Document Number: IEEE C80216m-09_0020r5 Date Submitted: 2009-01-12 Interference Mitigation using Conjugate Data Repetition for Cell Edge Users Document Number: IEEE C80216m-09_0020r5 Date Submitted: 2009-01-12 Source: 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 59 Topic 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 SDD 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 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) BS-1 BS-2 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 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*

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

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

CDR Frequency Partition CDR 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

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 range SINR= 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 Users CDF 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 51% gain 75% gain Cell edge throughput gain of CDR 50-75%

System Level Simulation for Cell Edge Users CDF 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 Users CDF 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 for cell edge users For interference mitigation for cell edge users, schemes such as conjugate-data-repetition (CDR) can be used