L4 multiplexing & multiple access 16

G.KARTHIGA M.SC(INFO TECH)
DEPARTMENT OF CS&IT
NADAR SARASWATHI COLLEGE OF ARTS AND
SCIENCE,THENI
1

 Multiplexing
 SDM
 FDM/FDMA
 TDM/TDMA
 CDM
 CDMA
2

 service location
 new applications, multimedia
 adaptive applications
 congestion and flow control
 quality of service
 addressing, routing,
device location
 hand-over
 authentication
 media access
 multiplexing
 media access control
 encryption
 modulation
 interference
 attenuation
 frequency
3
Application layer
Transport layer
Network layer
Data link layer
Physical layer

 Multiple transmitters sending signals
 at the same time through the shared
medium “air”
 How to share the medium (common
channel) with
 other transmitters?
 Multiplexing
 Goal: Minimize the degree of interferences
and
 maximize the bandwidth for data
transmissions
4

5
•Capacity of transmission medium usually exceeds
capacity required for transmission of a single signal
•Multiplexing - carrying multiple signals on a single
medium
•More efficient use of transmission medium

 Cost per kbps of transmission facility declines with
an increase in the data rate
 Cost of transmission and receiving equipment
declines with increased data rate
 Most individual data communicating devices require
relatively modest data rate support
6

 Frequency-division multiplexing (FDM)
Takes advantage of the fact that the useful bandwidth of the
medium exceeds the required bandwidth of a given signal
 Time-division multiplexing (TDM)
Takes advantage of the fact that the achievable bit rate of
the medium exceeds the required data rate of a digital signal
7

8
FDM
frequency
time
TDM
frequency
time
4 users
Example:

Multiplexing: Multiple transmitters send
signals at the same time
Multiplexing in 4 dimensions
 space (si)
 time (t)
 frequency (f)
 code (c)
Goal: supporting multiple users on a
shared medium (more channels)
 Maximize channel utilization
(higher total bandwidth)
Important: guard spaces needed
What will be the problem if the separation is
small and large? Small, the receiver cannot distinguish signals/noises. Large,
a waste of bandwidth
11
s2
s3
s1
f
t
c
k2 k3 k4 k5 k6k1
f
t
c
f
t
c
channels ki
Fr. Schiller

 Use space division multiplexing
Frequency reuses to increase the total system
bandwidth
 Segment space into sectors
 Use directed antennas or limited communication
range signals from base stations
 Mobile stations may receive signals from base
stations with different quality (select the best one =>
it is the closet one)
 May combine with other schemes, i.e., FDM
12

Separation of the whole spectrum into smaller frequency bands (consider
the whole spectrum as the multiple lanes of a road)
The same station uses different frequencies for sending signals for
different users
A channel gets a certain band of the spectrum for the whole time
Advantages:
 Simple
 No dynamic coordination
necessary
Disadvantages:
 Waste of bandwidth
if the traffic is
distributed unevenly
 Inflexible
 Guard spaces
(adjacent channel interference)
13
k2 k3 k4 k5 k6k1
f
t
c

 Assign a certain frequency to a transmission
channel between a sender and a receiver (use
frequency division multiplexing)
 Channels can be assigned to the same
frequency at all times (permanent), i.e., in radio
broadcast
 Channel frequency may change (hopping)
according to certain pattern
 Slow hopping (e.g., GSM) and fast hopping
(FHSS, Frequency Hopping Spread Spectrum)
 Frequency division duplex (FDD): simultaneous
access to medium by base station and mobile
station using different frequencies14

A channel gets the whole spectrum for a certain amount of time
Advantages:
 Only one carrier in the
medium at any time (constant time period)
 Throughput high even
for many users (RR)
Disadvantages:
 Time quantum normally very small
 Precise synchronization
necessary (timing)
15
f
t
c
k2 k3 k4 k5 k6k1

Combination of both methods (time & frequency)
A channel gets a certain frequency band for a certain amount of time
Example: GSM (a 2G cellular network)
Advantages:
 Better protection against
tapping (more complicated)
 Protection against frequency
selective interference
But: precise coordination
required
16
f
t
c
k2 k3 k4 k5 k6k1

