MIMO Features In WiMAX and LTE

ET4275 :Advanced Topics in Digital Wireless
Communications: Case Study
Technological University of Delft

MIMO Features in WiMAX
and LTE: An Overview
PRESENTED BY :

ANANTHAKRISHNAN RAMKUMAR - 4119568
&
PRAVEEN KALYANASUNDARAM - 4118863

January 2011

Trends in Mobile Communications

Property of Praveen Kalyansundaram & Ananthakrishnan Ramkumar

4G

Higher data rates, mobility, spectral efficiency and better
cell coverage compared to 3G.
High quality of service for next generation multimedia
support (real time audio, high speed data, HDTV video
content, mobile TV, etc.)
High usability: anytime, anywhere, and interoperability
with existing wireless standards
Based on an all-IP packet switched network.
Two technologies that are expected to fulfill the 4G
requirements are: Mobile WiMAX and 3GPP LTE


Mobile WiMAX (IEEE 802.16m)

IEEE 802.16m known as Mobile WiMAX is an extension of IEEE
802.16-2004 (fixed WiMAX). Mobile WiMAX introduces new features
to support enhanced Quality of Service to provide high mobility at very
high data rates.
Rapidly growing broadband wireless access technology that replaces
the current existing systems such as Wi-Fi and 3G.
Key features are :
Use of MIMO Technology
OFDMA with CP (both uplink and downlink)
Scalable bandwidth
Adaptive Modulation and Coding
Support for TDD and FDD
Hybrid-ARQ, Integrated Security for voice and data transmission


Long Term Evolution(3GPP-LTE)

3GPP Long Term Evolution (LTE)-developed by the 3rd Generation
Partnership Project is the latest standard in the mobile network
technology targeting 4G.
LTE-Advanced (Release 10) is still being developed primarily to meet
the requirements of the International Telecommunications Union
(ITU) for IMT fourth generation (4G).
Key Features
Use of MIMO Technology
OFDMA with CP in downlink, SC-FDMA in uplink
Scalable bandwidth
Adaptive Modulation and Coding
Support for TDD and FDD
Hybrid-ARQ, Integrated Security for voice and data transmission


Table of features
Aspect WiMAX LTE
Legacy GSM/GPRS/EDGE/UMTS/HSPA IEEE 802.16 a through e
Access Technology DL-OFDMA, UL-SC-FDMA DL-OFDMA, UL-OFDMA
Antenna Scheme MIMO: MIMO
DL 2x2,4x2,4x4 2x2,4x2,4x4
UL 1x2,1x4, 2x2 1x2,1x4,2x2 ,
Frequency Band Existing (800,900,1800,1900 MHz) and 2-11 GHz
new bands (Range 800 MHz to 2.62
GHz)
Mobility: Up to 350 Km/h with inter-cell soft Up to 120 Km/h with optimized hard
handovers handovers
Channel Bandwidth Scalable from 1.25 to 20 MHz with Scalable from 1.25 to 20 MHz with
system profiles 1.25,1.4,2.5,3.5,10, 15 system profiles 1.25,2.5, 5, 10and 20
and 20 MHz MHz
DL Spectral Efficiency 1.57 bps/Hz/Sector (2x2) MIMO2 1.59 bps/Hz/Sector (2x2) MIMO

UL Spectral Efficiency 0.64 bps/Hz/Sector (1x2) SIMO2 0.99 bps/Hz/Sector (1x2) SIMO
Radio Access Mode TDD and FDD TDD and FDD


MIMO Technology: An Introduction

• A MIMO is a technique to improve communication performance by
using multiple antennas at the transmitter and receiver
Features:
• Exploits multipath effects instead of mitigating it
• High spectral efficiency
• High speed data transmission
• Higher Channel capacity (i.e., min (M,N))
•Diversity in both the transmit and receive sides

Channel
011001 - - 011001
Matrix ‘H’
- -
- -

‘N’ Transmitters ‘M’ Receivers


Classification of MIMO Techniques


Spatial Multiplexing

A high data rate stream is split into several low data rate streams where each of
the individual streams is transmitted by a different transmit antenna in the
same frequency channel.
At the receiver the data streams can be separated by the equalizer provided
each of the data stream has undergone fading independent of each other.

This technique is used to increase the overall capacity of the channel. In a
system with N transmit antennas and M receive antennas the maximum spatial
multiplexing order is given by Ns = min (M,N) where Ns is the number of
streams which can be transmitted in parallel


Transmit Diversity

Improvement of link reliability by adding more redundancy to information
going through the channel.
Space Time block Code (STBC) or Alamouti Code: the data stream to be
transmitted is encoded in blocks, which are distributed among spaced antennas
and across time.
The frequency domain version of STBC is called Space Frequency Block Code
(SFBC).Pair of adjacent subcarriers are coded together instead of time slots.


