Interference Cancellation in UWB Systems

ByDr.M.Jayasheela, Professor/ECE

KIT-Kalaignarkarunanidhi Institute of Technology

Outline

• Zero Correlation Zone codes• Ultra Wide Band Technology• UWB Channels• Multiuser detection• Interference cancellation in UWB using m-zcz

sequences.

M (Maximal Length)Sequences

• An m-sequence contains, one more one, than zero. The number of ones is (N+1)/2.

• A run is defined as a sequence of a single type of binary digits within the m-sequence. The length of this subsequence is the length of the run.

• The periodic autocorrelation function θb(k) of an m-sequence is two valued .

• Let b(D) represent a m-sequence. Let this sequence be shifted by j chips represented by b(D+j). Then the modulo-2 sum of the two sequences results in a sequence b(D+k) which is the same m-sequence shifted by k chips.

Zero Correlation Zone Codes

• ZCZ codes are interference free in any wireless system as long as the one-side ZCZ length longer than the maximum single delay spread, which is caused by multipath propagation .

• The ZCZ length of conventional binary ZCZ code is restricted to 2k, where k is an integer.But, in practical applications the ZCZ length has to be chosen much larger than required.

m-ZCZ Squences

• A binary ZCZ code based on m-sequences, called m-ZCZ code.

• Its length can be flexibly selected to match the maximum time delay with the great degree-of-freedom, such that constructed interference resistant UWB system can support more no of users than that using conventional binary ZCZ codes.

ZCZ Generation

• The m-ZCZ code set is denoted as m-ZCZ(CL,S,Wmin)={(zi1,zi2)},where i=0, 1…S-1,

• CL represents code length, S- set size of the code set, Wmin –minimum length of one-side ZCZ.

• zi1= Zi1 is first sub code

minw.ia

minw.ia

where a0 is an m-sequence with its period N, and a iWmin stands for the sequence generated by cyclically leftward shifted a 0 with i. Wmin chips. This can be represented as

Naa owin

win modmin).(min.

• The second sub-code zi2={+1} contains only one “+1” chip. The congregated code length is CL = N +1.

• The set size of the code is S = [N / Wmin].

Auto correlation & Cross correlation

• The periodic auto-correlation function (ACF) of an m-ZCZ code and periodic cross- correlation function (CCF) of any two m-ZCZ codes can be calculated as follows:

2j2iminw.jkn

1N

0n

minw.inj,i zzaa)k(R

220

min1

1

0

0min modmod ji)j.w(n

N

n)i.w(n zzNNaa

where integer k denotes the relative time shift.

Correlation Property

• Using above equation ACF of any m-ZCZ code is zero when 0<|k|<Wmin and the CCF between any two codes is zero |k|<Wmin. Thus, there exists a ZCZ with minimum one-side length being wmin.

• The wmin can be flexibly controlled by adjusting the number of cyclic shifted chips.

• Example: Given L=16, wmin=4 we can get a set of m-ZCZ codes denoted by (16,3,4) containing J=[15/4]=3 codes as

(a01, a02)=(+1 +1 +1 -1 -1 -1 -1 +1 -1 +1 -1 -1 +1 +1 -1, +1)

(a11, a12)=( -1 -1 -1 +1 -1 +1 -1 -1 +1 +1 -1 +1 +1 +1 -1, +1)

(a21, a22)=( -1 +1 -1 -1 +1 +1 -1 +1 +1 +1 -1 -1 -1 -1 +1,+1)

where sub-codes are separated by commas.

Parameters

• When 1<wmin<N, the set size S satisfies 1<S= [N/wmin] <N. Therefore, length of m-ZCZ can be increased by reducing the set size.

• Spreading code is formed at the transmitter side by two sub-codes of an m-ZCZ code with a cyclic prefix and a cyclic postfix inserted at both sides of each sub-code. The lengths of the cyclic prefixes and cyclic postfixes should be at least equal to the maximum multipath time delay, so that the two sub codes will not overlap with each other after being transmitted via multipath channels.

• Despreading code is formed at the receiver side by the two sub-codes of an m-ZCZ code, with zeroes inserted in the guard intervals of identical length which should be equal to that of the cyclic prefixes and postfixes in the spreading code.

• To make decisions, aperiodic correlation is performed between the received signal and the local despreading code. Due to the effects of cyclic prefix and postfix the aperiodic correlation automatically becomes periodic correlation.The m-ZCZ code could be more suitable for wireless systems with a relatively short delay spread.

Spreading Codes

DeSpreading Codes

0…0 Sub-code 1 0…0 0…0 1 0……0

In order to reduce multipath interference the one-side ZCZ length wmin must be larger than the maximum delay spread ζmax

Comparison of Correlation b/w m & m-ZCZ Sequences

Sequence Length Correlation of m-Sequences

Correlation of m-ZCZ Sequences

3 0.333 0.01

7 0.1428 0.009

15 0.0667 0.0003

31 0.0322 0.0001

63 0.015873 0

BER Performance m- Sequences/m-ZCZ Sequences in CDMA System

Comparison of BER performance of m- Sequences and m-ZCZ Sequences for Eb/No=20 dB in Rayleigh

BER Performance m- Sequences/m-ZCZ Sequences in CDMA System

Comparison of BER performance of m-Sequences and m-ZCZ Sequences for 1000users in Rayleigh Channel

UWB Technology

• It transmits very short pulses on the order of a fraction of a nanosecond thereby spreads the energy from near D.C to a few gigahertz.

