‫دانشکده مهندس ی کامپیوتر‬
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‫مخابرات سیار (‪)40-626‬‬
‫روشهای دسترس ی چندگانه‬
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‫ل‬
‫نیمسال او ‪91-92‬‬
‫افشین ّ‬
‫همتیار‬
What is Multiple Access?
 Multiple users want to use the same media
to transmit and receive information.
 In wired systems, the media can be an
Ethernet cable.
 In wireless systems, the media is free
space. Although might have to pay a lot of
money to get the right of using it.
2
Multiple Access vs Multiplexing
 Multiplexing: One wants to use one
media to transmit and receive information
of many users.
 All of the users in each side of the channel
are connected to one node.
 Main multiplexing schemes are:
1. FDM: Frequency Division Multiplexing
2. TDM: Time Division Multiplexing
3. CDM: Code Division Multiplexing
3
Multiple Access vs Duplexing
 Duplexing: How transmit and receive paths
are separated from each other.
 Duplexing is still an issue for single user
scenarios in which Multiple Access is not
required.
 Main duplexing schemes are:
1. FDD: Frequency Division Duplexing
2. TDD: Time Division Duplexing
4
Frequency Division Duplexing
 Transmit and receive links use different
frequency channels, so each duplex
channel consists of two simplex channels
in two frequencies.
 Example: GSM
o Tx: 890-915MHz
o Rx: 935-960MHz
o Tx and Rx frequencies for each link are
separated by 45MHz.
 In order to separate Tx and Rx paths,
circuits known as duplexer are required at
analog front end of each transceiver.
5
Time Division Duplexing
 Transmit and receive links use same
frequency channel, but occupy different
time slots, so each duplex channel consists
of two simplex channels at two time slots.
 Simpler RF circuits (no duplexer),
but delays should be handled properly
 not easy to implement for highly
moving users.
 Examples:
o Cordless phones (DECT)
o WLAN (802.11)
6
Multiple Access Techniques (1)
 Fixed: used for circuit switched application
such as voice.
o
o
o
o
Frequency Division MA (FDMA)
Time Division MA (TDMA)
Code Division MA (CDMA)
Orthogonal Frequency Division MA (OFDMA)




