104 Multiple Access

The Mobile Radio Channel and the Cellular Principle Outline
Multiple access techniques are used to allow sharing of a finite amount of radio spectrum by many simultaneous users.
Conflict-free Protocols
Direct Sequence CDMA
Frequency Hopping CDMA
Contention based Protocols
MSs (nodes) have to compete for a shared channel
Each node is attached to a transmitter/receiver which communicates via a channel shared by other nodes
Transmission from any node is received by other nodes in the neighborhood
Shared Multiple
Access Control
Introduction (Cont’d)
Multiple access issues
If more than one node transmit at a time on the control channel to BS, a collision occurs
How to determine which node can transmit to BS?
Multiple access protocols
Contention protocols resolve a collision after it occurs. These protocols execute a collision resolution protocol after each collision
Collision-free protocols ensure that a collision can never occur.
Mobile Communication System
Multiple access protocols
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Frequency Division Multiple Access (FDMA)
The frequency band is divided into channels of equal bandwidth such that each conversation is carried on a different frequency.
Best suited to analog mobile radio.
Single channel per carrier.
BS dynamically assigns a carrier frequency to each active MS.
Used in All first generation cellular systems and early cordless telephones.
Separation of whole spectrum into smaller frequency bands
A channel gets a certain band of the spectrum for the whole time
Advantages:
f
t
c
k2
k3
k4
k5
k6
k1
A channel gets the whole spectrum for a certain amount of time
Advantages:
medium at any time
Time Division Multiple Access (TDMA)
TDMA systems divide the radio spectrum into time slots, and in each slot only one user is allowed to either transmit or receive.
TDMA systems transmit data in a buffer-and-burst method, thus the transmission for any user is noncontinuous.
TDMA is a more expensive technique
needs a highly precise synchronization between transmitter and receiver
Most of second generation systems use TDMA
Features of TDMA
TDMA shares a single carrier frequency with several users, where each user makes use of nonoverlapping time slots
TDMA uses different time slots for transmission and the reception.
Data transmission for users of TDMA system occurs in bursts.
Because of a discontinuous transmission, the handoff process is simpler for a mobile station, as it can listen to the other base stations during idle time slots.
The guard time between the slots is required.
High synchronization overhead is required because of burst transmissions.
Combination of both methods
A channel gets a certain frequency band for a certain amount of time (e.g. GSM)
Advantages:
All channels use the same spectrum
at the same time
Advantages:
good protection against interference and tapping
Disadvantages:
Features of CDMA
All the subscribers in a cell use the same frequency band simultaneously
The subscribers are assigned orthogonal codes to separate the signals.
Some second generation systems use CDMA
Most of third generation systems use CDMA
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Guard time
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MS #1
MS #2
MS #n
Frequency f ’
Note: Ci’ x Cj’ = 0, i.e., Ci’ and Cj’ are orthogonal codes, Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes
Frequency f
Direct Sequence Spread Spectrum(DSSS)
Each user is assigned a unique signature sequence (e.g. pseudorandom sequence), 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
Frequency Hopping Spread Spectrum(FHSS)
A pseudorandom sequence is used to change the radio channel frequency, across a broad frequency band in a random fashion.
CDMA Encoding Example(DSSS)
Encoder Output
CDMA Decoding(DSSS)
Without interfering users, the receiver would receive the encoded bits, Zi,m, and recover the original data bit, di, by computing:
CDMA Decoding Example(DSSS)
(–1,–1,–1,–1,–1,–1,–1,–1)
di = –1
CDMA (Single-user Scenario)
Multi-user Scenario
If there are N users, the signal at the receiver becomes:
How can a CDMA receiver recover a user’s original data bit?
CDMA (Multi-user Scenario)
2-Senders example
Space Division Multiple Access (SDMA)
Space division multiple access (SDMA) controls the radiated energy for each user in space.
SDMA serves different users by using directed antennas (sectorized antennas).
In the future, adaptive antennas will be used to simultaneously steer energy in the direction of many users
ALOHA
Developed in the 1970s for a packet radio network by Hawaii University.
Whenever a station has a data, it transmits. Sender finds out whether transmission was successful or experienced a collision by listening to the broadcast from the destination station. Sender retransmits after some random time if there is a collision.
Slotted ALOHA
Improvement: Time is slotted and a packet can only be transmitted at the beginning of one slot. Thus, it can reduce the collision duration.
Node 2 Packet
Node 3 Packet
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n
The probability that n packets arrive in two packets time is given by
where G is traffic load. (the mean transmission-attempts per frame-time)
The probability P(0) that a packet is successfully received without collision is calculated by letting n=0 in the above equation. We get
We can calculate throughput S with a traffic load G as follows:
The Maximum throughput of ALOHA is
The population of stations attempts to transmit (both new frames and old frames that collided) according to a Poisson distribution.
2
G
Nodes 2 & 3 Packets
In slotted ALOHA, time is slotted and packets must be transmitted within a slot.
From the figure, we can see a collision in slotted ALOHA is a full collision and there is no partial collision.
So the probability of the collision in slotted ALOHA is less than in pure ALOHA.
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The throughput S is
Max throughput achievable by slotted ALOHA is 0.368.
CSMA gives improved throughput compared to Aloha protocols.
Listens to the channel before transmitting a packet (avoid avoidable collisions).
Improvement: Start transmission only if no transmission is ongoing
CSMA/CD (CSMA with Collision Detection)
Improvement: Stop ongoing transmission if a collision is detected
CSMA/CA (CSMA with Collision Avoidance)
Improvement: Wait a random time and try again when carrier is quiet. If still quiet, then transmit
CSMA/CA with ACK
CSMA/CA with RTS/CTS
CSMA/CD (CSMA with Collision Detection)
In CSMA, if 2 terminals begin sending packet at the same time, each will transmit its complete packet (although collision is taking place).
Wasting medium for an entire packet time.
CSMA/CD
Step 1: If the channel is idle, transmit
Step 2: If the channel is busy, continue to listen until the channel is idle then transmit
Step 3: If a collision is detected during transmission, cease transmitting
Step 4: Wait a random amount of time and repeats the same algorithm
All terminals listen to the same channel as CSMA/CD.
Terminal ready to transmit senses the channel.
If channel is busy it waits until the end of current transmission.
It again waits for an additional predetermined time period DIFS (Distributed inter frame Space).
Then picks up a random number of slots (the initial value of backoff counter) within a contention window to wait before transmitting its frame.
If there are transmissions by other terminals during this time period (backoff time), the terminal freezes its counter.
It resumes count down after other terminals finish transmission + DIFS. The terminal can start its transmission when the counter reaches to zero.
station ready to send starts sensing the channel
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Immediate Acknowledgements from receiver upon reception of data frame without any need for sensing the channel.
ACK frame transmitted after time interval SIFS (Short Inter-Frame Space) (SIFS < DIFS)
Receiver transmits ACK without sensing the channel.
If ACK is lost, retransmission done.
Time
CSMA/CA with RTS/CTS
Transmitter sends an RTS (request to send) after channel has been idle for time interval more than DIFS.
Receiver responds with CTS (clear to send) after channel has been idle for SIFS.
Then Data is exchanged.
RTS/CTS is used for reserving channel for data transmission so that the collision can only occur in control message.
CSMA with RTS/CTS
station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the channel, other stations hear the reservation parameter in RTS and set their waiting time)
acknowledgement via CTS after SIFS by receiver (if ready to receive)
sender can now send data at once, acknowledgement via ACK
other stations store channel reservations distributed via RTS and CTS
t
SIFS
DIFS
data
ACK

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104 Multiple Access