Multiple Access Techniques for w ireless c ommunication. Multiple access schemes allow many mobile users to share a finite amount of radio spectrum High quality of communications must be maintained during the sharing process. Multiple Access Techniques. PR. Packet Radio - PowerPoint PPT Presentation
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1 1 Multiple Access Multiple Access Techniques Techniques for wireless for wireless communication communication Multiple access schemes Multiple access schemes allow many mobile users to allow many mobile users to share a finite amount of share a finite amount of radio spectrum radio spectrum High quality of High quality of communications must be communications must be maintained during the maintained during the sharing process sharing process
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Multiple Access Techniques Multiple Access Techniques for wireless communicationfor wireless communicationMultiple Access Techniques Multiple Access Techniques for wireless communicationfor wireless communication
Multiple access schemes allow many mobile Multiple access schemes allow many mobile users to share a finite amount of radio users to share a finite amount of radio spectrumspectrum
High quality of communications must be High quality of communications must be maintained during the sharing processmaintained during the sharing process
Multiple access schemes allow many mobile Multiple access schemes allow many mobile users to share a finite amount of radio users to share a finite amount of radio spectrumspectrum
High quality of communications must be High quality of communications must be maintained during the sharing processmaintained during the sharing process
Cellular SystemCellular System MA TechniqueMA Technique AMPS ( Advanced Mobile Phone AMPS ( Advanced Mobile Phone system ) system )
FDMA / FDD FDMA / FDD
GSM ( Global System for Mobile )GSM ( Global System for Mobile ) TDMA / FDD TDMA / FDD
US DC ( U. S Digital Cellular )US DC ( U. S Digital Cellular ) TDMA / FDD TDMA / FDD
JDC ( Japanese Digital Cellular ) JDC ( Japanese Digital Cellular ) TDMA / FDD TDMA / FDD
IS – 95 ( U.S Narrowband Spread IS – 95 ( U.S Narrowband Spread Spectrum ) Spectrum )
CDMA / FDD CDMA / FDD
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Capacity of Cellular SystemsCapacity of Cellular SystemsCapacity of Cellular SystemsCapacity of Cellular Systems
Channel capacity of a wireless system is the Channel capacity of a wireless system is the maximum number of users possible in the maximum number of users possible in the systemsystem
Channel capacity depends on:Channel capacity depends on: Bandwidth availableBandwidth available Signal to Noise ratio (SNR) in the channelSignal to Noise ratio (SNR) in the channel
Channel capacity of a wireless system is the Channel capacity of a wireless system is the maximum number of users possible in the maximum number of users possible in the systemsystem
Channel capacity depends on:Channel capacity depends on: Bandwidth availableBandwidth available Signal to Noise ratio (SNR) in the channelSignal to Noise ratio (SNR) in the channel
Principle of FDMA OperationPrinciple of FDMA OperationPrinciple of FDMA OperationPrinciple of FDMA Operation
Each user is allocated a unique frequency band Each user is allocated a unique frequency band or channel. These channels are assigned on or channel. These channels are assigned on demand to users who request servicedemand to users who request service
In FDD, the channel has two frequencies – In FDD, the channel has two frequencies – forward channel & reverse channelforward channel & reverse channel
Each user is allocated a unique frequency band Each user is allocated a unique frequency band or channel. These channels are assigned on or channel. These channels are assigned on demand to users who request servicedemand to users who request service
In FDD, the channel has two frequencies – In FDD, the channel has two frequencies – forward channel & reverse channelforward channel & reverse channel
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Cell capacity of FDMA SystemCell capacity of FDMA System
Capacity/cell NCapacity/cell N
Cell capacity of FDMA SystemCell capacity of FDMA System
Capacity/cell NCapacity/cell N
gB gB
tB
ratio ceInterferen toSignal required minimum the
exponent losspath theis
36
2
min
/2
min2/
isI
S
nwhere
IS
B
BBN n
nc
gt
88
ExampleExampleExampleExample
In the US, each cellular carrier has 416 channelsIn the US, each cellular carrier has 416 channels Assuming negligible noise input:Assuming negligible noise input:
In the US, each cellular carrier has 416 channelsIn the US, each cellular carrier has 416 channels Assuming negligible noise input:Assuming negligible noise input:
PreamblePreamble Information Information messagemessage
Trail BitsTrail Bits
Slot 1Slot 1 Slot 2Slot 2 Slot NSlot N
Trail BitTrail Bit Sync BitSync Bit Information Information BitBit
Guard BitsGuard Bits
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Components of TDMA FrameComponents of TDMA FrameComponents of TDMA FrameComponents of TDMA Frame
Preamble Preamble Address and synchronization Address and synchronization information for base station and subscriber information for base station and subscriber identificationidentification
Guard times Guard times Synchronization of receivers Synchronization of receivers between different slots and framesbetween different slots and frames
Preamble Preamble Address and synchronization Address and synchronization information for base station and subscriber information for base station and subscriber identificationidentification
Guard times Guard times Synchronization of receivers Synchronization of receivers between different slots and framesbetween different slots and frames
Data Transmission for user of TDMA system Data Transmission for user of TDMA system occurs in discrete burstsoccurs in discrete bursts
o The result is low battery consumption.The result is low battery consumption.o Handoff process is simplerHandoff process is simpler
Since different slots are used for T and R, Since different slots are used for T and R, duplexers are not required.duplexers are not required.
