8/2/2019 Cdma & Ofdm Operation

1/18

CDMA & OFDM OPERATIONAND COMPARISON

By

L NARESH

08L61A0453

8/2/2019 Cdma & Ofdm Operation

2/18

Overview

Introduction

CDMA

OFDM Comparison between CDMA & OFDM

Future works on OFDM

8/2/2019 Cdma & Ofdm Operation

3/18

Introduction

MULTIPLEXING is a method by which multiple

analog message signals or digital data streams

are combined into one signal over a shared

medium.

The aim is to share an expensive

resource(spectrum)

8/2/2019 Cdma & Ofdm Operation

4/18

CDMA (Code Division Multiple Access)

unique code assigned to each user

used mostly in wireless broadcast channels (cellular,satellite, etc)

all users share same frequency, but each user has own

chipping sequence (i.e., code) to encode data encoded signal= (original data) X (chipping sequence)

decoding: inner-product of encoded signal and chippingsequence

allows multiple users to coexist and transmitsimultaneously with minimal interference (if codes areorthogonal)

8/2/2019 Cdma & Ofdm Operation

5/18

CDMA Encode/Decode

8/2/2019 Cdma & Ofdm Operation

6/18

CDMA: two-sender interference

8/2/2019 Cdma & Ofdm Operation

7/18

Orthogonal Frequency Division

Multiplexing(OFDM)

It is a special kind of FDM

The spacing between carriers are such that

they are orthogonal to one another Therefore no need of guard band between

carriers.

8/2/2019 Cdma & Ofdm Operation

8/18

Example of OFDM

Lets we have following information bits

1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1,

Just converts the serials bits to parallel bitsC1 C2 C3 C4

1 1 -1 -1

1 1 1 -1

1 -1 -1 -1

-1 1 -1 -1

-1 1 1 -1

-1 -1 1 1

8/2/2019 Cdma & Ofdm Operation

9/18

Example of OFDM cont..

Modulate each column with corresponding sub-carrier using BPSK

Modulated signal for C1 Modulated signal for C2

Modulated signal for C3 Modulated signal for C4

8/2/2019 Cdma & Ofdm Operation

10/18

Example of OFDM cont..

Final OFDM Signal = Sum of all signal

)2sin()()(1

0

nttItVN

nn

T

!

!

Generated OFDM signal, V(t)

8/2/2019 Cdma & Ofdm Operation

11/18

OFDM Tx & Rx system

8/2/2019 Cdma & Ofdm Operation

12/18

8/2/2019 Cdma & Ofdm Operation

13/18

Advantages of OFDM over CDMA

OFDM can combat multipath interference withgreater robustness and less complexity.Equalisation can be undertaken on a carrier bycarrier basis.

OFDM can achieve higher spectral efficiency withMIMO than CDMA using a RAKE receiver.

Can be used to provide a single frequencynetwork.

It is relatively easy to aggregate spectrum.

It can be scaled according to the requirementsrelatively easily.

8/2/2019 Cdma & Ofdm Operation

14/18

Advantages of CDMA over OFDM

Not as complicated to implement as OFDM

based systems

The CDMA system showed a substantialadvantage over the OFDM system concerning

the transmit power as well as the power

spectral density which influences the radiated

emissions

8/2/2019 Cdma & Ofdm Operation

15/18

Future Issues of OFDM

Peak-to-average power reduction in OFDM

Timing and Frequency Synchronization

Efficient digital signal processing Implementation

of OFDM

Multiple input/Multiple output (MIMO) OFDM

8/2/2019 Cdma & Ofdm Operation

16/18

CONCLUSION

Specially designated frequency bands for

communications would favour the OFDM

system

where as lower and constant limit for large

parts of the frequency band of interest would

recommend the CDMA system.

8/2/2019 Cdma & Ofdm Operation

17/18

Thank You

8/2/2019 Cdma & Ofdm Operation

18/18

Queries?