FDMA used for analog systems.
TDMA, CDMA and OFDMA used for digital Systems.
MA and Duplexing are two different issues.
A system such as GSM can be TDMA but FDD.
 Statistical: used for packet switched
applications such as data
7
Multiple Access Techniques (2)
 FDMA
o Example: old AMPS systems with analog 30KHz channels.
 TDMA
o One of important issues is synchronization
o Special pattern in each frame used to correlate
and synchronize named Preamble
o Overhead such as guard bits between frames
and coding bits should be taken into account to
compute throughput.
o Example:
for GSM each time slot consists of 6 trailing bits,
26 training bits, and 116 information bits so:
bT = 8*(6+8.25+26+116) = 1250bits
bOH = 8*(6+8.25+26) = 322bits
Efficiency = (1-bOH/bT)*100% = 74%
8
Multiple Access Techniques (3)
 CDMA
o Based on Direct Sequence Spread Spectrum
technique
o Use more BW
o Frequency is the most valuable asset in a
wireless system!, but
 More resistance to multipath and fading
effects
 Multiplexing many users on the same BW,
using proper codes
o Another Spread Spectrum technique is
Frequency Hopping
9
DSSS vs FH
10
DSSS (1)
Example
Code for A =
<1,-1,-1,1,-1,1>
Code for B =
<1,1,-1,-1,1,1>
Code for C =
<1,1,-1,1,1,-1>
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DSSS (2)
PN code generation
12
DSSS (3)
DSSS Tx
DSSS Rx
13
Rake Receiver (1)
An integral part of DSSS systems
14
Rake Receiver (2)
 DSSS removes most of the energy from multipath.
 The received signal components typically experience
fading. The system normally synchronizes to the
strongest multipath component.
 A Rake receiver has N branches that synchronize
to N different multipath components.
 Different multipath components are combined
using
• Scanning
• Selection
• Equal Gain
• Maximal Ratio
 Rake is a diversity combining technique, with
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diversity branches provided by the environment.
Rake Receiver (3)
 Channel Impulse Response
 When the chip time Tc is much less than the rms
delay spread, each branch has independent fading
(assuming uncorrelated scattering), and Rake
provides diversity gain.
 When chip time Tc is greater than the rms delay
spread, multipath components can not be
resolved, and there is no diversity gain.
16
Rake Receiver (4)
Performance in fading channel
17
CDMA (1)
 Spread Spectrum originally aimed at single user
applications (mainly for military purposes).
 But later found to be useful for multiple access
schemes where we use different codes for different
users.  CDMA
 Other users appear as noise
 System highly interference limited
 Duplexing can be either FDD or TDD
18
CDMA (2)
Design Goals
1) Make the interference look as much like Gaussian
noise as possible:
o Spread each user’s signal using a pseudo-noise
random sequence
o Tight power control for managing interference
within the cell
o Averaging interference from outside the cell
as well as fluctuating voice activities of users
2) Apply point-to-point design for each link
o extract all possible diversity in the channel
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CDMA (3)
Point-to-point link design
 Very low SINR per chip: can be less than -15dB
 Diversity is very important at such low SINR.
 Time Diversity is obtained by interleaving across
different coherence times.
 Frequency diversity is obtained by Rake combining
of the multipaths.
 Transmit diversity in 3G-CDMA systems (multiple
base stations and antennas).
20
CDMA (4)
Power control
 In the reverse path, signals coming from different
users will experience wide range of variations.
 Since, cross-correlation of codes of different users
is not completely zero, we will experience large
interference if interferer’s signal is strong.
 This will lead to the so-called “Near-far” problem.
 The solution to Near-far problem is using proper
“Power Control” algorithms, to ensure received
power at base station coming from different users
is almost the same.
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CDMA (5)
Power control
 Maintains equal received power for all users in the cell.
 Tough problem since the dynamic range is very wide
(users’ attention can be differ by many 10’s of dB).
 Consists of both open-loop and closed–loop.
 Closed loop is needed since IS-95 is FDD.
 Consists of 1-bit up-down feedback at 800Hz.
 Not cheap: consumes about 10% of capacity for voice.
 Power control is one the most difficult parts of CDMA
systems.
 For a long time it was believed to be impossible, but
“Qualcomm” proved that it works.
22
CDMA (6)
Interference averaging
 The received signal-to-interference-plus-noise ratio for a
user is defined as:
 In a large system, each interferer contributes a small
fraction of the total out-of-cell interference.
 This can be viewed as providing interference diversity.
 Same interference-averaging principle applies to voice
activity and imperfect power control.
23
CDMA (7)
Strengths
 Multipath friendly
o Using spread spectrum techniques (DS or FH) creates
some sort of frequency diversity that will improve
system performance against deep fading.
o In addition, in time domain, different paths can be
resolved and properly added together to improve
performance
o Since some paths can be in fade, but others not in fade,
Rake receiver improves performance by proper
combining of paths (some sort of time diversity).
24
CDMA (8)
Strengths
 Soft Capacity
o Unlike FDMA and TDMA that have fixed number of slots
in time or frequency domain and therefore put a hard
limit on system capacity, in CDMA number of users can
be increased without hard limits.
o In this case, more users will show up as additional noise
and decrease system performance gradually.
25
CDMA (9)
Strengths
 Soft Handoff
o Unlike FDMA and TDMA in which neighbor base stations
can not transmit at the same frequency, in CDMA
neighbor stations can use same frequencies and talk to a
mobile at the same time.
o In this way, when a user cross the boundary of two
stations, it can simultaneously talk to two stations and
even add their signals together in the same way they
combine multipath signals.
o Therefore, instead of switching one base to another,
during handoff multiple signals are used and some sort
of macro-diversity is achieved.
o This procedure is known as “Soft Handoff” and will
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improve handoff quality.
CDMA (10)
Example: IS-95
 Most wide-spread 2G CDMA system
o
o
o
o
Channel Bandwidth: 1.25MHz
Processing Gain: 128
Bit Rate: 9.6Kbps
Data rate reduced to 1.2Kbps during silence times, so Tx
power can be reduced during vacant bits.
 FDD used with 45MHz separation
o Forward link band: 869 – 894MHz
o Reverse link band: 824 – 849MHz
27
CDMA (11)
Example: IS-95 Forward link
 In forward link 64 Walsh-Hamard codes are used to
differentiate up to 63 users.
 Each code is multiplied by I and Q PN codes, unique for
each base station, to ensure spreading of signals and to
reduce interference from neighbor cells.

Orthogonality preserved since users are added
synchronously at base.
 However, multipath can cause un-orthogonal signals
arriving at receiver to be combined by Rake receiver.
 Pilot signal used to ensure users can properly use coherent
detection and also detect proper base signals during
handoffs.
28
CDMA (12)
Example: IS-95 Forward link
29
CDMA (13)
Example: IS-95 Reverse link
 6 Symbols mapped to one of 64 Walsh codes which are the
same for all users, used for modulation and spreading.
 Then, user specific codes of length 242-1 used to separate
users and base stations from each other.
 In this way, the robostness to in-cell interference increases
compared with a short code.
 Open-loop and fast, closed loop power control used to
control transmit power of each user.
 Fast closed loop power control essential in fading
environments.



