1. Beamforming for Interference Mitigation in TDD System IEEE 802.16 Presentation Submission Template (Rev. 9) Document Number: IEEE S802.16m-08/653r2 Date Submitted: July 16, 2008 Source: Wendy C Wong, Shilpa TalwarVoice:408-765 2429 Intel CorporationE-mail:wendy.c.wong@intel.com 2200 Mission College Blvd, Santa Clara, CA 95054 * http://standards.ieee.org/faqs/affiliationFAQ.html Venue: IEEE 802.16m-08/024 Call for Comments and Contributions on Project 802.16m System Description Document (SDD), on the topic of “Interference mitigation”. Base Contribution: IEEE C802.16m-08/653r2 Purpose: Discussion and approval. 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: and.http://standards.ieee.org/guides/bylaws/sect6-7.html#6http://standards.ieee.org/guides/opman/sect6.html#6.3 Further information is located at and.http://standards.ieee.org/board/pat/pat-material.htmlhttp://standards.ieee.org/board/pat

2. Beamforming for Interference Mitigation in TDD System

3. Outline –Motivation –Beamforming with Nulling Cell-edge classification Resource allocation for cell edge users Distributed scheduling –Simulation results –Conclusions –Standards impact –Proposed SDD text

4. 4 Motivation for DL Transmit Beamforming with nulling –802.16m requirements 2x gain in cell-edge user throughput –DL is usually interference limited for cell edge users –DL Transmit beamforming with nulling can improve cell-edge capacity by by significantly reducing the interference from neighboring BSs and increase SINR. –Beamforming with active interference mitigation is proposed 2x2 BF nulling increases average cell edge system throughput rate by 37% relative to 2x2 STBC/SM 4x2 BF with nulling increases average cell edge system throughput rate by 97% relative to 2x2 STBC/SM

5. 5 BS1BS2 MS1 MS2 BS1 forms beam towards MS1 & null towards MS2; RRM Manager Beamforming with IM Concept: Why does it work? BS1 & BS2 serve ‘spatially’ independent MS1 and MS2 simultaneously using same resource; BS1 assist BS2 by forming a null to its MS2 while BS2 assist BS1 by forming a null to its MS1; If BS1 enables BF with nulling while BS2 does not. MS2 will experience good SINR while MS1 will not. Resources shall be set aside for all BSs to apply BF with nulling in order to help all cell edge users. BS1 forms beam towards MS1 & null towards MS2 BS2 forms beam towards MS2 & null towards MS1

6. Beamforming with Nulling: Implementation 3 steps 1.BS selects cell-edge users that can benefit from BF with nulling Example: MS1, MS2 2.RRM unit determines resources to serve cell edge users only Inter BS co-ordination 3.Independent scheduling performed by all BSs to serve their cell edge users Special UL Sounding

7. Step 1: Cell-edge Classification Goal Goal: BS selects cell-edge users that can benefit from BF with nulling –Multiple ways to identify cell-edge users –One example: use CINR or RSSI caused by neighbor BS as metric –Measurement reports by MS can be sent to serving BS in MOB_SCN_REP of 802.16e –BS decide if MS is cell edge user on metric value and knows number of significant interfering BSs from the MOB_SCN_REP

8. Step 2: Resource allocation to cell edge users Goal: allocate resources for cell edge users only. When number of interferers exceeds system nulling capacity, FFR can be used to reduce interferers. All BS report number of its cell edge users and maximum number of BS interferers to RRM unit over backhaul; RRM unit allocate resources to serving cell edge user only and send resource allocation info back to BSs over backhaul; When system nulling capacity is not exceeded, all BSs share same resources for cell edge users. Under rare circumstances when the system nulling capacity is exceeded, use FFR as to reduce interferer number.

