Nokia Networks white paper
Multi-antenna Optimization in LTE

Page 2 networks.nokia.com

Multi-antenna techniques have become an attractive solution for boosting user data rates, cell coverage areas and network capacity. Radio performance can be substantially enhanced by using the latest antenna technologies, while retaining existing base station sites and backhaul connections. The target is to bring more capacity and improve the network quality without increasing capital expenditure.
The technological framework of LTE is very well prepared for making use of advanced antenna configurations. The first LTE version in 3GPP, Release
8, included MIMO2x2 in the downlink, 4-antenna downlink transmission
(MIMO4x2) and multi-antenna uplink reception for four and eight antennas.
Also uplink Coordinated Multipoint (CoMP) with Centralized RAN (CRAN) can be supported with Release 8 terminals. All these features can be found in today’s commercial LTE networks.
Releases 9 and 10 added more advanced capabilities for 8-antenna
Transmission Modes (TM) 8 and 9. Release 11 defined the downlink CoMP feature with ideal backhaul, while Release12 introduced enhanced CoMP with both ideal and non-ideal backhaul. A new 4-transmit antenna downlink codebook was also in Release 12 and it is expected to achieve a 10% gain in system performance.
Release 13 is currently studying the use of larger numbers of MIMO antennas, also known as Massive MIMO, as well as using the vertical dimension for
MIMO transmission. The evolution of multi-antennas in 3GPP specifications is shown in Figure 1. All today’s terminals have two antennas for reception and one antenna for transmission. The first four-antenna terminals are already available in the market in the form of LTE + Wi-Fi routers.
Release 8
• 2x2MIMO
• 4TX downlink
• 4/8RX uplink
• Uplink CRAN
Release 9
• 8TX TM8
Release 10
• 8TX TM9
Release 11
• Downlink CoMP
(TM10)
Release 12
• Downlink eCoMP
• New 4TX codebook
Release 13
• Massive MIMO
Figure 1. 3GPP evolution of multi-antenna solutions (TM = Transmission Mode).
Based on the field deployments of multi-antennas, this white paper discusses site solutions, antennas and performance benefits. How antenna solutions will evolve further is also discussed. The inter-site interference coordination solutions – CoMP and eCoMP – are discussed in more detail in the Nokia Smart
Scheduler white paper.
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A compact site solution is essential to make multi-antennas work in practice.
The size of the antenna and the size and number of the RF units should be kept to a minimum. Typical cross-polarized multi-branch antennas are shown in Figure 2: the four-branch antenna uses two columns of cross-polarized antennas in the same radome as the eight-branch antenna, which comprises four columns. Co-polarized elements within a column are phased at RF to form the vertical elevation pattern needed to cover the sector. This approach helps to minimize the site space needed and means that the antennas could have remote electrical downtilt.
Antenna size is a function of wavelength: the size of the antenna is smaller at higher frequencies. Therefore, a four-branch antenna is quite practical at a frequency of around 2 GHz and an eight-branch antenna at higher frequency bands. The relative size of the antennas in different frequency bands is shown in Figure 3. Multi-band antennas are commonly used today - such antennas often include two-branch at low frequencies and four-branch at higher frequencies.
Figure 2. Typical multi-branch antenna solutions
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26 cm
32 cm
2.3 m
40 cm
1.3 m
0.7 m
Figure 3. Multi-branch antennas practical at high frequency bands
Minimizing the size of the RF units also helps ensure a compact site solution.
The most compact RF solution is the Nokia Flexi RF module, shown in Figure 4.
This module delivers 6x60W output power in a form-factor of less than 25 liters. A single module can provide MIMO2x2 for a three-sector base station.
The high power is beneficial for the coverage and indoor penetration, for both multi-radio and multi-carrier sharing.
Antennas
RF module
Figure 4. Compact Nokia RF module site solutions
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An LTE MIMO2x2 network can be enhanced by using four-antenna base station transmission and reception (4TX4RX). All LTE terminals support a four antenna solution since the capability was defined in Release 8. This makes the fourantenna approach a simple upgrade to the network, even with large numbers of legacy devices. There is no need to change devices to benefit from the full gain. It is expected that 4TX4RX can improve user data rates, especially at the cell edge, by around 50% and average data rates by some 20% in the downlink and even more in the uplink. 4TX4RX can also extend the cell size and improve indoor penetration. The expected gains from 4TX4RX are summarized in Figure 5.
