Modulo Precoding for 11ac

1. January 2011 Modulo Precoding for 11ac Authors: Date: 2011-01-17

2. January 2011 Introduction Precoding Techniques for MU-MIMO • Linear precoding - Low complexity, Low data rate (capacity) • Channel Inversion • MMSE Precoding • Block Diagonalization • Non-linear precoding – High Complexity, High data rate (capacity) • Vector Perturbation • Sphere encoding (NP-hard) • Lattice reduction aided (Suboptimal but still full diversity) • Tomlinson Harashima Precoding • Equivalent to a low complexity vector perturbation but the diversity order is the same as Linear Precoding

3. January 2011 Introduction • The main disadvantage of Linear Precoding is that the normalization constant g (to maintain constant average power) is often very large. • This happens when the transmit vector lies along the singular vectors of the precoding matrix H-1 with high singular value [1]. • Solution • Make sure the transmit signal does not lie along the singular vectors associated with large singular value of H-1. • Vector Perturbation can provide such transmit signal • This presentation is a proposal for non-linear precoding method like Vector Perturbation. • This technique can minimize the quantity γ

4. January 2011 Simulation Result • Vector Perturbation : BER Performance (Channel :Rayleigh) Note: Perfect CSI, Ideal Conditions

5. January 2011 Simulation Result • Vector Perturbation : Signal Power Modulation =64 QAM Channel Model = Rayleigh 4 x [1 1 1 1] MU-MIMO case Vector Perturbation can reduce the unnormalized transmit power which increases the effective receive SNR.

6. AP User 1 MIMO Decoder modulo VP algorithm User 2 MIMO Decoder modulo User k MIMO Decoder modulo User K MIMO Decoder modulo January 2011 Vector PerturbationMU-MIMO System Model • VP is analogous to doing MLD in the receiver. • The AP is free to use any algorithm to find the perturbing vector. Complexity can range from a simple O(n) modulo operation to MLD-like complexity • Regardless of the algorithm used by the transmitter, the receiver needs • only a predefined modulo lattice operation. STAs doesn’t need to know how the perturbing vector was generated Manufacturer specific

7. Review-Vector Perturbation- January 2011 Im -1+1j 0+1j 1+1j τ Re -1+0j 0+0j 1+0j τ n th stream’s decoded symbol : -1-1j 0-1j 1-1j Modulo operation : • This technique can minimize the quantity γ • Choose l in such a way that x=H-1 has smaller unnormalized transmit power than x=H-1u • τ: positive real value • l : Perturbation vector • each element is composed of a + jb (a and b are integers) • Receiver use modulo operationbecause a constellation is expanded

8. January 2011 Proposals for TGac • Allow the AP to perform vector perturbation to prevent huge attenuation of transmit signals • Add one bit in the VHT-SIG-A Field to indicate that vector perturbation was performed. • all modulo decode-capable STAs will modulo decode if the bit is set. • The element of l corresponding to modulo decode-incapable STAs canbe forced to zero. • Alternatively, the AP will not use VP when one user doesn’t support modulo lattice decoding.

9. January 2011 Reference • [1] Christian B. Peel, Bertrand M. Hochwald and A. Lee Swindlehurst, “A Vector-Perturbation Technique for Near-Capacity Multiantenna Multiuser Communication -Part II: Perturbation, ” IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 53, NO.3, MARCH 2005

10. January 2011 Strawpoll #1 • Would you support to allot one bit in the VHT-SIG-A to indicate that modulo lattice decoding is needed after channel equalization? • Yes • No • Abstain

11. January 2011 Strawpoll #2 • Would you support allowing the AP to have the capability of reducing the signal attenuation by doing vector perturbation ? • Yes • No • Abstain