LTE MIMO - AFRALTI · 2019. 9. 20. · signal power is maximized at the receiver input (based on the channel state information CSI) • Spatial Multiplexing: a high rate signal is

LTE MIMO

AGENDA

• MIMO Overview

• LTE MIMO Schemes

• Benefits of MIMO

• UE Considerations

• Antenna Configurations

MIMO BASICS • A set of multi-antenna transmission techniques

– Transmit (& Receive) diversity – Spatial multiplexing – Beamforming

• Available in all major wireless technologies – 802.11n (wifi) – WiMAX – HSPA Rel-6 & Rel-7 – LTE Rel-8 (Rel-9 & Rel-10)

• LTE vs 3G: – MIMO works better with high SINR – Thus OFDMA is well suited for MIMO (users are orthogonal in the cell); better suited than WCDMA

• LTE release 8 & 9 can go up to 4 X 4 MIMO (DL): but more antennas mean more overhead for the reference signals and more complex transmit/receiver systems

• Both downlink (DL) and uplink (UL): both with only one transmit antenna in the UE, the single-UE data rate cannot be increased in UL; but 2 UEs can be allocated orthogonal reference signals doubling the cell-level data rate (also called multi-user MIMO). Multi-user MIMO doesn’t increase UE complexity.

Multi-antenna Techniques • Conventional phased-array beamforming introduces phase and amplitude offsets to the

whole of the signal feeding each transmitting antenna. In LTE, the amplitude and phase of individual resource blocks can be adjusted, making beam steering far more flexible and user- specific.

• Receive diversity at the mobile device. Based on RSSI, the best antenna source is selected for signal reception.

• Transmit diversity : The signal to be transmitted is forwarded and sent over all antennas, the same signal that is sent on all transmit antennas reaches the receiver, and the combined signal level will be higher if only one transmit antenna was used, making it more interference resistant

– Using Space/Frequency Block Coding (SFBC) at the eNB. The transmitters send the same underlying user data, but in different parts of the RF frequency space.

– Using Cyclic Delay Diversity (CDD) at the eNB. CCD introduces deliberate delays between the antennas to create artificial multipath. Used in conjunction with spatial multiplexing.

• MIMO spatial multiplexing: Different signals are passed to different transmit antennas. • Adaptive MIMO Switching: This technique allows switching between transmit diversity and

spatial multiplexing based on the environment conditions

AGENDA

• MIMO Overview





Multi-antenna Schemes

Directivity Beamforming Gain

One signal transmitted in the

best directions based on channel Knowledge

Diversity Reduce Fading

One signal transmitted in all

directions

Multiplexing Capacity

Multiplication

Different signals transmitted in all

directions

Multi-antenna transmission • OFDM works particularly well with MIMO

– MIMO becomes difficult when there is time dispersion – OFDM sub-carriers are flat fading (no time dispersion)

• 3GPP supports one, two, or four transmit Antenna Ports • Multiple antenna ports Multiple time-frequency grids • Each antenna port defined by an associated Reference Signal

Multi-antenna transmisson

Single-antenna transmisson

Antenna port #4 Antenna port #3 Antenna port #2 Antenna port #1

The MIMO Family

How Does MIMO Work? • MIMO takes advantage of multi-path. • MIMO uses multiple antennas to send

multiple parallel signals (from transmitter).

• In an urban environment, these signals will bounce off trees, buildings, etc. and continue on their way to their destination (the receiver) but in different directions.

• “Multi-path” occurs when the different signals arrive at the receiver at various times.

• With MIMO, the receiving end uses an algorithm or special signal processing to sort out the multiple signals to produce one signal that has the originally transmitted data.

Improving vs Sharing SINR • Improving SINR

– Classical Beamforming: antenna array with phase adjustments to constructively add-up signals => improve average SINR

– Transmit Diversity : does not improve average SINR, but reduces variations in SINR

• Sharing SINR – Spatial Multiplexing : OFDMA is

better suited for MIMO (users are orthogonal in the cell) than WCDMA

C = log2 (1+SINR)

SINR: Signal to Interference plus Noise Ratio)

Beamforming – The Gains

• Specific phase adjustments are performed per antenna, for the same symbol

• The phase adjustments are such that the signals from the different antenna add-up constructively

• SINR improves with the number of antennas

• No gains near the cell-center (where the SINR is high), but gains at the edge translating into increased coverage

Spatial Multiplexing – The Gains

• Spatial Multiplexing increases the peak rate where the SINR is high (i.e. near cell center)

• Spatial Multiplexing decreases the peak rate where the SINR is low (i.e. near cell edge)

• The resulting trade-off is one between coverage and peak rate

Functions of MIMO • Precoding : spatial processing that occurs at the transmitter, such that the

signal power is maximized at the receiver input (based on the channel state information CSI)

• Spatial Multiplexing: a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel

• Diversity Coding techniques are used when there is no channel knowledge at the transmitter. The signal is emitted from each of the transmit antennas with full or near orthogonal coding.

