Report :- MIMO features In WiMAX and LTE: An Overview

MIMO Techniques in WiMAX and LTE: A Feature Overview 1

MIMO Techniques in WiMAX and LTE: A
Feature Overview
Ananthakrishnan Ramkumar (Student #4119568) & Praveen Kalyanasundaram (Student #4118863)

In this report, we give an overview of the various MIMO
Abstract—IEEE 802.16m and 3GPP LTE are the two techniques employed in Mobile WiMAX and 3GPP LTE, and
evolving wireless standards targeting 4G wireless systems. provide a comparison.
They make use of Multiple Input Multiple Output
(MIMO) technologies in order to meet the requirements of
4G wireless systems. A large number of MIMO techniques
have been developed and employed in these two standards
which greatly enhance the data rates and spectral
efficiency compared to 3G. In this case study we provide
an overview of the MIMO techniques including Open-loop
(OL), Closed-loop (CL), Single user and Multiuser MIMO
in the two standards. The MIMO features of the two
standards are surveyed.

I. INTRODUCTION
Since the launch of 3G mobile communication services, high
speed wireless access services that provide high speed data
transmission in a mobile environment have come to be used in Figure1: Trends in Mobile Communications
diverse applications including email, web access etc. Figure 1
shows the trends in wireless communications. It is evident that The report is divided as follows: In Section II MIMO
the next generation (4G) is expected to provide much higher technology is introduced and the techniques are discussed.
data rates and mobility compared to the current one. Section III and Section IV provide an overview of WiMAX
Multiple-input multiple-output (MIMO) technology is and LTE technologies respectively. In section V the various
serving as a breakthrough in the design of wireless MIMO techniques used in both technologies are analyzed.
communication systems. Exploiting multi-path scattering,
MIMO techniques deliver significant performance II. MIMO – INTRODUCTION AND TECHNIQUES
enhancements in terms of data transmission rate and MIMO is a technique to improve communication
interference reduction. The rapidly growing demand for performance by using multiple antennas at the transmitter and
bandwidth in mobile services makes it essential to the receiver.
industries to deliver cost effective, highly performing wireless
broadband systems with key technologies such as OFDM,
advanced antenna techniques such as MIMO and
beamforming.
The IEEE 802.16e (WiMAX Profile 1.0) and Third
Generation Partnership Project (3GPP) Evolved Universal
Terrestrial Radio Access (E-UTRA) Long Term Evolution
(LTE) (Releases 8 and 9) standards have been developed and
are part of the IMT-2000 third generation (3G) technologies
[1]. IEEE 802.16m (WiMAX Profile 2.0) [2] and 3GPP E-
UTRA LTE-Advanced (LTE-A) (Release 10) [3] are still
being developed primarily to meet or exceed the requirements
of the International Telecommunication Union (ITU) for IMT
Advanced fourth generation (4G) technologies.
Figure2: MIMO System


In MIMO systems, a transmitter sends multiple streams using
multiple transmit antennas. The transmit streams go through a
channel which consists of all M*N paths between the N
transmit antennas and M receive antennas. The receiver
decodes the received signal vectors into the original where ‘*’ denotes complex conjugate. Here C1 and C2 within
information. A narrowband flat fading MIMO system is the matrix represent the two complex modulation symbols that
modeled as: are transmitted in two timeslots. There are also higher order
space time codes that provide a better error-rate performance
Y=Hx+n -- (1) [4]. At the receiver maximum likelihood decoding is
Where Y and x are the receive and transmit vectors, performed with only linear processing. Space time coding
respectively, and H and n are the channel matrix and the noise assumes perfect CSI at the receiver.
vector, respectively. In receive diversity the independently faded signals received
at the antennas are used to provide diversity gain using
It has been proven that the maximum capacity achievable techniques such as Selection Diversity, Switching, Maximal
with a MIMO configuration consisting of N transmit and M Ratio Combining (MRC) ,Equal Gain Combining (EGC).
receive antennas is “min (M, N)” times the capacity of the
corresponding Single Input Single Output (SISO) system. [5]
B. Spatial Multiplexing:
There are three main aspects in MIMO:
Spatial multiplexing is a very useful transmission technique
A. Diversity: in MIMO systems to increase the overall capacity of the
channel at high SNR values. Here a high data rate stream is
Diversity leads to improvement of link reliability by adding
split into several low data rate streams where each of the
more redundancy to information going through the channel. In
individual streams is transmitted by a different transmit
this technique, several copies of the same signal are
antenna in the same frequency channel. If these signals arrive
transmitted using multiple antennas into the air interface where
at the receiver antenna array with sufficiently different spatial
they may experience fading independent of each other. In such
signatures, the receiver can perform processing to separate
scenarios there will be high probability that only some signals
these streams.
will undergo deep fades while others may not. This can be
used to obtain diversity gain.
Transmit and receive diversity are the common schemes in a
MIMO system and both of these do not require channel
knowledge at the transmitter.
Transmit diversity refers to the use of techniques such as
Space-time coding wherein each antenna transmits a
differently encoded, fully redundant version of the same signal
leading to redundancy in space and time.

