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Multiple Access Techniques for Wireless Communication
FDMA
TDMA
SDMA
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Introduction
• many users at same time
• share a finite amount of radio spectrum
• high performance• high performance
• duplexing generally required
• frequency domain
• time domain
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Frequency division duplexing (FDD)
• two bands of frequencies for every user
• forward band
• reverse band
• duplexer needed
• frequency seperation between forward band and reverse band is constant
frequency seperation
reverse channel forward channel
f
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Time division duplexing (TDD)
• uses time for forward and reverse link
• multiple users share a single radio channel
• forward time slot• forward time slot
• reverse time slot
• no duplexer is required
time seperationt
forward channelreverse channel
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Multiple Access Techniques
• Frequency division multiple access (FDMA)
• Time division multiple access (TDMA)
• Code division multiple access (CDMA)• Code division multiple access (CDMA)
• Space division multiple access (SDMA)
• grouped as:
• narrowband systems
• wideband systems
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Narrowband systems
• large number of narrowband channels
• usually FDD
• Narrowband FDMA• Narrowband FDMA
• Narrowband TDMA
• FDMA/FDD
• FDMA/TDD
• TDMA/FDD
• TDMA/TDD
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Logical separation FDMA/FDD
user 1forward channel
reverse channel
f
t
user n
reverse channel
forward channel
reverse channel
...
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Logical separation FDMA/TDD
user 1
forward channel reverse channel
f
t
user n
forward channel reverse channel
...
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Logical separation TDMA/FDD
forwardforward
f
t
user 1 user n
channel
reverse
channel
channel
reverse
channel
...
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Logical separation TDMA/TDD
user 1 user n
f
t
forward
channel
reverse
channel
forward
channel
reverse
channel
...
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Wideband systems
• Transmission b/w of a single channel is much larger than the coherence b/w of the channel.
• large number of transmitters on one channel
• TDMA -time slots to many tx’s on one channel and • TDMA -time slots to many tx’s on one channel and only one channel is allowed to acces channel.
• CDMA –allows all tx’s to access channel at same time
• FDD or TDD multiplexing techniques
• TDMA/FDD and TDMA/TDD
• CDMA/FDD and CDMA/TDD
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Logical separation CDMA/FDD IS-95
US narrow band spread spectrum
user 1
forward channel reverse channel
code
f
user n
forward channel reverse channel
...
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Logical separation CDMA/TDD(W-CDMA)
user 1
forward channel reverse channel
code
t
user n
forward channel reverse channel
...
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Multiple Access Techniques in use
Multiple Access
TechniqueAdvanced Mobile Phone System (AMPS) FDMA/FDD
Cellular System
Advanced Mobile Phone System (AMPS) FDMA/FDD
Global System for Mobile (GSM) TDMA/FDD
US Digital Cellular (USDC) TDMA/FDD
Digital European Cordless Telephone (DECT) FDMA/TDD
US Narrowband Spread Spectrum (IS-95) CDMA/FDD
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Frequency division multiple access FDMA
• one phone circuit per channel
• idle time causes wasting of resources
• simultaneously and continuously • simultaneously and continuously transmitting
• usually implemented in narrowband systems
• for example: in AMPS is a FDMA bandwidth of 30 kHz implemented
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FDMA compared to TDMA
• fewer bits for synchronization
• fewer bits for framing
• higher cell site system costs• higher cell site system costs
• higher costs for duplexer used in base station and subscriber units
• FDMA requires RF filtering to minimize adjacent channel interference
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Nonlinear Effects in FDMA
• many channels - same antenna
• for maximum power efficiency operate near saturationsaturation
• near saturation power amplifiers are nonlinear
• nonlinearities causes signal spreading
• intermodulation frequencies
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Nonlinear Effects in FDMA
• IM are undesired harmonics
• interference with other channels in the FDMA systemFDMA system
• decreases user C/I - decreases performance
• interference outside the mobile radio band: adjacent-channel interference
• RF filters needed - higher costs
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Number of channels in a FDMA system
N= Bt - Bguard
Bc
• N … number of channels
• Bt … total spectrum allocation
• Bguard … guard band
• Bc … channel bandwidth
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Example: Advanced Mobile Phone System
• AMPS
• FDMA/FDD
• analog cellular system• analog cellular system
• 12.5 MHz per simplex band - Bt
• Bguard = 10 kHz ; Bc = 30 kHz
N= 12.5E6 - 2*(10E3)30E3 = 416 channels
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Time Division Multiple Access
• time slots
• one user per slot
• buffer and burst method• buffer and burst method
• noncontinuous transmission
• digital data
• digital modulation
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Repeating Frame Structure
Preamble Information Message Trail Bits
One TDMA Frame
Slot 1 Slot 2 Slot 3 … Slot N
Trail Bits Sync. Bits Information Data Guard Bits
The frame is cyclically repeated over time.
