Multi-Antenna System Design for 3GPP LTE Amitava Ghosh 1, Weimin Xiao 2, Rapeepat Ratasuk 3, Alan Rottinghaus 4, Brian Classon 5

Motorola Inc 1421 West Shure Drive, Arlington Heights, IL 60004, USA

1 amitava.ghosh@motorola.com, 2 weiminxiao@motorola.com, 3 ratasuk@motorola.com, 4 rottinghaus@motorola.com, 5 abc007@email.mot.com

Abstract—Standardization work is nearly complete on long term evolution (LTE) of the UMTS Terrestrial Radio Access and Radio Access Network which is aimed for commercial deployment in 2010. Goals for the evolved system include support for improved system capacity and coverage, high peak data rates, low latency, reduced operating costs, multi-antenna support, flexible bandwidth operations and seamless integration with existing systems. To reach these goals, a new design for the air interface including state-of-art multi-antenna technology needs to be deployed. This paper provides a look at different multi-antenna schemes for LTE downlink and uplink. The paper also discusses various Node-B antenna configurations and summarizes the performance of different multi-antenna schemes under various scenarios.

I. INTRODUCTION With the emergence of packet-based mobile broadband

systems such as 802.16e, it is evident that a comprehensive long term evolution (LTE) of UMTS is required to remain competitive in the long term. As a result, work was started on Evolved UMTS Terrestrial Radio Access (E-UTRA) aimed at commercial deployment around 2010 timeframe. Long term goals for the system include support for high peak data rates (100 Mbps downlink and 50 Mbps uplink), low latency (10ms round-trip delay), improved system capacity and coverage, reduced operating costs, multi-antenna support, efficient support for packet data transmission, flexible bandwidth operations (up to 20 MHz) and seamless integration with existing systems [1]. To reach these goals, a new design for the air interface is adopted. This includes a state-of-art multi-antenna scheme (MAS) for both downlink (DL) and uplink (UL). The multi-antenna scheme for downlink comprises of transmit diversity, open-loop and closed-loop spatial multiplexing, multi-user MIMO (MU-MIMO), beam-forming etc. while for the UL MU-MIMO and max-ratio/interference rejection combining (MRC/IRC) are supported.

The paper is organized as follows. In Section II, an overview of E-UTRA DL and UL MAS is provided. This is followed, in Section III, by a discussion on control signaling which goes with MAS. In Section IV, possible Node-B transmit/receive antenna configurations are introduced. Section V then presents the system performance of various MAS schemes. Finally, conclusions are drawn in Section VI.

II. SUMMARY OF MULTIPLE ANTENNA SCHEMES In the downlink, OFDM is selected as the air-interface for

E-UTRA. With OFDM, it is straightforward to exploit frequency selectivity of the multi-path channel with low-complexity receivers. This allows frequency selective in

addition to frequency diverse scheduling and one cell reuse of available bandwidth. Furthermore, due to its frequency domain nature, OFDM enables flexible bandwidth operation with low complexity.

In the uplink, Single-Carrier Frequency Division Multiple Access (SC-FDMA) is selected to efficiently meet E-UTRA performance requirements. SC-FDMA has many similarities to OFDM, chief among them for the uplink is that frequency domain orthogonality is maintained among intra-cell users to manage the amount of interference seen at the base station. SC-FDMA also has a low power amplifier de-rating requirement, thereby conserving battery life or extending range. A comprehensive overview of LTE air-interface may be found in [8].

Table I lists the available downlink and uplink physical channels and their purpose.