Figure 4: Spatial multiplexing
If we consider a system with N transmit antennas and M
receive antennas then the maximum spatial multiplexing order
is given by the expression:
Ns = min (M,N) --(2)
where Ns is the number of streams which can be transmitted
in parallel [5]. 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. In spatial
multiplexing there is no necessity for additional bandwidth
and power.
Figure 3: Transmit diversity using Space-time code
One such code that was designed for a two-transmit antenna C. Beamforming:
system is the Alamouti code [4]. These codes are orthogonal in Beamforming is a signal processing technique that takes
nature and hence it is used to provide full diversity gain. advantage of the fading channels. It primarily improves the
It is described with the coding matrix received signal gain and coverage of the communication
system. In this technique the transmission radiation pattern


from an array of antennas is focused in the direction of specific The precoding may be channel dependant or independent.
user by constructively interfering in that specific direction. In With channel dependant precoding, also referred to as closed-
order to achieve this, it is required to have reliable knowledge loop precoding, the precoder matrix is chosen to match the
of the channel. characteristics of the MIMO channel. With channel
Based on the amount of channel knowledge gained various independent precoding, also known as open loop precoding,
types of beamforming can be implemented. Three different channel characteristics are not considered in the selection of
scenarios are possible namely: the precoder matrix.
(i) Full CSI: Statistical Eigen vector beamforming is a
E. Open Loop and Closed Loop Transmission
reliable technique.
(ii) Limited CSI: Grassmannian beamforming is used. In Open Loop (OL) transmission technique, the transmitter
(iii) No CSI: Blind beamfomring technique is used where has limited or no knowledge of the channel. To obtain
the CSI is blindly estimated from the received signal statistics. knowledge of the channel, an open-loop transmission scheme
uses the idea of the channel reciprocity available in TDD
because both downlink and uplink are using the same
frequency channel. That is why FDD is not used in an open-
loop transmission since downlink and uplink channels do not
use the same frequency. Each of these channels is totally
different and hence not reciprocal. OL transmission is suitable
for high mobility scenario, where channel varies rapidly and
feedback from receiver is not very useful. Some of the OL-
MIMO techniques include Spatial Multiplexing and Space-
Time codes which were discussed earlier in this report.
Closed loop MIMO (CL-MIMO) system on the other hand
uses feedback from the receiver to obtain Channel State
Information (CSI) and hence uses it for increasing throughput
or coverage. The major challenge in CL-MIMO is efficiently
Figure 5: Beamforming obtaining the CSI which is then used to construct the
beamforming or precoding matrix.
In MIMO systems multilayer beamforming is also supported
with the help of precoding which is explained in the later F. Single User vs Multi user MIMO
section. In such a case the full channel matrix (CSI) must be In SU-MIMO transmissions, time-frequency resources are
known to the system. By applying Singular Value dedicated to a single terminal/user with the aim of achieving
Decompostion (SVD) the channel matrix is diagonalized as peak user spectral efficiency. In MU-MIMO time-frequency
shown below: resources are shared by multiple users. Multi User MIMO
combines the high capacity achievable with MIMO with the
benefits of Space division Multiple Access (SDMA).

