Wireless Communications: System Design Dr. Mustafa Shakir

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

Modern cellular standards 1979: NTT (Japan), FDMA, FM, 25 kHz channels, MHz 1983: AMPS (US), FDMA, FM, 30 kHz channels, MHz 1985: TACS (Europe), FDMA, FM, 25 kHz channels, 900 MHz 1990: GSM (Europe), TDMA, GMSK, 200 kHz channels, MHz 1991: USDC/IS-54 (US), TDMA, p/4 DQPSK, 30 kHz channels, MHz 1993: IS-95 (US), CDMA, BPSK/QPSK, 1.25 MHz channels, MHz and GHz 1993: CDPD (US), FHSS, GMSK, 30 kHz channels, Mhz 2001: UMTS/IMT-2000 (3rd generation European cellular standard), supports data and voice (up to 2 Mbps), MHz and Mhz : LTE Advanced and Mobile WiMAX

Evolution Of Cellular Mobile Just 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  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 Mobile Just 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’

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

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 60 0 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)

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 = = 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