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1 Multiple Access Techniques for wireless communication l Multiple access schemes allow many mobile users to share a finite amount of radio spectrum l.

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Presentation on theme: "1 Multiple Access Techniques for wireless communication l Multiple access schemes allow many mobile users to share a finite amount of radio spectrum l."— Presentation transcript:

1 1 Multiple Access Techniques for wireless communication l Multiple access schemes allow many mobile users to share a finite amount of radio spectrum l High quality of communications must be maintained during the sharing process l Multiple access schemes allow many mobile users to share a finite amount of radio spectrum l High quality of communications must be maintained during the sharing process

2 2 Multiple Access Techniques SDMA PR Packet Radio Packet Radio Frequency Division Multiple Access Frequency Division Multiple Access Time Division Multiple Access Time Division Multiple Access Code Division Multiple Access Code Division Multiple Access Space Division Multiple Access Space Division Multiple Access Packet Radio Packet Radio Frequency Division Multiple Access Frequency Division Multiple Access Time Division Multiple Access Time Division Multiple Access Code Division Multiple Access Code Division Multiple Access Space Division Multiple Access Space Division Multiple Access

3 3 Multiple Access (MA) Technologies Multiple Access (MA) Technologies Cellular System MA Technique AMPS ( Advanced Mobile Phone system ) FDMA / FDD GSM ( Global System for Mobile ) TDMA / FDD US DC ( U. S Digital Cellular ) TDMA / FDD JDC ( Japanese Digital Cellular ) TDMA / FDD IS – 95 ( U.S Narrowband Spread Spectrum ) CDMA / FDD

4 4 Frequency Division Multiple Access (FDMA) code time frequency C1C1C1C1 C2C2C2C2 CNCNCNCN CNCNCNCN C2C2C2C2 C1C1C1C1 frequency

5 5 Principle of FDMA Operation l Each user is allocated a unique frequency band or channel. These channels are assigned on demand to users who request service l In FDD, the channel has two frequencies – forward channel & reverse channel l Each user is allocated a unique frequency band or channel. These channels are assigned on demand to users who request service l In FDD, the channel has two frequencies – forward channel & reverse channel

6 6 Properties of FDMA l Bandwidth of FDMA channels is narrow (30 KHz) l No equalization is required, since the symbol time is large compared to average delay spread l FDMA systems have higher cost o Costly band pass filters to eliminate spurious radiation o Duplexers in both T/R increase subscriber costs l Bandwidth of FDMA channels is narrow (30 KHz) l No equalization is required, since the symbol time is large compared to average delay spread l FDMA systems have higher cost o Costly band pass filters to eliminate spurious radiation o Duplexers in both T/R increase subscriber costs

7 7 Number Of channels in FDMA System

8 8 ExampleExample In the US, each cellular carrier is allocated 416 channels,

9 9 Time Division Multiple Access (TDMA) code time frequency C1C1C1C1 CNCNCNCN CNCNCNCN C2C2C2C2 C1C1C1C1 time

10 10 TDMA Operating principle l TDMA systems divide each FDMA channel into time slots l Each user occupies a cyclically repeating time slot. l TDMA can allow different number of time slots for separate user l TDMA systems divide each FDMA channel into time slots l Each user occupies a cyclically repeating time slot. l TDMA can allow different number of time slots for separate user

11 11 TDMA Frame Structure Preamble Information message Trail Bits Slot 1 Slot 2 Slot N Trail Bit Sync Bit Information Bit Guard Bits

12 12 Components of TDMA Frame l Preamble  Address and synchronization information for base station and subscriber identification l Guard times  Synchronization of receivers between different slots and frames l Preamble  Address and synchronization information for base station and subscriber identification l Guard times  Synchronization of receivers between different slots and frames

13 13 TDMA properties l Data Transmission for user of TDMA system occurs in discrete bursts o The result is low battery consumption. o Handoff process is simpler l Since different slots are used for T and R, duplexers are not required. l Equalization is required, since transmission rates are higher than FDMA channels l Data Transmission for user of TDMA system occurs in discrete bursts o The result is low battery consumption. o Handoff process is simpler l Since different slots are used for T and R, duplexers are not required. l Equalization is required, since transmission rates are higher than FDMA channels

