoding and equalization are done in software 21 Uplink user multiplexing Two ways of mapping the output of the DFT Consecutive carriers: Localized DTFS-OFDM Distributed carriers: Distributed DTFS-OFDM Distributed OFDM has benefit of frequency diversity
Note 1: Mapping between output of the OFDM and carriers is performed by MAC scheduler Note 2: Spectrum bandwidth may be allocated in dynamic fashion Localized DFTS-OFDM 22 Distributed DFTS-OFDM Uplink frame format Need for two different CP: 1. To accommodate environments with large channel dispersion 2. To accommodate MBSFN (MultiCast Broadcast Single Frequency Network) transmission Note: UL and DL frame formats are identical TCP: 160Ts (5.1us) for first symbol, 144Ts (4.7us) for other six symbols
TCP-e: 512 Ts (16.7 us) for all symbols 23 Modulation and coding on the UL Categories supported in LTE Rel 8 LTE defines UE categories Higher category = better performance Categories differ by TX power, Supported MCS Supported MIMO modes Example: iPhone 6 features a Qualcomm MDM9625M LTE Category 4 modem that can offer peak download speeds of up to 150Mbps and peak upload speeds of up to 50Mbps in
20MHz LTE. 24 Example UL throughput calculation in TDD Estimate PHY throughput range of UL in a TDD system using 15MHz of spectrum, TDD configuration 4:1 SISO antenna system and normal CP. The UE is category 4. 15MHz = 75 resource blocks = 75 * 12 = 900 sub-carriers In 4:1 TDD there are 2 downlink sub-frames per frame For normal CP there are 14 OFDM symbols perf subframe Number of resource elements in a frame: 2*14*900 = 25,200 Lower bound: All resource elements are carrying QPSK (2 bits/symbol) Date rate: 25,200*2 bits / 10ms = 5.04Mbps Upper bound:
All resource elements are carrying 16QAM (4 bits/symbol) Date rate: 25,200*4 bits / 10ms = 10.08 Mbps Note: Both lower and upper bounds are slightly lower due to overhead 25 Uplink reference signals (1) Used for uplink channel estimation Two types of sequences Data demodulation Reference Signal (DM-RS) Sounding Reference Signal (SRS) DM-RS
Sent on each slot transmission to help demodulate data Occupies center part of the slot transmission (symbols 4) in both transmission slots Use same bandwidth as the UL data (multiples of 12 carrier RBs) Properties of DM-RS sequences 26 Small power variations in frequency domain Small power variations in time domain Uplink reference signals (2) SRS
Allow network to estimate channel quality across entire band Used by MAC scheduler to perform frequency dependent scheduling Optional implementation UE can be configured to send SRS sequence at time intervals from 2ms to 160ms Two modes of operation 27 Wideband SRS UE send the sequence across the entire spectrum Hopping SRS UE sends narrowband sequence that hops across different parts of the spectrum
Review questions 28 What is the PAPR? What is the UE category? iPhone 6s is a category 5 device. What is its maximum supported data rate on DL? How about UL? Consider 10MHz TDD deployment in 7:3 frame configuration. Estimate aggregate UL throughput range for 2 by 2 MIMO configuration and extended CP. What are two types of reference sequences used on the UL?
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