TD-LTE and MIMO Beamforming
Principles and Test Challenges
August 2012
Rev. A 08/12
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TD-LTE and MIMO Beamforming
Principles and Test Challenges
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
MIMO Beamforming - Exploiting the Spatial Domain . . . . . . . . . . . . . . . . . . . . 3
How the Spatial Domain Facilitates MIMO Beamforming . . . . . . . . . . . . . 4
A Brief Review of Beamforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
MIMO and Beamforming in a Single System . . . . . . . . . . . . . . . . . . . . . . . 7
TD-LTE – Exploiting the Time Domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Testing MIMO Beamforming Receivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
The Challenges of RF Testing in TD-LTE MIMO Beamforming . . . . . . . . . . 12
Phase Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Antenna Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Channel Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
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TD-LTE and MIMO Beamforming
Principles and Test Challenges
Introduction
Why TD-LTE?
corresponding literature
Time-Division Long Term Evolution (TD-LTE) is
one of two variants of 3GPP LTE technology.
Development of the TD-LTE standard has been
spearheaded by China as an evolution path
for its TD-SCDMA 3G technology. However,
POSTER
TD-LTE and Beamforming
the popularity of TD-LTE is growing rapidly
in other markets, thanks to its high level of
commonality with FDD LTE (the other LTE
WEBINAR
Understanding Beamforming in
TD-LTE Deployments
variant), with the resulting economies of
scale, as well as the compelling economics
of the unpaired spectrum needed by TD-LTE.
Instead of duplexing uplink and downlink
stream in the frequency domain, TD-LTE (as well as TD-SCDMA) duplexes the uplink and
downlink in the time domain.
WiMAX technology has also been deployed in single-band spectrum rather than in
unpaired bands. The global momentum of LTE technology makes it highly likely that it
will also form the 4G evolution path for many WiMAX operators, with the TD-LTE variant
being ideally suited to deployment in unpaired WiMAX spectrum.
Why MIMO Beamforming?
MIMO beamforming is a combination of two related but different antenna techniques:
MIMO, which enables increased data rates in a given spectral bandwidth and
beamforming, which helps operators increase system coverage. A variation of MIMO
called MU-MIMO can share the higher available data rates among multiple subscribers,
further increasing network efficiency.
The Relationship Between TD-LTE and MIMO Beamforming
Technically, MIMO beamforming is not exclusive to TD-LTE systems, but there are
distinct advantages to deploying beamforming alongside a Time-Domain-Duplexed
(TDD) technology. These advantages will be discussed in detail in this paper.
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TD-LTE and MIMO Beamforming
Principles and Test Challenges
The business drivers for tying MIMO beamforming to TD-LTE are a function of TD-LTE’s
initial markets. Both China and India are planning to deploy TD-LTE on a staggering
scale; to give one reference point, a Chinese TD-LTE trial includes planned deployment
of over 200,000 TD-LTE base stations by the end of 2013. These countries must
optimize coverage due to pockets of incredibly high subscriber density. While much
of the initial focus for TD-LTE is on Asia, the technology will be adopted worldwide by
operators with unpaired spectrum (WiMAX and TD-SCDMA) who are looking to get in on
the growing LTE ecosystem.
This white paper provides:
•
A brief review of MIMO and beamforming antenna techniques
•
A description of the TD-LTE technology being deployed
•
An overview of the advantages of deploying MIMO beamforming in TD-LTE and
the corresponding testing challenges to ensure success in these markets
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Principles and Test Challenges
MIMO Beamforming - Exploiting the Spatial Domain
Traditional radio technologies have used the frequency and/or time domains in which to
differentiate between streams of data. These differentiated data streams could be used
for:
•
Multiplexing – differentiating between users
•
Duplexing – differentiating between uplink and downlink communications
•
Flexibility in data rates – modifying data rates by re-allocating resources in the
time or frequency domains
Figure 1: Legacy wireless technologies separated data streams in the time domain (e.g. TDMA),
the frequency domain (AMPS), or both (GSM).
Other domains (e.g. the code domain used in CDMA multiplexing) have been
implemented but all were limited by the availability of finite time and frequency
resources.
MIMO technology exploits space as a domain in which to increase data rates or share
time/frequency resources between users.
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How the Spatial Domain Facilitates MIMO Beamforming
A Brief Review of MIMO
Imagine a 2x2 MIMO system deployed in a “rich scattering” environment… one in which
there are a large number of reflections causing fading on radio signals. If two separate
data streams are transmitted from separate antennas, each will encounter different
fading effects en route to a pair of receiving antennas.