Where σM is the mth non-negative singular value from a set
of σ1 ≥ σ2 ≥ …. ≥ σM, U and V are the corresponding singular
vector unitary matrices. The two unitary matrices are removed
through pre- and post-multiplication at the transmitter and
receiver, respectively.
Figure 6: Single user vs Multi-user MIMO
Once SVD is applied, one data stream per singular value
can be transmitted with appropriate power in a defined G. MIMO Receiver Design
direction without creating any interference [7]. A number of MIMO receiver algorithms are used, depending
on the receiver complexity: Zero-Forcing (ZF), Minimum
D. Precoding Mean Square Error (MMSE) and MIMO Maximum
Precoding is generalized beamforming which permits to Likelihood Detector (MLD).
maximize the received signal level. It improves the capacity of The receiver receives signals transmitted by more than one
the system and also limits the transmit power. transmit antenna. To separate the mixed data streams at the
Precoding is done to exploit beamforming and for spatial receiver, the received mixed-signal is multiplied by the inverse
multiplexing. The process of precoding creates some of the MIMO channel matrix. This is called a ZF receiver.
redundancy into the data sequence before the transmission. For an output given by


receivers, conversely, can both suffer sharp performance losses
when MIMO channels are correlated.
Another advantage is diversity gain. The MLD receiver not
only can separate the transmit data streams but also can
achieve the receive diversity gain for multiple receive
The data obtained at the receiver end can be expressed as
antennas. Further, the receiver can support a very high
mobility environment. But MLD receiver has higher
Ŝ = H #X -- (3)
complexity load compared to MMSE receiver.
Where H# = (H*H)-1 H*
Each MIMO technique has advantages and disadvantages.
Here ‘H’ is the channel matrix and ‘X’ is the output at the
When designing a wireless system, appropriate MIMO
receiver. It is a simple MIMO receiver that suffers
technique is chosen by considering the service type, channel
performance loss at higher noise and interference levels at the
condition and complexity.
receivers.
In a MMSE receiver on the other hand, in order to minimize
III. WIMAX
performance loss, the inversion of the MIMO channel matrix
operation is adjusted according to the interference or noise A. Introduction
level given as:
WiMAX which stands for Worldwide Interoperability for
Ŝ = H*(HH* + Rn)-1x -- (4) Microwave Access is the trade name for the IEEE 802.16
international standards. It is a rapidly growing broadband
where Rn is the noise or interference covariance wireless access technology that replaces the current existing
systems such as Wi-Fi and 3G.
Improved reception of spatial multiplexing MIMO IEEE 802.16-2005 or IEEE 802.16m known as Mobile
transmission requires an exhaustive search of MIMO signal WiMAX is an extension of IEEE 802.16-2004 or IEEE
constellation combinations. The MIMO transmission process 802.16e (fixed WiMAX). Mobile WiMAX introduces new
is emulated at the receiver in such a way that a specific features to support enhanced Quality of Service to provide
complex modulation constellation is generated for each high mobility at very high data rates.
transmit antenna stream. The constellations are then applied to
the MIMO channel input. At the MIMO channel output, the B. Features
corresponding MIMO reception signal is generated for each
a) Access Technology: OFDMA with Cyclic Prefix in
receive antenna. At this point, the Euclidean distance is
computed for the emulated MIMO output signal against the both UL and DL
received signal. In this way, the different modulation b) TDD and FDD as the duplexing modes
constellations at MIMO channel input constitute different c) Adaptive modulation and coding (AMC)
hypothesis tests. - QPSK, 16QAM, and 64QAM
The minimum Euclidian distance associated with the - Forward Error Correction (FEC)
constellation combination hypothesis provides the most likely d) MIMO Matrix A (Space Time Block Coding) and
decoding. This is the principle behind MLD receiver. Optimal Matrix B (Spatial Multiplexing) support
receiver performance, however, comes at the cost of receiver e) Open Loop and Closed Loop configurations including
complexity.
Transmit Diversity, Spatial Multiplexing and Beam
The advanced receiver can be further extended by a
Successive Interference Cancellation (SIC) receiver forming techniques.
architecture. The idea behind the SIC receiver is that the signal C. Requirements
quality of the multiple transmit data streams is not the same,
because of the fading variations of the MIMO channel. We a) High peak data rates: max 74Mbps in 20MHz wide
can successfully demodulate the first data stream, re- spectrum
modulate it and then subtract the first data stream from b) Carrier Frequency: Unlicensed band-2.5 and 3.5
the receive input mixture, absent interference from the GHz, Licensed band under 6GHz
first data stream. We can then demodulate the second data c) Operating bandwidth: 1.25-20MHz
stream successfully with the simplest maximum ratio d) Mobility support – with appropriate pilot design
combining (MRC) receiver.
and H-ARQ
Here we give a comparison of the various receivers that can
be used for MIMO. MLD-based MIMO receivers have many e) Flexible Frequency Reuse
advantages compared with ZF and MMSE receivers. The f) Flexible bandwidth allocation
MLD receiver's performance advantages are significant when g) Quality of Service (QoS) support - allowing video
the MIMO channels are correlated. The ZF and MMSE calls, mobile entertainment, multimedia chat and
high speed internet access.


