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Mobile Computing COE 446

Wireless Multiple AccessTarek Sheltami

KFUPMCCSECOE

http://faculty.kfupm.edu.sa/coe/tarek/coe446.htm

Principles of Wireless Networks

K. Pahlavan and P. Krishnamurth

Outline CDMA capacity calculations

With/without sectorization gain, voice of active interference reduction factor and interference increase factor

Comparison of the Capacity of different 2G Systems

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How many users can simultaneously use a CDMA System before the system collapses?

CDMA systems are implemented based on the spread spectrum technology

A spread spectrum transmitter spreads the signal power over a spectrum N times wider than the spectrum of the message

An information BW of R occupies a transmission of BW W

W= NR (1) The spread spectrum receiver processes

the received signal with a processing gain of N

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How many users can simultaneously use a CDMA System before the system

collapses..?

During the processing at the receiver, the power of received signal having the code of that particular receiver will be increased N times beyond the value before processing

Let us consider a single cell with M simultaneous users on the uplink channel

Let us assume that we have an ideal power control enforced on the channel, so that the received power of signals from all MTs has the same value of P

Then, the received power from the target user after processing at the receiver is NP, and the received power interference from M-1 other MTs is (M-1)P

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How many users can simultaneously use a CDMA System before the system

collapses..?

Let us further assume that a cellular system is limited and the background noise is dominated by interference noise from other users, then, the receiver Signal-to-interference ratio for a target receiver:

r

NS

M 1

(2)

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How many users can simultaneously use a CDMA System before the system

collapses..?

All users always have a requirement for the acceptable error rate of the received data stream

For a given modulation and coding specification of the system, the error rate requirement will be supported by minimum of Sr requirements

From EQs #1 & 2

(3)r

W 1M

R S

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Using QPSK modulation and convolution coding, the IS-95 digital cellular systems require 3 dB < Sr

< 9 dB. The bandwidth of the channel is 1.25 MHz, and the transmission rate is R = 9600 bps. Find the capacity of a single IS-95 cell.

Solution:Using Equation (4.3) we can support:

1.25MHz 1 1.25MHz 1M 16 M 65 users

9600 bps 8 9600 bps 2 to

Problem 2: Capacity of One Carrier in a Single-Cell CDMA System

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Practical Consideration

In practice, design of digital cellular systems, there are other parameters affect the number of users that can be supported by the system

1. Number of Sectors: The use of sectored antenna is an important

factor in maximizing BW efficiency Cell sectorization using directional antennas

reduces the overall interference and increasing M Sectorization gain = GA

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Practical Consideration .. With ideal sectorization the user in one sector of a BS

antenna do not interfere with the users operating in other sectors

GA = Nsec, Nsec is the number of sectors in the cell In practice antenna patterns can not be designed to

have ideal characteristics, and due to multipath reflection, users in general communicate with more than one sector

The sector BS gain assumed to be GA = 2.5 (4 dB)2. The Voice of activity interference reduction

factor (Gv): The ratio of the total connection time to the active

talkspurt On the average, in two-way conversation, each user

talks roughly 50% of the time

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Practical Consideration ..

The short pause in the flow of natural speech reduce the activity factor further to about 40% of connection time in each direction

As a result, the typical number used for Gv = 2.5 (4 dB)

3. The Interference Increase Factor (Ho) Accounts for the users in other cells in CDMA system Because all neighboring cells in a CDMA cellular

network operate at the same frequency, they will cause additional interference

This interference is relatively small due to the processing gain and the distance involved

Ho = 1.6 (2 dB), is commonly used in industry

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Practical Consideration ..

From EQs 1, 2 and 3

A v

r o

G GW 1M

R S H (4)

The last part of EQ.# 4 is called Performance improving factor: K=(GAGV/Ho) = 4 (6 dB)

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Determine the multi-cell IS-95 CDMA capacity with correction for sectorization and voice activity. Use the numbers from Problem 2.

Solution:If we continue the previous example with the new correction factor included, the range for the number of simultaneous users becomes 64 < M < 260.

Problem 3: Capacity of One Carrier in a Multi-Cell CDMA System with Correction Factors

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Compare the capacity of IS-95 CDMA with AMPS FDMA and IS-136 TDMA systems. For the CDMA system, assume an acceptable signal to interference ratio of 6 dB, data rate of 9600 bps, voice duty cycle of 50 percent, effective antenna separation factor of 2.75 (close to ideal 3-sector antenna), and neighboring cell interference factor of 1.67.

Problem 4: Comparison of the Capacity of Different 2G Systems

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For the IS-95 CDMA using Equation (4.4) for each carrier with W = 1.25 MHz, R = 9600 bps, Sr= 4 (6dB), Gv= 2 (50 percent voice activity). GA = 2.75, and Ho = 1.67 we have:

For the IS-136 with a carrier bandwidth of Wc = 30 kHz, the number of users per carrier of Nu = 3, and frequency reuse factor of K = 4 (commonly used in these systems), each W = 1.25 MHz of bandwidth provides for

For the AMPS analog system with carrier bandwidth of Wc = 30 kHz, and frequency reuse factor of K = 7 (commonly used in these systems), each W = 1.25 MHz of bandwidth provides for

A V

r o

G GW 1M 108 users / cell

R S H

u

c

NWM 31.25 users / cell

W K

c

W 1M 6 users / cell

W K

Solution

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Determine the capacity of GSM for K = 3.

Solution:For the GSM system with a carrier bandwidth of Wc = 200 kHz, the number of users per carrier of Nu = 8, and frequency reuse factor of K = 3 (commonly used in these systems), each W = 1.25MHz of bandwidth provides for

Problem 4: Comparison of the Capacity of Different 2G Systems

u

c

NWM 16.7 users / cell

W K