If the receiver is able to use those differences in fading to distinguish between streams
and disaggregate them from each other, the system should be able to transmit and
demodulate two independent data streams in the same frequency band and at the
same time. The process is analogous to solving two equations for two unknown
variables... neither of the receiver’s antenna elements has enough information on its
own to demodulate multiple streams, but the combination of information from multiple
antenna elements does include sufficient information.
Figure 2: MIMO enables the use of space as an “extra” domain in which data streams can be differentiated
Since signal fading is a function of distance and location (i.e. a function of spatial
parameters) space can be treated as a domain in which data streams can be
disaggregated… even when neither stream is giving up resources in the time and
frequency domain.
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Principles and Test Challenges
Figure 3: 2x2 MIMO channel (conceptual)
Intuitively the resulting MIMO system can be thought of as a pair of data “pipes” known
as eigen channels (since the channels are based on the eigenvalues of the MIMO
channel’s characteristic transfer matrix). The eigen channel capacity, or “sizes” of the
pipes are proportional to the eigenvalues, and will therefore almost never be identical.
In practice the two “pipes” can be allocated to a single user (increasing data rates) or,
in the case of Multi-User MIMO (MU-MIMO), divided between two separate users to
increase capacity.
One other concept that must be understood intuitively is the idea of correlation.
Doubling the capacity of a 2x2 MIMO channel requires that the four radio links (shown
in Figure 3 as h11, h12, h21 and h22) exhibit a high degree of difference from each other
based on experiencing different fading. This is called a low correlation between links. In
a more rigorous discussion, correlation would be quantified as a scalar value between 0
and 1 (sometimes expressed as a percentage). Figure 4 depicts the concept of high and
low correlation (based on fading) of two radio links.
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Principles and Test Challenges
Figure 4: High (top) and low (bottom) degrees of correlation
Since a MIMO system depends on fading differences experienced by separate data
streams, it can best increase capacity when the channel (all four radio links) exhibits a
low degree of correlation.
A Brief Review of Beamforming
Beamforming, on the other hand, relies on a high degree of correlation. In beamforming,
multiple antennas transmit radio signals that are identical except for one thing: a beam
is created and steered by adjusting the phase angles of the transmissions so that they
are in phase (delivering high Signal-to-Noise Ratios, or SNRs) where good reception
is desired. In this case the eigen channels are such that the system delivers one large
data pipe alongside one or more very small data pipes. The latter go unused by the
system as shown in Figure 5.
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TD-LTE and MIMO Beamforming
Principles and Test Challenges
Figure 5: Beamforming
Critical points:
•
Beamforming is not necessarily characteristic of a MIMO system or a timedomain-based system. It can be and has been used in frequency-division based
systems.
•
In order for the system to successfully create a beam, it has to have
“knowledge” of the downlink RF channel. In a frequency-division based system
this requires a complicated measurement-and-feedback process, making it a
less attractive option.
MIMO and Beamforming in a Single System
It was noted earlier that a MIMO system can most effectively increase data rates and
capacity when there is a low degree of correlation in the MIMO channel. In the earliest
MIMO systems, low correlation was created by physically separating antenna elements.
One MIMO “rule of thumb” is that a system can deliver low-correlation channels
when antenna elements on both the transmitter and receiver sides of the system are
separated by a distance of more than half the signal wavelength. This is not an option
in cellular systems; at 700 MHz, for example, half a wavelength is greater than 20 cm…
much too large to be implemented in a mobile phone form factor.
Another way of ensuring low correlation in a MIMO system is to cross-polarize antenna
elements on both the transmitter and receiver sides of the system… in other words, to
physically orient antenna elements at right angles to each other.
This not only enables MIMO in the cellular world, it enables the addition of beamforming
to MIMO systems.
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Figure 6: Antenna element orientation in a MIMO beamforming system
In Figure 6, four antenna elements at the transmitter (shown in orange in the left side of
the figure) are designed and placed to create high correlation between their transmitted
signals. By adjusting the phase characteristics of each of their signals, a beam is
formed, creating areas of high SNRs.
A second set of four antenna elements (shown in blue) are cross-polarized in relation to
the first four. They also create a beam, but the two beams exhibit low correlation with
each other. The result is a MIMO beamforming system that can deliver multiple data
streams to a specific physical location.
Transmission Modes
Just as a non-beamforming MIMO system can deliver multiple modes (i.e. increasing
data rates to a single user or splitting data “pipes” so they are used by different users)
so can a MIMO beamforming system.
Figure 7: MU-MIMO and SU-MIMO
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Principles and Test Challenges
The left side of Figure 7 depicts a MIMO beamforming system being used to serve
multiple user signals at the same time, from the same cell and in the same frequency
space. On the right side a Single User MIMO (SU-MIMO) system delivers multiple data
streams or “layers” to a single user. Note that in cases where beamforming is used by
the receiver rather than the transmitter, the “nulls” between lobes can be steered to
reject interfering signals from known sources.
