Interference Mitigation using Conjugate Data Repetition for Cell Edge Users

Document Number: IEEE C80216m-09_0020r4

Date Submitted: 2009-01-12Source:

Kiran Kuchi, J. Klutto Milleth, Vinod R, Dileep M K, Divagar , Padmanabhan M S, Bhaskar R, Giridhar K Voice:CEWiT, India E-mail: kkuchi@cewit.org,in,

klutto@cewit.org.in Venue:

San Diego, USA. In response to the TGm Call for Contributions and Comments IEEE 802.16m-08/052 for Session 59Topic TGm SDD - section 20 (Interference Mitigation)

Base Contribution: IEEE C80216m-09_0020

Purpose: Presentation associated with comment tgmsdd_Kuchi_Kiran.cmtb associated with section 20 in SDDTo discuss in TGm for appropriate action. Notice:

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Presentation Outline

• Motivation• Conjugate Data Repetition (CDR)

– Improving cell edge performance • For both data and common control channels

• Conclusions• Proposed SDD Text

Motivation• Limited Spectrum

– Frequency reuse-1 is the most likely deployment mode– Typical urban cell size has 200-300 m radius– Interference limited in both uplink and downlink

– Very low cell edge data rates

• Provide uniform data rates throughout the cell

• Improve control channel coverage

• High Spectrum Efficiency

Interference Mitigation• Poor cell edge SINR in reuse-1 networks

– Severe co-channel interference (CCI)– Multiple CCI up to 4-dominant interferers– Typically cell edge SINR is in the [-6 0] dB range– Nearly 30% of users in the sector are cell edge

users• Re-use 1:3 is spectrally inefficient• Gain of soft re-use is not adequate for cell-edge

users

Interference Suppression in Legacy WiMax Systems

• 2-antenna MMSE receiver nulls a single interferer• Pre-whitened MLD can suppress two interferers• Use single stream transmission i.e., POD/CL-div• However, cell edge users typically have 3-4 dominant interferers• Existing solutions

– For low SINR cell edge users– Rate ½ QPSK and bit-level data repetition up to 6-times

• Bit level data repetition is spectrally inefficient

Conjugate Data Repetition (CDR) • Signal repetition in time/frequency• Allow interference and suppress it• Conjugate symbol repetition across

adjacent subcarriers or adjacent OFDM symbols

• All cells synchronously transmit data in conjugate symbol pairs for select users

– The network assigns a CDR frequency partition in which the RBs from different BSs overlap

• BS co-ordination not required• MMSE filtering of complex, and

complex-conjugate signals provides a high IC gain

• MMSE IC for each RB independently

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

• 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– Receiver does not estimate interference channel. MMSE needs interference covariance only

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CDR-MMSE Receiver

• CDR signal model

• CDR-MMSE filtering,

• In Rayleigh fading channels, conjugation ensures that, the signal and interference channel vectors are linearly independent with probability 1

• Linear independence ensures that MMSE provides full IC up to 2N-1 interferers • MMSE filter is applied to each RB independently• In CDR mode, allow interference and suppress it

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CDR Frequency PartitionCDR Partition

Sector 1

Sector 2

Sector 3

Low SINR users High SINR users

• In CDR frequency partition• Allow interference• Exploit the structure in the interference and suppress it using the noise+plus interference

covariance• The CDR region is dedicated for cell edge users• CDR implementation

• In a frequency reuse-1 system, the SINR of all the users in a cell are ranked in descending order• All cells in the network synchronously allocate the bottom x% users to a CDR frequency partition

which is fixed for the entire network• CDR can be implemented using the existing FFR framework• Operator can define a CDR frequency partition

CDR-MMSE Pilot Processing

• Use BPSK pilots and preferably pilot-on-pilot mode. – Step 1: Collect complex, and complex conjugate copies of the received pilots

– Step 2: Estimate channel coefficients of desired signal using 2D-MMSE• Assume uniform power-delay-profile• Knowledge of interferer pilot sequences improves 2D-MMSE

– Step 3: Subtract signal contribution from the received samples

– Step 4: Estimate noise-plus-interference covariance matrix

• Each RB can be processed independently to obtain high IC gain

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Conjugate Data Repetition

• With N rx antennas, CDR provides 2N observations– CDR doubles the number of copies of signal

• Full interference suppression up to 2N-1 interferers• Typically 3-4 dominant interferers in re-use 1:1 systems

– Use POD, CDR, MMSE for cell edge users• With 2-receiver antennas CDR-MMSE nulls 3-interferers

• CDR does not require active cooperation between BSs• CDR does not require channel estimates of interferers

Conjugate Data Repetition Contd.

