ECE 8708 Wireless Communications : Multiple Access Techniques
Chapter 9
Multiple Access Techniques
for Wireless Communications
Yimin Zhang, Ph.D.
Department of Electrical & Computer Engineering
Villanova University
http://yiminzhang.com/ECE8708
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ECE 8708 Wireless Communications : Multiple Access Techniques
Outlines
Duplexing
Time Division Duplexing (TDD)
Frequency Division Duplexing (FDD)
Multiple Access
FDMA
TDMA
Spread Spectrum Multiple Access
- Frequency Hopped Multiple Access (FHMA)
- Code Division Multiple Access (CDMA)
Packet Radio
ALOHA
Carrier Sense Multiple Access (CSMA)
Capacity
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ECE 8708 Wireless Communications : Multiple Access Techniques
Brief Historical Overview
•
•
•
•
•
•
•
•
Late 1940’s: Push to Talk FM Systems
– Half Duplex Mode
Improved Mobile Telephone Service (ITMS)
– Fully Duplex, Auto-dial, Auto-trunking Phone Systems
Techniques and Theory Developed in 1950’ & 60’s
AT&T Proposes Cellular System to FCC in 1968
1983 FCC Authorizes US Advanced Mobile Phone System
(AMPS)
– Deployed in Chicago
– 666 Duplex Channels (40 MHz in the 800MHz band)
Late 1991- U.S. Digital Cellular (USDC) Implemented
Code Division Multiple Access Developed by Qualcomm.
New Personal Communication Service
– Licenses in the 1800/1900 MHz Band Auctioned
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ECE 8708 Wireless Communications : Multiple Access Techniques
Duplex Methods of Radio Links
Base Station
Forward link
Reverse link
Mobile Station
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ECE 8708 Wireless Communications : Multiple Access Techniques
Frequency Division Duplexing
•
Frequency Division Duplexing (FDD) provides two distinct bands
of frequencies for every user.
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ECE 8708 Wireless Communications : Multiple Access Techniques
Time Division Duplexing
•
Time Division Duplexing (TDD) uses time instead of frequency to
provides both a forward and reverse links.
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ECE 8708 Wireless Communications : Multiple Access Techniques
Trade-offs Between FDD and TDD
•
Frequency Division Duplexing
– Geared toward Individual Channels for each User
– Frequency Separation must use Inexpensive Technology
•
Time Division Duplexing
– Eliminates Need for Forward and Reverse Channels
– Time Latency
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ECE 8708 Wireless Communications : Multiple Access Techniques
Multiple Access Techniques
Base Station
Forward link
Reverse link
Mobile Station
Mobile Station
Mobile Station
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Mobile Station
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ECE 8708 Wireless Communications : Multiple Access Techniques
Multiple Access Techniques
Multiple Access
• FDMA
• TDMA
• Spread Spectrum Multiple Access
- Frequency Hopped Multiple Access (FHMA)
- Code Division Multiple Access (CDMA)
Packet Radio
• ALOHA
• Carrier Sense Multiple Access (CSMA)
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ECE 8708 Wireless Communications : Multiple Access Techniques
Multiple Access Techniques
•
•
•
•
Term has its Origin in Satellite Communications
System of Earth Stations and a Satellite
Used to Mean Sharing a Communications Channel (of W Hz) among a
Group of Users
Signal Space of Time Bandwidth
TW
– Where T = k/R
– Signal Space D = 2TW
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ECE 8708 Wireless Communications : Multiple Access Techniques
Multiple Access Techniques
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ECE 8708 Wireless Communications : Multiple Access Techniques
Frequency Division Multiple Access
•
FDMA channel carries only one phone at a time.
•
Bandwidth is relatively narrow
(30 kHz).
•
Since FDMA is a continuous
transmission scheme,
fewer bits are needed
for overhead purpose
(e.g., sync) as
compared to TDMA.
•
Requires tight RF
filtering.