9. Step 3: Independent scheduling Goal Goal: Each BS schedule its cell edge MSs over the allocated resources independently. –Each BS schedule its cell edge MSs over the allocated resources independently using whatever scheduler it prefers; –However, to form the proper nulls to the cell edge users of other BSs, BS needs to know the scheduling decision of neighboring BSs. –For BSs to inform each other of their scheduling decision, special UL sounding is used.

10. Special UL Sounding to infer scheduling decision made by other BSs and form beamforming weight for data transmission. Special Uplink sounding region to infer scheduling decision made by other BSs and form beamforming weight for data transmission. BSs that allocate transmission to its cell edge users in resource block (RB) n will require those MSs to perform UL sounding in sounding block (SB) n.BSs that allocate transmission to its cell edge users in resource block (RB) n will require those MSs to perform UL sounding in sounding block (SB) n. Each sounding region in the UL will have the same interference environment as the corresponding resource block in the DL.Each sounding region in the UL will have the same interference environment as the corresponding resource block in the DL. Each BS can use sounding region to perform channel measurements and form covariance matrix for beamforming weight calculation.Each BS can use sounding region to perform channel measurements and form covariance matrix for beamforming weight calculation. RB 1 RB 2 RB 3 SR 1 SR 2 SR 3 DL subframe n DL subframe n+1 UL subframe n DL data transmission region UL sounding Region A BS 1 will transmit to MS_1_1 in RB 1 BS 2 will transmit to MS_2_3 in RB 1 BS 3 will transmit to MS_3_2 in RB 1 MS_1_1 will perform UL sounding in SR 1 MS_2_3 will perform UL sounding in SR 1 MS_3_2 will perform UL sounding in SR 1

11. Simulation Results Performance Improvement –Use 7 cells network deployment with 3 sectors and 5 cell edge users per sector; –This result is for mobile with travelling at 3kmph. –Simulation parameters compliant to EVM document; –CDF of average BS throughput rate and average SS throughput rate over at least 10 trials with 300 frames per trial are presented. Some trials have less than 300 frames. –A comparison of average BS throughput rate per trial for center 3 sectors: System Mode average BS throughput rate (Mbps) % increase relative to baseline Baseline, 2x2, STBC/SM 5.6394 0 IM BF with nulling, 2x2 7.7451 37.34 IM BF with nulling, 4x2 11.1253 97.28

12. Conclusions –Downlink BF with nulling is a simple, effective technique to reduce BS interference for cell edge users operating in an interference limited environment. –Can be used for low to moderate mobility user –FFR can be used to enhance BF performance when system nulling capacity is exceeded

13. Standard impact There are two standard impacts 1.Information exchange to/from RRM unit for information regarding resources allocated to cell edge users; 2.Special UL sounding to infer scheduling decision of interfering BSs and calculate beamforming weights;

14. Proposed SDD text - 1 Insertion into 8.1 The IEEE 802.16m Protocol Structure The inserted text is marked as italics: Interference Management block perform functions to manage the inter-cell/sector interference. The operations may include: –MAC layer operation Interference measurement/assessment report sent via MAC signaling Interference mitigation by resource allocation for beamforming with nulling, scheduling and flexible frequency reuse

15. Proposed SDD text - 2 Insertion into 11 Physical Layer Insert the following text into SDD Section 10 – Physical Layer [3] 11.x. UL sounding A special UL sounding region is needed for DL transmit beamforming with nulling. This special UL sounding region will enable all BSs to infer scheduling information of neighboring BSs and form proper beamforming weights during DL data transmission to cell edge users. 11.x.y Special UL sounding region Special UL sounding region is divided into N sounding blocks where N is the number of resource blocks allocated to cell edge users. BSs that allocate transmission to cell edge users in resource block n shall require those cell edge users to perform UL sounding in sounding block n as depicted in Figure xyz. Hence, each sounding region in the UL will have the same interference environment as the corresponding resource block in the DL. RB 1 RB 2 RB 3 SR 1 SR 2 SR 3 DL subframe n DL subframe n+1 UL subframe n DL data transmission region UL sounding Region A BS 1 will transmit to MS_1_1 in RB 1 BS 2 will transmit to MS_2_3 in RB 1 BS 3 will transmit to MS_3_2 in RB 1 MS_1_1 will perform UL sounding in SR 1 MS_2_3 will perform UL sounding in SR 1 MS_3_2 will perform UL sounding in SR 1

