Chapter 7 Multiple Access Techniques

Multiple Access Techniques
The transmission from the BS in the downlink can be heard by each and every mobile user in the cell, and is referred as broadcasting. Transmission from the mobile users in the uplink to the BS is many-to-one, and is referred to as multiple access.Multiple access schemes to allow many users to share simultaneously a finite amount of radio spectrum resources.Should not result in severe degradation in the performance of the system as compared to a single user scenario.Approaches can be broadly grouped into two categories: narrowband and wideband.

Multiple Access Techniques
Multiple Accessing Techniques : with possible conflict and conflict- freeRandom accessFrequency division multiple access (FDMA)Time division multiple access (TDMA)Spread spectrum multiple access (SSMA) : an example is Code division multiple access (CDMA)Space division multiple access (SDMA)

Multiple Access Techniques -- Narrowband Systems
Transmission experiences nonselective fading. This means that when fades occur, all of the information (i.e. the whole channel) is affected.Channel system : generally total spectrum is divided into a number of relatively narrow radio channels (e.g. FDMA). Occurrence of call blocking if channels are all being used. Unused bandwidth in each channel cannot be used by other users.

Narrowband Systems (Interleaving + coding techniques are used)
Data generated in source 1 2 3 4 5 8 9 10 11 12 15 16 17 18 Transmitted data 1 8 15 22 29 2 9 16 23 3 10 17 24 Received data 1 8 15 22 29 2 9 16 23 30 3 10 17 24 31 with the shaded data bits undergo deep fadeDeinterleaved data 1 2 3 4 5 8 9 15 16 17 18

Multiple Access Techniques -- Wideband Systems
The main feature of wideband systems is that either all the spectrum available (e.g. CDMA, TDMA) or a considerable portion of it is used by each user (e.g. TDMA+FDMA).The advantage of wideband systems is that the transmission bandwidth always exceeds the coherence bandwidth for which the signal experiences only selective fading. That is, only a small fraction of the frequencies composing the signal is affected by fading.Signal can be distorted and therefore equalization is needed but unlikely that a total signal fade will occur.

Duplexing
For voice or data communications, must assure two way communication (duplexing, it is possible to talk and listen simultaneously). Duplexing may be done using frequency or time domain techniques.Forward (downlink) band provides traffic from the BS to the mobileReverse (uplink) band provides traffic from the mobile to the BS.

Frequency division duplexing (FDD)
Provides two distinct bands of frequencies for every user, one for downlink and one for uplink.A large interval between these frequency bands must be allowed so that interference is minimized.
Reverse

Channel

Forward

Channel

frequency

fc,R

fc,,F

Frequency separation

Frequency separation should be carefully decided

Frequency separation is constant

Time division duplexing (TDD)
In TDD communications, both directions of transmission use one contiguous frequency allocation, but two separate time slots to provide both a forward and reverse link.Because transmission from mobile to BS and from BS to mobile alternates in time, this scheme is also known as ping pong.As a consequence of the use of the same frequency band, the communication quality in both directions is the same. This is different from FDD.

Time division duplexing (TDD)

F

R

R

R

R

0 1 2 3 4 5 6 7

.

Reverse

Channel

Forward

Channel

time

Ti

Time separation

Ti+1

channel

Slot number

F

F

F

Random Access -- Aloha systems
Aloha is a packet-switching system. The time interval required to transmit one packet is called a slot.When transmissions from two or more users overlap, they destroy each other, whether it is complete overlap or partial overlap collision takes place.The maximum interval over which two packets can overlap and destroy each other is called the vulnerable period.The mode of random access in which users can transmit at anytime is called pure Aloha. In this case, the vulnerable period is two slot times.A version in which users are restricted to transmit only from the instant corresponding to the slot boundary is referred to as slotted Aloha. The alignment of transmission to coincide with the slot boundary means that packets can only experience complete overlap, so the vulnerable period is only one slot time.

Random Access -- Aloha systems

Throughput of Aloha systems
In the Aloha systems, a transmitting user upon hearing a collision, backs off for a random delay interval and transmits again, until success is achieved. This is known as collision resolution.We assume that the total traffic generated for transmission (including re-transmission) obeys a Poisson distribution.The transmission is successful when there is no other packet transmission during the vulnerable period.

P (success) =P (no other packet transmission occurs within a vulnerable period)Let S be the throughput, defined as the successfully transmitted traffic load, and G be the total offered channel traffic load. Assuming that traffic generated for transmission obeys a Poisson distribution, then
P (no other packet transmission occurs) = exp(G)

where is the vulnerable period.

Hence P (success) = exp (G)

By definition P (success) = S /G,

therefore S = G exp (G)

Delay
The delay experienced by a packet in the system is measured from the instant of the packets arrival until the instant the sender receives confirmation.It is a function of the number of transmission, the retransmission delay and the time required for the sender to receive confirmation of successful transmission.

Delay
The mean number of transmission until successThe average packet delay for pure AlohaThe average packet delay for slotted AlohaThe mean re-transmission delay depends on the collision resolution algorithm used.