TABLE I PHYSICAL CHANNELS IN LTE

Channel Purpose

Physical Downlink Shared Channel (PDSCH) Carry user data (downlink)

Physical Broadcast Channel (PBCH) Carry broadcast information

Physical Multicast Channel (PMCH) Carry multicast services

Physical Control Format Indicator Channel (PCFICH)

Indicate the size of the control region in number of OFDM symbols

Physical H-ARQ Indicator Channel (PHICH)

Carry ACK/NACK associated with uplink transmission

DL

Physical Downlink Control Channel (PDCCH)

Carry downlink scheduling assignments and uplink scheduling grants

Physical Uplink Shared Channel (PUSCH) Carry user data (uplink)

Physical Uplink Control Channel (PUCCH)

Carry ACK/NACK associated with downlink transmission, scheduling request, and feedback of downlink channel quality and precoding vector

UL

Physical Random Access Channel (PRACH)

Carry random access transmission

The downlink subframe structure is shown in Fig. 1 with support for four transmit antennas and normal cyclic prefix. Each subframe consists of two slots of length 0.5ms. Within each slot, reference symbols are located in the 1st and 5th OFDM symbols for antenna ports 0 and 1, and in the 2nd

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OFDM symbols for antenna ports 2 and 3. This structure allows simple channel estimator to be used as well as other excellent performance, low-complexity techniques such as MMSE-FIR and IFFT-based channel estimators. Downlink control signaling (PDCCH) is located in the first n OFDM symbols with n ≤ 3 and with the earliest data transmission start after OFDM symbol where the control signaling ends [1].

In LTE, the following multi-antenna schemes are currently supported:

A. Space Frequency Block Code (SFBC) The SFBC operation is defined for 2 and 4 transmit

antennas and is also termed as pre-coding for transmit diversity and is used for rank-1 transmission. In the case of four transmit antennas the SFBC transmit matrix is given below. Each row represents one transmit antenna and different columns represent different resource elements. Each element in the time and frequency resource grid is called a resource element.

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

−

−

*34

*43

*12

*21

0000

0000

SSSS

SSSS

B. Open-loop Spatial Multiplexing The large-delay CDD operation is specified only as an

open-loop spatial multiplexing defined by

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

=⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

−− )(

)()()(

)(

)(

)1(

)0(

)1(

)0(

ix

ixUiDiW

iy

iy

P υMM

where W(i) is the precoding matrix of size P×υ. The diagonal size-υ×υ matrix D(i) supporting cyclic delay diversity and the size-υ×υ matrix U are both given in Table 6.3.4.2.3-1 or 6.3.4.2.3-2 of [1] for different numbers of layers υ. It may be noted that Node-B cyclically assigns different precoding matrices taken from a fixed codebook subset, to different data sub-carriers i in the scheduled subband. This feature is generally useful at higher values of vehicle speeds.

C. Closed-loop spatial multiplexing (Single-user MIMO) In this scenario, the UE feedbacks the channel quality

indicator (CQI) and precoding matrix indicator (PMI) information which is used at the Node-B transmitter. LTE supports 2x2, 4x2, and 4x4 DL SU-MIMO configurations

with rank adaptation. Although up to 4 layers are supported, with fixed mapping between layers and codewords as specified in [1], only one (1) or two (2) codewords or data streams may be simultaneously transmitted. Accordingly, the rank field indicates 1 or 2 codeword transmission, and the PMI indicator can select from a 2-Tx (size 8) or 4-Tx (size 16) precoding matrix codebook. In more detail, for 2x2, 4x2 and 4x4 mode, the supported DL MIMO configuration enables support for single-stream (υ=1), dual-stream (υ=2) using the codebook specified in [1]. SU-MIMO is illustrated in Fig. 2.

101000 101001HMod/code

C1 C2 CN

C1C2

CN

index feedback

H channelestimation

Base station Mobile station

codebook

101000 101001HMod/code

C1 C2 CN

C1C2

CN

index feedback

H channelestimation

Base station Mobile station

codebook

Fig. 2. Single-user MIMO.

D. Multi-user (MU) MIMO In MU-MIMO, a pair of UE uses the same time-frequency

resource with Rank-1 transmission. MU-MIMO is most useful when the Node-B antennas are correlated. In LTE, the MU-MIMO precoding vector uses a subset of SU-MIMO codebook. The CQI computation at UE is similar to that of SU-MIMO. However some details are still for further study in the specification e.g. signaling of interference vectors. The MU-MIMO is illustrated in Fig. 3.