h) Integrated Security for voice and data transmission OFDM technology has been incorporated into LTE because
using Advanced Encryption Standard (AES) it enables high data bandwidths to be transmitted efficiently
while still providing a high degree of resilience to reflections
and interference. The access schemes differ between the uplink
and downlink: OFDMA is used in the downlink; while SC-
FDMA is used in the uplink. SC-FDMA is used in view of the
fact that its peak to average power ratio is small and the more
constant power enables high RF power amplifier efficiency in
the mobile handsets - an important factor for battery power
equipment. MIMO technologies have been widely used to
improve downlink peak rate, cell coverage, as well as average
cell throughput.
LTE-Advanced has recently started in 3GPP wherein the
existing SU-MIMO technologies are extended to support
configuration with up to eight transmit antennas in the
downlink, and up to four transmit antennas in the uplink.

Figure 7: WiMAX multi-antenna implementation C. LTE Requirements
a) Higher performance
IV. LONG TERM EVOLUTION - 100 Mbit/s peak downlink, 50 Mbit/s peak uplink
- 1G for LTE Advanced
A. Introduction - Better cell edge performance
Long Term Evolution (LTE) is an upcoming technology - Reduced latency (to 10 ms) for better user
targeting 4G wireless systems. The objective of LTE is to experience
provide technical benefits to cellular technologies in terms of - Scalable bandwidth: 1.25-20 MHz
better spectral efficiency (i.e. higher data rates with available b) Backwards compatible:
bandwidth) and cell coverage as compared to 3G. - Works with GSM/EDGE/UMTS systems
3rd Generation Partnership Project (3GPP) Evolved - Utilizes existing 2G and 3G spectrum and new
Universal Terrestrial Radio Access (E-UTRA) Long Term spectrum
Evolution (LTE releases 8 and 9) standard has been developed - Supports hand-over and roaming to existing mobile
as part of the IMT 2000 third generation technologies (3G). networks
LTE-A (Release 10) is still being developed primarily to meet c) Wide application
or exceed the requirements of the International - Mobility up to 350kmph
Telecommunications Union (ITU) for IMT fourth generation - Large range of terminals (phones and PCs to
(4G). LTE is used to denote 3GPP releases 8 and 9, LTE-A to cameras)
denote 3GPP release 10 and E-UTRA for releases 8 to 10.
B. Features
a) Multiple access schemes:
- DL: OFDMA with Cyclic Prefix (CP)
- UL: Single Carrier FDMA (SC-FDMA) with Cyclic
Prefix (CP)
b) Adaptive modulation and coding Table 1: Wimax and LTE features summary
- DL/UL modulations: QPSK, 16QAM, and 64QAM
- Convolutional code and Rel-6 turbo code Aspect 3GPP-LTE Mobile
c) Advanced MIMO spatial multiplexing techniques
WiMAX
- (2 or 4)x(2 or 4) downlink and uplink supported.
802.16m
- Multi-user MIMO also supported.
Legacy GSM/GPRS/EDGE IEEE 802.16
d) Support for both Frequency Division Duplexing (FDD) and
Time division Duplexing (TD) /UMTS/HSPA a through e
e) Hybrid-ARQ, mobility support, rate control, security. Access
Technology
The main technologies used in LTE are: Orthogonal DL OFDMA OFDMA
Frequency Division Multiple Access (OFDMA), Single UL SC-FDMA OFDMA
Carries Frequency Division Multiple Access (SC-FDMA) and Radio Access TDD and FDD TDD and FDD
Multiple Input Multiple Output (MIMO). Mode


