cellular communications 7. multiple access. multiple access radio spectrum is shared among number...
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CELLULAR COMMUNICATIONS
7. Multiple Access
Multiple Access
Radio spectrum is shared among number of transmissions Uplink and downlink voice and data
transmission from the single handset Duplexing methods
Unrelated communications sessions Many voice conversations by different parties
Random Access Requests New handset /session requests
Traditional MAC Protocol Classification
Contention Protocols Transmit when you feel like transmitting Retry if collision, try to minimize collisions,
additional reservation modes Problem: Receiver must be awake as well
Scheduling Protocols Use a “pre-computed” schedule to transmit
messages Distributed, adaptive solutions are difficult
Quality of Service
Data networks are usually “best-effort” networks No guarantee on data delivery time Usually use packet switching(routing) Decision when and how to send data for each packet No resource allocation for session
Guaranteed quality Make promises that certain amount of data will be
deliveries within specified time Usually use circuit switching Route (circuit) is established at the beginning of the
session with all required resources (e.g. spectrum/bandwidth) allocated
Call Admission Control (CAC) Decide if to accept request for new
session call
Admin call only if QoS constrains could be met without affecting existing sessions Network is busy
Depends on type of required service
UMTS Quality of Service (QoS) Classes3GPP (3rd Generation Partnership Project) defines four classes for UMTS Conversation Class: Delay Constrained / Connection Oriented/ Constant Bit Rate Streaming Class: Delay Constrained / Connection Oriented / Variable Bit rate Interactive Class: Longer Delay Constraints / Connectionless Background Class: Best Effort Connectionless Services
Contention Protocol
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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
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 clock synchronization
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Slotted Aloha efficiency
Suppose N nodes with many frames to send, each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that there is 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 are 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]
12.13
Figure 12.4 Procedure for pure ALOHA protocol
12.14
The stations on a wireless ALOHA network are a maximum of 600 km apart. If we assume that signals propagate at 3 × 108 m/s, we find Tp = (600 × 105 ) / (3 × 108 ) = 2 ms. Now we can find the value of TB for different values of K .
a. For K = 1, the range is {0, 1}. The station needs to| generate a random number with a value of 0 or 1. This means that TB is either 0 ms (0 × 2) or 2 ms (1 × 2), based on the outcome of the random variable.
Example 12.1
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b. For K = 2, the range is {0, 1, 2, 3}. This means that TB
can be 0, 2, 4, or 6 ms, based on the outcome of the random variable.
c. For K = 3, the range is {0, 1, 2, 3, 4, 5, 6, 7}. This means that TB can be 0, 2, 4, . . . , 14 ms, based on the outcome of the random variable.
d. We need to mention that if K > 10, it is normally set to 10.
Example 12.1 (continued)
12.16
Figure 12.5 Vulnerable time for pure ALOHA protocol
12.17
A pure ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the requirement to make this frame collision-free?
Example 12.2
SolutionAverage frame transmission time Tfr is 200 bits/200 kbps or 1 ms. The vulnerable time is 2 × 1 ms = 2 ms. This means no station should send later than 1 ms before this station starts transmission and no station should start sending during the one 1-ms period that this station is sending.
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Pure Aloha efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [t0,t0+1]
.
P(no other node transmits in [t0-1,t0]
= 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 frame If channel sensed busy, defer transmission
Human analogy: don’t interrupt others!
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CSMA collisions
collisions can still occur:propagation delay means two nodes may not heareach other’s transmissioncollision:entire packet transmission time wasted
spatial layout of nodes
note:role of distance & propagation delay in determining collision probability
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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
12.23
Figure 12.10 Behavior of three persistence methods
12.24
Figure 12.11 Flow diagram for three persistence methods
12.25
Figure 12.14 Flow diagram for the CSMA/CD
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CSMA/CD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame
Efficiency goes to 1 as Tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA, but still decentralized, simple, and cheap
transprop tt /51
1efficiency
12.27
Figure 12.16 Timing in CSMA/CA
12.28
In CSMA/CA, the IFS can also be used to define the priority of a station or a frame.
Note
12.29
In CSMA/CA, if the station finds the channel busy, it does not restart the timer of
the contention window;it stops the timer and restarts it when the
channel becomes idle.
Note
12.30
Figure 12.17 Flow diagram for CSMA/CA
CSMA and Wireless
Hidden Terminal Problem
A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS
fails) collision at B, A cannot receive the collision (CD fails) A is “hidden” for C
BA C
Exposed Terminal Problem
B sends to A, C wants to send to D C has to wait, CS signals a medium in
use since A is outside the radio range of C
waiting is not necessary C is “exposed” to B
BA C D
Motivation - Near and Far Terminals
Terminals A and B send, C receives the signal of terminal B hides A’s signal C cannot receive A
This is also a severe problem for CDMA networks precise power control required
A B C
Controlled Access
12.36
12-2 CONTROLLED ACCESS
In controlled access, the stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations. We discuss three popular controlled-access methods.
ReservationPollingToken Passing
Topics discussed in this section:
12.37
Figure 12.18 Reservation access method
12.38
Figure 12.19 Select and poll functions in polling access method
12.39
Figure 12.20 Logical ring and physical topology in token-passing access method
Channalization
12.41
12-3 CHANNELIZATION
Channelization is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations. In this section, we discuss three channelization protocols.