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ECE 8708 Wireless Communications : Multiple Access Techniques
Nonlinear Effects in FDMA
•
•
•
Antenna at base station shared by channels
Nonlinearities of power amps and combiners
Results in signal spreading
– Generates intermodulation
– Causes adjacent channel interference and adjacent service
interference
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ECE 8708 Wireless Communications : Multiple Access Techniques
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ECE 8708 Wireless Communications : Multiple Access Techniques
Advanced Mobile Phone System (AMPS)
•
•
•
•
First U.S. analog cellular system
Based on FDMA/FDD
NBFM modulates the carrier
Total number of channels is given by:
N =
B t − 2 B guard
Bc
where
Bt : total spectrum allocation
Bguard : guard band allocated at the edge of the spectrum band
Bc : channel bandwidth
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ECE 8708 Wireless Communications : Multiple Access Techniques
Time Division Multiplex Access
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ECE 8708 Wireless Communications : Multiple Access Techniques
Time Division Multiplex Access
•
Data transmitted in a buffer-and-burst method
Preamble Æ Address
and synchronization
info for base station
and subscriber
identification
Guard times Æ
Synchronization of
receivers between a
different slots and
frames
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ECE 8708 Wireless Communications : Multiple Access Techniques
Time Division Multiple Access
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ECE 8708 Wireless Communications : Multiple Access Techniques
Time Division Multiple Access
•
TDMA shares a single carrier frequency with several users, where
each user makes use of nonoverlapping time slots.
•
Data transmission in TDMA systems is not continuous, but in
bursts.
•
TDMA uses different time slots for transmission and reception, thus
duplexers are not required. If FDD is used, TDMA/FDD systems
intentionally induce several time slots of delay between the two
links so that duplexers are not required in the mobile unit.
•
Equalization is usually necessary, since the rates are generally
high.
•
High synchronization overhead is required in TDMA.
•
Guard times are necessary to separate users; Guard times should
be minimized.
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ECE 8708 Wireless Communications : Multiple Access Techniques
Efficiency of TDMA
Frame efficiency
ηf
⎛
b
= ⎜⎜ 1 − OH
bT
⎝
⎞
⎟⎟ × 100 %
⎠
Number of overhead bits per frame
bOH = N r b r + N t b p + N t b g + N r b g
Nr : number of reference bursts per frame
Nt : number of traffic bursts per frame
br : # of overhead bits per reference burst
bp : # of overhead bits per preamble in each slot
bg : # of equivalent bits in each guard time interval
Total number of bits per frame
bT = T f R
Tf : frame duration; R : channel bit rate
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ECE 8708 Wireless Communications : Multiple Access Techniques
Number of Channels in TDMA System
•
Number of channels: total number of slots multiplied by the
channels available
N =
m ( B tot − 2 B guard )
Bc
m: maximum number of TDMA users supported on each radio
channel
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ECE 8708 Wireless Communications : Multiple Access Techniques
Example
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ECE 8708 Wireless Communications : Multiple Access Techniques
Example
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ECE 8708 Wireless Communications : Multiple Access Techniques
Example
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ECE 8708 Wireless Communications : Multiple Access Techniques
Outlines
Spread Spectrum Multiple Access
- Frequency Hopped Multiple Access (FHMA)
- Code Division Multiple Access (CDMA)
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ECE 8708 Wireless Communications : Multiple Access Techniques
Frequency Hopping
Typical frequency-hopping waveform pattern
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ECE 8708 Wireless Communications : Multiple Access Techniques
Direct Spread
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ECE 8708 Wireless Communications : Multiple Access Techniques
Wideband and Narrowband CDMA
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ECE 8708 Wireless Communications : Multiple Access Techniques
Code Division Multiple Access
•
Many users of a CDMA system share the same frequency band.
•
Either TDD or FDD may be used.
•
Unlike TDMA and FDMA, CDMA has a soft capacity limit.
•
Multipath fading may be substantially reduced. Small-scale fading is
mitigated if the spread spectrum bandwidth is greater than the
coherence bandwidth of the channel.
•
RAKE receiver can be used to achieve path diversity.
•
Single frequency reuse can be used and soft handover can be
performed to achieve diversity.
•
Near-far problem exists and power control is required.
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ECE 8708 Wireless Communications : Multiple Access Techniques
Summary of Multiple Access
power
FDMA
tim
e
cy
n
e
qu
fre
CDMA
power
e
power
tim
TDMA
ncy
e
u
q
fre
tim
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e
ncy
e
u
q
fre
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ECE 8708 Wireless Communications : Multiple Access Techniques
Space Division Multiple Access
•
Adaptive antennas are used at
the base station to direct the
beam towards the desired
user and form a null in the
directions of other users.
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ECE 8708 Wireless Communications : Multiple Access Techniques
Packet Radio
3
2
4
1
Shared Multiple
Access Medium
5
M
…
Packet Radio access, many subscribers attempt to access a single channel
in an uncoordinated (or minimally coordinated) manner.
