DS CDMA

Direct Sequence CDMA

In Direct Sequence spread spectrum transmission, the user data signal is multiplied by a code sequence. Mostly, binary sequences are used. The duration of an element in the code is called the "chip time". The ratio between the user symbol time and the chip time is called the spread factor. The transmit signal occupies a bandwidth that equals the spread factor times the bandwidth of the user data.

Figure: A DS-CDMA signal is generated by multiplication of a user data signal by a code sequence.
Source: Jack Glas, T.U. Delft.
In the receiver, the received signal is again multiplied by the same (synchronized) code. This operation removes the code, so we recover the transmitted user data.

Different CDMA users use different codes. In this example the receiver sees the signal from user 1, while the signal from user 2 is heavily attenuated by the correlator (multiplier and integrator) in the receiver.

A CDMA receiver can retrieve the wanted signal by multiplying the receive signal with the same code as the one used during transmission. We find:

where c₁ is the code sequence used by user 1, T_c is the chip duration, t_d is a common time offset, shared between transmitter and receiver and N is the length of the code sequence.. Note that the receive code must be perfectly time aligned with the transmit code.
Popular code sequences are

• Maximum length or Pseudo Noise (PN) sequences.
• Walsh Hadamard Codes.
• Gold codes, and
• Kasami codes.

Spreading gain

Figure: DS-CDMA signal with a jamming signal
Source: Jack Glas, T.U. Delft. As despreading is the same operation as spreading a possible narrowband jammer signal in the radio channel is spread before the detector. Thus, the jam signal is attenuated by the spread factor ("spreading gain").

Near-Far Effect

A major difficulty in Direct Sequence transmission is the Near-Far effect. If more than one user is active, the incoming interference power is suppressed by the cross correlation between the code of the reference user and the code of the interferer. In the event that the interferer is closer to the receiver then the reference user, the interference components may not be sufficiently attenuated by the despreading process. In cellular CDMA systems, (adaptive) power control is needed to avoid this problem

Inherent Diversity

In a multipath fading channel, delayed reflections interfere with the direct signal. However, a DS-CDMA signal suffering from multipath dispersion can be detected by a rake receiver. This receiver optimally combines signals received over multiple paths.

Different reflected waves arrive with different delays. A rake receiver can detect these different signals separately. These signals are then combined, using the diversity technique called maximum ratio combining.

Multipath self-interference is attenuated, because one can choose codes such that

wirelesscommunication_dscdma

AMPS (PART 1)

Advanced mobile phone system(AMPS):

TACS

MCS-L1

D-AMPS

Cells   > 10 – 20 km amps

Small cell --- less power

Frequency reuse

Overloaded cells --- microcells

Center of cells – base station

Base station – computer + transmitter/ receiver  connected to antenna

MTSO / MISO  : all base stations are connected to .

CDMA

RF

GSM vs CDMA

GSM

GSM is a ‘cellular’ technology, that is, the entire coverage area is divided into various hexagonal shaped cells (hence the popular name ‘cell phones’). Every cell has a corresponding network tower, which serves the mobile phones in that cellular area. For example: Imagine a honeycomb on a tree in a hexagonal shaped garden. The garden has many flowers. The honeybees collect the nectar from the flowers, and deposit it in the honeycomb. Your mobile phones are like the flowers, the network tower is like honeycomb, and the bees are the signals.

CDMA

As the name suggests (Code Division Multiple Access), there are many devices which use the same spread spectrum (hence multiple access). There is one physical channel, and a special code for every device in the coverage network. Using this code, the signal of the device is multiplexed, and the same physical channel is used to send the signal (the codes may or may not change). For example: There is a street on which many buses ply. Obviously, the bus will have many commuters, and they will have to buy the tickets to travel on the bus. In CDMA, the commuter is like your mobile phone, the tickets are your codes, the bus is a multiplexed carrier signal, and the street is the spread spectrum.

SOURCE:      CDMA vs GSM

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