101111H2

C1 C2 CN

H2 channelestimation

UE#2

codebook

101001H1

C1 C2 CN

H1 channelestimation

UE#1

codebook

Index #2 feedback

Index #1

feedback

101000Mod/code C1

C2

CN

Base station

MUprocessor

Mod/code101111

101111H2

C1 C2 CN

H2 channelestimation

UE#2

codebook

101111H2

C1 C2 CN

H2 channelestimation

UE#2

codebook

101001H1

C1 C2 CN

H1 channelestimation

UE#1

codebook

101001H1

C1 C2 CN

H1 channelestimation

UE#1

codebook

Index #2 feedback

Index #1

feedback

101000Mod/code C1

C2

CN

Base station

MUprocessor

Mod/code101111

101000Mod/code C1

C2

CN

Base station

MUprocessor

Mod/code101111

Fig. 3. Multi-user MIMO.

Table II lists the supported multi-antenna modes for all physical channels. In the downlink, the data channel is able to utilize all spatial multiplexing techniques presented above. Currently, the common reference signal design allows for 4 transmit antennas at the Node-B. Multi-antenna transmission employing more than 4 transmit antennas (e.g. beamforming) will be supported using dedicated reference signals. Unlike the data channel, all downlink common control channels (PCFICH, PHICH, and PBCH) are transmitted using open-loop transmit diversity. Currently user-specific control information transmitted on the PDCCH also uses open-loop transmit diversity. Although in [4] it was shown that closed loop precoding can substantially improvement user-specific control channel performance, this potential feature has been deferred for study in a later LTE release. In the uplink, there is currently no support for multi-antenna transmission as only one transmit antenna is available at the UE. However, with multiple receive antennas at the Node-B, MU-MIMO can be supported on the PUSCH. In addition, at the Node-B’s receiver, max-ratio combing (MRC)/interference rejection combining (IRC) is supported.

Fig. 1. Downlink reference signal structure – normal cyclic prefix, four transmit antennas.

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TABLE II SUPPORTED MULTI-ANTENNA MODES

Channel Supported Multi-Antenna Mode

PDSCH

Tx-diversity, Open loop spatial multiplexing, closed loop spatial multiplexing, MU-MIMO, Beamforming

PMCH - DL

PCFICH, PHICH, PDCCH, PBCH Transmit diversity (SFBC)

PUSCH MU-MIMO UL

PUCCH, PRACH None

III. E-UTRA CONTROL SIGNALLING FOR MAS The design of control signaling for both the uplink and

downlink are critical for the corresponding downlink and uplink multiple antenna schemes to operate properly.

Uplink control signaling not associated with data is transmitted independently of uplink data packet. The control signaling associated with downlink multiple antenna schemes include ACK/NACK, CQI, Rank and PMI feedback. When users have simultaneous uplink data and control transmission, control signaling is multiplexed with data prior to the DFT to preserve the single-carrier property in uplink transmission. In the absence of uplink data transmission, this control signaling can either be transmitted periodically in the PUCCH or in a periodic or a-periodic fashion using the PUSCH.

The CQI, PMI, and rank reporting can be periodic or a-periodic. A UE transmits CQI, PMI, and rank reporting on a PUCCH for sub-frames with no PUSCH allocation and on a PUSCH for those sub-frames with PUSCH allocation for scheduled PUSCH transmissions with or without an associated scheduling grant. The CQI reported on PUCCH and PUSCH can either be frequency selective or frequency non-selective. The various scheduling modes are summarized in Table III. It may be noted that a-periodic CQI report takes priority over periodic CQI reports.

