Transcript Wireless and Mobile System Infrastructure

Multiple Division
Techniques
FDMA, TDMA, and CDMA
1
Outline

Introduction

Concepts and Models for Multiple Divisions

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA)

Comparison of FDMA, TDMA, and CDMA
2
User n
…
Frequency
Frequency Division Multiple Access
(FDMA)
User 2
User 1
Time
• Single channel per carrier
• All first generation systems use FDMA
3
Basic Structure of FDMA
f1 ’
MS #1
f2’
f2
…
…
…
MS #2
f1
f n’
MS #n
Reverse channels
(Uplink)
fn
Forward channels BS
(Downlink)
4
Forward and Reverse channels in
FDMA and Guard Band
f1’
f2’
f n’
f1
f2
…
…
Reverse channels
Protecting
bandwidth
2
Forward channels
Sub Band
Wc
Guard
Band Wg
1
fn
3
4
…
N
Frequency
Total Bandwidth W =
NWc
5
…
User n
User 2
User 1
Frequency
Time Division Multiple Access
(TDMA)
Time
• Multiple channels per carrier
• Most of second generation systems use TDMA
6
The Concept of TDMA
Slot
…
…
t
…
MS #n
#n
…
#n
…
MS #2
Frame
Reverse channels
(Uplink)
…
#1
…
t
…
…
t
Frame
t
…
…
#2
#2
#2
…
…
…
#n
t
MS #1
…
#1
…
#2
…
#1
#1
…
Frequency f
#n
Frequency f ’
…
t
Frame
Frame
BS
Forward channels
(Downlink)
7
TDMA/FDD: Channel Structure
#n
…
t
#n
#2
#1
…
Frame
#n
#1
…
Frame
#n
#2
#1
Frame
#2
f
t
(a). Forward channel
f’
#2
#1
…
Frame
#n
#2
#1
…
Frame
#n
#2
#1
Frame
…
(b). Reverse channel
8
Forward and Reverse
Channels in TDMA
Forward
channel
Reverse
channel
Forward
channel
…
Reverse
channel
#n
#2
#1
…
#n
#2
#1
…
#n
#2
Frame
#1
…
#n
#2
#1
Frequency Frame
f=f’
Time
Channels in TDMA/TDD
9
…
#n
#2
#1
…
Frame
#n
…
#1
#2
Frame
#n
Frame
#2
#1
Frequency
Frame Structure of TDMA
Time
Guard
time
Head
Data
10
Code Division Multiple Access (CDMA)






used in several wireless broadcast channels (cellular,
satellite, etc) standards
unique “code” assigned to each user; i.e., code set
partitioning
all users share same frequency, but each user has
own “chipping” sequence (i.e., code) to encode data
encoded signal = (original data) X (chipping
sequence)
decoding: inner-product of encoded signal and
chipping sequence
allows multiple users to “coexist” and transmit
simultaneously with minimal interference (if codes are
“orthogonal”)
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Code Division Multiple Access
(CDMA)
...
User 2
User 1
User n
Frequency
Time
Code
• Users share bandwidth by using code sequences that are orthogonal to each other
• Some second generation systems use CDMA
• Most of third generation systems use CDMA
12
Structure of a CDMA System
Frequency f ’
Frequency f
C1
MS #2
C2’
C2
Cn’
Cn
…
…
C1’
…
MS #1
MS #n
Reverse
channels
(Uplink)
Forward
channels
(Downlink)
BS
Ci’ x Cj’ = 0, i.e., Ci’ and Cj’ are orthogonal codes,
Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes
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Code-Division Multiple Access (CDMA)

Basic Principles of CDMA (a multiplexing scheme
used with spread spectrum)


D = rate of data signal
Break each bit into k chips
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Chips are a user-specific fixed pattern
This pattern is called the User’s Code
The codes are orthogonal (limited set)
Chip data rate of new channel = kD chips/sec
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CDMA Encode/Decode
channel output Zi,m
d =1
data d = -1
bits
1 1 1
1
1 1 1
1
code -1 -1 -1 -1 -1 -1 -1 -1
0
Zi,m= di.cm
sender
slot 1
1 1 1 1 1 1
1
1
-1 -1 -1
-1
slot 1
channel
output
slot 0
1
-1
-1 -1 -1
slot 0
channel
output
M
Di =m=1S Zi,m.cm
received -1 -1 -1 1 -1 1 1 1 1 1 1 -1 1 -1 -1 -1
input
1 1 1
1
1 1 1
1
code -1 -1 -1 -1
-1 -1 -1 -1
receiver
slot 1
M
slot 0
6: Wireless and Mobile Networks
d0 = 1
d1 = -1
slot 1
channel
output
slot 0
channel
output
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CDMA: two-sender interference
6: Wireless and Mobile Networks
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CDMA
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If k = 6 and code is a sequence of 1’s and -1’s

For each ‘1’ bit, A sends user code as a chip pattern
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For each ‘0’ bit (-1), A sends complement of user code
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<c1, c2, c3, c4, c5, c6>
<-c1, -c2, -c3, -c4, -c5, -c6>
Receiver knows sender’s code and performs
decode function (assume synchronized so that the receiver
knows when to apply the user code )
Su d   d1 c1  d 2  c2  d 3 c3  d 4  c4  d 5  c5  d 6  c6