Beamforming

Signal processing technique that takes advantage of the fading channels
to improve received signal gain and coverage.
Transmission radiation pattern from an array of antennas is focused in
the direction of specific user by constructively interfering in that
specific direction.
Using feedback from the receiver, a precoder matrix is constructed to
achieve beamforming


Pilot/Reference Signal

Used for measuring the spatial channel and helps in coherent demodulation at the
terminal. Perform the operation of supervision, control, equalization,
synchronization within a transmission system.
The reference signals can be classified as
Common reference signal (CRS):used by all UEs within a cell.
Dedicated reference signal (DRS): used by a specific UE within the cell
Further classified as precoded or non-precoded. If the pilot/RS is also multiplied by
the precoding matrix before transmission then it is called precoded pilot.
In WiMAX and LTE specific sub-carriers are allocated as pilots/RS. Their location is
changed from symbol to symbol.


Open Loop MIMO

In Open Loop (OL) transmission technique, the transmitter
has limited or no knowledge of the channel.
To obtain knowledge of the channel, an open-loop
transmission scheme uses the idea of the channel
reciprocity available in TDD
OL transmission is suitable for high mobility scenario
OL-MIMO techniques include
-Spatial Multiplexing
-Transmit Diversity (e.g Space-Time codes)
-Random Beamforming


Random Beamforming/Precoder Cycling

Random beamforming is a method to increase channel selectivity by
changing beams within allocated time/frequency resources. It is used in
open loop MIMO scheme where there is no feedback available.
Precoder cycling is a random beamforming technique.
A predefined set of precoders are chosen from a predefined codebook
and are cyclically allocated to a group of adjacent subcarriers.
The Chordal distance (separation between beams) property of the set
of precoders must be good in order to increase the order of the
diversity.
Used to provide
Beam diversity gain :Predefined precoders in each localized
frequency band form different beams
Beam selection gain: Localized resource allotted to user terminal
based on the CQI feedback for preferred subbands


WiMAX and LTE: Overview of OL-MIMO

SFBC adopted for transmit diversity using pair of sub-carriers
- Fast changing channel destroys orthogonality of code in STBC
In 802.16m SFBC with precoder cycling is employed with precoded
pilots
Precoder cycling creates a fixed set of two virtual antennas (streams)
across all subcarriers and changes the virtual antennas by using
different precoder weights
In LTE, SFBC with FSTD is employed with non-precoded CRS.
FSTD cycles transmissions over pairs of transmit antennas
- Tradeoff: Reduced overhead using precoded pilots vs wider range of
interpolation in the frequency domain for finer channel estimation
using non-precoded CRS.


Closed Loop MIMO

An important MIMO scheme that uses feedback from the receiver to
obtain Channel State Information (CSI) and hence uses it for increasing
throughput or coverage.
The challenge here lies in efficiently obtaining the CSI in order to
construct a precoder matrix.
For overhead reduction, the whole beamforming matrix is quantized by
a vector codebook. The factors to be considered for the base codebook
design are performance gain, overhead, robustness and complexity.
User
Base Station Channel
Equipment/
Receiver
CQI - Channel Quality Indicator
CQI,PMI,RI PMI - Preferred Matrix Indicator
RI - Rank Indicator

Feedback

WiMAX and LTE: Overview of CL-MIMO

The index of the selected quantization codeword is fed back. An L bit codebook
consists of 2L codewords, where L is the required number of bits for indexing
each codeword.
Precoding codebook for transmission on two antennas

.
802.16m defines 3-bit feedback for 2-transmit antennas (2-Tx) as well as 4-bit
and 6-bit feedbacks for 4-transmit antennas (4-Tx), while LTE defines 2-bit
and 4-bit feedbacks for 2-Tx and 4-Tx, respectively.
- More the number of bits in codebook index, more the codewords. This gives
wider range for choosing the best precoder at the cost of signaling overhead.

Overview of CL-MIMO. . Contd

Base codebooks can be dynamically generated using a few parameters.
Also, high rank codewords with more columns include the low rank
codewords with a few columns as subset.
Reduces the storage complexity and the complexity of searching for
the best number of spatial streams.
Adaptive codebook is defined in 802.16m:codeword distribution
changed according to long-term channel statistics.
More codewords are steered in the ideal beamforming directions
802.16m has adopted differential feedback, where the correlation
between consecutive beamforming reports is exploited- the incremental
change between the current and previous matrices is fed back.
- Adv: Lower feedback overhead
- Disadv: Error propagation effect


Block diagram for downlink MIMO


Block diagram for Uplink MIMO


Single User MIMO: An Introduction

• The Time/Frequency resources are dedicated to a single User Equipment (UE)/
Advanced Mobile Station (AMS) so as to achieve peak user spectral efficiency.
• Encompass techniques such as transmit diversity, spatial multiplexing and
beamforming
• Based on the MIMO channel conditions and modulation type, the user
terminals need to have an appropriate receiver mechanism so as to reduce the
average processing power.