1. Any transmission from any station can be heard by any other stations
2. If two or more stations transmit at the same time, collision occurs
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Figure 6.1
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ECE 8708 Wireless Communications : Multiple Access Techniques
Packet Radio Protocols
•
Three types of random accesses:
– ALOHA, slotted ALOHA, and CSMA-CD
•
Throughput (Traffic occupancy)
R = λτ
λ : mean arrival rate
τ : packet duration
•
To obtain a reasonable through put, the rate at which new packets are
generated must lie within 0 < R <1.
•
Normalized throughput
T = R ⋅ Pr[ no collision ] = λτ ⋅ Pr[ no collision ]
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ECE 8708 Wireless Communications : Multiple Access Techniques
Probability of Collision
•
The probability that n packets are generated by the user population
during a given packet duration interval is assumed to be Poisson
distributed and is given by
R ne−R
Pr( n ) =
n!
•
Letting n =0 results in probability of no collision
Pr( 0 ) = e − R
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ECE 8708 Wireless Communications : Multiple Access Techniques
Collisions
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Figure 6.1
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ECE 8708 Wireless Communications : Multiple Access Techniques
ALOHA
•
Basic idea:
– let users transmit whenever they have data to be sent.
– When collision occurs, wait a random time ( why? ) and retransmit
again.
•
Differences between regular errors &collision
– Regular errors only affect a single station
– Collision affects more than one
– The retransmission may collide again
– Even the first bit of a frame overlaps with the last bit of a frame
almost finished, then two frames are totally destroyed.
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ECE 8708 Wireless Communications : Multiple Access Techniques
ALOHA
•
The probability that n packets are generated by the user population
during a duration of 2 packet time intervals
( 2 R ) n e −2 R
Pr( n ) =
n!
•
Probability of no collision
Pr( 0 ) = e − 2 R
•
Normalized throughput
T = R ⋅ Pr[ no collision ] = R e − 2 R
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ECE 8708 Wireless Communications : Multiple Access Techniques
ALOHA
1.peak value at R=0.5
with T=0.184
2.for any given T, there
are two values of R,
corresponding to
two modes:
occasional collision
mode with R ≈ T and
frequent collision mode
with R >> T
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Figure 6.17
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ECE 8708 Wireless Communications : Multiple Access Techniques
Slotted ALOHA
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Figure 6.1
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ECE 8708 Wireless Communications : Multiple Access Techniques
Slotted ALOHA
•
Synchronize the transmissions of stations
All stations keep track of transmission time slots and are allowed to
initiate transmissions only at the beginning of a time slot.
•
The probability that n packets are generated by the user population
during a given packet duration interval is assumed to be Poisson
distributed and is given by
R ne−R
Pr( n ) =
n!
Probability of no collision
•
Pr( 0 ) = e − R
•
Normalized throughput
T = R ⋅ Pr[ no collision ] = R e − R
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ECE 8708 Wireless Communications : Multiple Access Techniques
ALOHA vs. Slotted ALOHA
Peak value at R=1 with
T=0.368 for slotted
ALOHA, double
compared with ALOHA.
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Figure 6.17
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ECE 8708 Wireless Communications : Multiple Access Techniques
CSMA (Carrier Sensing Multiple Access)
•
•
Problem with ALOHAs: low throughput because the collision
wastes transmission bandwidth.
Solution: avoid transmission that are certain to cause collision, that
is CSMA. Any station listens to the medium, if there is some
transmission going on the medium, it will postpone its
transmission.
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ECE 8708 Wireless Communications : Multiple Access Techniques
CSMA (Carrier Sensing Multiple Access)
Suppose tprop is propagation delay from one extreme end to the other
extreme end of the medium. When transmission is going on, a station
can listen to the medium and detect it.
After tprop, A’s transmission will arrive the other end; every station will
hear it and refrain from the transmission.
Vulnerable period = tprop in CSMA, compared to τ and 2τ in ALOHAs.
Station A
begins
transmission at
t=0
A
sense
sense
Station A
captures
channel
at t=tprop
A
sense
sense
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Figure 6.19
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ECE 8708 Wireless Communications : Multiple Access Techniques
Three different CSMA schemes
•
Based on how to do when medium is busy:
–
–
–
–
1-persistent CSMA
Non-persistent CSMA
p-persistent CSMA
CSMA/CD
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ECE 8708 Wireless Communications : Multiple Access Techniques
1-persistent CSMA
sense channel when want to transmit a packet, if channel is busy,
then sense continuously, until the channel is idle, at this time,
transmit the frame immediately.