TABLE III PHYSICAL CHANNELS FOR CQI REPORTING

Scheduling Mode Periodic CQI A-periodic CQI

Frequency non-elective PUCCH, PUSCH PUSCH

Frequency selective PUCCH, PUSCH PUSCH

The transmission of a-periodic CQI, PMI and Rank reports on PUSCH are triggered after receiving an indication sent in the scheduling grant whereas the periodic CQI reports on PUSCH and PUCCH are configured using higher layer signaling. An UE supports dynamic rank adaptation between all applicable transmission ranks for closed-loop spatial multiplexing. An UE also supports dynamic rank adaptation between rank-1 transmit diversity and rank>1 large delay CDD for open-loop spatial multiplexing.

IV. NODE-B ANTENNA CONFIGURATIONS The design of Node-B for LTE depends upon various

factors such as: a) number of transmit and receive elements which in turn depends on the supported multiple antenna

schemes, b) LPA power, c) tower bottom, tower top or roof-top configurations of the radio head, d) linear array or cross-polarized etc, e) carrier-frequency, f) range and cell-edge data rate requirements, g) type of duplexing scheme (FDD/TDD), etc. A few possible Node-B configurations are summarized in Table IV for a 10 MHz LTE system.

TABLE IV NODE-B ANTENNA CONFIGURATIONS @ 10 MHZ

Configuration Type #Tx/Rx Elements

Tx power per path (W)

Feeder Loss (dB)

Tower top remote radio head

Tx:2/4/8 Rx:2/4/8 2 - 5 Zero

Roof top remote radio head

Tx:2/4/8 Rx:2/4/8 ~10 ~1

Tower bottom traditional indoor frame

Tx:2/4 Rx:2/4 ~20 >3

In general, Motorola’s e-Node-B solution consists of baseband digital modem which is connected to remote radio head consisting of transceiver/PA using digital baseband interface as shown in Fig. 4.

Remote Radio HeadRemote Radio Head

Remote Radio HeadRemote Radio Head

Baseband Modem

Transceiver / PA

Fig. 4. Motorola’s eNode-B diagram.

The choice of antenna configurations at the Node-B is highly dependent on the requirements for target UL and DL cell edge data rates. Table V and Table VI show the link budgets for different antenna configurations at AWS and 700 MHz band respectively. The detailed link budget parameters are given in Table VIII. The following conclusions can be drawn from the tables:

A. Generally 700 MHz deployment has better overall coverage since it has much lower propagation loss although the antenna gain is lower @ 700 MHz and penetration loss slightly higher.

B. The type of antenna configurations depends on the target UL and DL cell edge data rates. As an example for cell size of 1.7km and target UL data rate of >500 Kbps @ 700 MHz, tower top configuration with 4 transmit and 8 receive antennas is preferred.

TABLE V CELL EDGE DATA RATES VS. NODE-B ANTENNA CONFIGURATION @ AWS

Cell Edge Rates

2Tx, 2Rx

2Tx, 2Rx w TTLNA 2Tx, 4Rx 4Tx, 4Rx 4Tx, 8Rx

(TT)

UL (Kbps) 176 176/264 176/264 176/264 176/528

DL (Mbps) 2.2 1.1/2.2 1.1/2.2 4.4/6.6 1.1/6.6

Radius (km) 1.22 1.43/1.26 1.43/1.26 1.43/1.26 1.93/2.23

480

TABLE VI

CELL EDGE DATA RATES VS. NODE-B ANTENNA CONFIGURATION @ 700 MHZ

Cell Edge Rates

2Tx, 2Rx

2Tx, 2Rx w TTLNA 2Tx, 4Rx 4Tx, 4Rx 4Tx, 8Rx

(TT)

UL (Kbps) 176 176/264 176/264 176/264 176/528

DL (Mbps) 2.2 1.1/2.2 1.1/2.2 4.4/6.6 1.1/6.6

Radius (km) 1.68 1.87/1.64 1.87/1.64 1.97/1.73 2.57/1.73

V. SYSTEM PERFORMANCE In this section, the system performance of FDD LTE for

different antenna configurations is presented. The parameters used in the system simulations are shown in Table IX. Capacity comparisons are done for the following cases:

