MULTIPLE ACCESS MULTIPLE ACCESS INTERFERENCE CANCELLATION INTERFERENCE CANCELLATION

IN AN OVERLOADED CDMA IN AN OVERLOADED CDMA SYSTEM SYSTEM

PROJECT GUIDE:

Mr G.Vairavel

71806106042 - Muthusathiyapriya.B

71806106047 - Nithia .D

71806106304 - Naghamanickam .N.J

ABSTRACTABSTRACT

CDMA- To accommodate more number of users.

Overloading - to extend capacity limitation of multiple access techniques.

Two sets of orthogonal signal waveforms.

MAI- Inability to maintain complete orthogonality.

MAI CANCELLATION – using Weighted Linear Parallel Interference Canceller.

BER Performance comparison of WLPIC with CLPIC and MF Detector.

INTRODUCTIONINTRODUCTION

FDMA – splitted into frequency bands and allocated to each user.

TDMA- full spectral occupancy but only for short duration of time.

CDMA- unique digital codes are used to differentiate subscribers.

DRAWBACKS OF TDMA AND DRAWBACKS OF TDMA AND FDMAFDMA

Only one subscriber at a time.

Voice data –burst- takes much time.

Limits the capacity.

ADVANTAGES OF CDMAADVANTAGES OF CDMA

No hard handoffNo guard timeLess fadingGreater capacityNo equalizer neededImproved security and privacyFewer dropped calls

PROCESSING GAINPROCESSING GAIN

Processing gain - ratio of the spreading rate to the information rate.

Expressed in decibels (dB).

PSEUDO SEQUENCESPSEUDO SEQUENCES

Noise like pseudo-random codes. Identification codes - balanced not to correlate to other

codes of itself. The most commonly used sequences

1. Maximal length sequences

2. Gold codes

3. Walsh codes

4. Hadamard codes Gold codes- better correlation property, more number of

codes can be generated.

NEAR FAR EFFECTNEAR FAR EFFECT

Users close to the base station – stronger signal.

Cross correlation between 2 PN codes is not zero.

To combat this effect - Power control.

SCRAMBLINGSCRAMBLING

To split the interference power evenly over the in-phase and quadrature components.

Two types

1.Periodic scrambling

2.Complex scrambling

MULTIPLE ACCESS MULTIPLE ACCESS INTERFERENCE INTERFERENCE CANCELLATIONCANCELLATION

The most common interferences 1. Multiple Access Interference (MAI) 2. Inter Modulation Interference (IMI) Inter Modulation Interference - undesired combining

of several signals.

MAI- inability to maintain complete orthogonality Cancellation of MAI is done.

Using MATLAB 7.0.1.

CANCELLATION METHODSCANCELLATION METHODS

1. Successive Interference Canceller

2. Parallel Interference Canceller

SUCCESSIVE INTERFERENCE SUCCESSIVE INTERFERENCE CANCELLERCANCELLER

Order of subtractions given by the relative strength of the users.

Ranking the signal strengths.

Strongest signal is cancelled out first.

PARALLEL INTERFERENCE PARALLEL INTERFERENCE CANCELLERCANCELLER

All the Interferences are cancelled parallely. Block diagram of PIC

CANCELLATION METHODS CANCELLATION METHODS USING PICUSING PIC

Matched Filter Detector Conventional LPIC Weighted LPIC

Cancellation of MAI is done using WLPIC and it’s performance is compared with MF detector and CLPIC.

MATCHED FILTER DETECTORMATCHED FILTER DETECTOR

MF- detect each user in parallel . The MF output of Kth user is given by

where A is the amplitude

b is the bit transmitted

ρ is the cross correlation coefficient

MAI CANCELLATION USING MAI CANCELLATION USING CLPICCLPIC

Optimum weight is 1 for all the users. Estimate of MAI is subtracted from the received signal. The mth stage output of CLPIC is given by

Where

ρ is the cross correlation coefficient

y is the received signal

MAI CANCELLATION USING MAI CANCELLATION USING WLPICWLPIC

MAI estimate is weighted by a factor before cancellation. Mth stage output of a desired user is given by,

where yk is the received signal

ρk is the cross correlation coefficient

EXPRESSIONS TO OBTAIN EXPRESSIONS TO OBTAIN OPTIMUM WEIGHTSOPTIMUM WEIGHTS

Where Pk,opt is the optimum weight for the kth user A is the amplitude ρij is the cross correlation between ith and jth user