The 3GPP has defined several downlink physical channel “transmission modes” to
support different types of beamforming. Table 1 lists the 3GPP transmission modes (7,
8 and 9) that support beamforming along with relevant Downlink Control Information
(DCI) and port assignments.
Transmission
Mode
7
7
8
8
9
9
DCI
Format
1A
1
1A
2B
1A
2C
Antenna Ports
port 0 or TX diversity
port 5 (virtual port)
port 0 or TX diversity
ports 7 and 8 (2-layer SU-MIMO); port 7 or 8 (1-layer MU-MIMO)
port 7
port 7 or 8
Table 1: 3GPP beamforming transmission modes
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TD-LTE – Exploiting the Time Domain
Just as MIMO beamforming can exploit the spatial domain to create multiple data
streams, TD-LTE itself exploits the time domain to perform duplexing.
The introduction to this paper noted that one original driver for TD-LTE was the lower
cost and global availability of single-band (unpaired) spectra as opposed to the dualband spectra required by Frequency-Division Duplexed (FDD) systems. However, timedivision duplexing is not just a matter of necessity; it offers several advantages over
FDD systems, especially when paired with MIMO beamforming.
By sharing a single frequency band for uplink and downlink, and by adjusting the
number of time slots available in each direction, TDD-based systems offer a degree of
flexibility in uplink/downlink resource allocation. For example, TD-LTE profiles include
seven different frame structures (shown in Figure 8). Theoretically, TDD systems can
dynamically re-allocate uplink/downlink resources on the fly, though initial TD-LTE
deployments are unlikely to include this feature (due to technical considerations beyond
the scope of this paper).
Figure 8: Seven frame structures available in TD-LTE systems.
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TD-LTE and MIMO Beamforming
Principles and Test Challenges
Another advantage that eases the implementation of MIMO beamforming in a TDD
system, is the concept of channel reciprocity. It was noted earlier that FDD systems
can implement beamforming as long as there is a feedback loop from the terminal that
informs the transmitter about the state of the downlink channel.
TDD systems do not require that feedback loop; the RF channel state is a function of
frequency, space and time. In TDD the uplink and downlink channels share the same
frequency, occupy the same physical space and are separated by relatively insignificant
slices of time. The TD-LTE uplink and downlink are “characteristically identical”. Figure
9 displays a close-up view (in terms of time) of a faded TDD radio channel. While the
overall range of the faded signal can be fairly substantial, the differences in channel
state from one time slot to the next are not.
Channel State
Channel State
Time
t
Figure 9: The reciprocal channel in TDD systems
As a result, channel estimation of the uplink can be used to make reasonable
assumptions regarding downlink channel characteristics. Channel reciprocity in a
single uplink/downlink frequency lends itself to a way of improving both coverage and
system quality: MIMO beamforming.
Early TD-LTE deployments plan to combine MIMO and beamforming, offering the
advantages of higher data rates as well as capacity and quality improvement. A typical
MIMO beamforming configuration can be thought of as a 2x2 MIMO system, except
that each of the two transmitted “layers” is actually a steered beam formed by four
transmitting antenna elements. This has led to growth in the study of 8 × n systems,
where each base station is equipped with eight antenna elements, as a cost-efficient,
spectrally-efficient alternative to the addition of cell sites or additional carriers. All of
this creates an incredibly complex RF environment with significant development and
test implications that must be addressed to ensure success in the rapidly emerging
TD-LTE markets.
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Principles and Test Challenges
Testing MIMO Beamforming Receivers
The Challenges of RF Testing in TD-LTE MIMO Beamforming
Given this fundamental understanding of the processes required in a TD-LTE MIMO
beamforming system, some of the potential pitfalls in testing become obvious:
1.The number of radio links involved
2.The creation of a realistically-reciprocal channel
3. Phase accuracy
4. Accurate replication of advanced antenna techniques
The Number of Radio Links
The TD-LTE MIMO beamforming systems that are being developed and tested in Asia
and elsewhere today are 8x2 systems. A plan exists to ramp up to 8x4 systems in the
relatively near future. Release 10 includes provisions for 8x8 beamforming.
An 8x2 system creates 16 separate radio links in each direction; an 8x4 system doubles
that; a bi-directional 8x8 system of the future will require 128. Each link must not
only be faithfully created on the test bench, the links must be managed so that test
operators can set specific values of correlation.
The Reciprocal Channel
Figure 10 depicts a live TDD channel, which is naturally reciprocal for reasons already
discussed. However, test equipment must deliver a level of control beyond that
of the live environment, which means that uplink and downlink channels must be
implemented separately. This implies that test equipment cannot deliver an accurate
reproduction of the live environment unless it can deliver nearly identical channel states
in both directions, at all times and on all the radio links being emulated.