Bene fi t of 4TX4RX Compared to 2TX2RX
Push average data rates +20% in downlink and +50% uplink
Boost cell edge data rates +50%
With legacy 2-antenna UEs
Increase cell range up to 3 dB
Cell edge with
2TX2RX
Cell edge with
4TX4RX
Figure 5. Benefits of 4TX4RX, compared to 2TX2RX (simulations, using legacy devices with 2 antennas)
We’ll next illustrate 4TX4RX benefits based on field measurements. Figure 6 shows stationary measurements with MIMO2x2 open loop (TM3) and closed loop (TM4) and MIMO4x2 with closed loop. Upgrading from two transmit antennas to four antennas produces clear link-level throughput gain, particularly in situations with low to medium Signal-to-Interference and
Noise Ratio (SINR). The total transmit power with four transmit antennas is
3 dB higher, so the result includes both power and diversity gain. The SINR shown in the horizontal axis is the value measured by the device. An even more important benefit of four antennas is the possibility of employing 4-way maximum ratio combining (MRC) or interference rejection combining (IRC) in the uplink. The uplink link budget is usually the limiting factor in practical networks.
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Field measurement, stationary UE
45
40
35
30
25
20
15
10
5
0
0 5 10 15
UE-measured SINR, dB
Figure 6. Downlink MIMO4x2 measurements
2x2 tm3
2x2 tm4
4x2 tm4
18 20
The measured cell throughput values from a live network with 2TX2RX and with 4TX4RX are shown in Figure 7.The uplink cell throughput increases by
100%, which is a very promising result and in fact uplink improvements are the major reason for deploying 4TX4RX technology.
The downlink cell throughput increases by 50%. In general, the downlink benefits consist of the multi-antenna transmission gain and the higher transmission power. In this case, the downlink power was doubled by using
4T4RX compared to 2TX2R. The downlink improvements are also beneficial since the 4RX antenna can also directly support 4TX.
Cell throughput in live network
16
14
12
10
8
6
4
2
0
Downlink Uplink
Figure 7. 4TX4RX gains in live FDD network
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2TX2RX
4TX4RX networks.nokia.com

Eight-antenna (8TX) beamforming can use Transmission Mode 8 or 9. TM8 uses device measurements based on downlink Common Reference Signals
(CRS) and base station measurements based on uplink Sounding Reference
Signals (SRS). The drawback of the uplink SRS based solution for downlink beamforming is that only one transmit antenna is used in the uplink, while two receive antennas are used. This means the uplink does not provide full channel information for the downlink direction. TM9 brings further improvement on top of TM8 by using device-specific Demodulation Reference Signals (DM-RS) for the downlink measurements to generate feedback to the network.
8TX has been deployed in TD-LTE networks. The beamforming evolution in
3GPP is shown in Figure 8. An example set of drive test results are shown in
Figure 9. These show that TM8 beamforming brings a clear benefit to user data rates compared to TM3. The peak rate does not increase and the data rate in the good signal areas also does not increase, while the average data rate and the cell edge data rate are improved considerably. TM9 offers a better performance than TM8, while the performance can also be enhanced with 4-antenna terminals. Using 4-antenna terminals instead of 2-antenna terminals, the benefit in cell capacity is expected to be 40-50% and 70-100% in the cell edge data rate.
3GPP Release 8
TM7/TM3
TM3
TM7
3GPP Release 9
TM8 SUand MU-MIMO
CQI
TM8
Figure 8. 3GPP Evolution for TD-LTE beamforming
3GPP Release 10
TM9 SUand MU-MIMO
TM9
3GPP Release 11
TM10 CoMP
PMI1 PMI2
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PDCP Thr’put DL (kb/s)
1.25
1
0.75
0.5
0.25
0
TM3 TM8
-0.25
0
TM3 Round0
TM8 Round1
10000 20000 30000
TM3 Round1
TM8 Round2
TM3 Round2
TM8 Round3
40000 50000 60000
TM3 Round3
Figure 9. 8TX Downlink gains in the field in TD-LTE
TM8 Round0
The performance of downlink TD-LTE can be further enhanced with Nokia
Intelligent Beamforming, in which the eNodeB algorithms also consider intercell interference. Nokia Intelligent Beamforming not only tries to maximize the signal level, but also attempts to maximize the signal-to-interference ratio, leading to substantially higher data rates for users at the cell edge. One such algorithm is shown in Figure 10.
Cell 1 2
C e l l 1
Figure 10. Nokia Intelligent Beamforming for avoiding inter-cell interference
Combining uplink 8RX with Multi-user MIMO (MU-MIMO) is an attractive solution for increasing uplink cell throughput and coverage. Figure 11 shows that, in mobility tests, uplink MU-MIMO gains exceed 40%.
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14
12
10
4
2
8
6
ff
0
Figure 11. Over 40% uplink throughput gain with Multi-user MIMO
Uplink multi-antenna reception can also be extended beyond a single cell. In a
Centralized RAN network, the data transmitted by a device is received by the antennas from multiple cells. While the basic concept is known as uplink CoMP, the Nokia Centralized RAN solution, as illustrated in Figure 12, allows the dynamic combination of uplink signals received by up to 12 antennas served by different base band units. The major benefit here is that uplink CoMP works with existing base station hardware and all existing LTE terminals.
