multiplexing techniques
DESCRIPTION
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Contents
1. Overview2. Multiple Access Protocols3. Multiplexing Techniques4. TDMA5. FDMA6. CDMA7. SDMA8. Others9. Example of GSM
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Multiple Access protocols single shared communication channel two or more simultaneous transmissions by nodes:
interference only one node can send successfully at a time
multiple access protocol: distributed algorithm that determines how stations share
channel, i.e., determine when station can transmit communication about channel sharing must use channel itself! what to look for in multiple access protocols:
• synchronous or asynchronous • information needed about other stations • robustness (e.g., to channel errors) • performance
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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 average rate R/M3. Fully decentralized:
no special node to coordinate transmissions no synchronization of clocks, slots
4. Simple
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MAC Protocols: a taxonomy
Three broad classes: Channel Partitioning TDMA, FDMA, CDMA
divide channel into smaller “pieces” (time slots, frequency)
allocate piece to node for exclusive use Random Access ALOHA, CSMA, CSMA/CD, CSMA/CA
allow collisions “recover” from collisions
“Taking turns” Polling, Token passing tightly coordinate shared access to avoid collisions
Goal: efficient, fair, simple, decentralized
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Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle
TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.
FDM (Frequency Division Multiplexing): frequency subdivided.
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Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle
TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.
FDM (Frequency Division Multiplexing): frequency subdivided.
frequ
ency
bands time
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Channel Partitioning (CDMA)
CDMA (Code Division Multiple Access) unique “code” assigned to each user; i.e., code set
partitioning used mostly in wireless broadcast channels (cellular,
satellite, etc) 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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CDMA Encode/Decode
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CDMA: two-sender interference
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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”, 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 CSMA, CSMA/CD, CSMA/CA
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Slotted Aloha
time is divided into equal size slots (= pkt trans. time)
node with new arriving pkt: transmit at beginning of next slot
if collision: retransmit pkt in future slots with probability p, until successful.
Success (S), Collision (C), Empty (E) slots
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Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots, time to transmit 1 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
Operation when node obtains fresh
frame, it transmits in next slot
no collision, node can send 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
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Slotted Aloha efficiency
Suppose N nodes with many frames to send, each transmits in slot with probability p
prob that 1st node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
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 to infinity, gives 1/e = .37
Efficiency is the long-run fraction of successful slots when there’s many nodes, each with many frames to send
At best: channelused for useful transmissions 37%of time!
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Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame first arrives
transmit immediately
collision probability increases: frame sent at t0 collides with other frames sent in [t0-
1,t0+1]
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Pure Aloha efficiencyP(success by given node) = P(node transmits) .
P(no other node transmits in [p0-1,p0] .
P(no other node transmits in [p0-1,p0]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
… choosing optimum p and then letting n -> infty ...
= 1/(2e) = .18 Even worse !
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CSMA: (Carrier Sense Multiple Access)
CSMA: listen before transmit:
If channel sensed idle: transmit entire pkt If collision occurs has to retransmit again
If channel sensed busy, defer transmission P-Persistent CSMA: (for slotted channels) retry immediately with
probability p when channel becomes idle (may cause instability) Non-persistent CSMA: (for nonslotted channels) retry after random
interval human analogy: don’t interrupt others!
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CSMA collisions
collisions can occur:propagation delay means two nodes may not hear each other’ transmissioncollision:entire packet transmission time wasted
spatial layout of nodes along ethernet
note:role of distance and propagation delay in determining collision prob.
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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:
easy in wired LANs: measure signal strengths, compare transmitted, received signals
difficult in wireless LANs: receiver shut off while transmitting
human analogy: the polite conversationalist
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CSMA/CD collision detection
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“Taking Turns” MAC protocolschannel partitioning MAC protocols:
share channel efficiently and fairly at high load
inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node!
Random access MAC protocols efficient at low load: single node can fully
utilize channel high load: collision overhead
“taking turns” protocolslook for best of both worlds!
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“Taking Turns” MAC protocolsPolling: master node
“invites” slave nodes to transmit in turn
concerns: polling overhead latency single point of
failure (master)
Token passing: control token passed
from one node to next sequentially.
