Interference Mitigation using Conjugate Data RepetitionIEEE 802.16 Presentation Submission Template (Rev. 9) Document Number:

IEEE S80216m-08_595Date Submitted:

2008-07-17Source:

Kiran Kuchi, J Klutto Milleth , Vinod Ramaswamy, Padmanabhan Suryasarman, Baskaran Dhivagar, K Giridhar, Bhaskar Ramamurthi

CEWiT E-mail: kkuchi@cewit.org.in, klutto@cewit.org.inIndia

Venue: Denver, USA

Purpose:To facilitate the choice of interference mitigation schemes during the IEEE 802.16 meeting in Denver, USA

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Outline

• Motivation• Interference Suppression

– Conjugate data repetition

• Orthogonal pilots• Results and proposed SDD text

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Interference Suppression• Poor cell edge link budget in reuse-1 networks

– In urban deployments in India, typical cell size has 200-300 m radius– Interference limited in both uplink and downlink– Severe co-channel interference (CCI)– Multiple CCI up to 3-dominant interferers

• 2-antenna MMSE receiver can reject only a single dominant CCI

• Existing solutions

– DL control uses rate ½ QPSK and bit-level data repetition up to 6-times• Yet, poor downlink control channel coverage

– At cell edge, UL also supports rate ½ QPSK and data repetition up to 6-times– Significant loss in spectrum efficiency

Interference Mitigation Using Conjugate Data Repetition (CDR)

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Conjugate Data Repetition (CDR) • Conjugate symbol repetition across

adjacent subcarriers• After complex de-conjugation, the

receiver has two copies of signal and interference with different channel states

– 2-antenna MMSE can suppress 3 dominant CCI

– An advanced receiver can reject up to 4 dominant CCI

• The proposal is suitable for common control channels which are always transmitted in predetermined locations

• Conjugate repetition is applied in localized slots of deterministic size

– Slots are distributed in entire frequency in diversity mode

• More flexible slot allocation compared to Panasonic proposal C802.16m-07/211

BS-1 BS-2

F1 Y1

F2 Y1*

F3 Y2

F4 Y2*

F5 P2

F6 Y3

F7 Y3*

F8 Y4

F9 Y4*

F1 X1

F2 X1*

F3 X2

F4 X2*

F5 P1

F6 X3

F7 X3*

F8 X4

F9 X4*

Conjugate Data Repetition

T1 T2 T3 T4

X1 X5 X5* X1*

X2 X6 X6* X2*

P1 X7 X7* P2

X3 X8 X8* X3*

X4 X9 X9* X4*

T1 T2 T3 T4

Y1 Y5 Y5* Y1*

Y2 Y6 Y6* Y2*

P3 Y7 Y7* P4

Y3 Y8 Y8* Y3*

Y4 Y9 Y9* Y4*

BS-1 BS-2

• In this example, pairs of conjugate data symbols are repeated in the same subcarrier but in different OFDM symbols•The concept can be applied to both control and data regions•For repetition factors greater than 2, a deterministic or random phase rotation is applied in addition to conjugation•2D-POD is the preferred OL transmit diversity mode

•The IC receiver works in virtual (single) antenna mode

Conjugate Data Repetition

• Basic Idea– Each BS transmits data in conjugate symbol pairs on a pair of subcarriers

– 1st BS transmits

– 2nd BS transmits

– Complex-conjugation on

– After complex de-conjugation, the receiver has two copies of signal and interference with different channel states

• Signal

• Interference

– Receiver jointly filters – At cell edge, 2-antenna MMSE can suppress up to 8 CCI with non-uniform power profile

