White Paper by Ralink on 802.11n Radio Beamforming Technology

January 2010 Ralink Technology

WHITE PAPER

1

Transmit Beamforming

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Improving Wireless System Performance in Multipath Environments using Transmit Beamforming

1 Introduction

Multipath is often a problem in wireless systems. It causes fading as radio signals travel

over multiple paths and interfere with each other when they arrive at the receiver.

Preprocessing the wireless signal at the transmitter using a technique known as

beamforming can overcome multipath effects to improve link throughput and robustness.

IEEE 802.11n specifies a number of MIMO (Multi-Input Multi-Output) techniques that

use multiple antennas to improve performance in a multipath environment. Transmit

Beamforming (TxBF) is one of these techniques. TxBF is an optional feature in IEEE

802.11n, but there is growing industry demand for this feature in various wireless

applications. The Wi-Fi Alliance has listed beamforming as one of the optional features

in its 11n certification program. Ralink offers both 11n standard beamforming and a

proprietary form of beamforming that is compatible with legacy 11a/g devices. Both

methods greatly improve link robustness and throughput.

This paper gives some background on transmit beamforming and presents simulated and

measured performance data.

2 Beamforming Basics

Beamforming is a wireless transmission technique in which an array of antennas are

“directed” at a desired target or source by adjusting the relative gain and phase of the

Transmit Beamforming and 802.11nRalink Confidential 2©2010 Ralink Technology Corp.

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array elements. By adjusting the relative gain and phase of the elements, the antenna

pattern, or beam, can be made to point in a favored direction for receiving or transmitting

data, or to attenuate other directions in order to reduce the effect of an interference

source. Using beamforming can improve reception quality, and increase data throughput

in a Multi-In, Multi-Out (MIMO) communication system. A basic requirement for

transmit beamforming is the use of multiple antenna elements at the transmitter, and the

use of the measured wireless channel between the transmitter and receiver.

The IEEE 802.11n standard defines several methods for MIMO transmit beamforming of

the OFDM signal. One method, referred to as “explicit beamforming”, requires the

downstream channel to be measured at the receiver, or beamformee, and relayed back to

the transmitter, or beamformer. The beamformer uses the measured channel information

to derive the transmit beamforming parameters. A second technique defined in the

standard is “implicit beamforming”. In this implementation, the upstream wireless

channel is measured by the beamformer, and the measurement used to derive the

parameters for subsequent downstream beamformed transmission. Implicit beamforming

has the advantage that the beamformee does not need to measure and send the channel

state information to the beamformer. However, 11n standard implicit beamforming

requires a calibration exchange between the beamformer and beamformee, which can

complicate the transceiver design. Ralink’s proprietary implicit design does not require

this exchange, making it backward compatible with existing legacy 802.11a and 802.11g

products.

The figures below illustrate the operation of transmit beamforming. Figure 1 shows a

conventional one transmitter (1T) and one-receiver (1R) system without beamforming.

In contrast, Figure 2 shows a two-transmitter (2T) one-receiver (1R) system with

beamforming. In both cases, OFDM modulation scheme is used for transmission.


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OFDM Encoder

OFDMDecoder

RX Data

TX Data TX RX

Wireless Channel

Figure 1: System of single antenna at transmitter and receiver.

OFDMEncoder

TxBFProcess

Tx1:a1

Tx2:a2

TX Data

OFDMDecoderRx RX

Data

1h

2h

Figure 2: System of 2 Tx Beamformer with 1R beamformee receiver.

In Figure 2, beamforming is applied to a single transmit stream to produce two encoded

streams for two transmitters. These streams travel across a multipath channel to the

receiver. The receiver processes the combined streams to recover the original transmit

stream. Assuming that the channel coefficients are known at the transmitter,

beamforming essentially allows the transmitter to phase the two transmissions so that the

signal is optimally combined at the input to the receiver, thus preventing any signal

cancellation that could occur in a random channel.

Note that the 802.11a/g/n standards all employ multi-carrier modulation using the

IFFT/FFT algorithm (OFDM). The effect of OFDM is to decompose the channel into

many orthogonal narrow-band channels in the frequency domain. In Figure 2, the

channels ih and beamforming parameters 1 2,a a , are vectors of frequency domain

coefficients. Because the channels are orthogonal, each can be evaluated independently

finally, that on each receive antenna, the noise


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y h h n h h x n

to assess the overall effect of beamforming. This idea is further developed below with a

simple numerical model and simulation of a scalar OFDM sub-channel.

2.1 Channel ModelAs a simple example, we will assume that a particular channel can be modeled as having

flat fading Rayleigh multipath channel, and the transmitted signal modulation is BPSK.

As shown in Figure 2, the simplest beamforming configuration is used, with 2

transmitters and 1 receive antennas, i.e., a 2T1R system.

For a flat fading channel model means that the multipath channel has only one tap. So,

the channel operation reduces to a simple multiplication. For this simulation, the channel

experienced by each receive antenna is randomly varying in time. For the thi receive

antenna, each transmitted symbol gets multiplied by a randomly varying complex number

ih . As the channel under consideration is a Rayleigh channel, the real and imaginary

parts of ih are Gaussian distributed having mean ih and variance 1

2.

