1 / 23

Wireless Communications: System Design

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, 870-940 MHz 1983: AMPS (US), FDMA, FM, 30 kHz channels, 824-894 MHz

janad
Download Presentation

Wireless Communications: System Design

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Wireless Communications:System Design Dr. Mustafa Shakir

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

  3. 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

  4. 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.

  5. 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.

  6. 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

  7. 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’

  8. 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

  9. Representation Of Cells Through BS

  10. 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

  11. 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

  12. 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

  13. 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.

  14. 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

  15. 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

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

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

  18. 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

  19. 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.

  20. 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.

  21. 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.

More Related