Each channel has a unique code (encoding and decoding)
=> d1 -> (encoding function f(d1,key)) -> p1
After encoding, noises can be identified as noises
All channels use the same spectrum
at the same time
Advantages:
 Bandwidth efficient
 No coordination and synchronization necessary
 Good protection against interference and tapping
(different coding schemes)
Disadvantages:
 Lower user data rates
 More complex signal regeneration
What is the guard space? Keys for coding
17
k2 k3 k4 k5 k6k1
f
t
c

18
f
t
124
1
124
1
20 MHz
200 kHz
890.2 MHz
935.2 MHz
915 MHz
960 MHz
Fr. Schiller
GSM: 900MHz
Uplink: 890.2MHz to 915MHz
Downlink: 935.2MHz to 960MHz
Each channel 0.2MHz separated. Totally 124 channels for
each direction

 Assign a fixed sending frequency to a transmission channel between a
sender and a receiver for a certain amount of time
 The receiver and transmitter use the same frequency all the times
(simplified the design of receivers)
 How to do the time synchronization is the problem? Fixed time slot or
assigned dynamically
 Fixed TDM:
Allocating time slots for channels in a fixed pattern
(fixed bandwidth for each channel)
Fixed time to send and get data from a channel
Fixed bandwidth is good for constant data traffic but
not for bursty traffic
 TDD (time division duplex): assign different slots for uplink and
downlink using the same frequency
 Dynamic TDM requires coordination but is more flexible in bandwidth
allocation
19

20
1 2 3 11 12 1 2 3 11 12
t
downlink uplink
417 µs
Fr. Schiller
417 x 12 = 5004
Fixed period of 5ms
10-6

 If one terminal can be heard by all others, this
“central” terminal can poll all other terminals
according to a certain scheme, i.e. round-
robin or random
Now all schemes known from fixed networks can be used
(typical mainframe - terminal scenario)
 Example: Randomly Addressed Polling
Base station signals readiness to all mobile terminals
Terminals ready to send can now transmit a random
number without collision with the help of CDMA or FDMA
(the random number can be seen as dynamic address)
The base station now chooses one address for polling from
the list of all random numbers (collision if two terminals
choose the same address)
The base station acknowledges correct packets and
continues polling the next terminal
21

 Current state of the medium is signaled via a
“busy tone”
The base station signals on the downlink (base station to
terminals) if the medium is free or not
Terminals must not send if the medium is busy
Terminals can access the medium as soon as the busy tone
stops
The base station signals collisions and successful
transmissions via the busy tone and acknowledgements,
respectively (media access is not coordinated within this
approach)
Mechanism used, e.g., for CDPD (USA, integrated into AMPS)
22

 All terminals send on the same frequency
probably at the same time and can use the
whole bandwidth of the transmission channel
 So, how the receivers identify the
data/signals for them?
 Each sender has a unique random number
(code), the sender XORs the signal with this
random number
Different senders use different codes
The codes separate the signals from different senders
 The encoded signals are concatenated
together for sending, i.e., as a signal stream
of signals 23

 Disadvantages:
Higher complexity of a receiver (receiver cannot just listen
into the medium and start receiving if there is a signal)
All signals should have the same strength at a receiver
 Advantages:
All terminals can use the same frequency, no planning
needed
Huge code space (e.g. 232) compared to frequency space
Interferences is not coded
Forward error correction and encryption can be easily
integrated
24

 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
 XOR of the signal with 25
user data
chipping
sequence
resulting
signal
0 1
0 1 1 0 1 0 1 01 0 0 1 11
XOR
0 1 1 0 0 1 0 11 0 1 0 01
=
tb
tc
tb: bit period tc: chip period
tc = 1/m x tb
0 : +1 1: -1
0 (1) X 0 (1) = 1; 0 (1) X 1 (-1) =
-1
1 (-1) X 0 (1) = -1; 1 (-1) X 1 (-1)
= 1
Fr. Schiller