Downlink: (a) 2Tx Tower Bottom: SU-MIMO w/Precoding, 20W,

40W, 80W @ AWS, 20W, 40W @ 700MHz (b) 4Tx Tower Bottom: SU-MIMO w/ Precoding, 40W,

80W (c) 4Tx Tower Top: SU-MIMO w/ Precoding, 8W

Uplink: (d) 2Rx Tower Bottom : w/o TTLNA, Handset w/ MRC,

2 GHz (e) 4Rx Tower Bottom w/o TTLNA, Handset MRC, MU-

MIMO

Fig. 5 and Fig. 6 show the downlink sector throughput and cell edge user throughput comparison for the various cases. Based on the results shown, there is a marginal difference in capacity between 700 MHz and AWS band. Significant improvement in sector and cell edge throughput is seen with 40W for both 2x2 and 4x2 configurations. Also, 4x2 configuration using tower top configuration (8W total power) provides good compromise in sector and edge throughput.

DL Sector Throughput (Kbps)

0

5000

10000

15000

20000

25000

2x2_

AR_20W_70

0MHz

2x2_

AR_40W_70

0MHz

2x2_

AR_40W_2G

Hz

2x2_

MMSE_20W_2G

Hz

2x2_

MMSE_40W_2G

Hz

2x2_

MMSE_80W_2G

Hz

4x2_

Rank1

_16W_2

GHz

4x2_

AR_16W_2G

Hz

4x2_

AR_40W_2G

Hz

4x2_

AR_80W_2G

Hz

Kbp

s

Fig. 5. DL Sector ThroughputAll paragraphs must be indented.

DL Cell Edge Throughput (Kbps)

0100200

300400500600

700800

2x2_

AR_20W_70

0MHz

2x2_

AR_40W_70

0MHz

2x2_

AR_40W_2G

Hz

2x2_

MMSE_20W_2G

Hz

2x2_

MMSE_40W_2G

Hz

2x2_

MMSE_80W_2G

Hz

4x2_

Rank1

_16W_2

GHz

4x2_

AR_16W_2G

Hz

4x2_

AR_40W_2G

Hz

4x2_

AR_80W_2G

Hz

Kbps

Fig. 6. DL Edge User Throughput.

Fig. 7 and Fig. 8 show the uplink sector throughput and cell edge user throughput comparison for the various cases. However, there is a significant gain with 4 RX antennas and MU-MIMO for both sector and cell edge throughput.

UL Spectral Efficiency

0

0.2

0.4

0.6

0.8

1

1.2

HSUPA LTE 1x2 MRC LTE 1x4_MRC LTE 1x4_SDMA

Technology

SE (b

ps/H

z/se

ctor

)

Fig. 7. UL Sector Throughput.

UL Edge Spectral Efficiency

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

HSUPA LTE 1x2 MRC LTE 1x4_MRC LTE 1x4_SDMA

Technology

SE

(bps

/Hz/

sect

or)

Fig. 8. UL Edge User Throughput.

Next, the performance of DL closed-loop spatial multiplexing schemes and MU-MIMO with different antenna spacing were investigated with 4 transmit antennas at Node-B and 2 receive antennas at UE. The results are summarized in Table VII.

481

TABLE VII PERFORMANCE AT 0.5-λ AND 10-λ ANTENNA SPACING

Half wavelength

spacing

Ten wavelength

spacing

Half wavelength

spacing

Ten wavelength

spacing 2 Tx-Ant MIMO

Sector throughput

5%-ile user throughput

Sector throughput

5%-ile user throughput

Rank 1 BF 19.394 Mbps 739 Kbps 16.042

Mbps 505 Kbps

SU-MIMO w/ MMSE

19.598 Mbps 745 Kbps 18.308

Mbps 516 Kbps

SU-MIMO w/ MMSE+SIC

20.130 Mbps 753 Kbps 19.427

Mbps 479 Kbps

MU-MIMO 19.975 Mbps 735 Kbps 15.994

Mbps 492 Kbps

Based on the results shown in Table VII., it is seen that MU-MIMO works well only with correlated antennas, while SU-MIMO with rank adaptation works well both for uncorrelated and correlated antennas. For Rank-1 beam-forming, correlated antennas provide better performance than un-correlated antennas.