14 14 Efficiency of TDMA Frame Efficiency

15 15 Frame efficiency parameters

16 16 Frame efficiency parameter definition

17 17 Number of channels in TDMA System

18 18 ExampleExample The GSM System uses a TDMA frame structure where each frame consist of 8 time slots, and each time slot contains bits, and data is transmitted at kbps in the channel. Time duration of a bit Time duration of a bit Time duration of a slot Time duration of a slot Time duration of a frame Time duration of a frame The GSM System uses a TDMA frame structure where each frame consist of 8 time slots, and each time slot contains bits, and data is transmitted at kbps in the channel. Time duration of a bit Time duration of a bit Time duration of a slot Time duration of a slot Time duration of a frame Time duration of a frame

19 19 SolutionSolution Time duration of a bitTime duration of a bit Time duration of a slotTime duration of a slot Time duration of a bitTime duration of a bit Time duration of a slotTime duration of a slot ms Time duration of a frame

20 20 ExampleExample If a normal GSM timeslot consists of 6 trailing bits, 8.25 guard bits, 26 training bits, and 2 traffic bursts of 58 bits of data, find the frame efficiency. Solution l Time slot has (58) = bits. l A frame has 8 * = 1250 bits / frame. If a normal GSM timeslot consists of 6 trailing bits, 8.25 guard bits, 26 training bits, and 2 traffic bursts of 58 bits of data, find the frame efficiency. Solution l Time slot has (58) = bits. l A frame has 8 * = 1250 bits / frame. The number of overhead bits per frame is: b OH = 8(6) + 8(8.25) + 8(26) = 322 bits Frame efficiency = (1250 – 322 )/1250 = %

21 21 Capacity of Cellular Systems l Channel capacity of a wireless system is the maximum number of users possible in the system l Channel capacity depends on: l Bandwidth available l Signal to Noise ratio (SNR) in the channel l Channel capacity of a wireless system is the maximum number of users possible in the system l Channel capacity depends on: l Bandwidth available l Signal to Noise ratio (SNR) in the channel

22 22 Calculation of cell capacity For a Cellular System l m = Capacity/cell = l B t = Total spectrum for the system l B C = Channel bandwidth l N= Number of cells / cluster For a Cellular System l m = Capacity/cell = l B t = Total spectrum for the system l B C = Channel bandwidth l N= Number of cells / cluster

23 23 Co-channel cell interference Co-channel cell interference

24 24 Channel capacity calculation

25 25 S/I for digital cellular system

26 26 Capacity of Digital Cellular CDMA l Capacity of FDMA and TDMA system is bandwidth limited. l Capacity of CDMA system is interference limited. l The link performance of CDMA increases as the number of users decreases. l Capacity of FDMA and TDMA system is bandwidth limited. l Capacity of CDMA system is interference limited. l The link performance of CDMA increases as the number of users decreases.

27 27 Number of possible users in CDMA  is the background thermal noise  is the background thermal noise S is the average user power S is the average user power W is the total RF bandwidth W is the total RF bandwidth R is the information bit rate R is the information bit rate  is the background thermal noise  is the background thermal noise S is the average user power S is the average user power W is the total RF bandwidth W is the total RF bandwidth R is the information bit rate R is the information bit rate

28 28 Techniques to improve capacity l Antenna Sectorization A cell site with 3 antennas, each having a beamwidth of 120 degrees, has one-third of the interference received by omni- directional antenna. This increases the capacity by a factor of 3 l Monitoring or Voice activity Each transmitter is switched off during period of no voice activity. Voice activity is denoted by a factor  l Antenna Sectorization A cell site with 3 antennas, each having a beamwidth of 120 degrees, has one-third of the interference received by omni- directional antenna. This increases the capacity by a factor of 3 l Monitoring or Voice activity Each transmitter is switched off during period of no voice activity. Voice activity is denoted by a factor 

29 29 SNR Improvement

30 30 SNR Improvement … )

31 31 ExampleExample l If W = 1.25 MHz, R= 9600 bps, and a minimum acceptable E b / N o is 10 dB, determine the maximum number of users that can be supported in a single cell CDMA system using l omni directional base station antennas and no voice activity detection 3 sectors at base station and  = 3/8. Assume the system is interference limited.  = 0. 3 sectors at base station and  = 3/8. Assume the system is interference limited.  = 0. l If W = 1.25 MHz, R= 9600 bps, and a minimum acceptable E b / N o is 10 dB, determine the maximum number of users that can be supported in a single cell CDMA system using l omni directional base station antennas and no voice activity detection 3 sectors at base station and  = 3/8. Assume the system is interference limited.  = 0. 3 sectors at base station and  = 3/8. Assume the system is interference limited.  = 0.

32 32 SolutionSolution (a)(a)

33 33 Solution … (b) Users per sector

34 34 Solution … Total users N in 3 sectors


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