Wireless Communications: System Design

Dr. Mustafa Shakir

Evolution of wireless in Europe and the US can be summarized in the following diagrams:

FM Technology

TDM A CDM A

M any S tandardsUntil 1988

G SM1989

DCS 18001991

AM PS(Analog)

1979

D ig ita l TechnologyLate 1980

U S Europe

Modern cellular standards 1979: NTT (Japan), FDMA, FM, 25 kHz channels, 870-940 MHz

1983: AMPS (US), FDMA, FM, 30 kHz channels, 824-894 MHz

1985: TACS (Europe), FDMA, FM, 25 kHz channels, 900 MHz

1990: GSM (Europe), TDMA, GMSK, 200 kHz channels, 890-960 MHz

1991: USDC/IS-54 (US), TDMA, p/4 DQPSK, 30 kHz channels, 824-894 MHz

1993: IS-95 (US), CDMA, BPSK/QPSK, 1.25 MHz channels, 824-894 MHz and 1.8-2.0 GHz

1993: CDPD (US), FHSS, GMSK, 30 kHz channels, 824-894 Mhz

2001: UMTS/IMT-2000 (3rd generation European cellular standard), supports data and voice (up to 2 Mbps), 1885-2025 MHz and 2110-2200 Mhz

2008 2009: LTE Advanced and Mobile WiMAX

Evolution Of Cellular MobileJust an overview

Engineering Research To full fill the necessity : As the requirement of wireless connections and required data rate increased

engineers tried to full fill the requirement. Simple Analog Mobile To Analog Cellular Mobile : First simple mobile system was upgraded to cellular in the form of AMPS

in 1983. Analog Cellular Mobile to Digital Cellular Mobile : Then GSM was introduced with TDMA approach having more capacity and

data rate. Digital Cellular Mobile To CDMA: After that to full fill the requirements of more data and more subscriber

CDMA was introduced by Qualcomm. CDMA supports a variable number of users in 1.25MHz wide channels using

direct sequence spread spectrum. Interference Affordability: CDMA system can operate at much larger interference levels because of

their inherent interference resistance properties.

Evolution Of Cellular MobileJust an overview Contd. Large Capacity of CDMA : The ability of CDMA to operate with a much smaller S/N ratio than

FM techniques allows CDMA systems to use the same set of frequencies in every cell which provides a large improvement in capacity.

Cell Clusters Service areas are normally divided into clusters of cells to

facilitate system design and increased capacity

Definition

a group of cells in which each cell is assigned a

different frequency

cell clusters may contain any number of cells, but

clusters of 3, 4, 5, 7 and 9 cells are very popular in

practice

Cell Clusters A cluster of 7 cells

the pattern of cluster is repeated throughout the network

channels are reused within clusters cell clusters are used in frequency planning for the

network Coverage area of cluster called a ‘footprint’

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Cell Clusters (1) A network of cell clusters in a densely populated Town

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Representation Of Cells Through BS

Frequency Plan Intelligent allocation of frequencies used

Each base station is allocated a group of channels

to be used within its geographical area of coverage

called a ‘cell’

Adjacent cell base stations are assigned completely

different channel groups to their neighbors.

base stations antennas designed to provide just the

cell coverage, so frequency reuse is possible

Frequency Reuse Concept Assign to each cluster a group of radio channels to be used

within its geographical footprint

ensure this group of frequencies is completely different

from that assigned to neighbors of the cells

Therefore this group of frequencies can be reused in a cell

cluster ‘far away’ from this one

Cells with the same number have the same sets of

frequencies

Frequency Reuse Factor Definition

When each cell in a cluster of N cells uses one of

N frequencies, the frequency reuse factor is 1/N

frequency reuse limits adjacent cell interference

because cells using same frequencies are

separated far from each other

Factors Affecting Frequency Reuse Factors affecting frequency reuse

include:

Types of antenna used

--omni-directional or sectored

placement of base stations

-- Center excited or edge excited.

Excitation of Cells Once a frequency reuse plan is agreed upon overlay the

frequency reuse plan on the coverage map and assign

frequencies

The location of the base station within the cell is referred to as

cell excitation

In hexagonal cells, base stations transmitters are either:

centre-excited, base station is at the centre of the cell or

edge-excited, base station at 3 of the 6 cell vertices

Finding the Nearest Co-Channel

After selecting smallest possible value of N we should see that N should

follow the following eq. N= i2+j2+ij

(1) Move i cells along any chain of hexagons

(2) Turn 600 counter-clockwise and move j cells, to reach the next cell

using same frequency sets

this distance D is required for a given frequency reuse to provide

enough reduced same channel interference

ie, after every distance D we could reuse a set of frequencies in a

new cell

Freq Reuse ( N=7 , i=2 j=1)

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Freq Reuse ( N=19 , i=3 j=2)

How frequency Reuse Increases Capacity Example: A GSM communication system uses a frequency

reuse factor of 1/7 and 416 channels available. If 21 channels

are allocated as control channels, compute its system capacity.

Assume a channel supports 20 users

Channels available for allocation = 416 - 21 = 395

Number of channels = 395 / 7 = 57

Number of simultaneous users per cell = 20 x 57 = 1140

Number of simultaneous users in system = 7 x 1140 = 7980

To satisfy the user, a channel needs to be available on request.

Reasonable probability of call blockage (GOS) is 2%. GOS fluctuate with location and time. The goal is to keep a

uniform GOS across the system. Reduction of variations in GOS allow more users – an increase

in capacity. Three types of algorithms for channel allocation:

Fixed channel allocation (FCA) Channel Borrowing Dynamic channel allocation (DCA)

Channel Allocation Techniques Targets to achieve through the different

channel allocation techniques.

Available spectrum is W Hz and each channel is B Hz. Total number of channels:

Nc = W/B For a cluster size N, the number of channels :

Cc = Nc/N To minimize interference, assign adjacent channels to

different cells.

Fixed Channel Allocation Techniques

FCA is the optimum allocation strategy for uniform traffic across the cells.

Impacts the performance of a system particularly as to managing calls when mobile user handed from one cell to another

A non uniform FCA strategy, when it is possible to evaluate GOS in real time and adjust the FCA accordingly. This requires a more complex algorithm.

Features of Fixed Channel Allocation Techniques