• Spatial multiplexing can also be combined with precoding when the channel is known at the transmitter or combined with diversity coding

Taxonomy of Antenna Configurations

Source: 3GPP Technical Specification 36.300

Downlink Transmission Modes – LTE Rel 8

Mode Mode Notes AC1 Single-antenna port; port 0 This is the simplest mode of operation with no pre-coding.

AC2 Transmit Diversity Transmit diversity with two or four antenna ports using SFBC.

AC3 Open-loop spatial multiplexing This is an open loop mode with the possibility to do rank adaptation based on the RI feedback (i.e. no precoding feedback). In the case of rank = 1 transmit diversity is applied similarly to transmission mode 2. With higher rank spatial multiplexing with up to four layers with large delay, CDD is used.

AC4 Closed-loop spatial multiplexing This is a spatial multiplexing mode with pre-coding feedback supporting dynamic rank adaptation.

AC5 Multi-user MIMO Transmission mode for downlink MU-MIMO operation.

AC6 Closed-loop Rank = 1 pre-coding Closed loop pre-coding similar to transmission mode 5 without the possibility of spatial multiplexing, i.e. the rank is fixed to one.

AC7 Single-antenna port; port 5; This mode can be used in a beam forming operation when UE specific reference signals are in use.

DL-MIMO Schemes vs Antenna Types

DL-MIMO vs Physical Channels

UL-MIMO Schemes • Receive diversity at the eNB

• SU-MIMO for single UE

• MU-MIMO for multiple UE

MU-MIMO in Uplink

Codebook-based Pre-coding

Regular UE Feedbacks

• Channel Quality Indicator (CQI): maximum UE- recommended CQI with < 10% BLER

• Rank Indicator (RI): UE’s recommendation for the number of layers i.e. streams for spatial multiplexing – MIMO-specific

• Pre-coded Matrix Indicator (PMI): preferred precoding matrix – Closed Loop MIMO-specific

Pre-coding Matrix

• The signal is pre-coded at the eNodeB before transmission (i.e. multiplied by the a precoding matrix)

• Optimum precoding matrix is selected from predefined “codebook”

• Selection is based on UE feedbacks • In multicode case, there is one CQI per layer, and

the rate is adapted on each layer

Regular UE Feedbacks

Codebook index

Number of layers υ = 1 υ = 2

0 1 1

2 1 1 2 0

0 1

1 1 1

2 − -

2 1 1

2 j -

3 1 1

2 − j -

Single-User (SU) MIMO

Tx Rec

NR NT

SAME TIME AND FREQUENCY RESOURCES –Capacity gains due to multi-antenna at both ends – LTE Rel 8&9 supports 1x2, 2x2, 4x2, 4x4 – Requires a multipath environment

SEPARATE DATA STREAMS – Pre-coding is used to control/reduce the interference among spatial multiplexing data flows. – Spatial Multiplexing decorrelates antennas and transmission paths

TWO TYPES OF SPATIAL MULTIPLEXING –In the closed-loop mode, the eNodeB applies the precoding based on the precoding matrix indicator (PMI) reported by the UE – Spatial Multiplexing decorrelates antennas and transmission paths

Multi-User (MU) MIMO

Uplink MU-MIMO Multi single antenna UEs (reducing UE complexity and costs) are associated to transmit in the UL MU- MIMO mode.

The scheduler assigns the same resource to multiple Ues, and each UE transmits data by single antenna.

The eNodeB separates the data by the specific MIMO demodulation scheme.

The interference of the multi user data can be controlled by the scheduler.

Sche

dule

r

Pre-

code

r

User Data 1

User Data 2

S1

S2

User 1

User 2

Pre-coding Vector

Channel Information

Sche

dule

r

Deco

der

User Data 1

User Data 2

S1

S2

User 1

User 2

Channel Information

Downlink MU-MIMO MIMO is supported in LTE downlink to achieve spatial multiplexing, including single user mode SU- MIMO and multi user mode MU-MIMO.