16. Backup

17. TDD : UL Receive/DL Transmit beamforming at BS: system layout UL received signal at BS p with M receive antennae from MS_p_m is: –k = subcarrier index; – = received vector at BS, Mx1; –S p_m (k) = QAM sybmol of MS_p_m; – channel response from MS_p_m to BS p, MxN; – weight vector applied to signal at MS_p_m, Nx1 –S q_n = QAM symbol of interfering user MS_q_n; – channel response from MS interferer MS_q_n to BS p; – noise with 0 mean and variance  2 I;

18. TDD : UL signal processing and UL Tx BF weight calculation at MS/BS With proper pilot design in the UL, the BS can measure H p_p_m for each of its cell edge user. Using SVD, we have Hence, the BS can request MS_p_m to set its UL transmit weight to be where v 1 (Nx1) is the right singular vector corresponding to the largest singular value of H p_p_m. The actual weight vector which is 2x1 can be calculated by the MS from its DL channel measurement. If selection diversity is chosen for the system, The BS gets to decide which MS antenna to use for its UL transmission.

19. TDD: UL signal processing and UL Tx BF weight calculation at BS With receive beamformer weight vector as (Mx1) to combine signals received at M antennas: The weight vector derived using MMSE with interference nulling or MRC is Assuming data and noise are uncorrelated, we have Equation 1 Equation 2 Equation 3

20. TDD: DL Tx beamforming at BS At the BS, apply BF weights calculated from UL signal to DL transmit signal as At the MS with N Rx antennae, the received signal is where due to TDD and H p_p_m is the channel matrix describing the channel from MS to its serving BS. Equation 4

21. TDD: DL signal processing at MS receive If the same number of antennae at the MS is used for its UL transmit and DL receive, the weight (1xN) applied at the MS is There are cases where the MS DL will use a different weight from the uplink. In this case One example is to use MMSE at the MS DL receive to calculate the MS BF weight. Equation 5

22. Simulation Setup Compliant to simulation parameters setup in EVM document; Some parameters highlight are # of cells7Channel modeleITU/-PedB # of sector per cell3mobile speed (kmph)0/3 # of cell center user0Number of BS antenna2 or 4 # of cell edge user5BS antenna gain (dBi)17 Cell radius (m)866# of MS antenna2 frequency reuse1channel bandwidth (MHz)10 schedulingPFMS noise figure (dB)7 baseline setup2x2, STBC/SMPHY abstraction modelMI Tx power of BS (dBm)46traffic modelbest-effort Antenna azimuth 3 sector antenna defined in 3GPPHARQ synchronous with retransmission maximum of 4, chase combining Permutation 2x3 AMC defined in 16e, full loadedMCS no repetition, QPSK/16QAM/64QAM with CTC BS cable loss (dB)2Channel estimationPerfect channel with delay

23. CDF of average BS throughput rate with cell edge users only

24. CDF of average user throughput rate

25. MCS distribution PDF

26. Overhead calculation and adjusted data rate increase If we take into account of special UL sounding overhead, the adjusted data rate increase will be: Size of scheduling quantum in time slot Number of scheduling quanta per DL per 5ms frame size of sounding symbols in time slot % overhead to add Adjusted Throughput improvement for 2x2 Adjusted Throughput improvement for 4x2 161.0016.6731.71 83.14 231.0016.6731.71 83.14 321.0016.6731.71 83.14 411.0025.0029.60 77.60 511.0020.0030.83 80.83 611.0016.6731.71 83.14 711.0014.2932.38 84.88