FDMA
In FDMA, each user is allocated a unique frequency band or channel. During the period of the call, no other user can share the same frequency band.

FDMA
All channels in a cell are available to all the mobiles. Channel assignment is carried out on a first-come first- served basis.The number of channels, given a frequency spectrum BT , depends on the modulation technique (hence Bw or Bc ) and the guard bands between the channels 2Bguard . These guard bands allow for imperfect filters and oscillators and can be used to minimize adjacent channel interference.FDMA is usually implemented in narrowband systems.

Main features
Continuous transmission : the channels, once assigned, are used on a non-time-sharing basis. This means that both subscriber and BS can use their corresponding allotted channels continuously and simultaneously.Narrow bandwidth : Analog cellular systems use 25-30 kHz. Digital FDMA systems can make use of low bit rate speech coding techniques to reduce the channel band even more.If FDMA channels are not in use, then they sit idle and cannot be used by other users to increase capacity.Low ISI : Symbol time is large compared to delay spread. No equalizer is required (Delay spread is generally less than a few s flat fading).

Main features
Low overhead : Carry overhead messages for control, synchronization purposes. As the allotted channels can be used continuously, fewer bits need to be dedicated compared to TDMA channels.Simple hardware at mobile unit and BS : (1) no digital processing needed to combat ISI (2) ease of framing and synchronization.Use of duplexer since both the transmitter and receiver operate at the same time. This results in an increase in the cost of mobile and BSs.FDMA required tight RF filtering to minimize adjacent channel interference.

Nonlinear effects in FDMA
In a FDMA system, many channels share the same antenna at the BS. The power amplifiers or the power combiners, when operated at or near saturation are nonlinear.The nonlinear ties generate inter-modulation frequencies.Undesirable harmonics generated outside the mobile radio band cause interference to adjacent services.Undesirable harmonics present inside the band cause interference to other users in the mobile system.

TDMA
TDMA systems divide the channel time into frames. Each frame is further partitioned into time slots. In each slot only one user is allowed to either transmit or receive.Unlike FDMA, only digital data and digital modulation must be used.Each user occupies a cyclically repeating time slot, so a channel may be thought of as a particular time slot of every frame, where N time slots comprise a frame.

Features
Multiple channels per carrier or RF channels.Burst transmission since channels are used on a timesharing basis. Transmitter can be turned off during idle periods.Narrow or wide bandwidth depends on factors such as modulation scheme, number of voice channels per carrier channel.High ISI Higher transmission symbol rate, hence resulting in high ISI. Adaptive equalizer required.

Features
High framing overhead A reasonable amount of the total transmitted bits must be dedicated to synchronization purposes, channel identification. Also guard slots are necessary to separate users.

TDMA Frame

Preamble

Information

Trail Bits

Slot 1

Slot 2

Slot 3

Slot N

Guard

Bits

Sync

Bits

CRC

Information

One TDMA Frame

One TDMA Slot

A Frame repeats in time

Control

Bits

Features
A guard time between the two time slots must be allowed in order to avoid interference, especially in the uplink direction. All mobiles should synchronize with BS to minimize interference.

Features
The use of digital technology permits the inclusion of several facilities in the mobile unit, increasing its complexity. One example is the use of slow frequency hopping to counteract multi-path fading.Flexible data rates by assigning multiple time slots to different users based on their demand.No duplexer used since uses different time slots for transmission and reception. Even if FDD is used, a switch rather than a duplexer is required.Lower BS cost : the use of multiple channels per carrier channel provides a proportional reduction of the equipment at the BS.

Features
Efficient power utilization : FDMA systems require a 3- to 6-dB power back off in order to compensate for inter-modulation effects.Efficient handoff : TDMA systems can take advantage of the fact that the transmitter is switched off during idle time slots to improve the handoff procedure. An enhanced link control, such as that provided by mobile assisted handoff (MAHO) can be carried out by a subscriber by listening to neighboring base station during the idle slot of the TDMA frame.

Efficiency of TDMA
Efficiency of TDMA is a measure of the percentage of bits per frame which contain transmitted data. The transmitted data include source and channel coding bits.bOH includes all overhead bits such as preamble, guard bits, etc.

TDMA/TDD and TDMA/FDD
In TDMA/TDD system, half of the time slots in the frame information message would be used for the forward link channels and half would be used for reverse link channels. Same channel conditions.In TDMA/FDD systems, same frame structure can be used for both forward and reverse transmission but carrier frequencies used are different.

TDMA/TDD and TDMA/FDD

Cellular Systems and MAC
Cellular SystemMultiple Access TechniqueAMPSFDMA/FDDGSMTDMA/FDDUSDC (IS-54 and IS-136)TDMA/FDDPDCTDMA/FDDCT2 Cordless PhoneFDMA/TDDDECT Cordless PhoneFDMA/TDDUS IS-95CDMA/FDDW-CDMACDMA/FDDCDMA/TDDcdma2000CDMA/FDDCDMA/TDD

Synchronous and asynchronous TDMA
Previously we mentioned the same slot of all the frames are assigned to the same user. The slot assignment can actually be fixed or dynamic.If the assigned slot is fixed from frame to frame for the duration of the connection, the users have to synchronize to their respective assigned slots. This mode of TDMA is referred to as STDMA.With packet-switched transmission, it is more efficient to allow a user to transmit only when it has packets to send. Transmission slots are dynamically assigned from frame to frame. This mode is known as ATDMA.A TDMA is implemented using a reservation access mechanism.