SIMULATION RESULTSSIMULATION RESULTS

BER PERFORMANCE BER PERFORMANCE COMPARISONCOMPARISON

USING RANDOMLY USING RANDOMLY GENERATED PN SEQUENCESGENERATED PN SEQUENCES

WITHOUT OVERLOADING

Comparision of MF o/p of single Comparision of MF o/p of single user with multiuseruser with multiuser

Comparison of MF o/p of single Comparison of MF o/p of single user with MF and CLPIC o/p of user with MF and CLPIC o/p of

multiusermultiuser

Comparison of MF o/p of single Comparison of MF o/p of single user with MF and WLPIC o/p of user with MF and WLPIC o/p of

multiusermultiuser

Comparison of MF o/p of single Comparison of MF o/p of single user with MF,CLPIC and WLPIC user with MF,CLPIC and WLPIC

o/p of multiusero/p of multiuser

Comparison of BER performance of Comparison of BER performance of MF and WLPIC without scrambling MF and WLPIC without scrambling

Comparison of BER performance of Comparison of BER performance of MF and WLPIC with scrambling MF and WLPIC with scrambling

No.of users Vs BER – without Near-No.of users Vs BER – without Near-Far effectFar effect

No. of users Vs BER – with Near-No. of users Vs BER – with Near-Far effectFar effect

USING RANDOMLY USING RANDOMLY GENERATED PN SEQUENCESGENERATED PN SEQUENCES

WITH OVERLOADING

Comparison of BER performance of Comparison of BER performance of MF and WLPIC with scrambling MF and WLPIC with scrambling

and 50% overloading and 50% overloading

Comparison of BER performance of Comparison of BER performance of MF and WLPIC with scrambling MF and WLPIC with scrambling

and 75% overloading and 75% overloading

USING GOLD CODES(Single USING GOLD CODES(Single set) AS PN SEQUENCESset) AS PN SEQUENCES

WITH OVERLOADING

Comparision of BER performance Comparision of BER performance of MF and WLPIC with scrambling of MF and WLPIC with scrambling

and 50% overloading and 50% overloading

USING GOLD CODES(2 sets) USING GOLD CODES(2 sets) AS PN SEQUENCESAS PN SEQUENCES

WITH OVERLOADING

Comparison of BER performance of Comparison of BER performance of MF and WLPIC without scrambling MF and WLPIC without scrambling

and with 50% overloadingand with 50% overloading

Comparison of BER performance of Comparison of BER performance of MF and WLPIC with scrambling MF and WLPIC with scrambling

and 50% overloading and 50% overloading

Comparison of BER performance of Comparison of BER performance of MF and WLPIC with scrambling MF and WLPIC with scrambling

and 75% overloading and 75% overloading

PERFORMANCE ANALYSISPERFORMANCE ANALYSIS

WLPIC performs better for two sets of gold codes.

WLPIC performs better than the MF Detector as well as CLPIC.

CONCLUSIONCONCLUSION

WLPIC performs better than CLPIC and MF.

WLPIC performs better for two sets of gold codes.

Works well under near far as well as non near far conditions also.

REFERENCESREFERENCES[1]Preetam Kumar and Saswat Chakrabarti ,”OCDMA/OCDMA overloading

scheme for cellular ds-cdma using orthogonal gold codes And Complex Scrambling ,” G S Sanyal School of Telecommunications ,IIT kharagpur,India,received on May 5,

2008 ;Revised and accepted on June 27,2008.

[2]V.Tikiya, S.Manohar and A.Chockalingam, “SIR Optimized Weighted Linear Parallel Interference Canceller on Fading Channels,“ IEEE transactions on wireless communications ,vol. 5,No. 8, August 2006 .

[3]F.Vanhaverbeke ,M.Moeneclaey and H.Sari ,”DS/CDMA with two sets of orthogonal sequences and Iterative-Detection ,“ IEEE Communication Lettters, vol.4 , pp.289-291,September 2000 .

 

 

 

[4]Preetam Kumar and Saswat Chakrabarti,” Overloading Scheme for Cellular DS-CDMA using Quasi Orthogonal Sequences and Iterative Interference Cancellation Receiver”.

[5]K. Yang, Y. K. Kim, and P. V. Kumar, “Quasi-orthogonal sequences for code-division multiple-access systems,”IEEE Transactions on Information Theory, Vol. 46, pp.982–993, May 2000.

[6]Elmustafa Erwa and Jeffery Andrews,”Performance comparison of Successive and Parallel Interference Canceller”, Dec 9, 2003.

[7]John Proakis, “Digital Communications”.

[8]Theodore S. Rappaport, “Wireless Communications Principles and Practice”, second edition.