In addition, it is assumed that the channel experience by each transmit antenna to receive

antenna is independent from the channel experienced by other transmit antennas, and

finally, that on each receive antenna, the noise has the Gaussian probability density

function with zero mean, and noise variance 2 0

2nN . As mentioned above, it is also

assumed that at each transmit antenna, the channel ih is known.

Note that the assumption of independence of ih , and random variation over time are not

very likely to occur in practice, but nevertheless, such a model is often used to assess new

communication technology, such as coding gains and modulation schemes.

2.2 Transmit BeamformingOn the receive antenna, the received signal is,

1 2 1 2

xx

= 0 = =

=

= = + + +

ih


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+

-

-

where y, is the received signal, ih are the channel coefficients seen from the thi transmit

antenna, x is the transmitted symbol and n is the noise at the receiver antenna. When

transmit beamforming is applied, we multiply the symbol from each transmit antenna

with a complex number corresponding to the inverse of the phase of the channel so as to

ensure that the signals add constructively at the receiver. The received signal can be

expressed as:

1

21 2

j

j

ey h h x n

e,

and the channel coefficients further expressed as:

1

2

1 1

2 2 .

j

j

h e h

h e h

To apply beamforming to this simple system, shown in Figure 2, set the two

beamforming parameters 11

ja e , and 22

ja e . Then the beamformed signal arriving at

the receiver is:

1 2y h h x n+

+

+ .

Note that the effective channel coefficients add in-phase, so that the effect of multiple

paths is used to an advantage, ultimately improving the strength of the received signal.

Assuming zero-forcing (ZF) equalization, we need to divide the received symbol y with

the new effective channel, i.e.,

1 2 1 2

y ny xh h h h

) .

=

=

=

=

=

= =

=

- -

+

+


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Transmit Beamforming effect on BERA simple Matlab script is used to evaluate the benefit of beamforming in the channel

model defined above. In particular, the script performs the following procedure:

a. Generate random binary sequence of +1’s and -1’s.

b. Multiply the symbols with the beam steering metrics – corresponding to the phase

of the channel

c. Compute ZF equalization at the receiver

d. Perform hard decision decoding (i.e., slicer) and count the bit errors

e. Repeat (a) through (d) for a range of signal to noise values of 0

bEN

, and plot the

resulting bit error rate (BER) results.

Figure 3: BER results comparing TxBF to MRC, and standard non-beamforming transmission.

The results are plotted in Figure 3, above. As can be seen, at a BER of 41e , transmit

beamforming provides 18 dB improvement in performance compared to non-

beamforming 1T1R or 2T1R configurations. Interestingly, adding a second transmitter

using the random channel model described above, does not add any benefit in

-

BER for BPSK modulation in Rayleigh channel

1tx-1rx (theory)2tx-1rx (no beamforming-sin)

1tx-2rx(mrc-theory)2tx-1rx(beamforming-sin)

0.1

0.01

0.001

1e-04

1e-050 5 10 15 20 25 30 35

Eb/No, dB

Bit

Err

or R

ate


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performance. That is because although some random channel angles align, other random

combinations cancel, causing the average BER to remain unchanged. At the same time,

2T1R beamforming is equivalent to adding a second receiver and performing maximum

ratio combining (MRC) equalization of the two Rx paths. In both the 1T2R MRC and

2T1R TxBF cases, the transmitted signal combines constructively at the receiver, thus

guaranteeing optimal performance.

Transmit beamforming improves performance by creating a transmitted signal that adapts

to the multipath fading. As can be seen in the figures above, the performance gain is

achieved at the cost of the additional transmit chain. Because no additional analog

circuitry is required at the receiver, TxBF is a particularly effective technique for small

handheld devices, as discussed below.

This section presents simulated and measured results for one of Ralink’s newest chipsets

implementing Transmit Beamforming, RT3883. The results are for a single spatial

stream using 20MHz of bandwidth. The simulated channel is IEEE channel model B,

corresponding to a typical home or small office environment, and the packet lengths are

1.5 Kbytes, utilizing 802.11n MCS7 (65 Mbps) modulation. The 802.11n, like the earlier

802.11a and 802.11g standards, specify OFDM modulation. OFDM is a form multi-

carrier modulation that uses the FFT to break the wide band channel into multiple

narrow-band channels. Using OFDM, the 20MHz channel is partitioned into 56

independent narrow-band channels. When TxBF is applied to OFDM, the beamforming

parameters are customized independently for each subcarrier, greatly improving the

performance of the overall reception quality.

Figure 4 shows the simulated performance of TxBF when transmitting to a device that

has one receive antenna. There are three curves. One curve indicates the packet error rate

Transmit Beamforming and 802.11nRalink Confidential 8

3 OFDM Beamforming Performance

©2010 Ralink Technology Corp.

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(PER) performance of a standard wireless link, with 1T1R configuration. The second

curve presents the performance of a link employing 802.11n STBC (space-time block

coding), and the last shows the performance of a 2T1R beamforming system.

