coding Xin Sheng Zhou Problem for Wireless Networks Traffic demand - - PowerPoint PPT Presentation

Cooperative networks and coding

Xin Sheng Zhou

Problem for Wireless Networks

• Traffic demand
• Main problem: Shared medium =

Interference

What we have done?

• Wireless throughput had doubled every 30

months over a period of 104 years, or a million-fold increase since 1957

– 25-fold from wider spectrum – 1600-fold from reduced cell size – 5-fold from dividing the spectrum into smaller slices – 5-fold from better modulation scheme

Why reduced cell size?

• Assume received SNR unchanged at the edge of the cell
• Reduced cell size means reduced transmitting power
• SINR does not change = Capacity unchanged for single user
• Network capacity increased because more users can be served
• When power is reduced compared to the noise level, the capacity

for single user also reduced

40W 40W 5W 5W 5W 5W 5W 5W 5W 5W

        N I P 1 log

How can we deal with interference?

• Do not care about other users and consider

them as interference/noise

– When we do not need that information, e.g. mobile user receives signals from other cells

• Try to understand/decode other users message

and cancel the interference

– When we need both of them. E.g. Uplink at the base station from multiple users

• If we do not need the interfering information,

would it be benefit if we decode the interference first? Especially in cooperative networks

Interference effect

• Total rate
• Average rate per

sender

FDMA

TDMA

• Naïve TDMA
• Modification

– If user uses half time, it can uses double power and still maintain the average power – Same capacity region as FDMA

Successive decoding

• Decode user 2 and

consider user 1 as noise

• Subtract signal

from user 2 and decode user 1

CDMA

CDMA (2)

Heterogeneous networks

• To improve network capacity

– Existing solution: Cell splitting – New approach: Heterogeneous networks

• Low power nodes are overlaid within a macro

network with the same frequency

Heterogeneous networks

• Heterogeneous networks

– Interference management

• Power control
• Time-frequency resource partitioning and allocation

– DL: OFDM, UL: SC-FDMA

– Cell range expansion

• Cell handover biasing for load balancing
• Adaptive resource partitioning among different node power

classes

– Interference cancellation receiver

Relay

• Relay node

– In-band: backhaul link and access link use the same frequency (UL and DL still use different frequency) – Half duplex: time division on backhaul link and access link – Full duplex: Spatial separation, or interference cancellation

Base station Relay node Mobile user fD fD fU fU B M R B M R B M R B M R DL UL

Half-duplex 2-phase 2-way RC

• Two source nodes communicate with each other with the

help of the relay node

• No direct link between two source nodes
• Example: Satellite communications
• Modeled as two phases

1

R

2

R

t n1 n2 n3

Half-duplex 2-phase 2-way RC

) ; , ( )} , ( ), | ; ( min{ )} , ( ), | ; ( min{

3 2 1 2 1 1 3 1 3 2 2 2 3 2 3 1 1

Y X X I R R Y X I X Y X I R Y X I X Y X I R         

• T. Oechtering, C. Schnurr, I. Bjelakovic, and H. Boche, “Achievable rate region of a two phase bidirectional

relay channel," in Proc. 41st Annu. Conf. Information Sciences Systems, Mar. 2007, pp. 408-413.

DMC AWGN

                                                        

3 2 1 2 1 1 3 3 2 2 2 3 3 1 1

, min , min N P P C R R N P C N P C R N P C N P C R     

Achievable rate region

LDPC codes factor graph for MAC

Z X X Y   

2 1 3

• Multiple access channels

Half-duplex 3-phase 2-way RC

• Direct link between two sources
• Example: Cellular networks
• Modeled as three phases

1

R

2

R

n1 n2 n3 t

Half-duplex 3-phase 2-way RC

)} ; ( ) ; ( ), ; ( min{ )} ; ( ) ; ( ), ; ( min{

1 2 1 3 3 2 2 2 1 2 3 3 1 1

Y X I Y X I Y X I R Y X I Y X I Y X I R           Achievable rate region

                                                               

1 2 1 3 3 2 2 2 1 2 3 3 1 1

, min , min N P C N P C N P C R N P C N P C N P C R       DMC AWGN

Joint encoding for 2-way RC

• Joint encoding at the relay

node

• Generate additional check

bits

Received from node 1 Received from node 2 Generated by node 3

Joint decoding for 3-phase 2-way RC

• Decoding based on three

parts

– Signal received from the source node – Signal received from the relay node – Its own message as side information

Joint decoding at node 1

Message 1 Message 2 Additional check bits

Simulation results for 3-phase 2-way RC

• (1920,640) LDPC codes
• Signal-to-noise ratios are

the same in two receiving phases

• Eb : Energy per bit from the

source node

Thank you!

Any questions and comments are welcome!