DL
UL
Figure 10: The reciprocal TDD channel
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TD-LTE and MIMO Beamforming
Principles and Test Challenges
Phase Accuracy
The beamforming aspect of MIMO beamforming is entirely dependent on phase
characteristics of the transmitted radio links. Therefore, accurate testing must include
a high degree of phase accuracy. This is an important distinction between MIMO
beamforming testing and testing non-beamforming MIMO receivers.
Antenna Techniques
As was discussed, polarization is highly critical in MIMO beamforming systems. The
low correlation required by the MIMO aspect of the system is dependent on crosspolarization of the beams. Furthermore, since space is now a multiplexing domain, the
angles at which the emulated links arrive at the receiver and are delivered from the
transmitter are now critical parts of the RF environment.
Another aspect that must be accurately replicated in a spatially-oriented system is the
antenna pattern. As shown in Figure 11, this is far from uniform and is critical to system
performance.
Figure 11: Typical antenna pattern in a MIMO beamforming system
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Channel Models
GSM, CDMA and most wireless technologies employed RF channel models known as
“classical models”. They served for many years as accurate ways to replicate a radiated
signal as seen by a radio receiver.
Figure 12: Classical (left) and geometric (right) channel models
Figure 12 shows why the classical model is insufficient for testing in MIMO or
beamforming scenarios. As seen by a receiver’s antenna element, the field surrounding
the element is uniform. Since a single-antenna narrow-band receiver can not make use
of spatial information, this model is as good as any other.
A wide-band MIMO system, on the other hand, must make use of spatial information in
order to work. For the RF emulation to realistically reflect the real world the system must
replicate all relevant angles of departure, angles of arrival and angle spreads, (i.e.,
geometric channel model in Figure 12).
Noise
A by-product of the polarization needed to reduce signal correlation is that it also
reduces the power of the received signal. In addition, the directivity resulting from a
realistic antenna pattern and the gain provided by beamforming must be accounted for
as well.
This has an effect on adding noise for testing purposes. Unlike an SISO environment,
noise levels for testing are not simply a matter of adding a fixed value to the emulated
inherent noise from the transmitter. Instead, additive noise used in testing must be
based on:
•
The actual power as measured at each transmitting antenna element
•
The calculated loss between each transmitting antenna element and each
receiving antenna element
For MIMO beamforming testing, the calculated virtual output power must be used as
the basis for adding noise.
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TD-LTE and MIMO Beamforming
Principles and Test Challenges
Conclusion
TD-LTE technology is an ideal upgrade wherever network operators own single-band
spectrum originally intended for TD-SCDMA or WiMAX, or where lower-cost unpaired
spectrum is available.
Operators who intend to deploy TD-LTE have resolved to use MIMO techniques to
exploit space as a domain in which to multiplex data streams. These MIMO systems
also employ beamforming, a technique which optimizes coverage by concentrating
power where coverage is needed most. This is especially important in Asian markets,
where operators must optimize coverage due to pockets of high population density.
This paper discussed the drivers behind MIMO beamforming. At a conceptual level this
paper examined the techniques involved and included information intended to help
developers ensure success in deploying MIMO beamforming base stations and UEs.
Receiver test requirements for MIMO beamforming present a series of unique technical
challenges as outlined in this paper. These challenges must be considered in the
development and testing phases of a product’s lifecycle. With the goal of isolating
performance issues as early as possible in research and development, device engineers
must have the ability to replicate the complete real-world spatial channel conditions
of even the most complex environments in MIMO beamforming. Automatic phase
calibration, accurate creation of spatial channel models and support for
8 x n bi-directional MIMO, including MU-MIMO, are essential features for testing MIMO
beamforming.
Spirent’s MB5 MIMO Beamforming Test System is specifically designed for this purpose.
With years of experience in creating realistic RF environments, Spirent is well positioned
to provide a test solution that addresses all areas required to ensure the successful
deployment of TD-LTE and MIMO beamforming.
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Principles and Test Challenges
Acronyms
AMPS
Advanced Mobile Phone System
CDMA
Code Division Multiple Access
DCI
Downlink Control Information
FDD
Frequency-Division Duplexing
GSM
Global System for Mobile Communications
MIMO
Multiple-Input Multiple-Output
MU-MIMO
Multi-User MIMO
RF
Radio Frequency
SISO
Single-Input Single-Output
SNR
Signal-to-Noise Ratio
SU-MIMO
Single-User MIMO
TDD
Time Division Duplexing
TD-LTE
Time Domain Long-Term Evolution
TDMA
Time Division Multiple Access
TD-SCDMA
Time Domain Synchronous Code Division Multiple Access
UE
User Equipment
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