Multiple Access Techniques for Wireless Communication
description
Transcript of Multiple Access Techniques for Wireless Communication
FDMA : Frequency division Multiple Access TDMA : Time division Multiple AccessCDMA : Code division Multiple AccessSDMA : Space division Multiple AccessPDMA : Polarization division Multiple Access
IntroductionMultiple access schemes are used to allow many
users to share simultaneously a finite amount of radio spectrum
Sharing of spectrum is required to increase capacity
For high quality communication this sharing of spectrum should not degrade performance of the system
high performanceduplexing generally requiredfrequency domaintime domain
DuplexingWhat is Duplexing?
to talk and listen simultaneously is called duplexingClassification of communication systems
according to their connectivity
Simplex
Half-duplex
Duplex
A B
A B
A B
Duplexing Contd…Duplexing may be done using
frequency domain techniquetime domain technique
Frequency division duplexing (FDD)two bands of frequencies for every user
forward band ( for traffic from Base station to mobile unit)
reverse band (for traffic from mobile unit to Base station)
duplexer neededfrequency separation between forward
band and reverse band is constant throughout the system
frequency separation/split
reverse channel forward channel
f
Time division duplexing (TDD)
uses different time slots for forward and reverse linkforward time slotreverse time slot
no duplexer is required (a simple switch can be used)
Communication is not full-duplex
time separation/splitt
forward channelreverse channel
Trade-offs b/w FDD and TDD
FDDProvides individual radio frequencies to each
user hence, transceiver should work on two frequency bands
Frequency allocation must be carefully coordinated with Out-of-band users
Duplexer neededTDD
Single frequency hence simple transceiverDuplexer not needed, a switch can do the job There is time latency, communication is not
full-duplex
Multiple Access Techniques in Wireless Communication System
Frequency division multiple access (FDMA)Time division multiple access (TDMA)Code division multiple access (CDMA)Space division multiple access (SDMA)grouped as:
narrowband systemswideband systems
Narrowband systemsBandwidth of the signal is narrow
compared with the coherence bandwidth of the channel
In NB systems available radio spectrum is divided into large number of narrowband channels usually FDD (large frequency split)Narrowband FDMANarrowband TDMA
Narrowband systemsNarrowband FDMA
a user is assigned a particular channel which is not shared by other users
if FDD is used then each channel has a forward and reverse link (called FDMA/FDD)
Narrowband TDMAAllows users to share the same channel but allocates
a unique time slot to each userFDMA/FDDFDMA/TDDTDMA/FDDTDMA/TDD
Narrowband systems
FDMA/FDDFDMA/TDDTDMA/FDDTDMA/TDD
Logical separation FDMA/FDD
f
t
user 1
user n
forward channel
reverse channel
forward channel
reverse channel
...
Logical separation FDMA/TDD
f
t
user 1
user n
forward channel reverse channel
forward channel reverse channel
...
Logical separation TDMA/FDD
f
t
user 1 user n
forward
channel
reverse
channel
forward
channel
reverse
channel
...
Logical separation TDMA/TDD
f
t
user 1 user n
forward
channel
reverse
channel
forward
channel
reverse
channel
...
Wideband systemsThe transmission BW of a single channel is
much larger than the coherence bandwidth of the channel
users are allowed to transmit in a large part of the spectrum
large number of transmitters on one channel
TDMA techniques allocates time slots to different transmitters
CMA techniques allows the transmitters to access the channel at the same time
Wideband systemsFDD or TDD multiplexing techniquesTDMA/FDDTDMA/TDDCDMA/FDDCDMA/TDD
TDMA
time
power
frequen
cy
CDMA
time
power
frequen
cy
FDMA
time
power
frequen
cy
Multiple Access
Technique
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
Cellular System
Frequency division multiple access FDMAone phone circuit per channelidle time causes wasting of resourcessimultaneously and continuously transmittingusually implemented in narrowband systemsComplexity of FDMA mobile systems is lower
compared to TDMAFDMA uses duplexersfor example: AMPS is a FDMA system with
bandwidth of 30 kHz
FDMA compared to TDMAfewer bits for synchronization fewer bits for framinghigher costs for duplexer used in base station
and subscriber unitsFDMA requires RF filtering to minimize
adjacent channel interference
Nonlinear Effects in FDMAmany channels - same antennafor maximum power efficiency operate near
saturationnear saturation power amplifiers are
nonlinearnonlinearities causes signal spreading intermodulation frequencies
Nonlinear Effects in FDMAIM are undesired harmonicsinterference with other channels in the
FDMA systeminterference outside the mobile radio band:
adjacent-channel interferenceRF filters needed - higher costs
Number of channels in a FDMA system
N … number of channels Bt … total spectrum allocationBguard … guard bandBc … channel bandwidth
N=Bt - 2Bguard
Bc
Example: Advanced Mobile Phone SystemAMPSFDMA/FDDanalog cellular system12.5 MHz per simplex band - BtBguard = 10 kHz ; Bc = 30 kHz
N=12.5E6 - 2*(10E3)
30E3= 416 channels
Time Division Multiple AccessTime slotsone user per slotBuffer and burst methodNon-continuous transmissionAdvantage:Total bandwidth is utilizedDisadvantage:Strict Burst Timing is required at the earth
station
Slot 1 Slot 2 Slot 3 … Slot N
Repeating Frame Structure
Preamble Information Message Trail Bits
One TDMA Frame
Trail Bits Sync. Bits Information Data Guard Bits
The frame is cyclically repeated over time.