VI. CONCLUSION In this paper, an overview of multi-antenna schemes for

LTE is provided. The paper also discusses various Node-B antenna configurations and summarizes the performance of multi-antenna schemes under various scenarios.

REFERENCES [1] 3GPP TS 36.211, Evolved Universal Terrestrial Radio Access (E-

UTRA); Physical Channels and Modulation, v.8.1.0, November 2007. [2] 3GPP TR 25.913, Requirements for Evolved UTRA (E-UTRA) and

Evolved UTRAN (E-UTRAN), v.7.3.0, March 2006. [3] 3GPP TR 25.814, Physical Layer Aspects for Evolved UTRA, v.2.0.0,

June 2006. [4] R1-073990, “Support of Precoding for E-UTRA DL L1/L2 Control

Channel,” Motorola, RAN1#50-Bis, Shanghai, China, Oct 2007. [5] R1-073983, “Beamforming for E-UTRA,” Motorola, RAN1#50-Bis,

Shanghai, China, Oct 2007. [6] R1-074613, “MU-MIMO for E-UTRA,” Motorola, RAN1#51, Jeju,

Korea, Nov 2007. [7] Ghosh, A. et al, “Uplink Control Channel Design for 3GPP LTE,”

IEEE 18th International Symposium on Personal, Indoor and Mobile Radio Communications, September 2007.

[8] Classon, B. et al, “Overview of UMTS Air-Interface Evolution,” IEEE 64th Vehicular Technology Conference, September 2006.

Note – 3GPP documents may be downloaded from ftp://ftp.3gpp.org

TABLE VIII SYSTEM SIMULATION PARAMETERS

Parameter Assumption

Cellular Layout Hexagonal grid, 19 cell sites, 3 sectors per site

Inter-site distance (ISD) 500m, 1732m

Distance-dependent path loss L=I + 37.6log10(.R), R in

kilometers I=128.1 – 2GHz

Lognormal Shadowing Similar to UMTS 30.03, B 1.41.4

Shadowing standard deviation 8 dB Correlation distance of Shadowing 50 m

Between cells 0.5 Shadowing correlation Between sectors 1.0 Penetration Loss 20dB Carrier Frequency 700 MHz and 2.0 GHz Channel model Typical Urban (TU) UE speeds of interest 3 km/h Total BS TX power various UE power class 24dBm

Inter-cell Interference modeling UL: Explicit modeling (all cells occupied by UEs)

Min distance between UE and cell >= 35 meters

TABLE IX

SYSTEM SIMULATION PARAMETERS

Parameter AWS 700 MHz

Erceg-Greenstein B PL = A + B×log10(r), r in kilometer

Propagation Model DL (2.1GHz) – A=123.63, B=4.33

UL (1.7GHz) – A=121.24, B=4.33

A=111.22, B=4.33

Penetration Loss 18dB Building, 2dB Body Loss

15dB Building, 2dB Body Loss

Tower Bottom Cable Loss 3 dB 2 dB

Node-B DL = 16.5dBi, UL = 15.5dBi

12.5dBi Antenna

Gain UE -2dBi -6dBi

Fast Fading Margin 2 dB

Log-Normal Fading Margin 4.9 dB

Interference Margin 3 dB

H-ARQ Gain 3 dB

Node-B Noise Figure 5 dB

UE Noise Figure 9 dB

Rx Diversity Gain 10×log(Number of Ants)

Transmit Power Tower bottom = 10W/branch, Roof top remote

radio head 10W/branch, Tower top remote radio head = 2W/branch

482