Frequency-Division Multiple Access (FDMA)Time-Division Multiple Access (TDMA)Code-Division Multiple Access (CDMA)
Topics discussed in this section:
Duplexing
Time Division Duplexing Frequency Division Duplexing
Multiple Access
FDMA Different frequency for different users Multicarrier(MC) FDMA: set of different frequencies
TDMA Different time slots for different users
FHMA Different frequency for different times for different users
governed by code
CDMA Same carriers for different users but modulated differently
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Some medium access control mechanisms for wireless
TDMA CDMAFDMASDMA
Fixed Aloha ReservationsDAMA
MultipleAccess withCollisionAvoidance
Polling
Pure
CSMA
• Used in GSM Slotted
Non-persistent p-persistent CSMA/CA
• Copes with hidden and exposed terminal• RTS/CTS• Used in 802.11 (optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11 (mandatory)
• Used in 802.11 (optional)
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS DSSS
• Used in GSM
Fixed
• Used in Bluetooth • Used in UMTS
Frequency Division Multiple Access (FDMA)
The frequency spectrum is divided into unique frequency bands or channels
These channels are assigned to users on demand
Multiple users cannot share a channel Users are assigned a channel as a pair of
frequencies (forward and reverse channels)
FDMA requires tight RF filtering to reduce adjacent channel interference
Channel-
1
Channel-
6
Channel-
5
Channel-
7
Channel-
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Channel-
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FDMATIM
E
FREQUENCY
Channel-
2
Channel-
3
Channel-
6
Channel-
4
Channel-
5
Channel-
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Channel-
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Channel-
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FDMA Guide Bands
Logical separation FDMA/FDD
f
t
user 1
user n
forward channel
reverse channel
forward channel
reverse channel
...
Logical separation FDMA/TDD
f
t
user 1
user n
forward channel reverse channel
forward channel reverse channel
...
Frequency division multiple access FDMA one phone circuit per channel idle time causes wasting of resources simultaneously and continuously
transmitting usually implemented in narrowband
systems for example: in AMPS is a FDMA
bandwidth of 30 kHz implemented
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FDMA (cont.)
Channels can be assigned on-demand when a user needs to communicate FDD requires user to be assigned forward &
reverse channel TDD only requires single channel per user
FDMA usually used in narrowband systems (e.g., 30 kHz frequency bands) Large symbol time compared to average delay
spread low ISI Little synchronization required because
transmission is continuous in FDMA less overhead than TDMA
Time Division Multiple Access (TDMA) TDMA systems divides the radio spectrum
into time slots, and in each time slot only one use is allowed to either transmit or receive
Transmission for any user is non-continuous
In each TDMA frame, the preamble contains the synchronization information
TDMA shares a single carrier frequency with several users
TDMA could allocate varied number of time slots per frame to different users
TDMA
TIME
FR
EQ
UEN
CY
Channel-
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Channel-
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Channel-
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Channel-
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Channel-
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Channel-
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Channel-
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Channel-
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Channel-
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Channel-
1
TDMA Guard Time
Logical separation TDMA/FDD
f
t
user 1 user n
forward
channel
reverse
channel
forward
channel
reverse
channel
...
Logical separation TDMA/TDD
f
t
user 1 user n
forward
channel
reverse
channel
forward
channel
reverse
channel
...
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TDMA (cont.)
Slot contains Preamble for addressing and synchronization Data Guard times between the slots to reduce cross-
talk between channels
Preamble Slot 1 Slot 2 Slot n Preamble
One Frame
Pream Data Guard Time
Trail Bits Slot 1
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TDMA Disadvantages
Requires guard time between time slots to separate users and accommodate Time inaccuracies due to clock instability Delay spread of transmitted symbols Transmission time delay
Requires signal processing techniques and high overhead for synchronization due to burst transmissions
CDMA
CDMA Family
Logical separation CDMA/FDD
code
f
user 1
user n
forward channel reverse channel
forward channel reverse channel
...
Logical separation CDMA/TDD
code
t
user 1
user n
forward channel reverse channel
forward channel reverse channel
...
Diversity
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Diversity
Fading : Rapid fluctuations of signal strength due to constructive and destructive interference between multi-paths.
Diversity : Technique to compensate for fading channel impairments. It can be obtained over:
Time - Interleaving of coded bits
Frequency – Spread spectrum & frequency hopping
Space – Multiple antennas
OFDM-FDMA and OFDM-TDMA OFDM-FDMA
Each user occupies a subset of subcarriers for a given time. The frequency bands assigned to a specific user is not changed over the time.
OFDM-TDMA Each user occupies more than one OFDM symbols, and transmits on
different time slots.
OFDMA
Each user occupies a subset of subcarriers for a given time. Users should not be overlapped in frequency domain at any given time. But, the frequency bands assigned to a specific user may change over the time.
OFDMA
Multiuser diversity achievable data rate of a given resource
varies from one user to another. assign each resource to the user who can
exploit it best → multiuser diversity. For example, consider an antenna with two
users:
OFDMA – multiuser diversity
Transmitter receives feedback from users
OFDMA – multiuser diversity
For OFDM-TDMA, the SINR on each subcarrier is the average of two users
For OFDMA with resource allocation, each subcarrier are allocated to the specific user that has the best channel frequency response. Thus the SINR for OFDMA is the maximum of two users.
SDMA
Space Division Multiple Access
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Antenna Diversity
Multiple antennas at the base station to transmit the same signal.Fundamental difference :
“Multi-user diversity takes advantage of rather than Compensate fading”
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Opportunistic Beam forming
The information bearing signal at each of the transmit antennais multiplied by a random complex gain.Formation of random beam.
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Dumb Antennas in Action: One User
Most of the time, the beam is nowhere near the user.
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Many users: Opportunistic Beamforming
•In a large system, there is likely to be a user near the beam at any one time.•By transmitting to that user, close to true beamforming performance is achieved, without knowing the locations of the users (but receiving feedback)
Summary