Equalization is required, since transmission Equalization is required, since transmission rates are higher than FDMA channelsrates are higher than FDMA channels
Data Transmission for user of TDMA system Data Transmission for user of TDMA system occurs in discrete burstsoccurs in discrete bursts
o The result is low battery consumption.The result is low battery consumption.o Handoff process is simplerHandoff process is simpler
Since different slots are used for T and R, Since different slots are used for T and R, duplexers are not required.duplexers are not required.
Equalization is required, since transmission Equalization is required, since transmission rates are higher than FDMA channelsrates are higher than FDMA channels
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Efficiency of TDMAEfficiency of TDMAEfficiency of TDMAEfficiency of TDMA
b Number of overhead bits per preamble in each slots
b Number of equivalent bits
in each guard time interval
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Cell capacity in TDMA SystemCell capacity in TDMA System
m is the number of TDMA users in one channelm is the number of TDMA users in one channel
Cell capacity in TDMA SystemCell capacity in TDMA System
m is the number of TDMA users in one channelm is the number of TDMA users in one channel
ratio ceInterferen toSignal required minimum the
exponent losspath theis
36
2
min
/2
min2/
isI
S
nwhere
IS
B
BBmN n
nc
gt
1818
ExampleExample ExampleExample
The GSM System uses a TDMA frame structure The GSM System uses a TDMA frame structure where each frame consist of 8 time slots, and where each frame consist of 8 time slots, and each time slot contains 156.25 bits, and data is each time slot contains 156.25 bits, and data is transmitted at 270.833 kbps in the channel.transmitted at 270.833 kbps in the channel. •Time duration of a bitTime duration of a bit•Time duration of a slotTime duration of a slot•Time duration of a frameTime duration of a frame
The GSM System uses a TDMA frame structure The GSM System uses a TDMA frame structure where each frame consist of 8 time slots, and where each frame consist of 8 time slots, and each time slot contains 156.25 bits, and data is each time slot contains 156.25 bits, and data is transmitted at 270.833 kbps in the channel.transmitted at 270.833 kbps in the channel. •Time duration of a bitTime duration of a bit•Time duration of a slotTime duration of a slot•Time duration of a frameTime duration of a frame
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SolutionSolution SolutionSolution • Time duration of a bit Time duration of a bit
• Time duration of a slot Time duration of a slot
• Time duration of a bit Time duration of a bit
• Time duration of a slot Time duration of a slot
b 3
1 1=T = 3.692 s
bit-rate 270.833 10
b 3
1 1=T = 3.692 s
bit-rate 270.833 10
slot bT 156.25 T 0.577 s ms• Time duration of a frame
slot8 T 4.615ms
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ExampleExampleExampleExample
If a normal GSM timeslot consists of 6 trailing bits, If a normal GSM timeslot consists of 6 trailing bits, 8.25 guard bits, 26 training bits, and 2 traffic bursts 8.25 guard bits, 26 training bits, and 2 traffic bursts of 58 bits of data, find the frame efficiency. of 58 bits of data, find the frame efficiency.
If a normal GSM timeslot consists of 6 trailing bits, If a normal GSM timeslot consists of 6 trailing bits, 8.25 guard bits, 26 training bits, and 2 traffic bursts 8.25 guard bits, 26 training bits, and 2 traffic bursts of 58 bits of data, find the frame efficiency. of 58 bits of data, find the frame efficiency.