token message concerns:
token overhead latency single point of failure
(token)
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Frame Relay (more)
Flag bits, 01111110, delimit frame address:
10 bit VC ID field 3 congestion control bits
• FECN: forward explicit congestion notification (frame experienced congestion on path)
• BECN: congestion on reverse path• DE: discard eligibility
addressflags data CRC flags
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Frame Relay -VC Rate Control Committed Information Rate (CIR)
defined, “guaranteed” for each VC negotiated at VC set up time customer pays based on CIR
DE bit: Discard Eligibility bit Edge FR switch measures traffic rate for each VC;
marks DE bit DE = 0: high priority, rate compliant frame;
deliver at “all costs” DE = 1: low priority, eligible for congestion discard
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Frame Relay - CIR & Frame Marking Access Rate: rate R of the access link
between source router (customer) and edge FR switch (provider); 64Kbps < R < 1,544Kbps
Typically, many VCs (one per destination router) multiplexed on the same access trunk; each VC has own CIR
Edge FR switch measures traffic rate for each VC; it marks (i.e. DE = 1) frames which exceed CIR (these may be later dropped)
Internet’s more recent differentiated service uses similar ideas
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Example: GSM
Frequency Band 935-960, 890-915 MHz Two pieces of 25 MHz band
(same as AMPS) AMPS has 833 user channels How about GSM?
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Different Generations
1G analog
2G digital
3G higher data rate for multimedia applications
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1G Cellular Systems
Many Different Standards: AMPS (US) NMT (Northern Europe) TACS (Europe) NTT (Japan) many others...
Spectrum around 800 and 900 MHz
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2G Cellular Systems
Four Major Standards: GSM (European) IS-54 (later becomes IS-136, US) JDC (Japanese Digital Cellular) IS-95 (CDMA, US)
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Frequency Division Duplex (FDD)
Forward Link
Reverse Link
Two separate frequency bands are used for forward and reverse links.
Typically, 25 MHz in each direction.
AMPS: 824-849 MHz (forward) 869-894 MHz (reverse)
mobile
base station
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Frequency Division Multiple Access (FDMA)
The spectrum of each link (forward or reverse) is further divided into frequency bands
Each station assigned fixed frequency band
frequ
ency
bands
idle
idle
idle
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Number of User Channels in AMPS Bandwidth allocated to each user in each link
(forward or reverse) is 30 KHz.
No. of user channels = Total bandwidth / user bandwidth = 25 MHz / 30 kHz
= 833 Is it enough?
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Frequency Reuse
f
f
The same frequency can be reused in different cells, if they are far away from each other
Radio coverage, called a cell.
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Cellular ArchitectureMS – Mobile StationBSC – Base Station ControllerMSC – Mobile Switching CenterPSTN – Public Switched
Telephone Network
MSC PSTN
BSC
segmentation of the area into cells
MS
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Time Division Multiple Access (TDMA)
The mobile users access the channel in round-robin fashion.
Each station gets one slot in each round.
Slots 2, 5 and 6 are idle
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FDMA/TDMA, example GSM
1 2 3 7 8
f
t
124
1
124
1
20 MHz
200 kHz
890.2 MHz
935.2 MHz
915 MHz
960 MHz
Each freq. carrier is divided into 8 time slots.
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Number of channels in GSM
Freq. Carrier: 200 kHz TDMA: 8 time slots per freq carrier
No. of carriers = 25 MHz / 200 kHz = 125
No. of user channels = 125 * 8 = 1000
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Capacity Comparison
Reuse factor 7 for AMPS 3 for GSM (why smaller reuse factor?)
What’s the capacity of GSM relative to AMPS?
A. one half of AMPS B. the sameC. 3 times larger D. 10 times larger
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Answer
AMPS reuse factor = 7 no. of users / cell = 833 / 7 = 119
GSM reuse factor = 3 no. of users / cell = 1000 / 3 = 333 almost 3 times larger than AMPS!
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Multiple Access Methods
Three major types: Frequency Division Multiple Access
(FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA)
• Frequency hopping (FH-CDMA)• Direct sequence (DS-CDMA)
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Frequency-Time Plane
Time
Frequency
Partition of signal space into time slots and frequency bands
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FDMA
Time
Frequency
Different users transmit at different frequency bands simultaneously.
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TDMA
Time
Frequency
Different users transmit at different time slots.
Each user occupy the whole freq. spectrum.
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Frequency Hopping CDMA
Frequency
Time
At each successive time slot, the frequency band assignments are reordered.
Each user employs a code that dictates the frequency hopping pattern.
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Synchronization
The previous figure implies that each signal synchronizes with each of the other signals.
In practice, this is not the case. Frequency hops may collide, but it does
not occur frequently. How often collisions occur depends on the
choice of codes.
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Direct Sequence CDMA
Time
Frequency
All users occupy the whole bandwidth all the time.
Signals of different users overlap with one other.
How can it be done?
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CDMA Encoding
Each user is assigned a unique signature sequence (or code), denoted by (c1,c2,…,cM). Its component is called a chip.
Each bit, di, is encoded by multiplying the bit by the signature sequence:
Zi,m = di cm
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Encoding Example
Data bit d1 = –1
Signature sequence (c1,c2,…,c8) = (+1,+1,+1,–1,+1,–1,–1,–1)
Encoder Output(Z1,1,Z1,2,…,Z1,8) = (–1,–1,–1,+1,–1,+1,+1,+1)
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Bandwidth
Note that the chip rate is much higher than the data rate.