• In 16m DL, basic unit for transmission is 1 RB• In the CDR region

– Data is repeated in conjugate pairs in each RB– CDR RBs can be allocated in localized, block

distributed or tone-wise distributed modes– Different sectors can use different permutations– CDR receiver processes each RB independently

Conjugate Data Repetition Contd.

• Compatibility with other cell edge features– Works with CL-diversity – Best band scheduling feasible– Restrict MIMO modes to

• Rank-1 pre-coding (2D-POD) and CL-beam forming

CDR Requirements

• Define a CDR resource block structure• MIMO support

– OL Rank-1 precoding– CL-diversity

• CDR can be implemented using the FFR framework– The operator can define a CDR frequency partition

CDR Implementation

• Define a CDR frequency partition• Allocate common control channels in CDR partition• Allocate low SINR cell edge users in the remaining CDR

partition• CDR frequency partition size can be decided by the operator

based on QOS requirements

Link Level Performance Evaluation of CDR

CDR-POD with MMSE, max: 4-interferers (12 pilots/RB)

•Simulation Assumptions•PED-Bchannel•Localized allocation•RB size 18x6•½ QPSK with 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

•Total SINR=-3 dB with 4-interferers•Good suppression up to 3-interferers, partial suppression with4-interferers

Link Level Performance Comparison of CDR and Bit-level

Data Repetition

Simulation Assumptions• Antenna scheme : 4 x 2, POD• Resource : Distributed LRU• Channel Model : eITU PED-B • Interference Profile

– Typical reuse-1 C/I profile = [0 3 6 9 12 14 14 15] ==> -2.75dB– Typical reuse-1 Cell edge C/I profile=[0 0.6 2 3.4 4.6 6.7 8.3] ==> -5.5dB– The profile consists of Signal to Interference Power Ratio up to 8-dominant interferers in

ascending order• BLER is evaluated as a function of SRIR which is defined as: SRIR= Signal Power/(Noise+Residual interference power excluding the dominant 8-interferers)

*In interference limited networks, operating SRIR is usually in 12-15 dB rangeSINR= Signal Power/(Noise+Total interference power)

• Channel Estimation – 2DMMSE within RB (PRU)

Typical reuse-1 Interference Profile

Typical reuse-1 cell edge interference profile

System Level Performance Evaluation of CDR

System Level Simulation for Cell Edge UsersCDF of Spectrum Efficiency

•Simulation Assumptions•PED-A channel•Localized allocation•RB size 18x6•2Tx 2Rx•Adaptive MCS selection, includes all MCS modes defined in 16e

•Results are obtained with a system level simulator in which actual link simulation is run for 1000 frames for each user. The CDF is obtained by randomly selecting 150 cell edge users

•Cell edge throughput gain of CDR 50-75%

51% gain

75% gain

System Level Simulation for Cell Edge UsersCDF of BLER for ½ QPSK with repetition factor =4

•BLER of CDR < 0.1 for 96% of users •BLER of conventional system is < 0.1 for 48% users•Doubles the control channel coverage

System Level Simulation for Cell Edge UsersCDF of BLER for ½ QPSK with repetition factor =6

•BLER of CDR < 0.05 for 98% of users •BLER of conventional system is < 0.05 for 40% users•The gain in control channel coverage is significant

Conclusions Due to limited spectrum, frequency re-use 1:1 is the expected deployment

mode in India Very important to improve cell edge performance

CDR and rank-1 transmission (POD/CL-div) ensures high cell edge performance

CDR can be implemented along with FFR in a frequency partition High cell edge throughput Reliable control channel decoding Use CDR for both common control channel and cell edge data users Allocate cell edge users in a CDR frequency partition

Define a CDR RB structure MIMO mode: Use OL or CL rank-1 precoding

Proposed SDD Text

• Add section 20.5• 20.5 Interference mitigation region for cell

edge users– An interference mitigation region for cell edge

users where schemes such as conjugate-data-repetition can be used