SolutionSolutionTime slot has 6+ 8.25+ 26 + 2(58) = 156.25 bits.Time slot has 6+ 8.25+ 26 + 2(58) = 156.25 bits.A frame has 8 * 156.25 = 1250 bits / frame.A frame has 8 * 156.25 = 1250 bits / frame.• The number of overhead bits per frame is:
bOH = 8(6) + 8(8.25) + 8(26) = 322 bits
Frame efficiency = (1250 – 322 )/1250 = 74.24 %
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Capacity of Digital Cellular CDMACapacity of Digital Cellular CDMACapacity of Digital Cellular CDMACapacity of Digital Cellular CDMA
Capacity of FDMA and TDMA system is Capacity of FDMA and TDMA system is bandwidth limited.bandwidth limited.
Capacity of CDMA system is interference Capacity of CDMA system is interference limited.limited.
The link performance of CDMA increases as The link performance of CDMA increases as the number of users decreases.the number of users decreases.
Capacity of FDMA and TDMA system is Capacity of FDMA and TDMA system is bandwidth limited.bandwidth limited.
Capacity of CDMA system is interference Capacity of CDMA system is interference limited.limited.
The link performance of CDMA increases as The link performance of CDMA increases as the number of users decreases.the number of users decreases.
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Number of possible users in CDMANumber of possible users in CDMANumber of possible users in CDMANumber of possible users in CDMA
b
o
WRN 1 SEN
Wwhere Pr ocessing GainR
• is the background thermal noiseis the background thermal noise•S is the average user powerS is the average user power•W is the total RF bandwidthW is the total RF bandwidth•R is the information bit rateR is the information bit rate
• is the background thermal noiseis the background thermal noise•S is the average user powerS is the average user power•W is the total RF bandwidthW is the total RF bandwidth•R is the information bit rateR is the information bit rate
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Techniques to improve capacityTechniques to improve capacityTechniques to improve capacityTechniques to improve capacity Antenna SectorizationAntenna Sectorization
Sectoral antennas at 120 degrees increases Sectoral antennas at 120 degrees increases the capacity by a factor of 3the capacity by a factor of 3
Monitoring or Voice activity Monitoring or Voice activity
Each transmitter is switched off during period of Each transmitter is switched off during period of no voice activityno voice activity
Antenna SectorizationAntenna Sectorization
Sectoral antennas at 120 degrees increases Sectoral antennas at 120 degrees increases the capacity by a factor of 3the capacity by a factor of 3
Monitoring or Voice activity Monitoring or Voice activity
Each transmitter is switched off during period of Each transmitter is switched off during period of no voice activityno voice activity
If = 3/8 and number of sector is equal to 3 , SNR increases by a factor o .f 8
)
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ExampleExampleExampleExample
If W = 1.25 MHz, R= 9600 bps, and a minimum If W = 1.25 MHz, R= 9600 bps, and a minimum acceptable Eacceptable Ebb/ N/ Noo is 10 dB, determine the is 10 dB, determine the maximum number of users that can be supported maximum number of users that can be supported in a single cell CDMA system using: in a single cell CDMA system using: omni directional base station antennas and no omni directional base station antennas and no voice activity detection voice activity detection 3 sectors at base station and 3 sectors at base station and = 3/8. Assume = 3/8. Assume the system is interference limited. the system is interference limited. = 0. = 0.
If W = 1.25 MHz, R= 9600 bps, and a minimum If W = 1.25 MHz, R= 9600 bps, and a minimum acceptable Eacceptable Ebb/ N/ Noo is 10 dB, determine the is 10 dB, determine the maximum number of users that can be supported maximum number of users that can be supported in a single cell CDMA system using: in a single cell CDMA system using: omni directional base station antennas and no omni directional base station antennas and no voice activity detection voice activity detection 3 sectors at base station and 3 sectors at base station and = 3/8. Assume = 3/8. Assume the system is interference limited. the system is interference limited. = 0. = 0.
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SolutionSolutionSolutionSolution(a)(a)(a)(a)
b
o
WRN 1 SEN
1.25 10
96001 010
1 13.02 14
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(b) Users per sector (b) Users per sector (b) Users per sector (b) Users per sector
1.25 101 96001 03 10
835.7
sb
o
WR1N 1 ,)0 1(SEN
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Total users N in 3 sectorsTotal users N in 3 sectorsTotal users N in 3 sectorsTotal users N in 3 sectors