If more than one station are sensing, then they will begin
transmission the same time when channel becomes idle, so
collision. At this time, each station executes a backoff algorithm to
wait for a random time, and then re-sense the channel again.
Problem with 1-persistent CSMA is “high collision rate”.
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ECE 8708 Wireless Communications : Multiple Access Techniques
Non-persistent CSMA
Sense channel when want to transmit a packet, if channel is idle, then
transmit the packet immediately. If busy, run backoff algorithm
immediately to wait a random time and then re-sense the channel
again.
This is popular for wireless LAN applications, where the packet
transmission intervals is much greater than the propagation delay to
the farthermost user.
Problem with non-persistent CSMA is that when the channel becomes
idle from busy, there may be no one of waiting stations beginning the
transmission, thus waste channel bandwidth.
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ECE 8708 Wireless Communications : Multiple Access Techniques
p-persistent CSMA
Sense channel when want to transmit a packet, if channel is busy,
then persist sensing the channel until the channel becomes idle. If
the channel is idle, transmit the packet with probability of p, and wait,
with probability of 1-p, additional propagation delay tprop and then resense again.
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ECE 8708 Wireless Communications : Multiple Access Techniques
CSMA-CD
•
When the transmitting station detects a collision, it stops its
transmission immediately (Not transmit the entire frame which is
already in collision).
•
The time for transmitting station to detect a collision is 2tprop.
•
In detail: when a station wants to transmit a packet, it senses
channel, if it is busy, use one of above three algorithms (i.e., 1persistent, non-persistent, and p-persistent schemes). The
transmitter senses the channel during transmission. If a collision
occurred and was sensed, transmitter stops its left transmission of
the current frame; moreover, a short jamming signal is transmitted to
ensure other stations that a collision has occurred and backoff
algorithm is used to schedule a future re-sensing time.
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ECE 8708 Wireless Communications : Multiple Access Techniques
CSMA-CD
The reaction time in CSMA-CD is 2tprop
A begins to
transmit at
t=0
A
B
A
B
A
B
A detects
collision at
t= 2 tprop-δ
B begins to
transmit at
t= tprop-δ;
B detects
collision at
t= tprop
It takes 2 tprop to find out if channel has been captured
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Figure 6.22
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ECE 8708 Wireless Communications : Multiple Access Techniques
Packet Radio: Comparison
1.
2.
3.
When a is small, i.e, tprop << τ, the CSMA-CD is best and all CSMAs are
better than ALOHAs.
When a is approaching 1, CSMAs become worse than ALOHA.
ALOHAs are not sensitive to a because they do not depend on reaction
time.
1
CSMA/CD
1-P CSMA
Non-P CSMA
0.8
Τmax
Slotted Aloha
0.6
0.4
Aloha
0.2
0
0.01
a
0.1
1
= tprop /τ
Maximum achievable throughput of random access schemes
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Figure 6.24
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ECE 8708 Wireless Communications : Multiple Access Techniques
Capacity of Cellular Systems
•
Channel capacity for a radio system is defined as the maximum
number of channels or users that can be provided in a fixed
frequency band Î spectrum efficiency of wireless system.
For a cellular system, the radio capacity is defined as
m=
•
•
•
Bt
Bc N
Bt : Total allocated spectrum for the system
BC : Channel bandwidth
N : Number of cells in frequency reuse pattern
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ECE 8708 Wireless Communications : Multiple Access Techniques
Capacity of Cellular Systems
Carrier to Interference ratio
D 0− n
C
=
I
6 D −n
For maximum interference D0 = R
D0
1⎛ R⎞
⎜ ⎟
6⎝D⎠
−n
⎛C ⎞
≥⎜ ⎟
⎝ I ⎠ min
⎡ ⎛C ⎞ ⎤
Q ≥ ⎢6⎜ ⎟ ⎥
⎣ ⎝ I ⎠ min ⎦
1/ n
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ECE 8708 Wireless Communications : Multiple Access Techniques
Capacity of Cellular Systems
Because
Q =
3N
The radio capacity becomes
m=
Bt
=
Bc N
Bt
=
2
Q
⎛ 6
Bc
B c ⎜⎜ n / 2
3
⎝3
Bt
⎛C ⎞
⎜ ⎟
⎝ I ⎠ min
⎞
⎟⎟
⎠
2/n
In particular, when n=4
Bt
m=
Bc
2⎛C ⎞
⎜ ⎟
3 ⎝ I ⎠ min
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ECE 8708 Wireless Communications : Multiple Access Techniques
Capacity of Cellular Systems
Bt
m=
Bc
2⎛C ⎞
⎜ ⎟
3 ⎝ I ⎠ min
Reducing (C/I)min can increase the capacity.
