s-38 3310 christos karaiskos

Altruistic Transmit Beamforming for Cross-layer Interference Mitigation in

Heterogeneous Networks (12.09.2012)

Christos Karaiskos

Supervisor: Prof. Jyri Hmlinen

Instructor: D.Sc. Alexis Dowhuszko Espoo 2012

Outline

Introduction/Problem Description

Background

Methodology

Results:

I. G-mode 1 with Infinite Phase Resolution

II. Finite Feedback- Multiple Dominant Interferers

III. Increasing Amplitude Resolution

IV. Increasing the Number of Transmit Antennas

Conclusions

Future Work

2

Introduction/Problem Description

3

Macro cell-splitting techniques cannot deal with the data deluge.

The future belongs to heterogeneous networks, where macro, pico, relay and femto nodes coexist.

The main challenge for co-channel overlaid deployments is cross-layer interference.

Background: Transmit Beamforming (1/2)

4

Adjust Phase &

Amplitude X X

Ideal Interference Mitigation for 2 antenna case:

Interference SNR 0

Altruistic TBF

Egoistic TBF

T

Ntwwww ]...[ 21

]...[ 21 tNhhhh

Transmitter Selection Combining (TSC):

Chooses the best channel after comparison of amplitudes

G-mode 1:Attempts to align phases of channel gains

G-mode 2: Attempts to align phases and adjust amplitudes of channel gains

Background: Transmit Beamforming (2/2)

5

We will present the codebooks for two Tx antennas:

1pN

3pN

1

3

a

p

N

N

if

if

egoistic (MBS)

altruistic

(FBSi)

6

Methodology (1/2)

For results in I, II:

Consider a highly interfered Macro User Equipment (MUE) and multiple interferer Femto Base

Stations (FBSs), of which k are altruistic and the rest egoistic. The received SINR at the MUE

is:

)(dLP

PE

I

Tx

includes egoistic FBSs power and noise

L(d) is path loss attenuation

is transmit power

IP

TxP

represents mean SINR from a given transmitter to the MUE.

7

Methodology (2/2)

We model the SINR as a random variable, which is a function of random variables.

Goal 1: Find cumulative distribution function (CDF) of Z analytically to measure MUE outage.

For results in III,IV:

Goal 2: Compute the mean instantaneous SNR gain of methods and compare it against no TBF.

2

wh

I. G-mode 1 with Infinite Phase Resolution (1/3)

8

System Model:

1 dominant altruistic FBS interferer

Infinite feedback capacity


9

What infinite phase resolution means:

Egoistic MBS-MUE (numerator of SINR):

maximum possible SNR with respect to finite

g-mode 1

Altruistic FBS-MUE (denominator of SINR:

minimum possible SNR with respect to finite

g-mode 1

So, SINR at MUE is the maximum possible

with respect to g-mode 1: Upper Bound

egoistic (MBS)

altruistic

(FBS)

Perfect phase feedback


10

where ,

Whittaker W

II. Finite FeedbackMultiple Altruistic Interferers (1/4)

11

System Model:

multiple dominant FBS interferers

12


Consider chi-squared approximations found in [1]:

Result:

[1] A.A. Dowhuszko, G. Corral-Briones, J. Hmlinen and R. Wichman, On Throughput-Fairness Tradeoff in Virtual MIMO Systems with Limited Feedback.

0 2 4 6 80

0.2

0.4

0.6

0.8

1

Spectral Efficiency (bps/Hz)

CD

F

FBSs apply g-mode 2 with 4 bits

0 2 4 6 80

0.2

0.4

0.6

0.8

1


CD

F

FBSs apply g-mode 1 with 2 bits

0 2 4 6 80

0.2

0.4

0.6

0.8

1


CD

F

FBSs apply TSC with 1 bit

All FBSs Egoistic

Dominant FBS Altruistic

2 Dominant FBSs Altruistic



All FBSs Altruistic

13


MBS is assumed to always apply egoistic g-mode 1 with

N=2 bits resolution.

14


0 5 10 15 20 250.5

1

1.5

2

2.5

3

Number of Altruistic Interferers

Spectr

al E

ffic

iency (

bps/H

z)

10th Percentile

g-mode2 (N=4)

g-mode2 (N=3)

g-mode1 (N=3)

g-mode1 (N=2)

TSC (N=1)

III. Increasing Amplitude Resolution

15

Altruistic g-mode 2 has 1 bit for amplitude feedback denoting the weakest antenna

Extended altruistic g-mode 2 has 2 bits for amplitude feedback:

1 bit denotes the weakest antenna 1 bit denotes if the difference between channel gain amplitudes is above or below a predefined threshold

After simulations, best threshold value was found approximately 6 dB.

Comparison of altruistic g-mode 2 and extended altruistic mode-2:

IV. Increasing the Number of Transmit

Antennas (1/2)

16

For cases of 4+ antennas:

(Randomly) Group antennas 2 by 2

Stage1: Perform altruistic TSC, g-mode 1 or g-mode 2 to each pair (same method/same

resolution)

Stage 2:Perform altruistic TSC, g-mode 1 or g-mode 2 with different or same resolution to the

outcome of the 2 pairs

Stage 1 Stage 2 Outcome

IV. Increasing the Number of Transmit

Antennas (2/2)

17

V. Conclusions

18

For 2 Tx Antennas:

For g-mode 1, it is not efficient to consider more than 4 bits of phase feedback, since improvements in the performance of the interfered MUE are almost

unnoticeable after that point.

When there are multiple dominant interferers, applying altruistic beamforming in clusters of interferers can provide the best gains for the interfered MUE and

significantly reduce outage probability for a given rate.

Increasing the amplitude resolution could provide further gains compared to g-mode 2, when the number of feedback bits is greater than 4. This is because g-mode

2 relies mainly on phase, so its performance becomes saturated after some point.

For more Tx Antennas:

Grouping the antennas in pairs and applying altruistic TBF methods in stages could produce a low complexity interference mitigation scheme for multi-antenna

systems.

V. Future Work

19

Analyze the case in which more than one beamforming vectors are proposed to the interferer. Then, the interferer is given a choice to choose the one that degrades as

less as possible the performance of its own user.

Produce analytical results for the extended g-mode 2 algorithm.

Investigate and analyze further techniques for interference mitigation with more than two transmit antennas.

20

Thank You!

s-38 3310 christos karaiskos

Documents

finite gmode

phases of channel gains

egoistic mbs altruistic

interference snr

mue outage

egoistic fbss power

rest egoistic

received sinr