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January 2011. Modulo Precoding for 11ac. Authors:. Date: 2011-01-17. January 2011. Introduction. Precoding Techniques for MU-MIMO Linear precoding - Low complexity , Low data rate (capacity) Channel Inversion MMSE Precoding Block Diagonalization

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introduction

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
introduction1

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
simulation result

January 2011

Simulation Result
  • Vector Perturbation : BER Performance (Channel :Rayleigh)

Note: Perfect CSI, Ideal Conditions

simulation result1

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.

vector perturbation mu mimo system model

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

review vector perturbation
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
proposals for tgac

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.
reference

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
strawpoll 1

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
strawpoll 2

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
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