 Data bit
 d1 = –1 (0: +1; 1 = -1)
 Signature sequence
 (c1,c2,…,c8) = (+1,+1,+1,–1,+1,–
1,–1,–1)
 Zi,m = di cm = (-1) x (+1), (-1) x (+1), …, (-1)
x (-1)
 Encoder Output
 (Z ,Z ,…,Z ) = (–1,–1,–1,+1,–26

 Without interfering users, the receiver would
receive the encoded bits, Zi,m, and recover the
original data bit, di, by computing:
27


M
m
mmii cZ
M
d
1
,
1

(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)x(-1) (-1)x(+1)

 (–1,–1,–1,–1,–1,–1,–1,–1)


M
m
mmii cZ
M
d
1
,
1
28
multiply
add and
divide by M
i = 1, m = 1 i = 1, m = 8
-8/ m, m = 8

 If there are N users, the signal at the receiver
becomes:

 How can a CDMA receiver recover a user’s
original data bit?
30


N
n
n
mimi ZZ
1
,
*
,



N
n
n
mimi ZZ
1
,
*
,
31
Multiplied by the signature
sequence of user 1
2-Senders
example

 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:
32

M
m
mkmj cc
M 1
,,
1

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.
33
Other values indicate a partial degree of correlation.

Orthogonal codes
All pair wise cross correlations are zero
Fixed- and variable-length codes used in
CDMA systems
For CDMA application, each mobile user
uses one sequence in the set as a
spreading code
Provides zero cross correlation among all users
Types
Welsh codes
Variable-Length Orthogonal codes
34

Set of Walsh codes of length n
consists of the n rows of an n x n
Walsh matrix:
W1 = (0)
n = dimension of the matrix
Every row is orthogonal to every other
row
Requires tight synchronization
Cross correlation between different shifts of
Walsh sequences is not zero 35
W2n =
Wn Wn
Wn Wn
æ
è
ç
ç
ö
ø
÷
÷

 Spread data rate by an orthogonal code
(channelization code)
Provides mutual orthogonality among all users in
the same cell
 Further spread result by a Pseudo-Noise
(PN) sequence (scrambling code)
Provides mutual randomness (low cross
correlation) between users in different cells
37

Approach SDMA TDMA FDMA CDMA
Idea segment space into
cells/sectors
segment sending
time into disjoint
time-slots, demand
driven or fixed
patterns
segment the
frequency band into
disjoint sub-bands
spread the spectrum
using orthogonal codes
Terminals only one terminal can
be active in one
cell/one sector
all terminals are
active for short
periods of time on
the same frequency
every terminal has its
own frequency,
uninterrupted
all terminals can be active
at the same place at the
same moment,
uninterrupted
Signal
separation
cell structure, directed
antennas
synchronization in
the time domain
filtering in the
frequency domain
code plus special
receivers
Advantages very simple, increases
capacity per km²
established, fully
digital, flexible
simple, established,
robust
flexible, less frequency
planning needed, soft
handover
Dis-
advantages
inflexible, antennas
typically fixed
guard space
needed (multipath
propagation),
synchronization
difficult
inflexible,
frequencies are a
scarce resource
complex receivers, needs
more complicated power
control for senders
Comment only in combination
with TDMA, FDMA or
CDMA useful
standard in fixed
networks, together
with FDMA/SDMA
used in many
mobile networks
typically combined
with TDMA
(frequency hopping
patterns) and SDMA
(frequency reuse)
still faces some problems,
higher complexity,
lowered expectations; will
be integrated with
TDMA/FDMA
38

 Schiller: Ch. 2.1, 2.2, 2.4, 2.5, 2.6.1-2.6.4
 Schiller: Ch 3.1, 3.2, 3.3, 3.4.1, 3.4.8 ,3.4.9,
3.4.10, 3.5, 3.6
 Schiller, Mobile Communications, sections 4.1
(except 4.1.7) and 4.4.2, 4.4.4 – 4.4.6 (except
the protocol stack)
 Wireless Communications & Networks, 2Edition,
Pearson, William Stallings (Ch 7)
39

L4 multiplexing & multiple access 16

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