Coded Transmit Macrodiversity:Block Space-Time Codes

over Distributed Antennas

Yipeng Tangand

Matthew ValentiLane Dept. of Comp. Sci. & Elect. Engg.

West Virginia University

mvalenti@wvu.edu

Overview

Block Space Time Codes Coded Transmit Macrodiversity

Space-time codes with widely separated antennas

Simulation Results 2-antenna case 3-antenna case

• Both 60 and 120 degree sectorized antennas

Block Space Time Codes

The original STC were trellis based. Tarokh 1998. Shifted burden of diversity from receiver to

transmitter. Rather complex.

Block space time codes later emerged as a lower complexity alternative. Block STC has no memory. Symbol-in, symbol out. Simple decoding structure.

Advantages and Disadvantages of Block STC

Advantages: Space-time block coding utilizes multiple antennas to

create spatial diversity, this allows a system to have better performance in a fading environment.

Good performance with minimal decoding complexity. Can achieve maximum diversity gain equivalent to

space-time trellis codes. Receivers that use only linear processing.

Disadvantages: Does not have as much coding gain as space-time

trellis codes. Can not always achieve the maximum data rates

allowed by the general theory of space-time codes.

Diagram of Block STC Transmission

X1 X2

0 T 2T

X1 -X2*

X2 X1*

0 T 2T

Ant 1

Ant 2

Data

STC encoder

Data STC encoder

Fading

AWGN

STC decoder

xc

ry

Modulation

Encoder matrix:

*1

*2

212 xx

xxG

Rate: 1/2

Block STC decoder

Each symbol in a block is decoded separately by minimizing the metric

The decoder outputs the hard-decisions on the data.

The more TXs and RXs the system has, the better performance the system can achieve.

2

1 1

l

t

n

i

itit cr

Decoding Block STC

2

12*212

2

22111 xxrxxr

*

2

1

2

1*1

*2

21*2

1

x

x

r

rr

2

1 1

l

t

n

i

itit cr

2

2

2

2

2

1

2

21*2

*21

2

1

2

2

2

1

2

12*2

*11 11 xxrrxxrr

Since |x1|=|x2| (PSK), we can get:2

12*2

*11 xrr

2

21*2

*21 xrr

The received signals are:

In order to minimize

it is equivalent to minimize

By using: *2ccc

we have:

and

Performance of Block STC

Two transmit antenna system has around 18 dB coding gain while three transmit antenna system has around 25 dB coding gain.

Most of diversity has been achieved just by two transmit antenna.

back

Coded transmit Macrodiversity

Previous research has only considered the case that all of the transmit antennas are located in the same general location. At a single base station in a cellular system. Microdiversity. Spatial correlation is an issue.

We consider combination of macrodiversity with block STC. Macrodiversity: Antennas are far apart. The array consists of the antennas of adjacent base stations. For edge excited cells with 120 or 60 degree sectorized antennas, can use the

three base stations at the corner of the cell.

Assumptions

Power control. We assume that each of the antennas transmits with

the same power.• TX power is controlled by closest base station.

However, due to different path lengths, the received signal powers will be different.

• Depends on geometry and path loss exponent (n). The best way to allocate power is an open problem.

Synchronization. The received signals are not phase synchronized. However, we assume the signals are time synchronous.

• i.e. aligned in time.• This may not be an accurate assumption.

Two antenna system

RX

d1 d2

TX1 TX2

rn

rn

d

dd

d

dd

2,2

1,1

Normalized distances:

Average received power at mobile station is:

nn

S

nn

S

dN

E

dN

E

)(

)(

,20

2

,10

1

Performance: Two transmit antenna system

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 115

20

25

30

Position of the mobile station

SN

R (

dB)

Block STC, TX = 2, RX = 1, 1bit/sec/Hz

BER = 1e-3

When the mobile station is close to either of the base station (to achieve the same BER), it requires more signal power than if it is halfway between the two transmit antennas.

Performance is the best at location 0.5 which is exactly halfway between the two transmit antennas.

Why? The center has the best diversity advantage.

Three transmit antenna system:Edge-excited Cell

TX1

RX

TX2

TX3

For the downlink, space time codes are transmitted from three base stations and received by one receive antenna in the mobile station.

Encoder matrix is:

*2

*3

*

*1

*4

*3

*4

*1

*2

*3

*2

*1

234

143

412

321

3

xxx

xxx

xxx

xxx

xxx

xxx

xxx

xxx

G

x

Edge-excited

120-degree sectorized antenna

Each antenna transmits over a 120-degree sector.

A

O C

B

Three base stations are located in A, B and C, where the highest required SNR values are. The center of the cell is O, where the lowest required SNR is. The BER is 10-3.

120-degree system Performance

Shown is the received SNR required to achieve a BER of 10-3

Again performance is best at center of cell.

-Best diversity advantage.

-Highest total SNR.

60-degree sectorized antenna

Three base stations are located on the corners of an equalateral triangle, and the block STC with encoder matrix G3 for the downlink communications is simulated.

Three base stations are located in A, B and C, where the highest SNR values are. The center of the cell is in O, where the lowest SNR is.

A

O

B C

60-degree system performance

Shown is the received SNR required to achieve a BER of 10-3

Again performance is best at center of cell.

Performance remains good at the midpoint between any two antennas.

Performance Comparison

120-degree system has better trunking efficiency, and less frequent handoff.

60-degree system has better energy efficiency, and larger system capacity.

Why?• Consider mobile at location x • With 120-degree sectorization is served by A-B-C• With 60-degree sectorization is served by A-B-D • D is closer than C.

A

B

C

D

Conclusion

There’s no reason that the antennas in a STC system must all be in the same general location. Spread the antennas out!

The antennas could be located at different base stations. Macrodiversity not microdiversity. This improves coverage in areas that are far from base stations. Could use three base stations and either 120 or 60 degree sectorized

antennas. Future work

Optimal power allocation strategies. Impact of channel estimation. Coping with signals that are not time-synchronous.