Consider our previous example. Suppose the original data signal occupies a
bandwidth of W. What is the bandwidth of the encoded
signal?
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Spread Spectrum Technique
Time
Frequency
Time
Frequency
Encoding
The bandwidth expands by a factor of M.
M is called spreading factor or processing gain.
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CDMA Decoding
Without interfering users, the receiver would receive the encoded bits, Zi,m , and recover the original data bit, di, by computing:
M
mmmii cZ
Md
1,
1
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CDMA Decoding Example
(c1,c2,…,c8) = (+1,+1,+1,–1,+1,–1,–1,–1)
(Z1,1,Z1,2,…,Z1,8) = (–1,–1,–1,+1,–1,+1,+1,+1)
(–1,–1,–1,–1,–1,–1,–1,–1)
di = –1
multiply
add and divide by M
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Multiuser Scenario
If there are N users, the signal at the receiver becomes:
How can a CDMA receiver recover a user’s original data bit?
N
n
nmimi ZZ
1,
*,
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Multiplied by the signature sequence of user 1
2-user example
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Signature Sequences
In order for the receiver to be able to extract out a particular sender’s signal, the CDMA codes must be of low correlation.
Correlation of two codes, (cj,1,…, cj,M) and (ck,1,…, ck,M) , are defined by inner product:
M
mmkmj cc
M 1,,
1
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The Meaning of Correlation
What is correlation? It determines how much similarity one sequence
has with another. It is defined with a range between –1 and 1.
Correlation Value
Interpretation
1 The two sequences match each other exactly.
0 No relation between the two sequences
–1 The two sequences are mirror images of each other.
Other values indicate a partial degree of correlation.
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Generation of Signature Sequences How to generate signature sequences of
low correlation?
There are two classes of signature sequences that are widely used in CDMA systems. Orthogonal Codes Pseudo Noise Sequences (PN Sequences)
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Orthogonal Codes
Two codes are said to be orthogonal if their correlation is zero. no interference between the two users.
In our previous two-user example, the codes are orthogonal.
How to generate orthogonal codes?
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Walsh Codes
The most common orthogonal codes used in CDMA systems.
A set of Walsh codes of length n is defined by the rows of an n n Hadamard matrix.
Hadamard matrix can be constructed by an iterative method.
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Iterative Construction
Example:
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1121 0
nn
nnn HH
HHHH
0110
1100
1010
0000
10
0042 HH
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Signature Sequences
The signature sequences can be found by Taking the rows out Replacing 0 by –1
10
002
H )1,1(
)1,1(
2
1
s
s
Are they orthogonal?
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IS-95 Forward Link
Walsh Codes of length 64 is used for spreading in the forward link (base-to-mobile) of IS-95.
It is NOT suitable for the reverse link (mobile-to-base). (Why?) PN sequences are used instead.
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PN Sequences
What is Pseudo-Noise Sequences? They are deterministic. But they look like random noise.
How to generate PN sequences? One common way is to use linear feedback
shift register.
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Shift Register Implementation: An Examplex1 x2 x3
x1 x2 x3 Output
1 0 0 ---
0 1 0 0
1 0 1 0
1 1 0 1
1 1 1 0
0 1 1 1
0 0 1 1
1 0 0 1
Initial state: 1 0 0
Output: 0 0 1 0 1 1 1 …
(Periodic with period 7)
The output sequence must be periodic (why?)
The period cannot be greater than 7. (why?)
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2.2 Multiple Access protocols
single shared communication channel two or more simultaneous transmissions by nodes: interference
only one node can send successfully at a time multiple access protocol:
distributed algorithm that determines how stations share channel, i.e., determine when station can transmit
communication about channel sharing must use channel itself!
type of protocols:• synchronous or asynchronous • information needed about other stations • robustness (e.g., to channel errors) • performance
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2.3 Multiple Access Control Protocols
Three broad classes: Channel Partitioning
divide channel into smaller “pieces” (time slots, frequency, code)
allocate piece to node for exclusive use TDMA, FDMA, CDMA
Random Access allow collisions “recover” from collisions CSMA, ALOHA
Taking turns tightly coordinate shared access to avoid collisions Token ring
Goal: efficient, fair, simple, decentralized
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2.4 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”, 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 CSMA and CSMA/CD
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2.5 CSMA: Carrier Sense Multiple Access
CSMA: listen before transmit: If channel sensed idle: transmit entire pkt If channel sensed busy, defer transmission human analogy: don’t interrupt others!
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2.6 CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage persistent or non-persistent retransmission
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 conversationalist
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CSMA/CD collision detection
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Thank You