Frequency Existing 2-11 GHz
Band (800,900,1800,1900
MHz) and new
bands (Range 800
MHz to 2.62 GHz)
Channel Scalable from 1.25 Scalable from
Bandwidth to 20 MHz with 1.25 to 20 MHz
system profiles with system Figure 8: OFDMA-uplink in WiMAX and SC-FDMA-uplink in
1.25,1.4,2.5,3.5,10, profiles 1.25,2.5, LTE (Note: Different colors indicate different users)
15 and 20 MHz 5, 10and 20
MHz
B. SU-MIMO: Spatial Multiplexing
Antenna MIMO MIMO
Scheme The major constraints in implementing the spatial
DL 2x2,4x2,4x4 2x2,4x2,4x4 multiplexing MIMO technology in 802.16m and LTE are the
UL 1x2,1x4, 2x2 1x2,1x4,2x2 , cost of multiple antennas, the size limitation of multiple
antennas for handheld devices, backward compatibility
Number of 2 1
constraints and receiver complexity. Reception of the spatial
code-words
multiplexing MIMO transmissions is optimized by selecting
Mobility:
the best receiver operation, based on the MIMO channel
Speed Up to 350 Km/h Up to 120 Km/h condition and the modulation type, to reduce the average
Handover Inter-cell soft Optimized hard processing power required.
handovers handovers 802.16m uses Vertical Encoding (VE)/Single Codeword
DL Spectral 1.57 1.59 (SCW) transmission for both uplink and downlink. The reason
Efficiency bps/Hz/Sector bps/Hz/Sector for this choice is that advanced receivers would be better
(2x2) MIMO2 (2x2) MIMO implemented with an optimal Maximum Likelihood Detector
UL Spectral 0.64 0.99 (MLD).The advantage of VE is that the implementation of
Efficiency bps/Hz/Sector bps/Hz/Sector HARQ process is simple and it requires only a single report of
(1x2) SIMO2 (1x2) SIMO channel quality indicator (CQI) for all multiplexed layers.
On the other hand, LTE uses Multiple Codeword (MCW)
transmission on the downlink. The reason for this choice is
V. SURVEY OF MIMO TECHNIQUES because of lower complexity and better performance of
MMSE-SIC receivers for MCW transmission in LTE.
A. Uplink and Downlink Modeling of the effective SNR for each codeword is much
WiMAX uses OFDMA in both uplink and downlink whereas more difficult in MLD than MMSE-SIC. The disadvantage of
LTE uses SC-FDMA in uplink and OFDMA in downlink. using MCW is that it requires one CQI report and one HARQ
Despite its many advantages, OFDMA has the disadvantage process for each FEC codeword. Each HARQ process requires
of high frequency (esp. Doppler spread) sensitivity and high an ACK/NAK feedback signaling on uplink.
peak-to-average power ratio (PAPR). PAPR occurs due to C. Reference Signal (RS)/Pilot
random constructive addition of sub-carriers and results in
Reference Signal (RS) also known as the pilot signal is used
spectral spreading of the signal leading to adjacent channel
for measuring the spatial channel and help in coherent
interference. It is a problem that can be overcome with high
demodulation at the terminal. They perform the operation of
compression point power amplifiers and amplifier linearization
supervision, control, equalization, synchronization or reference
techniques.
purposes within a transmission system. It is possible to make
While these methods can be used on the base station, they
an estimate of the channel response at various frequencies by
become expensive on the User Equipment (UE). Hence, LTE
comparison with the known reference pilot subcarrier.
uses Single Carrier FDMA (SC-FDMA) with cyclic prefix on
The reference signals can be classified into Common
the uplink which reduces PAPR as there is only a single carrier
Reference Signal (CRS) and Dedicated Reference Signal
as opposed to N carriers.
(DRS). The cell common reference signal is a reference signal
used by all UEs within a cell. The DRS or UE specific
reference signal is a reference signal used by an UE within the
cell or used by a UE group. The RS can be 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.
Precoded pilots offer lesser overhead compared to non-
precoded pilots. This is because in case of non-precoded pilots


the RS has to be transmitted by each of the transmit antennas, D. Multi-User MIMO
whereas in precoded pilots the number of RS to be transmitted MU-MIMO allocates multiple users in one time-frequency
is given by the number of spatial streams which is bounded by resource to exploit multi-user diversity in the spatial domain,
m= min (M, N), where N and M are the number of transmit which leads in significant gains over SU-MIMO.
and receive antennas. Moreover in non-precoded RS, the In the uplink scenario, users transmit to the base station over
spatial precoder chosen from the codebook has to be indicated the same channel i.e multiple access. The challenge here is for
to the terminal in each transmission assignment which adds to the base station to separate the signals transmitted by the users
the overhead. But the advantage of using non-precoded pilots using Multi-User Detection (MUD) or other techniques. In the
is that it enables finer channel estimation in the frequency downlink, the base station transmits simultaneously to a group
domain. of users i.e broadcast. Here the challenge is to overcome the
The RS signal used in the uplink for both 802.16m and E- inter-user interference to detect the signals.
UTRA are similar. Non-precoded DRS is used in both
standards for channel adaptation and beam selection in the
uplink. Moreover, precoded DRS is used for coherent
demodulation in the uplink.
However different designs have been adopted for downlink
pilots in the two technologies. 802.16m uses both non-
precoded common pilots and precoded dedicated pilots for
channel measurements and coherent demodulation, supporting
up to eight transmit antennas. On the other hand, non-precoded
CRS’s supporting up to four transmit antennas have been
defined for LTE release 8. LTE release 9 and LTE-A have
chosen DL precoded DRS (UE-Specific RS).Also, eight
antenna port transmissions supporting up to eight layers
(spatial) has been proposed for LTE-A. Figure 10: MU-MIMO Downlink

