Wireless Communications: System Design

2. Wireless Communications:System Design Dr. Mustafa Shakir

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

4. 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

5. 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.

6. 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.

7. 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

8. 2 3 7 1 6 4 5 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’

9. 2 2 2 2 2 2 2 3 3 3 3 3 3 3 7 7 7 7 7 7 7 1 1 1 1 1 1 1 6 6 6 6 6 6 6 4 4 4 4 4 4 4 5 5 5 5 5 5 5 Cell Clusters (1) • A network of cell clusters in a densely populated Town

10. Representation Of Cells Through BS

11. 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

12. 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

13. 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

14. 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.

15. 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

16. 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

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

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

19. 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

20. Channel Allocation Techniques • 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) • Targets to achieve through the different channel allocation techniques.

21. Fixed 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.

22. Features of 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.