INTERFERENCE CANCELLATION ALGORITHM WITH PILOT IN 3GPP/LTEResearchers: Professor Otgonbayar Bataa (Ph.D)

Buyanhishig Ulziinyam

Erdenebayar Lamjav

Technology Mongolian University of Science and Technology

School of Information and Communication

OUTLINE

• Introduction• The Analysis of ICS Requirement for 3GPP/LTE• Overview of IC• Channel Estimation Techniques

• IC algorithm with pilot signal• Channel Simulation• Conclusions

Introduction

• The major challenges for LTE terminal implementation are efficient channel estimation (CE) method as well as equalization.

• We are assuming the basic CE techniques and future direction for research in CE fields.

• General interference cancellation methods work in the frequency and time domain. The algorithms use in the time domain and is used methods named LMS, LMMSE and MLSE.

• Minimum LSE technique has been proposed for general MIMO-OFDM systems with pilot signal.

1.1 The Analysis of ICS Requirement for 3GPP/LTE

• This task includes following subtasks:• Applications and features of ICS repeater• Analysis of ICS requirement for 3GPP/LTE

• Repeater communication is one promising candidate solution in future cellular networks because of its ability to increase throughput, data rate and coverage.

• ICS repeater is one of the candidate technology in future 3GPP/LTE-A to increase capacity.

1.1 The Analysis of ICS Requirement for 3GPP/LTE

• Traditionally repeaters have been active continuously and perform blind forwarding without knowing the signal.

• However the repeater in LTE Advanced is likely to include some advanced functionalities such as: • frequency selectivity, • gain controllability, • multi antenna ability, • advanced antenna processing, • optimum power control algorithm, etc.

1.1 The Analysis of ICS Requirement for 3GPP/LTE

• The ICS Repeater is a new kind of single-band RF repeater that can automatically detect and cancel the interference signals caused by oscillation of RF feedback between the Donor and Coverage Antennas in real time by adopting DSP (Digital Signal Processing) technology.

• It can continuously and stably cancel the interference signals and be adapted to any changes in the surrounding RF environment (including fixed and mobile objects).

1.1 The Analysis of ICS Requirement for 3GPP/LTE

ICS

Delay+

Gain

Receiver Antenna

Transmit Antenna

From Base Station

Feedback Signal

1.2 Features of ICS Repeater

• High-speed, large dynamic A/D, D/A technology• The adaptive filter design based on the modern digital signal

processing technology • Real-time cancellation of interference signal (incl. multi-path

fading, feedback signal)• ICS function to prevent self-oscillation, enhance gain and

coverage range, and reduce isolation requirement between donor antenna and coverage antenna

• Highly selective digital channel selector • No interference to BTS by adopting linear amplifier with high

gain and low noise • Adopting filter with highly selectivity and low insertion loss

eliminates interference between uplink and downlink

2.1 Overview of IC• Successive and Parallel IC can be iteratively (iterative

IC) ,that is, after one iteration over all streams we can further improve the quality of the streams by cancelling the streams estimated in the first iteration.

Successive IC (SIC) Parallel IC (PIC)

2.2.1 Channel equalization

• There are three categories of equalization techniques. • Frequency-domain technique

• Frequency-domain technique, which applies the conventional equalization algorithm for single-carrier MIMO systems to each subcarrier, the design complexity is rather high, and the memory required to store the equalizer coefficients is large.

• Time–frequency domain technique• Time-domain technique

• A time-domain equalizer, which is designed using the second-order statistics (SOS) of the shifted received OFDM symbols, is applied to partially cancel the ICI and ISI.

2.2.2 Channel Estimation Techniques

• We proposed to study the performance of the two linear estimators under the effect of the channel length.

• Channel estimation algorithms can be generally separated into two methods, • Pilot-based channel estimation • Non-Pilot channel estimation.

• Adaptive channel estimation methods are typically used for rapidly time-varying channel.

• Channel estimation is used two-dimensional time and frequency domain structure of OFDM system.

2.4.1 Pilot based system architecture

Interference canceller

CP Removal

CP Removal

FFTGuard

Removal

FFTGuard

Removal

Soft DeMapper

.

.

.

.

.

.