There are two schemes in MU-MIMO: Linear and Non-
linear. In the linear case, the data symbols are precoded with
the pseudo inverse of the channel, so at the receiver the
interference due to other users is cancelled. Zero-forcing (ZF)
MU-MIMO technique is one linear MU-MIMO technique. [6]
Non-linear MU-MIMO uses Dirty Paper Coding, in which
precoding is done, given the interference is known at the
transmitter. It was shown in [8] that the capacity of a channel
where the transmitter knows the interfering signal, is the same
as if there were no interference.
Although non-linear MU-MIMO with dirty paper coding
theoretically offers the best performance, the practical
implementation is difficult, and hence linear MU-MIMO has
been adopted by both standards for its simplicity.

Figure 11: Inverse Channel multiplication in ZF MU-MIMO

In 802.16m and LTE downlink scenario, a scheduler is
Figure 9: Reference signal transmission by two transmit used, which selects several users with good spatial separation
antennas and performs 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. 802.16m also supports OL MU-MIMO. Here
a unitary precoding matrix is preset for each frequency domain
resource.
For uplink MU-MIMO, both WiMAX and LTE allow
multiple users to transmit simultaneously in the same uplink
resource. The base station (ABS/eNB) 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. Figure 12: Layer permutation with four codewords
E. Open Loop MIMO
In LTE, since non-precoded CRS is used, the predefined
(i) Space Time/Frequency Code:
precoders can be changed within the subcarriers of a resource
This is an Open loop technique wherein transmit diversity
block (RB) so that beam diversity gains are fully used. Layer
technique provides spatial diversity gain. Both WiMAX and
permutation is performed along with precoder cycling in E-
LTE-A have adopted the frequency domain version of the
UTRA to further increase diversity gain from virtual antennas
Alamouti Code [4] as the basic transmit diversity MIMO
with MCW transmission. This combination of precoder
technique, where coding is performed to pairs of adjacent
cycling and layer permutation is called large-delay CDD and
subcarriers rather than two adjacent time slots. The main
has been adopted as OL-SM technique in LTE.
reason for this is to sustain the orthogonality of the code under
high mobility of the terminals. Hence SFBC outperforms
(iii) SFBC and FSTD
STBC in high speed scenarios. In MIMO modes where more
Space Frequency Block Coding (SFBC) is used along with
than two transmit antennas are required, the application of
Frequency Switched Transmit Diversity (FSTD).In SFBC the
precoders become necessary. This technique that is adopted in
codeword symbols are mapped across frequency. The FSTD
both the standards makes effective use of all the spatial
mainly cycles the transmissions over pairs of transmit antennas
degrees of freedom over a set of subcarriers by limiting the
across subcarriers within a Resource Unit (RU).
transmission of SFBC to a pair of subcarrier. As a result it
improves the robustness against spatial correlations in the
channel.

(ii) Precoder Cycling
Random beamforming is a method to increase channel - (a)
selectivity by changing beams within allocated time/frequency
resources. Precoder cycling is a random beamforming
technique.
Here 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.
- (b)
In both 802.16m and LTE standards, precoder cycling is
employed to achieve beamforming. This technique in 802.16m
Figure 13: (a) SFBC with two transmit antennas on downlink
provides both beam diversity gain and beam selection gain.
(b) SFBC+FSTD with four transmit antennas on downlink
Beam diversity gain is achieved by distributing the resources
within a wide frequency band. Here the predefined precoders
In case of two antennas only SFBC is used whereas in cases
form different beams in each localized frequency band. At the
where four transmit antennas are used, a combination of SFBC
receiver, all the resources that are added up will benefit from
and FSTD are employed. Figure 12 shows how SFBC and
beam diversity gain.
FSTD is implemented, where in SFBC is limited to
On the other hand beam selection gain is obtained based on
transmission over a pair of subcarriers. By using different
the CQI feedback, by allocating a localized resource to a
antennas to transmit over the two subcarriers, spatial diversity
terminal for its preferred sub-bands. According to the sub-
is obtained. This provides robustness against spatial
bands the precoders are cyclically changed and thus
correlations in the channel.
opportunistic beamforming gain can be achieved by allocating
Both 802.16m and LTE make use of precoders in order to
the preferred sub-bands as reported by the terminal.
achieve spatial diversity gain. However there is a slight
variation in the use of precoders in both these standards. The
variation lie in the design of DL demodulation pilots. In IEEE
802.16m, SFBC with precoder cycling is employed with