In MU-MIMO, the data flows are scheduled to multi users and the resources are shared within users.

Multi user gain can be achieved by user scheduling in the spatial domain.

Pre-coding is used in both modes to control/reduce the interference among spatial multiplexing data flows.

LTE MIMO Schemes – REL 10

• Downlink Dual-layer Beamforming

• SU-MIMO – Up to 8-layers for DL and 4-layers for UL

• MU-MIMO – DL Improvments

• CoMP - Coordinated multi-point transmission/reception

COMP • DL COMP

– Coordinated scheduling and/or beamforming

– joint processing/transmission

• UL COMP COMP is a technology where antennas of multiple cell sites are utilized in a way such that the transmit/receive antennas of the serving cell as well as the neighboring cells can contribute in improving quality of the received signal at the UE/eNodeB, as well as in reducing the cochannel interferences from neighboring cells.

AGENDA

• MIMO Overview





Expected Throughput (Simplified)

The MIMO Throughput Calculator

DL Scheduling Block (SB) -> Bit calculation (Normal CyclicPrefix)

dlCyc lic Prefix = 15 KHz => 7 OFDM symbols Tx Diversity 2x2 MIMO Resourc e Elements (RE) per Resourc e Bloc k (7 OFDM symbols x 12 SubCarriers)

84 168

RE per SB 2 x RB

168 336

RS RE (per RB) 8 16 RS RE (per SB) 16 32 Control Region Size (CRS) in OFDM symbols nrOfSymbolsPdc c h

1 2 3 1 2 3

RE per CRS (OFDM*12 - 4 RS Tx) (OFDM*12 - 8 RS MIMO)

8 20 32 16 40 64

Tot Num RE per SB available for PDSCH (best case w/o SCH/BCH)

144 132 120 288 264 240

Bits per SB - QPSK (2) 288 264 240 576 528 480 Bits per SB - 16QAM (4) 576 528 480 1152 1056 960 Bits per SB - 64QAM (6) 864 792 720 1728 1584 1440 Max Theoretical L1 Thrpt (Mbps)

20 MHz => 100 RB (64 QAM) 86.4 79.2 72 172.8 158.4 144

15 MHz => 75 RB (64 QAM) 64.8 59.4 54 129.6 118.8 108

10 MHz => 50 RB (64 QAM) 43.2 39.6 36 86.4 79.2 72

5 MHz => 25 RB (64 QAM) 21.6 19.8 18 43.2 39.6 36

Tot Num RE per SB available for PDSCH (worst case with SCH/BCH in SB) SCH = 24, BCH = 4 x 12 - 4 per CW

76 64 52 152 128 104

Bits per SB - (QPSK) 152 128 104 304 256 208 Bits per SB - (16QAM) 304 256 208 608 512 416 Bits per SB - (64QAM) 456 384 312 912 768 624

MIMO RATE

SIMULATION RESULTS • N × L = N eNodeB transmit antennas and L UE receive

antennas Linear antenna arrays with antenna spacing 10 λ are assumed for eNodeB transmit antennas, where λ denotes the wavelength of the carrier frequency. interference-limited small urban macrocell environments carrier frequency is 2 GHz inter-site distance is 500 m bandwidth is 10MHz UE speed is 3 km/h The 2-tier cell layout with 57 cells in total was considered 10 users per cell receiver methods used:

•

•

• • • • •

• •

– – – –

maximal ratio combining (MRC) and interference rejection combining (IRC). MMSE and MMSE with successive interference cancellation (MMSE-SIC) receivers are used.

Source: MIMO Technologies in 3GPP LTE and LTE-Advanced EURASIP Journal onWireless Communications and Networking Volume 2009, Article ID 302092

AGENDA

• MIMO Overview





UE Considerations

• Size of the device

– 700 MHz: MIMO antennas very challenging

– <1 GHz MIMO antennas challenging

– >1 GHz: MIMO antennas relatively easy

• Battery Consumption

multi-antenna implementation requires dedicated components, resulting in increased battery consumption.

MIMO vs UE categorie

AGENDA

• MIMO Overview





ANTENNA CONFIGURATIONS

THANK YOU

Documents

LTE MIMO - AFRALTI · 2019. 9. 20. · signal power is maximized at the receiver input (based on the channel state information CSI) • Spatial Multiplexing: a high rate signal is