Fiber
Heavy cell overlapping in open area
RF
Baseband
Figure 12. Uplink CoMP implementation with Centralized RAN
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Centralized RAN offers a major gain in uplink throughput and capacity by turning inter-cell interference into a useful signal. Interference is a major problem, particularly in open areas such as stadiums. The gain from
Centralized RAN is illustrated in Figure 13. Live network deployments typically show an increase in uplink data rates and capacity of between 100 and
200%. The uplink improvements are important for mass events where people typically want to share pictures and videos and where networks are uplink limited.
Uplink throughput at stadium
120
100
80
60
40
20
CRAN o ff
CRAN on
0
Open loop power control
Closed loop power control
Figure 13. Uplink CoMP gains in live network with Centralized RAN (CRAN)
Using a 6-sector solution instead of a conventional 3-sector solution can improve network capacity and coverage. The 6-sector configuration increases system capacity by minimizing the inter-cell interference and increases coverage through higher antenna gain. A 3-sector antenna typically has a
65 degree 3dB beam width, while a 6-sector antenna typically has a 35 degree beam width. Many Nokia networks have used 6 sectors, mainly for increasing network capacity in congested areas. The configurations are shown in
Figure 14.
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3 sectors
6 sectors: less interference and more coverage
Figure 14. 3 Sector and 6 sector configurations
The physical size of a 6-sector site solution can be minimized by using dual beam antennas where the single antenna creates two beams for two adjacent sectors. The complete 6-sector site solution with MIMO2x2 consists of three antennas, two Nokia RF modules and one system module, which is significantly more compact than traditional solutions. This is shown in Figure 15.
Figure 15. Compact 6-sector site solution
Nokia Smart Dual Beam solution makes it possible to take benefit of 6 sector solution also in GSM technology.
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The evolution of Nokia RF technology makes it practical to integrate RF units and antennas together, as shown in Figure 16. Such integration means that the site solution becomes even more compact, minimizing the number of separate units required. Since most operators will soon run networks on up to five different frequency bands, integrated solutions must be capable of supporting this. Nokia’s integrated solution is called Radio Antenna
System (RAS). The next phase in the evolution is the Active Antenna System
(AAS), where a large number of small RF units are distributed inside the antenna. Active antennas also enable flexible beamforming in the vertical plane, as shown in Figure 17. Nokia is the clear leader with active antenna implementations - live network testing has been completed in both 3G and in
LTE networks with major operators, with gains exceeding 50%.
3GPP Release 13 brings even better support for active antennas with the introduction of the Massive MIMO (Full dimension MIMO or 3D MIMO) work item. Massive MIMO enables transmission using up to 64 antenna ports.
With multiband site solutions, we also need to consider the identification and avoidance of Passive Intermodulation (PIM) effects, for example, in the case of shared 700 MHz and 800 MHz deployments. Nokia RF products can run PIM measurements remotely, determining the reasons for PIM and allowing any uplink desensitization to be avoided. Nokia also supports frequency domain interference avoidance using the Smart Scheduler and Nokia will also develop an active PIM cancellation solution.
Figure 16. Nokia integrated antenna solutions
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Figure 17. Vertical beamforming with active antennas
Base station multi-antenna solutions enhance network capacity and coverage and offer higher data rates while continuing to use legacy two-antenna devices. The 4TX4RX solution is already widely deployed in commercial FDD and TDD networks, with 8TX8RX in use in TDD networks. 4TX4RX antennas at
2 GHz band and 8TX8RX at higher bands are quite compact and well suited to urban deployments. 4TX4RX can also be used in bands below 1 GHz, particularly in rural towers where larger antennas are allowed. Field results indicate major gains from 4TX4RX and 8TX8RX, particularly in cell edge performance in the downlink, with even greater gains in the uplink. Nokia’s
TD-LTE Intelligent Beamforming algorithm further improves the benefits of
8TX solutions.
Four-antenna devices can enhance the network’s average throughputs and capacity, and also increase the peak data rate with MIMO4x4. Nokia Flexi base station offers a smooth evolution based on software upgrades to support
MIMO4x4 in the downlink.
The reception of uplink multi-antennas can be extended to several cells with the Nokia Centralized RAN solution. This has proven very powerful in increasing uplink capacity during mass events held at stadiums and at similar gatherings. Nokia Centralized RAN works with any terminals and with current base station hardware.
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• Nokia Centralized RAN summary
• Nokia Centralized RAN press release
• Nokia TD-LTE Intelligent Beamforming
• Nokia High Performance 6-sector Site white paper
• Nokia Active Antenna System white paper
• Nokia Flexi Multiradio 10 base station
• Nokia Smart Scheduler white paper
• Nokia Liquid Radio Software Suites summary
Page 15 networks.nokia.com

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