CL-MIMO Enhancements for 802.16e Standard

2. Introduction • This presentation covers two related CL-MIMO contributions: • C802.16maint-08/192r1 describes an enhanced CQICH feedback mechanism to support CL-MIMO • C802.16maint-08/93r3 describes a 4Tx 4 bit codebook and 2Tx 3 bit codebook

3. Problem Statement • The current 802.16e standard doesn’t provide a complete CL MIMO solution • The feedback mechanisms are not very well defined or missing essential elements • The current CQICH feedback mechanism doesn’t provide flexibility for feeding back multiple bands • Requires the use of Feedback Header signalling which has robustness/overhead limitations • The codebooks provided for 4 antenna BS are: • Too small (3bit) with suboptimal performance • Complex to implement (6bit) with small performance gain • Not suitable for correlated antennas

4. REV 2 Design Goal for CL-MIMO • Add codebook based CL-MIMO with the following attributes: • Provides good DL performance for several known antenna configurations • 4 vertically polarized closely spaced antenna array • 2 closely spaced cross-polarized array • 2 widely spaced cross-polarized array • Reduced MS search complexity • Efficient and robust UL feedback mechanism

5. Modified CQICH Feedback Mechanism • Modify the CQICH to support two new CQICH types: • 6 bit even/odd where 6 bits are mapped onto 3 tiles (half slot) • 18 bits where each 6 bits are mapped onto 2 tiles (full 6 tiles slot) • Benefits • Double (or triple) the payload of the CQICH channel at the expense of marginally reducing the reliability. • Note that the reliability of these CQI channels is still better than the Feedback Header at QPSK 1/2 • Another advantage of mapping to half slot is increased granularity – users can occupy only half a slot or multiples of half slot and allow better utilization of the fast feedback channel by more users.

6. Modification to the CQICH_Enhanced_Alloc_IE • Reserved bits in the CQICH type fields are reclaimed to indicate the new CQICH 6 and 18 bit mapping

7. Mapping of 6 bits onto CQICH tiles • Uses the same principle mapping of Table 389 – Enhanced fast-feedback channel subcarrier modulation with the following modifications: • The mapping of Fast-feedback vector indices per tile shall be CQICH type dependant as shown below: • CQICH type 0b110 (even): Tile(0), Tile(2), Tile(4) • CQICH type 0b111 (odd): Tile(1), Tile(3), Tile(5) • CQICH type 0b001 (18 bit mapping in a full slot): • Bit17 – Bit12: Tile (0), tile (3) • Bit11 – Bit6: Tile (1), tile (4) • Bit5 – Bit0: Tile (2), tile (5)

8. 6 Bits Encoding • 4 antenna BS • PMI (4b) and differential CINR (2b) • 2 antenna BS • PMI (3b), RI (1b), and differential CINR (2b) • Note that in a 2x2 channel the smallest eigenvalue fluctuates much more than in a 2x4 channel and requires more frequent rank adaptation 6 bits MSB LSB PMI (4b) Differential CINR (2b) 6 bits MSB LSB PMI (3b) RI (1b) Differential CINR (2b)

9. Feedback operation • PMI and differential CINR is typically reported every frame for up to three bands using for example: • 6 bits half slot even/odd to report one band • 18 bits full slot to report 3 bands • etc. • Flexible design enables high geometry users to feedback 3 bands and low geometry users one or two preferred bands • FH type 0110 to feed back typically every 8 frames • Band bitmap (12b) + 3x(best bands CINR (5b) + Rank (1b) ) = 30 • Whether rank is common or per band is determined by the feedback type of the Enhanced allocation IE

10. Feedback operation – cont’d • N precoders for the different bands, are signaled in the CQICH channels based on the allocated CQICH types and CQICH_Num when feedback type is set to 0b100 (8.4.5.4.16). • If CQICH type = 0b100 and CQICH type = 0b101 (6bit primary / 4 bit secondary), (as per current R2/D4) • 6 bits carry best common precoder for the 3 selected bands and differential CINR for lowest numbered band • 4 bits carry differential CINR for second and third bands. • If one or multiple CQICH Type = 0b000 (6 bit full slot), 0b110 or 0b111 (6 bit in half slots) • If N=3, the MS feeds back PMI (4b) and Differential CINR (2b) for all 3 bands in the order of lowest band to highest as indicated in the 12b band bitmap. • If N<3, the MS reports PMI and differential CINR on the allocated band and best band other then allocated band • If CQICH Type = 0b001 (18 bit CQI), the MS feeds back the PMI and differential CINR in the following order: • The first group of 6 bits carries the lowest AMC band , • The second group of 6 bits carries the second AMC band • The third group of 6 bits carries the third AMC band

11. Modified CQICH 4Rx Performance • Feedback header message under the same configuration and QPSK rate ½ requires SNR=4dB for payload error rate = 10-2. • Note that here we assume that the MS doesn’t transmit anything but the FH in the same OFDM symbol.

12. Modified CQICH 2 Rx Performance • Feedback Header for 2-antenna BS requires SNR=11dB for payload error rate = 10-2.

13. Codebook Design Principles • The underlying idea is to design codebooks with values drawn from a simple alphabet i.e. QPSK • The search complexity at the MS becomes substantially lower since no multiplications are required • Comparing to the 16e 6 bit codebooks, only 3% of the number of multiplications are actually required. Note that some metrics exist that don’t require any multiplications at all for the QPSK codebooks. • The codebooks were also designed to have constant modulus property ensuring power amplifier balance • In addition the 4 bit codebook reduces UL overhead and provides much better overall balance between DL performance, UL overhead and MS search complexity

14. 2 Tx 3-bit Codebook Design

15. 4 bit – 4 TX Codebook

16. 4 antenna simulation results – Precoding per Band AMC Scheme 1 assumes optimal precoding per subcarrier