precoded pilots. These precoded pilots reduce the overhead The precoding operation for the closed-loop spatial
when compared to non-precoded pilots. The precoder cycling multiplexing is defined by
creates a fixed set of two virtual antennas across all subcarriers
within a RU and uses various precoded weights to change y = Wx - (5)
these virtual antennas. Where y = [y0. . . yN-1]T, yn denotes the complex symbol
On the other hand LTE makes use of SFBC with FSTD and transmitted on the nth antenna,
uses non-precoded CRS. Although non-precoded CRS leads to x = [x0. . . xM-1]T, xm denotes the modulation symbol
higher overhead, it provides a wider range of interpolation in transmitted on the mth layer, and W denotes the N × M
the frequency domain for finer channel estimation. precoding matrix.
F. Closed Loop MIMO For transmission on two antennas, the precoding matrix W is
Feedback is required when channel reciprocity is unavailable selected from Table 2, where each column vector is in the
(e.g., in frequency-division duplex systems). The major form of [ 1 e^ j(θ+kπ)]T multiplied by a scaling factor [9].
challenge lies in how to report the preferred beamforming Based on the codebook index the precoder is chosen.
matrix, which is used for the transmitter to compute the actual The factors to be considered for the base codebook design
precoder over a limited feedback. For overhead reduction, the are performance gain, overhead, robustness and complexity
whole beamforming matrix is quantized by a matrix or vector First, 802.16m defines 3-bit for 2-transmit antennas (2-Tx) as
codebook. The index of the selected quantization codeword is well as 4-bit and 6-bit feedbacks for 4-transmit antennas (4-
fed back. An L bit codebook consists of 2L codewords, where Tx), while LTE defines 2-bit and 4-bit feedbacks for 2-Tx and
L is the required number of bits for indexing each codeword. 4-Tx, respectively. Besides the preferred beamforming matrix,
In the closed-loop spatial multiplexing mode, the base station an indication of the preferred number of spatial streams is also
(also known as eNodeB) applies the spatial domain precoding defined. More the number of bits in codebook index, more the
on the transmitted signal taking into account the precoding codewords. This gives wider range for choosing the best
matrix indicator (PMI) reported by the User Equipment(UE) precoder at the cost of signaling overhead.
so that the transmitted signal matches with the spatial channel
experienced by the UE. To support the closed-loop spatial The high rank codewords with more columns include the low
multiplexing in the downlink, the UE needs to feedback the rank codewords with a few columns as subset. This reduces the
rank indicator (RI), the PMI, and the channel quality indicator complexity of searching for the best number of spatial streams.
(CQI) in the uplink. The RI indicates the number of spatial Also, in each LTE codeword and most of 802.16m codewords
layers that can be supported by the current channel equal power is given to all antennas. This reduces the
experienced at the UE. complexity of power amplifier.
The base station decides the transmission rank, M, taking
into account the RI reported by the UE as well as other factors Table 2: Precoding codebook for transmission on two
such as traffic pattern, available transmission power, etc. The antennas.
CQI feedback indicates a combination of modulation scheme
and channel coding rate that the eNodeB should use to ensure
that the block error probability experienced at the UE will not
exceed 10%.[9]

Since the optimal codebook varies with the deployment
scenario, adaptive codebook is defined in 802.16m. The
adaptive codebook changes its codeword distribution
according to long-term channel statistics. By doing this,
codewords are transmitted in the ideal beamforming
directions.
Also, for overhead reduction, 802.16m has adopted
differential feedback, where the correlation between
consecutive beamforming reports is exploited. Each feedback
Figure 14: Closed loop spatial multiplexing with N antennas specifies only the incremental change between the current and
and M layers previous matrices. But the down side of this is the error
propagation effect.


