LTE MIMO Schemes
Benefits of MIMO
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 doesnt 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
LTE MIMO Schemes
Benefits of MIMO
Directivity Beamforming Gain
One signal transmitted in the
best directions based on channel Knowledge
Diversity Reduce Fading
One signal transmitted in all
Different signals transmitted in all
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
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
Regular UE Feedbacks
Channel Quality Indicator (CQI): maximum UE- recommended CQI with < 10% BLER
Rank Indicator (RI): UEs recommendation for the number of layers i.e. streams for spatial multiplexing MIMO-specific
Pre-coded Matrix Indicator (PMI): preferred pre- coding matrix Closed Loop MIMO-specific
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
Number of layers = 1 = 2
0 1 1 2
1 1 2 0
1 1 1 2
2 1 1 2 j
3 1 1 2 j
Single-User (SU) MIMO
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.
User Data 1
User Data 2
User Data 1
User Data 2
Downlink MU-MIMO MIMO is supported