A 800bps forward channel used for closed loop signals sent back
to the mobile (1dB step changes).
-50 to 23dBm dynamic range, accuracy = 1.5 – 2dB
30
3 finger Rake used at base station
CDMA (14)
Example: IS-95 Reverse link
31
CDMA (15)
Issues
 Main advantages
o
o
o
o
Allows interference averaging across many users
Soft capacity limit
Allows soft handoff
Simplified frequency planning
 Challenges
o Very tight power control to solve the near-far problem
o We can only keep users in the cell orthogonal
o More sophisticated coding/signal processing to extract
the information of each user in a very low SINR
environment
o Synchronization issues
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CDMA (16)
Synchronization issues
 Carrier vs. Chip synchronization
 Acquisition vs. Tracking
 Matched filter vs. Correlator for acquisition
 Early-Late gates for tracking
 PLL vs. DLL
33
Multicarrier Modulation (1)
Frequency selective channel with no equalizer
 Earlier saw that this is possible with DSSS and
Rake receiver.
 Another option: Multicarrier or OFDM modulation
34
Multicarrier Modulation (2)
 Breaks data into N non-overlapping substreams
 Substream modulated onto separate carries
o Substream bandwidth is BN = B/N for B total
bandwidth
o BN < Bc implies flat fading on each subcarrier
(no ISI)
 Use BPF of width BN to separate signals at
receiver
35
OFDM (1)
 It is quite clear how an ideal OFDM system works
in frequency domain if we use continuous time
non-overlapping subcarriers.
 Actually, in this case, these functions (cos(2πfit))
are “eigen-functions” of the LTI system.
 But, one main problem with this approach is that
in reality infinite-length subcarriers can not be
used.
 Also it is not practical to generate continuous
time signals and multiply them with input
signals.
 So, the main question is how to implement an
OFDM system practically in discrete-time
domain?
36
OFDM (2)
 If we only transmit a finite number of N symbols,
d[0] to d[N-1], sinusoids are no longer eigenfunctions of the system.
 One way to restore this property is by adding a
cyclic prefix (CP) to the symbols (of length L-1):
x = [d[N-L+1], . . . d[N-1], d[0], . . . d[N-1]]
 Now, if only look at channel output for m = L to
N+L-1 and define channel vector of length N as
h = [h0, h1, . . . hL-1, 0, . . . 0], then it can be
easily verified that the new output is equal to
“cyclic convolution” of d and h.
37
OFDM (3)
 Consequently, for cyclic convolution in time
domain, the DFT of output is given by
multiplication of DFT of d and DFT of channel
vector:
Yi = Di.Hi
 So, by using the cyclic prefix and transmitting
the DFT of signal through channel, we again get
each component separately at the output (eigenfunctions are back).
 I this way channel will be transformed into a set
of independent parallel channels.
38
OFDM (4)
 Efficient IFFT structure at transmitter:
 Reverse structure (remove CP and use FFT) at
receiver.
39
OFDM (5)
Challenges
 Peak-to-average power ratio
o
o
o
Adding multiple substreams can result in high peak
signal values
Impacts amplifier efficiency
Solutions include clipping, coding, and tone reservation.
 Inter-carrier Interference
o
o
Subcarrier orthogonality compromised by timing jitter,
frequency offset, and fading.
Frequency and timing offset causes interference
between carriers.
40
OFDM (6)
High PAR
 For single carrier we have:
 In multicarrier, assuming coherent addition of
subcarriers, peak power increases linearly with N2,
while average power increases linearly with N.
 It can be shown that PAR increases approximately
linearly with number of subcarriers N.
41
OFDM (7)
High PAR
42
OFDM (8)
High PAR
43
OFDM (9)
Inter-carrier interference
 Mismatched oscillators, Doppler shift or errors in
timing synchronization cause subchannel interference
(loss of subcarrier orthogonality).
 Mitigating by minimizing number of subchannels and
using pulse shapes robust to timing errors.
44
OFDM (10)
Effects of phase/frequency imperfections
45
OFDM (11)
Fading across subcarriers
 Leads to different BERs for subcarriers
o Performance limited to worst subcarrier status
 Compensation techniques
o Frequency equalization
o
o

