CDMA
CODE DIVISION MUTIPLE ACCESS
CDMA
CDMA is one method for implementing a multiple
access communication system.
CDMA Versions:
MUTIPLE ACCESS
Technique where many subscribers can share
the use of a communication channel at the same
time
FDMA: (FREQUENCY DEVISION MUTIPLE
ACCESS):
TDMA(TIME DEVISION MUTIPLE ACCESS):
CDMA: (CODE DEVISION MUTIPLE ACCESS):
DATA SIGNAL:
DATA SIGNAL + CODE:
AFTER MIXING,WIDE BAND SPREAD SPECTRUM
THIS IS HOW CALLS THROUGH CDMA ARE:
NOW HOW CAN WE ACHIEVE THIS?
Simply by using CODES!
1. Data signal is multiplied with a spreading /chip code data
stream(often by using XOR function).
The resulting data stream has a higher data rate than the data itself
CODES:
Walsh Code:
Orthogonal in nature(zero correlation)
It is used for forward channelizing.
IS 95A and IS 95B 64 Walsh codes are used
PN codes:
State machines generating PN Codes consists of shift registers &
XOR gates.
Short PN code(16 bit):
It is use to identify BS.
Identifies cells and sectors.
Long PN code(42 bit code)
It is use to identify MS.
Used in Reverse
channelizing
2. The wideband data stream is then modulated onto a
carrier
1. To decode the signal the CDMA signal is first
demodulated from the carrier to reconstitute the high
speed data stream.
2. This is multiplied with the spreading code to regenerate
the original data.
CDMA TRANSCEIVER
Due to this CDMA Spread Spectrum Technique, it is possible to
transmit several sets of data independently on the same carrier and
then reconstitute them at the receiver without mutual interference.
CDMA spread spectrum encode process
Lets show this by an example:
Data to be transmitted is 1001
The chip/spreading code is 0010
1
0
0
1
Data to be transmitted
0010 0010 0010 0010 Chip or spreading code
1101 0010 0010 1101 Resultant spread data output
CDMA spread spectrum decode process
1101 0010 0010 1101
0010 0010 0010 0010
Incoming CDMA signal
Chip or spreading code
1111 0000 0000 1111
Result of de-spreading
Integrated output
Spreading codes may be 64 bits or 128 bits long
Types of Spread Spectrum Communications
1. DIRECT SEQUENCE SPREAD SPECTRUM:
Allows each station to transmit over the entire frequency.
Each user is assigned a chip sequence for multiple
simultaneous transmission over same channel
The system works with 64 Kbits/sec data, but can accept input
rates of 8, 16, 32,
2. FREQUENCY HOPPING SPREAD SPECTRUM:
The narrow band information signal is rapidly switched
between different frequencies within the hopping
Can be implemented over a much larger frequency band than
it is possible to implement DS- spreading bandwidth
Do not require power control to prevent Near-Far problem
Synchronous CDMA
Uses property of orthogonality
Use in base-to-mobile links
IS-95 uses 64 bit Walsh codes
THIS IS HOW IT IS DONE
ASynchronous CDMA
Use in base-to-mobile links
Use pseudo-random" or "pseudo-noise" (PN)
Sequences for spreading
Can use spectrum more efficiently then TDMA,FDMA
and even CDMA
THIS IS HOW IT IS DONE
CDMA CHANNELS:
Forward Traffic Channel :
CDMA Cell Site
Pilot
Forward Traffic Channel
Forward Traffic Channel
Sync
Forward Traffic Channel
Paging
Forward Traffic Channel
Pilot Channel(W0):
Used by the mobile station for initial system
acquisition
Transmitted constantly by the base station
Every sector of every cell site has a unique pilot
channel.
Key factor in performing soft handoffs
Paging Channel(W1-W7):
To inform mobile units of incoming calls.
Used to set up a call
To exchange short messages (SMS)
Data Rates can be 2.4,4.8 or 9.6 Kbps.
Synchronization Channel (W32):
Used along with the pilot channel
Provide information to the mobile unit like Network
ID, system time, Paging ch. data rate etc.
Tx at 1200 bps
Traffic Channel ( W8-W31 & W33-W63):
Used to send and receive data or/and voice
It carries the actual call.
TX upto 9.6kbps/14.4kbps
CDMA Reverse Traffic Channels
Response to commands/queries from the base
station
A convolutional encoder and interleaver is used to
mitigate burst errors during transmission
Reverse Traffic Channel
CDMA MS Call Processing
Power-Up
Power-Up
Initialization
Initialization
Mobile station is in idle
handoff or is unable to
receive Paging Channel
Message
Mobile station
has fully
acquired
system timing
Idle
Idle
Mobile station receives a
Paging Channel message
requiring ACK or
response, originates a call.
System
System
Access
Access
Mobile station is
directed to a Traffic
Channel
Mobile station receives an
ACK to an Access Channel
transmission other than an
Origination Message or a
Page Response Message
Traffic
Traffic
Mobile station ends
use of the Traffic
Channel
PROBLEMS IN CDMA SYSTEM
1.
MULTI-PATH FADING:
Fluctuation or randomness received at the receiver
Time delays are introduced when signal comes from
different paths
SOLUTION:
RAKE receiver uses signals from different paths
arriving at the receiver at different time instances to
get a stronger signal.
Rake Receiver Contd.
One of the receivers constantly searches for different
multipath and helps to direct the other three
fingers(receivers) to lock onto strong multipath signals.
Provides both path diversity and frequency diversity.
2.
MUTI -USER INTERFACE:
Signals from other users appears as an interface to
desired user due to non-zero cross-correlation values
arising due to time delay
SOLUTION:
Spreading the signal by orthogonal codes which have
zero cross co-relation.
The mobile user can be time-aligned by a
synchronization method.
3. NEAR-FAR PROBLEM:
Handsets may be anywhere within the particular cell
boundaries. Some handsets will be close to the base
station,
Signal = k x 1 / d2
The effects of objects and other obstructions in the
signal propagation path
How to solve these problems?
Using Power Control:
1. Forward-Link Power Control:
Power is fixed at initiation of the call
Mobile unit reports the quality of the received forward-link signal
and the sector responds accordingly
Mobile unit reports frame errors via a Power Measurement Report
Message
When a PMRM is not received, it then increase its transmit power
by a larger increment (1-2dB up to a maximum) from 0.2-0.5dB
2. Reverse-Link Power Control:
Every 1.25 ms, the mobile unit will either power up or power down
in response to the various power control commands that it receives
from the serving sectors.
ADVANTAGES
Frequency reuse:
In a CDMA system the same frequency can be used in every
cell because channelization is done using the pseudorandom
codes
Soft handoff:
Since adjacent cells use the same frequencies, CDMA systems
have the ability to perform soft handoffs
ME can communicate simultaneously with two or more cells.
The best signal quality is selected until the handoff is
complete.
Recovering lost data:
CDMA can also effectively reject interference. Convolution
encoding and interleaving can be used to assist in
recovering this lost data.
Reduce deployment and operating cost because
fewer cell sites are needed
CDMA is also resistant to jamming.
Spectral capacity increase
Power Control:
MeanPowerMaxPower
GSM:125mW2W
CDMA:2mW200mW