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Features of TDMA
• a single carrier frequency for several users
• transmission in bursts
• low battery consumption
• handoff process much simpler for subscriber unit
• FDD : switch instead of duplexer as tx and rx are done on different time slots
• very high transmission rate
• high synchronization overhead
• guard slots necessary
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Number of channels in a TDMA system
N= m*(Btot - 2*Bguard)
Bc
• N … number of channels
• m … number of TDMA users per radio channel
• Btot … total spectrum allocation
• Bguard … Guard Band
• Bc … channel bandwidth
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Example: Global System for Mobile (GSM)
• TDMA/FDD
• forward link at Btot = 25 MHz
• radio channels of Bc = 200 kHz• radio channels of Bc = 200 kHz
• if m = 8 speech channels supported, and
• if no guard band is assumed :
N= 8*25E6200E3 = 1000 simultaneous users
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Efficiency of TDMA
• percentage of transmitted data that contain information
• frame efficiency f • frame efficiency f
• usually end user efficiency < f ,
• Data tx has source and channel coding bits
• How get f ?
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Efficiency of TDMA
• bOH … number of overhead bits
• Nr … number of reference bursts per frame
bOH = Nr*br + Nt*bp + Nt*bg + Nr*bg
• Nr … number of reference bursts per frame
• br … reference bits per reference burst
• Nt … number of traffic bursts per frame
• bp … overhead bits per preamble in each slot
• bg … equivalent bits in each guard time intervall
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Efficiency of TDMA
bT = Tf * R
• bT … total number of bits per frame
• Tf … frame duration
• R … channel bit rate
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Efficiency of TDMA
f = (1-bOH/bT)*100%
• f … frame efficiency
• bOH … number of overhead bits per frame
• bT … total number of bits per frame
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Space Division Multiple Access
• Controls radiated energy for each user in space
• using spot beam antennas
• base station tracks user when moving• base station tracks user when moving
• cover areas with same frequency by using
TDMA or CDMA systems
• cover areas with same frequency by using
NB-FDMA systems
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Space Division Multiple Access
• primitive applications are “Sectorized antennas”
• in future adaptive antennas simultaneously steer energy in the direction of many users at once
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Reverse link problems
• general problem
• different propagation path from user to base
• dynamic control of transmitting power from • dynamic control of transmitting power from each user to the base station required
• limits by battery consumption of subscriber units
• possible solution is a filter for each user
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Solution by SDMA systems
• adaptive antennas promise to mitigate reverse link problems
• limiting case of infinitely fast track ability• limiting case of infinitely fast track ability
• thereby unique channel that is free from interference
• all user communicate at same time using the same channel
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Disadvantage of SDMA
• perfect adaptive antenna system: infinitely large antenna needed
• compromise needed• compromise needed
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Multiple Access protocols
• Single shared broadcast channel • Two or more simultaneous transmissions by nodes: interference
– Collision if node receives two or more signals at the same time
Multiple Access ProtocolMultiple Access Protocol
• Distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit
• Communication about channel sharing must use channel itself! – No out-of-band channel for coordination
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Channel Partitioning
• Frequency Division Multiplexing– Each node has a frequency band
• Time Division Multiplexing• Time Division Multiplexing– Each node has a series of fixed time slots
• What networks are these good for?
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Computer Network Characteristics
• Transmission needs vary– Between different nodes
– Over time– Over time
• Network to be fully utilized
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Ideal Multiple Access Protocol
Broadcast channel of rate R bps1. When one node wants to transmit, it can send at
rate R.2. When M nodes want to transmit, each can send at 2. When M nodes want to transmit, each can send at
average rate R/M3. Fully decentralized:
– no special node to coordinate transmissions– no synchronization of clocks, slots
4. Simple
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Random Access Protocols
• When node has packet to send– transmit at full channel data rate R.– no a priori coordination among nodes
• two or more transmitting nodes ➜ “collision”,• two or more transmitting nodes “collision”,• random access MAC protocol specifies:
– how to detect collisions– how to recover from collisions (e.g., via delayed retransmissions)
• Examples of random access MAC protocols:– slotted ALOHA– ALOHA, – Reservation Protocols-Reservation ALOHA, PRMA– CSMA, CSMA/CD, CSMA/CA
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Pure (unslotted) ALOHA
• unslotted Aloha: simpler, no synchronization
• when frame first arrives– transmit immediately
• collision probability increases:• collision probability increases:– frame sent at t0 collides with other frames sent in
[t0-1,t0+1]
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Pure Aloha efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [t0-1,t0] .
P(no other node transmits in [t0,t0+1] 0 0
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
… choosing optimum p and then letting n -> ¥ ...