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< d1, d2, d3, d4, d5, d6 > = received chip pattern
< c1, c2, c3, c4, c5, c6 > = sender’s code
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CDMA Example
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User A code = <1, –1, –1, 1, –1, 1>
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User B code = <1, 1, –1, – 1, 1, 1>
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To send a 1 bit = <1, –1, –1, 1, –1, 1>
To send a 0 bit = <–1, 1, 1, –1, 1, –1>
To send a 1 bit = <1, 1, –1, –1, 1, 1>
Receiver receiving with A’s code
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(A’s code) x (received chip pattern)
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User A ‘1’ bit decoded results: + 6 which translates into 1
User A ‘0’ bit: - 6  binary 0
User B ‘1’ or ‘0’ bit decoded result: 0  signal ignored, SA
signal decode results in a value of 0 which is different from a
decode value of +/- 6 for transmitted bits ‘1’ or ‘0’ from User A
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CDMA Example Continued
B Sends (data bit = 1)
1
1
-1
-1
1
1
Receiver codeword A (decode)
1
-1
-1
1
-1
1
Multiplication
1
-1
1
-1
-1
1
=0
SA(1,1,-1,-1,1,1) = 1 X 1 + 1 X (-1) + (-1) X (-1) + (-1) X 1 + 1 X (-1) + 1 X 1 = 0
B Sends (data bit = 0) -1
-1
1
1
-1
-1
1
-1
-1
1
-1
1
-1
1
-1
1
1
-1
Receiver codeword A
Multiplication
=0
SA(-1,-1,1,1,-1,-1) = (-1) X 1 + (-1) X (-1) + 1 X (-1) + 1 X 1 + (-1) X (-1) + (-1) X 1 = 0
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B (data bit = 0)
-1
-1
1
1
-1
-1
C (data bit = 1)
1
1
-1
1
1
-1
Combined signal
0
0
0
2
0
-2
Receiver codeword B
1
1
-1
-1
1
1
Multiplication
0
0
0
-2
0
-2
Top Case B sends a 1  SB = 8
= -4
Bottom Case B sends a 0  SB = - 4
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Transmissions from B and C, receiver attempts
recovery using A’s codeword (an error situation)
B (data bit = 0)
-1
-1
1
1
-1
-1
C (data bit = 1)
1
1
-1
1
1
-1
Combined signal
0
0
0
2
0
-2
Receiver codeword A
1
-1
-1
1
-1
1
Multiplication (SA)
0
0
0
2
0
-2
=0
Decode result: SA = 0 for this case where B and C have sent data and we attempt
to recover a transmission from A . Different receiver codeword can result in Sx that
is non-zero but much less than the correct orthogonal result.
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Near-far Problem
MS2
BS
MS1
Received signal strength
Distance
0
MS2
d2
Distance
BS d1 MS1
22
Received Signals at BS
Reception of CDMA signals at BS
(reverse link) requires equal power
levels from all MSs in the cell.
Power
MS1 is causing adjacent channel
interference to MS2
MS1
MS2
f1
f2
Solution ?
Frequency
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Power Control in CDMA
Controlling transmitted (effective) power affects the CIR
1
Pr
=
Pt 4df


 c 
Pr =
Pt =
d =
f =
c =
=
Received power in free space (Units of Pr and Pt must be the same)
Transmitted power
(usually dB or dBm)
Distance between receiver and transmitter
Frequency of transmission
Speed of light (3 X 108 m/s if d is in meters and f is in Hz)
Attenuation constant (2 to 4)
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CDMA Advantages for a Cellular Network

Transmission is in the form of a Direct Sequence
Spread Spectrum (DSSS) which provides
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Frequency diversity – frequency-dependent transmission
impairments (e.g. fading) have less effect on signal which
is spread over a large bandwidth
Multipath resistance – chipping codes used for CDMA
exhibit low cross correlation and low autocorrelation thus
a signal delayed by more than 1 chip interval does not
interfere with it’s own stronger/direct signal.
Privacy – privacy is inherent. For DSSS, each user has a
unique code resulting in spread spectrum/noise-like signals
Graceful degradation – system only gradually degrades
(SNR  error rate increase) as more users access the
system up to the point of an unacceptable error rate
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Drawbacks of CDMA Cellular
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Self-jamming – unless all of the MS are perfectly
synchronized, the arriving transmissions will not be
perfectly aligned on chip boundaries  interference.
Requires very accurate timing sources (GPS receiver
disciplined clock). No time or frequency guard bands as in
TDMA and FDMA.
Near-far problem – signals closer to the receiver are
received with less attenuation than signals farther away
and given lack of complete orthogonality, distant stations
more difficult to recover – power control very important
Soft handoff – requires that the mobile acquire the new cell
before it relinquishes the old; this is more complex than
hard handoff used in FDMA and TDMA schemes
26
Comparison of various Multiple
Division Techniques
Technique
FDMA
TDMA
CDMA
SDMA
Concept
Divide the frequency
band into disjoint
subbands
Divide the time into
non-overlapping
time slots
Spread the signal
with orthogonal
codes
Divide the space in to
sectors
Active terminals
All terminals active
on their specified
frequencies
Terminals are active
in their specified
slot on same
frequency
All terminals active
on same frequency
Number of terminals
per beam depends on
FDMA/
TDMA/CDMA
Filtering in
frequency
Synchronization in
time
Code separation
Spatial separation
using smart antennas
Hard handoff
Hard handoff
Soft handoff
Hard and soft
handoffs
Advantages
Simple and robust
Flexible
Flexible
Very simple, increases
system capacity
Disadvantages
Inflexible, available
frequencies are
fixed, requires
guard bands
Requires guard
space,
synchronization
problem
Complex receivers,
requires power
control to avoid
near-far problem
Inflexible, requires
network monitoring to
avoid intracell
handoffs
Radio, TV and
analog cellular
GSM and PDC
2.5G and 3G
Satellite systems,
other being explored
27
Signal separation
Handoff
Current
applications