08-11-2012, 05:56 PM
CDMA
cdma hh.pptx (Size: 506.43 KB / Downloads: 29)
INTRODUCTION
Over the past few years the wireless technology industry has seen massive growth with ever increasing mobile subscriber rates. CDMA is the second most widely deployed technology in the world with more than 100 million subscribers worldwide
Multiple Access Schemes
FDMA Frequency Division Multiple Access
* Allocates discrete amount of Bandwidth
to each user.
TDMA Time Division Multiple Access
* Allocates unique time slot to each user.
* Each user has specific time interval to
transmit information.
CDMA Code Division Multiple Access
* All users share the same frequency all the
time.
* A unique code assigned to each user allows
it to be distinguished from other users.
* CDMA is a form of Direct Sequence Spread
Spectrum communications
Spread Spectrum
Definition: A Transmission technique in which a pseudo noise code, independent of the information data, is employed as a modulation waveform to spread the signal energy over a Bandwidth much greater than the signal information Bandwidth. At receiver the signal is despread using a synchronous replica of the pseudo noise code.
Thus,
signal occupies a bandwidth >> Bandwidth required to send the information.(benefits: immunity to interference and jamming and multi-user access)
The bandwidth is spread by means of a code which is independent of the data.
The receiver synchronizes to the code to recover the data. (The use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time. )
Types of Spread Spectrum Communications
Three ways to spread the bandwidth of the signal:
Frequency hopping. The signal is rapidly switched between different frequencies within the hopping bandwidth pseudo-randomly, and the receiver knows before hand where to find the signal at any given time.
Time hopping. The signal is transmitted in short bursts pseudo-randomly, and the receiver knows beforehand when to expect the burst.
Direct sequence. The digital data is directly coded at a much higher frequency. The code is generated pseudo-randomly, the receiver knows how to generate the same code, and correlates the received signal with that code to extract the data.
Pseudo-Random Codes
PN sequence is a deterministic periodic binary sequence with a noiselike waveform.
Periodic binary sequences can be conveniently generated using linear feedback shift registers (LFSR).
Feedback shift-register contains of ordinary shift register made up of m flip-flops and a logic circuit that are interconnected to form a multiloop feedback circuit
If the number of stages in the LFSR is r, P 2r - 1 where P is the period of the sequence
However, if the feedback connections satisfy a specific property, P = 2r - 1. Then the sequence is called a Maximal Length Shift Register (MLSR) or a PN sequence.
Summary
• DSSS is the core of CDMA techniques
• PN sequence is crucial to DSSS
• PN sequence realizes
– Multiple Access
– Message Privacy
– Suitable to real-world communication channels
Multipath fading reduced by diversity reception.
The co - channel and adjacent channel interference are tending to increase as more number of users starts using the system.
Multiple antennas multiple Rake finger receiver structure provided increased SNR making it suitable for efficient communication.