Features of TDMAa single carrier frequency for several userstransmission in burstshandoff process much simpler (can listen
when idle)Low battery consumptionBandwidth can be supplied on demandFDD : switch instead of duplexerhigh synchronization overhead
Number of channels in a TDMA system
N … number of TDMA channel slotsm … number of TDMA users per radio channelBtot … total spectrum allocationBguard … Guard BandBc … channel bandwidth
N=m*(Btot - 2*Bguard)
Bc
Example: Global System for Mobile (GSM)
TDMA/FDDforward link at Btot = 25 MHz radio channels of Bc = 200 kHzif m = 8 speech channels supported, andif no guard band is assumed :
N= 8*25E6200E3
= 1000 simultaneous users
Efficiency of TDMApercentage of transmitted data that contain
informationframe efficiency fusually end user efficiency < f ,because of source and channel codingHow get f ?
Slot 1 Slot 2 Slot 3 … Slot N
Repeating Frame Structure
Preamble Information Message Trail Bits
One TDMA Frame
Trail Bits Sync. Bits Information Data Guard Bits
The frame is cyclically repeated over time.
Efficiency of TDMA
bOH … number of overhead bitsNr … number of reference bursts per framebr … reference bits per reference burstNt … number of traffic bursts per framebp … overhead bits per preamble in each slotbg … equivalent bits in each guard time
interval
bOH = Nr*br + Nt*bp + Nt*bg + Nr*bg
Efficiency of TDMA
bT … total number of bits per frameTf … frame durationR … channel bit rate
bT = Tf * R
Efficiency of TDMA
f … frame efficiencybOH … number of overhead bits per framebT … total number of bits per frame
f = (1-bOH/bT)*100%
Spread Spectrum Multiple Access (SSMA)SSMA uses Signals have transmission BW
that is several orders of magnitude greater than the minimum required BW
A Pseudo-noise sequence converts a narrow band signal to a wideband noise-like signal before transmission
SSMA not BW efficient when used by a single user
Many users can share the same BW without interfering with one another
Type of SSMA techniques: frequency hoped multiple access (FHMA)Direct sequence multiple access (DS) or Code
division multiple access (CDMA)
Pseudo-noise (PN) sequencePN code sets can be generated from linear
feedback shift registersExample:
Modulo 2 adder
Stage 1 Stage 2 Stage 3RegisterOutput
Shift Register
Clock
Correlation matrix of 31-bit PN sequence
5 10 15 20 25 30
5
10
15
20
25
30 -10
-5
0
5
10
15
20
25
30
Frequency Hopped Multiple Access (FHMA)It is digital multiple access systemCarrier frequencies of individual users varied in
pseudorandom fashion with in a wideband channelDigital data broken into uniform sized bursts which
are transmitted on different carriersInstantaneous BW of any one transmission burst is
much smaller than the total spread BWLocally generated PN code is used to synchronize the
receiver frequency with that of transmitter.Provides a High level of security
Fast frequency hopping systemSlow frequency hopping
FHMAErasures can occur when two or more users
transmit on the same channel at the same time
FH(Frequency hopped) signal is immune to fading so error control coding can be combined to guard against erasures .
CDMANarrowband signals is multiplied by a very large
bandwidth signal called the spreading signal.The spreading signal is pseudo noise code
sequence that has a chip rate which is orders of magnitudes greater than data rate of the message
All users use the same carrier frequency and transmit simultaneously
Each user has its own pseudo random code word which is approximately orthogonal to other codewords
CDMAReceiver performs time co-relationAll other codewords appear as noise Receiver needs to know the code word used
by transmitterMany users same frequency, TDD or FDDSoft capacity(capacity increases linearly)Self-jammingNear far problem
Packet RadioMany subscribers attempt to access a single
channel in an uncoordinated or minimally coordinated manner
Collisions from simultaneous transmissions are detected at the BS
ACK and NACK is broadcasted by BS (its like perfect feedback)In case of NACK the signal retransmitted
PR easy to implementInduces delays
Packet Radio ProtocolsVulnerable period Vp: time interval during
which the packets are susceptible to collisions with transmission from other usersIf packet length in time is then a packet will suffer collision if other
terminals transmit packets during the period t1 to t1+2
Packet B
Packet A
t1 t1+2
Packet Radio contd …
Subscribers use contention techniquesALOHA protocol example of contention
techniquesAdvantage: ability to serve many users without
any overhead
48
Pure ALOHAUncoordinated stations are contesting for the
use of a single shared channelAlgorithm
Stations transmit whenever they have something to send
Colliding frames will be destroyedIf destroyed, the sender waits a random amount of time and sends it again
Maximum channel utilization is 18.4%81.6% of the total available bandwidth is essentially
wasted due to losses from packet collisions
49
In pure ALOHA, frames are transmitted at completely arbitrary times
50
Slotted ALOHA A method for doubling the capacity of pure
ALOHA systems Divide up time into discrete interval slots
Stations share the same time by synchronizing with a master Master periodically broadcasts a synchronization
pulse (clock tick packet) to all stations Stations are only allowed to send their packets
immediately after receiving a clock tick Each interval corresponding to one frame time
Maximum channel utilization is 36.8%
51
52
ALOHA ProtocolsAdvantages
Simple (due to distributed control)Flexible to fluctuations in the number of hostsFair
DisadvantagesLow channel efficiency with a large number of
hostsNot good for real-time traffic (e.g., voice)Cannot support priority traffic