111 ˆ nxghxy

2*1

*2 ˆ nxghxy

*21

***2 ˆ nxgxhy

)ˆ,ˆ( *11 xx

),( *xx

2y

),( *hh

),( *gg

),( *21 yy

Simulation Results with 3-Dominant CCI

LOS Channel

12 14 16 18 20 22 24 260

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

SINR of MMSE

prob

abili

ty

LOS, Nintf=3, Nr=2, R=2, Conjugate Repetition

22 24 26 28 30 32 34 360

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

SINR of MMSE

prob

abili

ty

Flat Fading, Nintf=3, Nr=2, R=2, Conjugate Repetition

Flat Fading

Cell Edge Performance with 8-Interferers

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10 15 20 250

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

SINR of MMSE

prob

abili

ty

Flat Fading, Nintf=8, Nr=2, R=2, Conjugate RepetitionSINR=-7 dB; SNR=30 dB; Intf Profile=[0 -3.5 -6.5 -9.0 -11.0 -12.5 -13.0 -13.5]

SINR of MMSE receiver with conjugate repetition in flat fading channel, 8-interferers, SNR=30 dB, input SINR=-7 dB

Interference Profile

Interference power

I0 0

I1 -3.5

I2 -6.5

I3 -9.0

I4 -11.0

I5 -12.5

I6 -13.0

I7 -13.5

Pilot SupportReference to S80216m-08_296r1.ppt

Quasi Orthogonal Pilot Design

• With PN pilots– Poor channel estimation quality and reduced interference

rejection gain

• We prefer pilots colliding with pilots – A cluster is defined as a collection of a group of sectors

• The group may be formed with sectors from same cell or different cells

– Same PN cover with orthogonal pilot codes with in a cluster • Within a cluster, interferers will have orthogonal pilots• At cluster boundaries, some interferers may have non-orthogonal

pilots– Different PN sequences in adjacent clusters

Quasi Orthogonal Pilots

• Cluster Pattern-1– Each cell has three sectors employing different orthogonal codes which are having the same color.

These three sectors forms a cluster which use the same PN sequence or share the same pilot locations

– Different colors depict different clusters which use different PN cover or shifted pilot locations• Cluster Pattern-2

– A cluster is formed by grouping sectors from adjacent cells. Sectors in the same cluster use the same color

Cluster Pattern-1 Cluster Pattern-2

Quasi Orthogonal Pilots

• 2D-MMSE channel estimation on slot by slot • Near interference free channel estimation

• With 2-receiver antennas, and conjugation repetition, cell edge interference rejection up to 8-interferers

• Significant gain in system performance

Link simulation with Quasi-Orthogonal Pilots

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•Simulation Assumptions•PED-A channel•Localized allocation•RB size 9x6•½ QPSKwith 360 bits•2Tx 2Rx•2D-POD transmit diversity•Total pilot density 11.11%•Quasi-orthogonal pilots•Interference power profile

•[0 -3 -6 -9] dB•Rest of the interference is modeled as AWGN•Conjugate repetition factor=2

•SNR is defined as signal to rest of the interference power

CDF of BLER of Cell Edge User Interference power profile in dB [0 -3.5 -6.5 -9 -11.0 -12.5 -13.0 -13.5]

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•Simulation Assumptions•PED-B channel•Distributed allocation•RB size 9x6•½ QPSK with 360 bits•2Tx 2Rx•2D-POD transmit diversity•Total pilot density 11.11%•Quasi-orthogonal pilots•Rest of the interference is modeled as AWGN•SNR= 35 dB•SIR=-3 dB•Conjugate repetition factor=2

•SNR is defined as signal to rest of the interference power

•This is a preliminary result with few simulation runs•Trend shows that quasi-orthogonal pilots (Walsh+PN cover) gives a high advantage over PRBS pilots

Conclusion

• We propose data repetition in conjugate pairs – Flexible slot allocation

• High interference suppression using MMSE• Suitable for common control channel

– With 2D Phase Offset Diversity (2D-POD)

• For data channels, this scheme shall be used in a cell edge FFR zone– Better spectrum efficiency compared to 1/3 reuse

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Proposed SDD text

• Interference mitigation techniques using conjugate data repetition (CDR) shall be supported for data and common control channels. CDR includes simple repetition of modulated data.

• Use sector specific orthogonal and quasi-orthogonal pilots

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