Document Number: IEEE C802.16m-08/1471r3 Date Submitted: 2008-11-06 Interference Mitigation using Conjugate Data Repetition for Cell Edge Users Document Number: IEEE C802.16m-08/1471r3 Date Submitted: 2008-11-06 Source: Kiran Kuchi, J. Klutto Milleth Voice: CEWiT E-mail: kkuchi@cewit.org,in, klutto@cewit.org.in Isamu YOSHII E-mail yoshii.isamu@panasonic.com Venue: Dallas, USA. In response to the TGm Call for Contributions and Comments 802.16m-08/040 for Session 58 Topic TGm SDD- Other Base Contribution: IEEE C802.16m-08/1471r1 Purpose: Presentation associated with comment tgmsdd_Kuchi_Kiran.cmtb To discuss in TGm for appropriate action. Notice: This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.16. Patent Policy: The contributor is familiar with the IEEE-SA Patent Policy and Procedures: <http://standards.ieee.org/guides/bylaws/sect6-7.html#6> and <http://standards.ieee.org/guides/opman/sect6.html#6.3>. Further information is located at <http://standards.ieee.org/board/pat/pat-material.html> and <http://standards.ieee.org/board/pat >.

Presentation Outline Motivation Conjugate Data Repetition (CDR) Improving cell edge performance Conclusions Proposed SDD Text 5/20/2019

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 High Spectrum Efficiency 5/20/2019

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 5/20/2019

Interference Suppression in Legacy WiMax Systems Cell edge users typically have 3-4 dominant interferers With 2-Rx antennas, conventional IC algorithms cannot provide adequate IC gain 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 5/20/2019

Conjugate Data Repetition (CDR) BS-1 BS-2 Signal repetition in time/frequency 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 is not required MMSE filtering of complex, and complex-conjugate signals provides a high IC gain MMSE IC for each RB independently F1 X1 F2 X1* F3 X2 F4 X2* F5 P1 F6 X3 F7 X3* F8 X4 F9 X4* F1 Y1 F2 Y1* F3 Y2 F4 Y2* F5 P2 F6 Y3 F7 Y3* F8 Y4 F9 Y4* 5/20/2019

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 doe snot estimate interference channel. MMSE needs interference covariance only 5/20/2019

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 High IC gain for each RB 5/20/2019

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 5/20/2019

Conjugate Data Repetition With N 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 cooperation between BSs CDR does not estimate channel of interferers 5/20/2019

Proposed Downlink RB Structure for Cell Edge Users Slot format with CDR in 18 X 6 resource block Pilot density with 2-Tx antennas 11.11% OL transmit diversity: Phase-Offset-Diversity (a.k.a rank1- precoding) CL transmit diversity: single stream MIMO For cell edge data users, CDR will be applied in an FFR region as a reuse-2 scheme Ensure that DL-MAP overlaps with CDR region to get full IC benefit CDR works with both localized, distributed resource allocation schemes 5/20/2019

CDR-POD with MMSE in PED-B 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 5/20/2019

CDF of SINR of 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. The above CDF is collected for bottom 33% of users. All cells in the network synchronously allocate the bottom 33% users to a CDR frequency partition which is fixed for the entire network CDR combined with bit level data repetition can be used for select very low SINR users 5/20/2019

System Level Simulation for Cell Edge Users CDF of SINR of MMSE Receiver with CDR-POD Simulation Assumptions PED-A channel Localized allocation RB size 18x6 ½ QPSK with 360 bits 2Tx 2Rx 2D-POD transmit diversity Total pilot density 11.11% Conjugate repetition factor=2 Results are obtained with a system level simulator in which the SINR at the output of MMSE is estimated during channel estimation; 400 users are simulated 6.0 dB IC gain 5/20/2019

System Level Simulation for Cell Edge Users Spectrum Efficiency Comparison with MMSE-CDR-POD Simulation Assumptions PED-A channel Localized allocation RB size 18x6 ½ QPSK with 360 bits 2Tx 2Rx 2D-POD transmit diversity Total pilot density 11.11% Conjugate repetition factor=2 MCS selection between R=1/2 QPSK and R=1/2 16-QAM 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 400 cell edge users Simulation done with two MCS levels : ½ QPSK and ½ 16-QAM Y-axis – Percentage of users whose throughput is greater than x-axis Maximum supported SE for simple repetition is slightly over 0.5 b/s/Hz and 1 b/s/Hz for CDR 80% of cell edge users always gets higher SE compared simple repetition PHY overheads are not taken into account in SE calculation 5/20/2019

Conclusion Due to limited spectrum, frequency re-use 1:1 is the preferred deployment mode Very important to improve cell edge performance CDR and rank-1 transmission (POD/CL-div) ensures high cell edge performance in both OL and CL-modes CDR and FFR complement each other CDR can be implemented along with FFR in a frequency partition CDR works with the following resource allocation schemes Localized , Distributed PRU based DRU Tone based distribution Use CDR for both control and cell edge data channels CDR does not require BS co-operation CDR works with a simple MMSE receiver CDR does not need to estimate channel of interferers Uses covariance based detection 5/20/2019

Proposed SDD Text Add section 20.5 20.5 Interference mitigation using conjugate data repetition (CDR) Interference mitigation using conjugate data repetition (CDR) will be supported for cell edge users for data and/or control channels. CDR includes simple repetition of modulated data 5/20/2019