• The various detectors that are considered
in SU-MIMO include
- Maximum Likelihood Detector (MLD),
- Minimum Mean Squared Error –
Successive Interference Cancellation
(MMSE - SIC)


WiMAX and LTE: Overview of SU-MIMO

WiMAX 802.16m LTE

• Uses Vertical Encoding • Uses Multiple Code Word (MCW)
(VE)/Single Code Word (SCW) transmission

• A MLD gives better performance • Lower complexity and better
compared to MMSE-SIC for the performance of MMSE-SIC receivers
advanced receiver with SCW with MCW transmission.
- Better in case of correlated channels - Modeling of effective SNR per
codeword is difficult in MLD
• HARQ process is simplified and
only a single report of the CQI is • One CQI report and one HARQ
required for all multiplexed layers process required for each FEC
codeword and hence more overhead


Multi User MIMO: An Introduction

• The time – frequency resources are shared by multiple users to exploit the
multi-user diversity in the spatial domain
• Provides enhanced throughput under heavy data traffic
• The multiple users are selected at the base station with the help of precoder
matrix that is constructed by the orthogonal PMI’s reported by different users.
Two schemes are observed in MU-MIMO:
• Linear scheme in MU-MIMO include Zero-
Forcing(ZF) technique wherein the data
symbols are precoded with the pseudo
inverse of the channel in order to cancel out
the interference of other users.
• Non-linear MU-MIMO uses Dirty Paper
Coding (DPC) that pre-cancels known
interference without additional power once
the transmitter is assumed to know the
interference signal regardless of the CSI at
the receiver.


WiMAX and LTE: Overview of MU-MIMO

DOWNLINK:
Both standards use a scheduler to select users with good spatial
separation and perform pseudo inversion of the combined channel
matrix to obtain the precoding matrix. The CQI reported by each user is
then adjusted at the base station to fit the channel quality after
precoding.


WiMAX and LTE: Overview of MU-MIMO

UPLINK:
For uplink MU-MIMO, both WiMAX and LTE allow multiple users to transmit
simultaneously in the same uplink resource. The base station distinguishes the
signals from different user terminals through the pilots/RSs allocated to each
terminal and separates them using an advanced receiver which is MLD in case of
802.16 and MMSE in LTE.


SUMMARY

Parameter LTE WiMAX Explanation

Capacity +++ +++ Use of MIMO Technology

Spectral Efficiency Use of Spatial Multiplexing,
Uplink ++ +++ Beamforming. OFDMA in downlink in
Downlink +++ +++ both. But LTE uses SC-FDMA in uplink.
Mobility +++ ++ Use of Open-Loop techniques,SFBC

Receiver ++ + MLD (higher computational complexity)
Complexity used in WiMax. MMSE in LTE.
Feedback ++ +++ Differential feedback in 802.16m leads to
lesser overhead compared to LTE
Power ++ ++ Transmit power control.
Consumption


Conclusions

802.16m and 3GPP LTE are both capable technologies to
meet the requirements of 4G, in terms of data rates,
spectral efficiency and mobility

Both standards are technically similar when it comes to
employing MIMO techniques

Two technologies however differ in terms of legacy and
time to market


References

Qinghua Li; Guangjie Li; Wookbong Lee; Moon-il Lee; Mazzarese, D.; Clerckx, B.; Zexian Li; ,
"MIMO techniques in
WiMAX and LTE: a feature overview," Communications Magazine, IEEE , vol.48, no.5, pp.86-
92, May 2010
Alamouti, S.M.; , "A simple transmit diversity technique for wireless communications ,"
Selected Areas in Communications, IEEE Journal on , vol.16, no.8, pp.1451-1458, Oct 1998
Q. H. Spencer et al., “An Introduction to the Multi- User MIMO Downlink” IEEE Commun.
Mag., vol. 42, no. 10, Oct.
2004, pp. 60–67.
Jim Zyren, Freescale Semiconductor, ”Overview of 3GPP Long Term Evolution Physical Layer“,
white paper.
Juho Lee, Jin-Kyu Han, and Jianzhong (Charlie) Zhang, “MIMO Technologies in 3GPP LTE and
LTE- Advanced,”
EURASIP Journal on Wireless Communications and Networking, vol. 2009
Qinghua Li; Xintian Lin; Jianzhong Zhang; Wonil Roh; , "Advancement of MIMO technology in
WiMAX: from IEEE 802.16d/e/j to 802.16m," Communications Magazine, IEEE , vol.47, no.6,
pp.100-107, June 2009


MIMO Features In WiMAX and LTE

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