In the case of a receiver having a single antenna, STBC using 2 transmitters has a clear

advantage over a 1T1R system, providing a 4 dB advantage at 10% PER. This

improvement can represent a 20 to 30% increase in range, for the ability to achieve

MCS7 PHY rate.

Transmit beamforming in the 2T1R configuration, as shown in Figure 4, can provide an

additional 5dB improvement over STBC in the given channel model (IEEE model B).

This benefit results due to the optimal nature of beamforming, in maximizing the SNR at

the receiver by guaranteeing coherent combination of the channel paths, and can mean

more than a 2X range extension when transmitting MCS7 data packets.

Figure 4: PER performance comparison of 1T1R, STBC and 2T1R TxBF.

1R ClientsIEEE B, 20MHz, RT3883

1.000

0.100

0.010

SNR (dB)

1x1 MCS7

2x1 TxBF MCS7

2x1 STBCMCS7

16 18 20 22 24 26 28

PER


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3.1 Transmit Beamforming Turntable Test Performance

The benefit of transmit beamforming can result in extended range of increased

throughput in a wireless link. It can also enhance the robustness of the connection as

well, in varying channel conditions. A standard WLAN test to assess link robustness

conducted on a turntable, where either the transmitter or receiver is rotated on a turntable

while monitoring the link throughput. As the angle sweeps between 0 and 360 degrees,

various orientations of antenna configuration can result in signal cancellations at the

receiver.

Figure 5 depicts the Ralink RT3883 3T3R AP linked with 1T1R client station, using

implicit beamforming and 40MHz channels, which is capable of peak throughput of

150Mbps. The client is a laptop placed on a turntable, and rotated through one full

revolution, with and without beamforming active.

Without beamforming, the throughput varies greatly, decreasing to 61 Mbps and 42 Mbps

at several points during the rotation, as shown in Figure 6. The steep drops result when

the signals add out-of-phase at the receive antenna, due to unfavorable reflections off the

surfaces of the laptop, and the surrounding environment. When beamforming is

activated, the peak throughput increases to 79Mbps, and stays there steadily throughout

the entire test. Beamforming improves robustness, nearly doubling throughput at several

angle points during the test; whereas the minimum sustained throughput without

beamforming is 42 Mbps, beamforming allows the system to maintain 79 Mbps

throughout the test.


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Figure 5: Turntable test set-up

0 20 40 60 80 100 120 140 160 180 20010

20

30

40

50

60

70

80

90

Time (second)

Throughput (Mbps)

iBF offiBF on, 0x41 0x11, 250ms update

360 degrees

Figure 6: TxBF turntable test using RT3883 3x3 AP

Ethernet CableFor remotecontroller

Put STANetebook

(Acer 5920G)under

Turn Table

RemoteController NB(Control STA

and Attenuator)

Ethernet STADell M4300

Ethernet CableConnect with AP

GPIB Cable

Cover withAbsorber

Antenna

Gate

Attenuator

RF Cable x 3

RT2880 APinside the

Shielding Box

2.5m

OTA0/360

Degree

90 Degree

USBextended

CableRT3071/RT2770

270 Degree

180 Degree


Thro

ughp

ut (M

bps)


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4.1 Transmit Beamforming Turntable Test PerformanceTransmit beamforming can be used to overcome the effect of multipath fading to

effectively improve data rate and communication range of any wireless system. In

particular, it significantly increases the robustness of a single spatial stream wireless link.

The feature allows single spatial stream devices to experience the diversity benefits of

multiple antennas at the AP. This will drive significantly higher throughputs, lower

power consumption, improved coverage range and increased network capacity.

The RT3883 can support transmit beamforming in different configurations including

2Tx1R, 2Tx2R, 3Tx2R, and 3Tx1R. There is a performance advantage for all these

systems. In particular, beamforming at the transmitter using multiple transmitters can

achieve diversity improvement equivalent to using the same number receive chains in the

client device. This an appealing advantage for many portable devices, such as smart

phones, media players, and mobile Internet devices. These devices often have only one

radio chain and antenna due to their small form-factor and power and cost constraints.

For these applications, in multipath channels, beamforming enhances link robustness and

can greatly extend the range of communication.

Finally, beamforming has been shown to enable the demanding application of wireless

high definition audio and video streaming. By extending range and throughput,

beamforming can allow a single set-top box equipped with a wireless AP to distribute

multiple video sources to several locations throughout the entire home.


4 Applications for Beamforming


Ralink Technology Corporation

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5F, No.36, Tai-Yuen Street, Jhubei City HsinChu Hsien 302,

Taiwan, R.O.CTel: +886-3-56 0-0868Fax: +886-3-560-0818 www.ralinktech.com

The content (text, images) of this white paper has been carefully reviewed and is worth believing. Whatever, this white paper is subject to update without notice. All the content can’t be transferred,reproduced, distributed or broadcasted publicly without the written agreement of Ralink.

Transmit Beamforming and 802.11nRalink Confidential ©2010 Ralink Technology Corp.

White Paper by Ralink on 802.11n Radio Beamforming Technology

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