In order to provide the same voice quality, (C/I)min may be lower in a
digital systems when compared to an analog system.
Typically, the minimum required C/I is about 12 dB for narrowband
digital systems, and 18 dB for narrowband FM systems.
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ECE 8708 Wireless Communications : Multiple Access Techniques
Capacity of Cellular Systems
Trade-off between channel bandwidth and power
Bt
m=
Bc
2⎛C ⎞
⎜ ⎟
3 ⎝ I ⎠ min
Bt
m=
Bc '
⎛ C ⎞ ⎛⎜ B c ⎞⎟
⎛C ⎞
⎜ ⎟ =⎜ ⎟ ⎜
⎝ I ⎠ eq ⎝ I ⎠ min ⎝ B c ' ⎟⎠
2⎛C ⎞
⎜ ⎟
3 ⎝ I ⎠ eq
2
Reduce bandwidth to half Æ need to increase C/I by a factor of 4.
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ECE 8708 Wireless Communications : Multiple Access Techniques
Example
8dB
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ECE 8708 Wireless Communications : Multiple Access Techniques
Capacity of Cellular Systems
In digital cellular systems,
E b Rb
E c Rc
C
=
=
I
I
I
⎛C ⎞
E c Rc
2
⎜ ⎟
⎛
⎞
B '
⎝ I ⎠ min
I
=
= ⎜⎜ c ⎟⎟
E c ' Rc ' ⎝ Bc ⎠
⎛C ⎞
⎜ ⎟
I'
⎝ I ⎠ eq
Because
Rc ∝ Bc
⎛ B '⎞
Ec
= ⎜⎜ c ⎟⎟
E c ' ⎝ Bc ⎠
3
Reduce bandwidth to half Æ need to increase bit energy by a factor of 8.
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ECE 8708 Wireless Communications : Multiple Access Techniques
Capacity Comparison: TDMA vs. FDMA
In FDMA, Bt is divided into M channels, each with bandwidth Bc.
Bt
m=
Bc
2⎛C ⎞
⎜ ⎟
3 ⎝ I ⎠ min
C = E b Rb
Bt
=
Bt
M
2⎛C ⎞
⎜ ⎟
3 ⎝ I ⎠ min
=
M
2⎛C ⎞
⎜ ⎟
3 ⎝ I ⎠ min
I = I 0 Bc
For TDMA with multiple time slots and occupies the same spectrum,
C ' = E b Rb '
I ' = I 0 Bc '
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ECE 8708 Wireless Communications : Multiple Access Techniques
Example
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ECE 8708 Wireless Communications : Multiple Access Techniques
Capacity of Digital Cellular CDMA
Denote N as the number of users. SNR in RF
SNR =
S
1
=
( N − 1) S
N −1
SNR in receiver after despreading
Eb
S/R
W /R
=
=
N0
( N − 1) S / W
N −1
With noise considered
Eb
W /R
=
(N − 1) + (η / S )
N0
Number of users that can access the system
N = 1+
W /R
− (η / S )
Eb / N 0
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ECE 8708 Wireless Communications : Multiple Access Techniques
Capacity of Digital Cellular CDMA
•
•
•
Capacity of FDMA and TDMA system is bandwidth limited.
Capacity of CDMA system is interference limited.
The link performance of CDMA increases as the number of
users decreases.
•
To increase the CDMA capacity
- use sectoring: lower interference and noise (N0 Æ N0’)
- switch off during periods with no voice activities (factor α)
•
When the number of user is large
N = 1+
1 W /R
α Eb / N 0 '
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ECE 8708 Wireless Communications : Multiple Access Techniques
Example
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ECE 8708 Wireless Communications : Multiple Access Techniques
Outlines
Duplexing
Time Division Duplexing (TDD)
Frequency Division Duplexing (FDD)
Multiple Access
FDMA
TDMA
Spread Spectrum Multiple Access
- Frequency Hopped Multiple Access (FHMA)
- Code Division Multiple Access (CDMA)
Packet Radio
ALOHA
Carrier Sense Multiple Access (CSMA)
Capacity
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