VI. SUMMARY OF FEATURES

Feature LTE WiMAX Explanation APPENDIX
Capacity +++ +++ Use of MIMO
A. List of Terminologies:
Technology
Spectral Use of Spatial AES Advanced Encryption Standard
Efficiency Multiplexing, AMC Adaptive Modulation Coding
Uplink + ++ Beamforming. ARQ Automatic Repeat Request
Downlink +++ +++ OFDMA in downlink CP Cyclic Prefix
in both. But LTE uses CQI Channel Quality Indicator
SC-FDMA in uplink. CSI Channel State Information
Mobility +++ ++ Use of Open-Loop DL Downlink
techniques E-UTRA Evolved-UMTS Terrestrial Radio Access
Receiver ++ + MLD (higher HARQ Hybrid Automatic Repeat Request
Complexity computational MLD Maximum Likelihood Detector
complexity) used in MMSE Minimum Mean Squared Error
WiMax. MMSE in SM Spatial Multiplexing
LTE. STBC Space Time Block Code
Pilot + ++ Precoded Pilots in STC Space Time Coding
Overhead WiMAX and non- UL Uplink
precoded RS used in ZF Zero Forcing Detector
LTE CRS Common Reference Signal
Feedback ++ +++ Differential feedback DRS Dedicated Reference Signal
in 802.16m leads to
lesser overhead
compared to LTE B. IEEE802.16m and 3GPP-LTE Terminologies
Power ++ ++ Adaptive power
Consumption control.

Note: + indicates a positive feature. More the + better it is.

VII. CONCLUSION
In this report we have provided an overview of the various
MIMO techniques that are implemented on the two standards
namely IEEE 802.16m Mobile WiMAX and 3GPP LTE.
Various MIMO schemes such as Open Loop, Closed Loop,
Single and Multi User adopted in these two technologies have
been studied and analyzed.
From the case study we conclude that IEEE 802.16m
(Mobile WiMAX) and 3GPP LTE are both capable
technologies designed to meet the requirements of the next
generation (4G) mobile wireless communication system in
terms of data rates, mobility and spectral efficiency. Both are
technically similar when it comes to employing MIMO
techniques. Both are based on the same fundamental elements,
namely OFDMA modulation, use of smart antenna techniques,
and flat all-IP networks. Although there are minor differences
in choices and approaches in the two standards, each having
subtle technical advantages and disadvantages i.e. tradeoffs,
performance wise they are both equally competent.
However from a market perspective, the two technologies REFERENCES
differ in terms of legacy and time to market. WiMAX has edge
Primary Reference:
over LTE due to its early deployments. LTE was designed Qinghua Li; Guangjie Li; Wookbong Lee; Moon-il Lee;
with mobility in mind from start, while 802.16m evolved from Mazzarese, D.; Clerckx, B.; Zexian Li; , "MIMO techniques in
standards based on fixed wireless networking.


WiMAX and LTE: a feature overview," Communications
Magazine, IEEE , vol.48, no.5, pp.86-92, May 2010
[1] ITU-R Rec. M.1457-8, “Detailed Specifications of the
Radio Interfaces of International Mobile Telecommunications-
2000 (IMT-2000),” May 2009.
[2] ITU-R SG WP 5D, “Acknowledgment of Candidate Submission
from IEEE under Step 3 of the IMT-Advanced Process (IEEE
Technology),” Doc. IMT-ADV/4-E, Oct. 23, 2009.
[3] ITU-R SG WP 5D, “Acknowledgment of Candidate
Submission from 3GPP Proponent (3GPP Organization Partners
of ARIB, ATIS, CCSA, ETSI, TTA AND TTC) under Step 3 of
the IMT-Advanced Process (3GPP Technology),”Doc. IMT-
ADV/8-E, Oct. 23, 2009.
[4] 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
[5] G. J. Foschini and M. J. Gans, “On Limits of Wireless
Communications in a Fading Environment When Using
Multiple Antennas,” Wireless Personal Commun., vol. 6, no. 3,
Mar. 1998, p. 311.
[6] 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.
[7] Feng Wang, Marek E.Bialkowski & Xia Liu,"Investigating the
Effect of Mutual Coupling on SVD Based Beamforming over
MIMO Channels", University of Queensland, An International
Journal (SPIJ) Book: 2009 Volume 3, Issue 4
[8] Costa, M.;"Writing on dirty paper (Corresp.)," Information
Theory, IEEE Transactions on , vol.29, no.3, pp. 439- 441, May
1983
[9] 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

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