EXPERIMENT NO 4

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BE(EXTC) SEM VIII (RCOE) Page 1

Name :

_______________________________________.

Branch :

Electronics & Telecommunication (SEM VIII)

Subject : Wireless Network

Experiment No: 04

Title : To find the Capacity and Spectral Efficiency of

DSCDMA

Date :

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EXPERIMENT NO: 04

AIM

:

To find the Capacity and Spectral Efficiency of DSCDMA

APPARATUS

: MATLAB 7

THEORY:

The capacity of a DS-CDMA system depends on the processing gain, Gp (a ratio of spreading

bandwidth, Bw, and information rate, R), the bit energy-to-interference ratio, Eb/I0, the voice

duty cycle, vf, the DS-CDMA omnidirectional frequency reuse efﬁciency, f, and the number of

sectors, G, in the cell-site antenna. The received signal power at the cell from a mobile is S = R x

Eb. The signal-to-interference ratio is

where:

Eb = energy per bit

Io = interference density

In a cell with Nu, mobile transmitters, the number of effective interferers is Nu - 1 because each

mobile is an interferer to all other mobiles. This is valid regardless of how the mobiles are

distributed within the cell since automatic power control (APC) is used in the mobiles. The APC

operates such that the received power at the cell from each mobile is the same as for every other

mobile in the cell, regardless of the distance from the center of the cell. APC conserves battery

power in the mobiles, minimizes interference to other users, and helps overcome fading.In a

hexagonal cell structure, because of interference from each tier is given as:

where:

Nu =number of mobile users in the band, Bw

ki = 1, 2, 3, . . . = the interference contribution from all terminals in individual cells in tiers 1, 2,

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the path loss to the center cell and the power reduction because of power control to an interfering

mobile’s own cell center. If we define a frequency reuse efficiency, f, then Eb/ Ib is given by

Capacity of a DS-CDMA System

This equation does not include the effect of background thermal and spurious noise (i.e., ) in

the spreading bandwidth Bw. Including this as an additive degradation term in the denominator

results in a bit energy-to-interference ratio of:

Note that from above Equation the capacity of the DS-CDMA system is reduced by /S which is

the ratio of background thermal plus spurious noise to power level.

For a ﬁxed Gp = Bw /R, one way to increase the capacity of the DS-CDMA system is to reduce

the required Eb/I0, which depends upon the modulation and coding scheme. By using a powerful

coding scheme, the Eb /I0 ratio can be reduced, but this increases system complexity. Also, it is

not possible to reduce the Eb /I0 ratio indeﬁnitely. The only other way to increase the system

capacity is to reduce the interference. Two approaches are used: one is based on the natural

behavior of human speech and the other is based on the application of the sectorized antennas.

From experimental studies it has been found that typically in a full duplex 2-way voice

conversation, the duty cycle of each voice is, on the average, less than 40%. Thus, for the

remaining period of time the interference induced by the speaker can be eliminated. Since the

channel is shared among all the users, noise induced in the desired channel is reduced due to the

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the FDMA or TDMA system because of the time delay associated with reassigning the channel Resource during the speech pauses. If we deﬁne vf as the voice activity factor (<1), then :

The equation to determine the capacity of a DS-CDMA system should also include additional

parameters to reflect the bandwidth efficiency factor, the capacity degradation factor due to

imperfect power control, and the number of sectors in the cell-site antenna. Equation is

augmented by these additional factors to provide the following equation for DS-CDMA capacity

at one cell:

N

U

=

N

U

=

where

f = frequency reuse efficiency <1

b = bandwidth efficiency factor <1

Cd = capacity degradation factor to account for imperfect APC <1

vf = voice activity factor < 1

Bw = one-way bandwidth of the system

R = information bit rate plus overhead

Eb = energy per bit of the desired signal

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= efﬁciency of sector-antenna in cell (< G, number of sectors in the cell-site antenna)

For digital voice transmission, Eb/I0 is the required value for a bit error rate (BER) of about 10-3

or better, and f depends on the quality of the diversity. Under the most optimistic assumption,

f <0.5. The voice activity factor, vf is usually assumed to be less than or equal to 0.6. Eb /I0

for a BER of 10-3 can be as high as 63 (18 dB) if no coding is used and as low as 5 (7 dB) for a system using a powerful coding scheme. The capacity degradation factor, Cd will depend on the

implementation but will always be less than 1.

PROGRAM:

bw_eff=0.9;

frequency_reuse_eff=0.45;

capacity_degradation_factor=0.8;

voice_activity_factor=0.4;

information_bit_rate=16.2*10^3;

signal_interference=10^0.7;

oneway_bw=12.5*10^6;

efficiency_antenna=1;

capacity=(frequency_reuse_eff*bw_eff*capacity_degradation_factor*efficiency_antenna*oneway_bw)/

(voice_activity_factor*information_bit_rate*signal_interference);

spectral_efficiency=(capacity*information_bit_rate)/oneway_bw;

RESULT:

capacity = 124.7039