Efficiency = 1/(2e) = .18 worse !
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Slotted ALOHAAssumptions• all frames same size• time is divided into equal
size slots, time to transmit 1 frame
Operation
• when node obtains fresh frame, it transmits in next slot
• no collision, node can send new frame in next slot
frame• nodes start to transmit
frames only at beginning of slots
• nodes are synchronized• if 2 or more nodes transmit
in slot, all nodes detect collision
new frame in next slot
• if collision, node retransmits frame in each subsequent slot with prob. p until success
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Slotted ALOHA
Pros
• single active node can continuously transmit at full rate of channel
• highly decentralized: only slots in nodes need to be in sync
• simple
Cons• collisions, wasting slots• idle slots• nodes may be able to
detect collision in less than time to transmit packet
• clock synchronization
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Slotted Aloha efficiency
• Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send
• Suppose N nodes with many frames to send, each • Suppose N nodes with many frames to send, each transmits in slot with probability p
• prob that node 1 has success in a slot = p(1-p)N-1
• prob that any node has a success = Np(1-p)N-1
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Optimal choice of p
• For max efficiency with N nodes, find p* that maximizes Np(1-p)N-1
• For many nodes, take limit of Np*(1-p*)N-1 as N goes • For many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives 1/e = .37
• Efficiency is 37%, even with optimal p
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Carrier Sense Multiple Access
CSMA: listen before transmit:
If channel sensed idle: transmit entire frame
• If channel sensed busy, defer transmission
• Human analogy: don’t interrupt others!
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Carrier Sense Protocols• Use the fact that in some networks you can sense the medium to check whether
it is currently free– 1-persistent CSMA– non-persistent CSMA– p-persistent protocol– CSMA with collision detection (CSMA/CD): not applicable to wireless systems– CSMA with collision detection (CSMA/CD): not applicable to wireless systems
• 1-persistent CSMA– when a station has a packet:
• it waits until the medium is free to transmit the packet• if a collision occurs, the station waits a random amount of time
– first transmission results in a collision if several stations are waiting for the channel
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Carrier Sense Protocols (Cont’d)• Non-persistent CSMA
– when a station has a packet:• if the medium is free, transmit the packet• otherwise wait for a random period of time and repeat the algorithm
– higher delays, but better performance than pure ALOHA
• p-persistent protocol– when a station has a packet wait until the medium is free:– when a station has a packet wait until the medium is free:
• transmit the packet with probability p• wait for next slot with probability 1-p
– better throughput than other schemes but higher delay
• CSMA with collision Detection (CSMA/CD)– stations abort their transmission when they detect a collision– e.g., Ethernet, IEEE802.3 but not applicable to wireless systems
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CSMA collisions
collisions can still occur:propagation delay means two nodes may not heareach other’s transmission
collision:entire packet transmission time wasted
note:role of distance & propagation delay in determining collision probability
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CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA– collisions detected within short time
– colliding transmissions aborted, reducing channel wastage
• collision detection:• collision detection:– easy in wired LANs: measure signal strengths, compare
transmitted, received signals
– difficult in wireless LANs: receiver shut off while transmitting
• human analogy: the polite communicator
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CSMA/CD collision detection
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Demand Assigned Multiple Access
• Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution for packet arrival and packet length)
• Reservation can increase efficiency to 80%– a sender reserves a future time-slot– sending within this reserved time-slot is possible without collision
Wireless Networks Spring 2005
– sending within this reserved time-slot is possible without collision– reservation also causes higher delays– typical scheme for satellite links
• Examples for reservation algorithms:– Explicit Reservation (Reservation-ALOHA)– Implicit Reservation (PRMA)– Reservation-TDMA
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Explicit Reservation•Explicit Reservation (Reservation Aloha):
– two modes: • ALOHA mode for reservation:
competition for small reservation slots, collisions possible • reserved mode for data transmission within successful reserved slots (no collisions
possible)
– it is important for all stations to keep the reservation list consistent at any point in
Wireless Networks Spring 2005
– it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time
Aloha reserved Aloha reserved Aloha reserved Aloha
collision
t
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PRMA•Implicit reservation (PRMA - Packet Reservation MA):
– a certain number of slots form a frame, frames are repeated
– stations compete for empty slots according to the slotted aloha principle
– once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send
– competition for this slots starts again as soon as the slot was empty in the last frame
Wireless Networks Spring 2005
frame
frame1
frame2
frame3
frame4
frame5
1 2 3 4 5 6 7 8 time-slot
collision at reservation attempts
A C D A B A F
A C A B A
A B A F
A B A F D
A C E E B A F D t
ACDABA-F
ACDABA-F
AC-ABAF-
A---BAFD
ACEEBAFD
reservation
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