Noise enhancement


Accurate channel estimate
Power inefficient

Works well for small BC (large delay spread) in order to use
codes over many subchannels and recover data
Precoding: compensate channel variations at transmitter
Coding across subcarriers
o Adaptive loading (power and rate)
 For small subcarrier bandwidths Doppler spread becomes
important (higher user mobility):
o
Should have fD<<BN in order to ignore Doppler spread
 Current OFDM-based wireless systems: 802.11a, 802.11g,46
802.16a, 802.20
OFDM (12)
Example: Flash OFDM
 Bandwidth = 1.25MHz
 Number of data subcarriers=113
 OFDM symbol = 128 samples = 100μS
 Cyclic prefix = 16 samples = 11μS delay spread
47
Multiuser OFDM (1)
 We have seen OFDM as a point-to-point
modulation scheme, converting the frequencyselective channel into a parallel channel.
 It can also be used as a multiple access technique
called “OFDMA”.
 By assigning different time/frequency slots to
users, they can be kept orthogonal, no matter
what the multipath channels are.
48
Multiuser OFDM (2)
In-cell Orthogonality
 The basic unit of resource is a virtual channel:
a hopping sequence
 Each hopping sequence spans all the subcarriers
to get full frequency-diversity.
 Coding is performed across the symbols in a
hopping sequence.
 Hopping sequences of different virtual channels in
a cell are orthogonal.
 Each user is assigned a number of virtual channels
depending on their data rate requirement.
49
Multiuser OFDM (3)
In-cell Orthogonality
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Multiuser OFDM (4)
Out-of-cell interference averaging
 The hopping patterns of virtual channels in
adjacent cells are designed such that any pair has
minimal overlap.
 This ensures that a virtual channel sees
interference from many users instead of a single
strong user.
 This is a form of interference diversity.
51
Statistical MA (1)
 So far discussed fixed MA, FDMA, TDMA, and CDMA in
which a fixed channel is assigned to a user during the
whole talk period.
 Another MA scheme used for data services is based on
packet nature of data transmitted.
 Therefore, channel is not assigned to a user and any
user only uses the channel while has bursty data to
transmit.
 Since multiple users can try to use the channel at the
same time, “collision” may naturally occur in packet
multiple access systems.
 In general, in order to avoid collision, some sort of ACK
mechanism is used.
 Main problems in packet systems are:
o Loss of packets (low throughput)
52
o Packet delay and jitter
Statistical MA (2)
 Main statistical MA techniques used in practice
are:
 Slotted aloha
 Carrier Sense MA (CSMA)
 These techniques are also commonly known as
MAC layer algorithms in multi-user networks.
53
Statistical MA (3)
Slotted ALOHA
 Lets assume users send packets of length τ
 Users are synchronized to send at intervals of about τ
 After sending a packet if collision occurs (no ACK received),
the sender waits for a random time and sends at next
available time slot.
 If we assume packet generation to have Poisson distribution
with mean arrival time of λ, then traffic occupancy without
collision will be R=λτ.
 Probability of no collision is also given by:
P(n) = Rne-R/n!  P(n=0) = e-R
 Throughput of slotted ALOHA is then given by:
T = Re-R
 Therefore, maximum throughput will be for R=1, and we will
get:
T = e-1 = 0.3679
54
Statistical MA (4)
Medium Access Control
 The Medium Access Control (MAC) is responsible for
scheduling data of users contesting for the same medium in
a suitable way. For a MAC protocol, the figures of interest are
data throughput and delay.
 Optimum MAC ,mechanisms for centralized and decentralized
wired and wireless systems are very different. Recent focus
is on decentralized wireless relaying protocols, which have
been successfully deployed in mobile ad-hoc networks
(MANETs)
 Due to the absence of central control and synchronization in
MANETs. Carrier Sense Multiple Access with Collision
Avoidance (CSMA/CA) is a highly efficient random access
scheme that is widely used in wireless communication
systems such as wireless LANs.
 The most widespread deployed protocol is the IEEE 802.11b
55
MAC which is based on CSMA/CA.
Statistical MA (5)
Carrier Sense MA
 In ALOHA schemes, users do not consider channel
status when they transmit over the channel.
 In CSMA, users sense the channel status by
listening to the channel to reduce chance of
collision and re-transmission.
 If no carrier is detected on the channel, then this
scheme has very good performance.
 In wireless systems, propagation delays may be
large and so the performance reduces, however,
still CSMA is better than ALOHA.
56
Statistical MA (6)
CSMA Techniques
 Two common CSMA techniques are:
1) CSMA/CD: used in wired Ethernet
o Users start transmission and then listen to
channel and abort transmission if collision is
detected, use exponential backoff for future
transmissions of collided packet.
57
Statistical MA (7)
CSMA Techniques
2) CSMA/CD: used in wireless LAN 802.11
o
In wireless networks CSMA/CD is not popular for
two reasons:


o
o
Implementation of CD requires a full duplex radio
that can sense the channel at the same time as
transmission
Due to hidden terminals, even if collision happens at
receiver, the transmitters can not detect collisions.
One solution for CA is that transmitter before
sending any data, broadcasts a signal onto the
network (RTS) in order to tell other devices not to
broadcast (directly or through Access Point).
If a user wants to send a packet and channel is
busy, it will set a counter and decrement it over
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time to try again.
Statistical MA (8)
MAC: Center of gravity in wireless networks
 Transmit power levels
 Error rates, Interference behavior
 Frame Lengths
 Throughput, Interference behavior
 Scheduling timings
 Delay, Interference behavior
 IP packet buffering
 QoS
59