Least Square

Equalization

MLSE using Viterbi

algorithm

Matched Filtering

Turbo receiver

Turbo receiver

CP Adder

CP Adder

IFFTGuard Adder

IFFTGuard Adder

Symbol Mapper

LTE MIMO Encoder

(Turbo/CC Codder)

.

.

.

.

.

.

Turbo transmitter

Turbo transmitter

Actual feedback channel hF()

c(n)

e(n)

yE(n)

c(n) – Input signalyE(n) - Feedbacke(n) – MMS error

LTE Channel Decoder

LTE Channel Encoder

HARQ Buffer

AWGN channel

Source

Soft Mapper

2.6 IC algorithm using pilot signal

• For pilot based channel estimation of OFDM system, following three are required. • Firstly, suitable pilot pattern needs to be considered. • Secondly, pilot-based channel estimation algorithm

with low complexity should be identified. • Thirdly, proper demodulation method toward

effective channel estimation has to be developed

2.6.1 How to use pilots for Channel Estimation

Methods/Estimation & Interpolation/

Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems

Based on Block type pilot arrangement

Based on Comb type pilot arrangement

LS /Least Square/ + +LMS /Least Mean Square/ +MMSE /Minimum Mean Square Error /

+ +

LMMSE /Linear MMSE/ or Wiener filtering

+

Decision Feedback Interpolation +Linear Interpolation +Second Order Interpolation +Low Pass Interpolation +Spline Cubic Interpolation +Time Domain Interpolation +Piecewise Constant Interpolation +

2.6.2 Pilot orientation for Channel Estimation in LTE

Comb Type:Part of the sub-carriers are

always reserved as pilot for each symbol

Block Type: All sub-carriers is used as

pilot in a specific period

Fre

qu

ency

Time

Block type pilot

Fre

qu

ency

Time

Comb type pilot

Pilot

Information

2.6.3 Pilot for Channel Estimation

Figure 2. Pilot allocation in EUTRAN (generic frame structure, normal cyclic prefix)

2.6.4 Comb-type pilot channel estimation

• Piecewise Constant Interpolation• Linear Interpolation• Second Order Interpolation• Cubic Spline Interpolation• These algorithms have very low complexities.

However, they are not precise enough or need some presumptions (e.g. the number of channel taps, the delay spread), which limit the application of these algorithms for LTE systems.

2.6.5 Block-type Channel Estimation

• LSE: Least Square Estimation• The goal of the channel least square estimator is to

minimize the square distance between the received signal and the original signal.

• LMMSE: Linear MMSE• the LMMSE estimator always needs some a priori

information of the channel, e.g. noise level and channel correlations.

2.6.6 Least Square (LS) Estimator

• The cost function of LS algorithm:

• The purpose of LS algorithm is to minimize the cost function J without noise. For the minimization of J, let

hXFhFXhYFXhXFYYY

hXFYhXFY

hXFYJ

HHHHHHHH

H

)(

2

.0

Hh

J

2.6.6 Least Square (LS) Estimator

• Then

• Then we could get

0

)()(00

hXFXFYXF

hXFhFXh

JhYFX

h

J

h

J

HHHH

HHH

H

HHH

HH

YXFYXFXFXFh HHHHLS

111)(

hFH

2.6.6 Least Square (LS) Estimator

• The LS estimators are known by its very low complexity because they not need the statistic information of channel.

• The least square estimates (LS) of the channel at the pilot subcarriers given in bellow can be obtained by the following equation:

represents the least-squares (LS) estimate obtained over the pilot subcarriers.

H

YXH 1)(ˆ

Max Threshold > Jmin

FFT

Counter = 0

Matched Filtering

MLSE using Viterbi Algorithm

Interference Cancellation

Output Data

NO

Soft DeMapper

YES

1

0

21

0

][][minˆN

n

L

kkm mxfntx

Soft Mapper

hXFYhXFYJH

min

2.6.6 Simulation results

• Figure The symbol error rate technique with pilot, that of a 2Tx−2Rx at the LS estimator based receiver

Figure The symbol error rate comparison of the proposed non-pilot and pilot technique with any modulation order at the receiver

CONCLUSION AND FUTURE WORKS

• Finally, in this project the Minimum LSE equalization used to pilot system, that is mainly considered as proved by any works. It would be very interesting to extend the ideas of the polynomial approach and transceiver/repeater designs to new practical system based channel interference cancellation methods.