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
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