• Blue tooth ,cordless phone and numerous other devices are related to the unlicensed frequency bands .Each occupy narrowband of frequences relative to what is allowed by UWB

UWB spectrum allocation

• Based on Shannon’s Capacity limit equation –the maximum channel capacity grows linearly with the channel bandwidth while grows logarithmically with signal to noise ratio, a greatly improved channel capacity can be achieved by UWB due to its ultra wide bandwidth.

Advantages &ApplicationsUWB PROPERTIES ADVANTAGES APPLICATIONS

Very wide fractional and absoulte RF Bandwidth

•High rate communications•Lowfrequencies penetrate walls and grounds

•High rate WPAN•Lowpower•Multiple Access

Very short pulses •Direct resolvability of discrete mutipath components•Diversity gain

Low power combined communications and Localizations

Persistence of multipath reflections

•Low fade margin•Low power

NLOS Communications indoors

Carrier less transmission •Hardware Simplicity Smart Sensor Networks

UWB Signal Model

• Time Hopping Spread Spectrum impulse Radio(TH-UWB)

In TH-UWB a pseudorandom sequence defines the time when the pulses are transmitted

• Direct Sequence Spread Spectrum impulse Radio(DS-UWB)

In DS-UWB the pulses are transmitted continuously using a pseudorandom sequence for the spreading of information bits.

Modulation

• Pulse Position Modulation• Pulse Amplitude Modulation• On-Off Keying

UWB Channel Modeling

4 types of Channel models• CM1 0-4 meters’ range with LOS• CM2 0-4 meters’ range with NLOS• CM3 4-10 meters’ range with LOS• CM4 greater than 10 meters’ range with NLOS

The IEEE Channel Characteristics for Four different Scenarios

Typical Channel response of CM1

Typical Channel response of CM2

Typical Channel response of CM3

Typical Channel response of CM4

System Model for Multiuser THPPM Based Ultra Wide Band Systems with m-ZCZ Sequences

Performance Comparison of m and m-ZCZ sequences using CM1

Performance Comparison of m and m-ZCZ sequences using CM4

Comparison of m/m-ZCZ Sequence in UWB using AWGN

5 10 15 20 25 30 35 40 45 5010

-5

10-4

10-3

10-2

10-1

users

Bit

Err

or R

ate

comp of ML AND M-ZCZ Sequence -in UWB SYSTEM in AWGN CHANNEL

ebno12-ML

ebno12-MZCZ

Multi-user Detection• Multiple Access Interference (MAI) due to the

simultaneous transmission of signals by different users is a serious problem in the wireless systems. One of the methods of suppressing this interference is multiuser detection.

• Multi user detection is signal processing intensive and they promise to give a large-scale increase in capacity of present 3G and future generation wireless communication systems.

CONVENTIONAL MATCHED FILTER

Multiuser Detection Schemes in UWB

• Decorrelating Detector (DD)The decorrelating detector suppresses the interference by a linear transformation of cross correlation matrix inversion on the soft output of the matched filter bank. The inverse of the correlation matrix R is applied to the MF output.

Decorrelating Detector

Minimum Mean Square Error (MMSE) detector

• The MMSE detector is a linear detector which considers both the thermal noise and MAI. It performs a transformation LMMSE which minimizes , the mean square error between the actual data and the soft output of the MAI.

Successive Interference Cancellation (SIC)

• SIC is a nonlinear type of MUD scheme in which users are decoded successively. The approach successively cancels strongest users by re-encoding the decoded bits and after making an estimate of the channel, the interfering signal is recreated at the receiver and subtracted from the received waveform.

Single stage of Successive Interference Cancellation scheme

Parallel Interference Cancellation (PIC)

• The data estimates obtained from the matched filter denoted by d1

^(0),…….,dK^(0) corresponding to all K users

are multiplied by the amplitude estimates, spreaded using corresponding spreading codes. Thus the received signal is regenerated. All the regenerated signals except the desired user are partially summed. This output is the MAI and is subtracted from the received signal. Thus the MAI cancelled output is passed on to a second bank of MF to produce second set of data estimates. This process is repeated for multiple stages till the desired bit error rate performance is achieved.

Parallel Interference Cancellation Multiuser Detector

MUD Based TH PPM UWB Simulation Model

. Performance of MUD Schemes in TH PPMUWB using m-ZCZ sequences for CM1 fading condition

Performance of MUD Schemes in TH PPMUWB using m-ZCZ sequences for CM3 fading condition

Application:Telemedicine(Transmission of bio medical images)

Input ImageDetected Image using CMF with m-ZCZ Sequences for LOS(0-4mt) with PSNR=22.79

Detected Image using SIC with m-ZCZ Sequences for LOS(0-4mt) with PSNR=24.5

Detected Image using PIC with m-ZCZ Sequences for LOS(0-4mt) with PSNR=24.82

Detected Image using MMSE with m-ZCZ Sequences for LOS(0-4mt) with PSNR=26.5

Detected Image using DD with m-ZCZ Sequences for LOS(0-4mt) with PSNR=28.9

Conclusion

BER performance of m-ZCZ is better than that of existing m Sequences

This Sequences implemented in UWB Systems for indoor applications and used for suppressing multiuser interference.

It is used in Telemedicine applications .

THANK YOU