- Slides: 23
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 n 1979: NTT (Japan), FDMA, FM, 25 k. Hz channels, 870 -940 MHz n 1983: AMPS (US), FDMA, FM, 30 k. Hz channels, 824 -894 MHz n 1985: TACS (Europe), FDMA, FM, 25 k. Hz channels, 900 MHz n 1990: GSM (Europe), TDMA, GMSK, 200 k. Hz channels, 890 -960 MHz n 1991: USDC/IS-54 (US), TDMA, p/4 DQPSK, 30 k. Hz channels, 824 -894 MHz n 1993: IS-95 (US), CDMA, BPSK/QPSK, 1. 25 MHz channels, 824 -894 MHz and 1. 8 -2. 0 GHz n 1993: CDPD (US), FHSS, GMSK, 30 k. Hz channels, 824 -894 Mhz n 2001: UMTS/IMT-2000 (3 rd generation European cellular standard), supports data and voice (up to 2 Mbps), 1885 -2025 MHz and 2110 -2200 Mhz n 2008 2009: LTE Advanced and Mobile Wi. MAX
Evolution Of Cellular Mobile Just an overview Ø § Ø § Ø Ø § 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. 25 MHz 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. : Ø 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 n Service areas are normally divided into clusters of cells to facilitate system design and increased capacity n Definition q a group of cells in which each cell is assigned a different frequency n 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 n A cluster of 7 cells 2 3 7 1 6 4 5 n n 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) n A network of cell clusters in a densely populated Town 2 3 2 7 3 1 1 6 4 2 2 7 5 3 1 2 7 1 6 4 5 3 7 3 1 6 4 5 5 2 3 5 3 7 1 6 4 5 6 4 2 7 1 7 6 4 5
Representation Of Cells Through BS
Frequency Plan n Intelligent allocation of frequencies used n Each base station is allocated a group of channels to be used within its geographical area of coverage called a ‘cell’ q Adjacent cell base stations are assigned completely different channel groups to their neighbors. q base stations antennas designed to provide just the cell coverage, so frequency reuse is possible
Frequency Reuse Concept n Assign to each cluster a group of radio channels to be used within its geographical footprint q ensure this group of frequencies is completely different from that assigned to neighbors of the cells n Therefore this group of frequencies can be reused in a cell cluster ‘far away’ from this one q Cells with the same number have the same sets of frequencies
Frequency Reuse Factor n Definition q When each cell in a cluster of N cells uses one of N frequencies, the frequency reuse factor is 1/N q frequency reuse limits adjacent cell interference because cells using same frequencies are separated far from each other
Factors Affecting Frequency Reuse n Factors affecting frequency reuse include: Ø Types of antenna used --omni-directional or sectored Ø placement of base stations -- Center excited or edge excited.
Excitation of Cells q Once a frequency reuse plan is agreed upon overlay the frequency reuse plan on the coverage map and assign frequencies q The location of the base station within the cell is referred to as cell excitation q In hexagonal cells, base stations transmitters are either: n centre-excited, base station is at the centre of the cell or n 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= i 2+j 2+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 q this distance D is required for a given frequency reuse to provide enough reduced same channel interference q 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 Ø Example: A GSM communication system uses a frequency Capacity 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 Ø 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
Channel Allocation Techniques v Targets to achieve through the different channel allocation techniques. Ø To satisfy the user, a channel needs to be available on request. Ø Reasonable probability of call blockage (GOS) is 2%. Ø GOS fluctuate with location and time. The goal is to keep a uniform GOS across the system. Ø Reduction of variations in GOS allow more users – an increase in capacity. v Three types of algorithms for channel allocation: Ø Fixed channel allocation (FCA) Ø Channel Borrowing Ø Dynamic channel allocation (DCA)
Fixed Channel Allocation Techniques Ø Available spectrum is W Hz and each channel is B Hz. Total number of channels: Nc = W/B Ø For a cluster size N, the number of channels : Cc = Nc/N Ø To minimize interference, assign adjacent channels to different cells.
Features of Fixed Channel Allocation Techniques Ø FCA is the optimum allocation strategy for uniform traffic across the cells. Ø Impacts the performance of a system particularly as to managing calls when mobile user handed from one cell to another Ø 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.