SSMA
Spread spectrum systems : The desired signal is transmitted over a bandwidth which is much larger than the Nyquist bandwidth. It is first developed for military applications forSecurityUndetectability: minimum probability of being detectedRobust against intentional jammers

Applications
SecurityRobust against unintentional interferenceIt is not bandwidth efficient when used by a single user but has the capability to overcome narrowband jamming signals (cannot overcome AWGN or wideband jamming signal) and multi-path.Providing multiple access If many users can share the same spread spectrum bandwidth without interfering with one another, bandwidth efficient improved but will affect the capability to overcome jamming.

Spread Spectrum Access
Two techniquesFrequency Hopped Multiple Access (FHMA)Direct Sequence Multiple Access (DSMA) Also called Code Division Multiple Access CDMA
*

Frequency Hopping (FHMA)
Digital muliple access techniqueA wideband radio channel is used. Same wideband spectrum is usedThe carrier frequency of users are varied in a pseudo-random fashion.Each user is using a narrowband channel (spectrum) at a specific instance of time. The random change in frequency make the change of using the same narrowband channel very low.
*

Frequency Hopping (FHMA)
The sender receiver change frequency (calling hopping) using the same pseudo-random sequence, hence they are synchronized. Rate of hopping versus Symbol rate If hopping rate is greather: Called Fast Frequency HoppingOne bit transmitted in multiple hops. If symbol rate is greater: Called Slow Frequency HoppingMultiple bits are transmitted in a hopping periodGSM and Bluetooth are example systems

EE 552/452 Spring 2007

Code Division Multiple Access (CDMA)
In CDMA, the narrowband message signal is multiplied by a very large bandwidth signal called spreading signal (code) before modulation and transmission over the air. This is called spreading. CDMA is also called DSSS (Direct Sequence Spread Spectrum). DSSS is a more general term. Message consists of symbols Has symbol period and hence, symbol rate
*

Code Division Multiple Access (CDMA)
Spreading signal (code) consists of chipsHas Chip period and and hence, chip rateSpreading signal use a pseudo-noise (PN) sequence (a pseudo-random sequence)PN sequence is called a codewordEach user has its own cordwordCodewords are orthogonal. (low autocorrelation)Chip rate is oder of magnitude larger than the symbol rate. The receiver correlator distinguishes the senders signal by examining the wideband signal with the same time-synchronized spreading code The sent signal is recovered by despreading process at the receiver.

CDMA Advantages
Low power spectral density. Signal is spread over a larger frequency bandOther systems suffer less from the transmitterInterference limited operationAll frequency spectrum is usedPrivacyThe codeword is known only between the sender and receiver. Hence other users can not decode the messages that are in transitReduction of multipath affects by using a larger spectrum
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CDMA Advantages
Random access possibleUsers can start their transmission at any timeCell capacity is not concerete fixed like in TDMA or FDMA systems. Has soft capacityHigher capacity than TDMA and FDMA No frequency managementNo equalizers neededNo guard time neededEnables soft handoff

CDMA Principle

1

0

1

1 1 1 0 1 0 1 1

1 1 1 0 1 0 1 1

Chip period

One bit period (symbol period)

Data

Coded

Signal

Input to the modulator (phase modulation)

Represent bit 1 with +1

Represent bit 0 with -1

*

Processing Gain
Main parameter of CDMA is the processing gain that is defined as:
Gp: processing gain

Bspread: PN code rate

Bchip: Chip rate

R: Data rate
IS-95 System (Narrowband CDMA) has a gain of 64. Other systems have gain between 10 and 100. 1.228 Mhz chipping rate1.25 MHz spread bandwidth
*

Near Far Problem and Power Control
At a receiver, the signals may come from various (multiple sources. The strongest signal usually captures the modulator. The other signals are considered as noiseEach source may have different distances to the base station
B

M

M

M

M

pr(M)

*

Near Far Problem and Power Control
In CDMA, we want a base station to receive CDMA coded signals from various mobile users at the same time. Therefore the receiver power at the base station for all mobile users should be close to eacother. This requires power control at the mobiles. Power Control: Base station monitors the RSSI values from different mobiles and then sends power change commands to the mobiles over a forward channel. The mobiles then adjust their transmit power.

DSSS Transmitter

+

PN Code

Generator

Chip Clock

Baseband

BPF

Oscillator

fc

Message

sss(t)

m(t)

p(t)

Transmitted

Signal

*

DSSS Receiver

IF Wideband

Filter

PN Code

Generator

Phase Shift Keying

Demodulator

Synchronization

System

Received

Data

Received

DSSS Signal

at IF

*

Spectra of Received Signal

Frequency

Frequency

Spectral
Density

Spectral
Density

Signal

Interference

Interference

Signal

Output of Wideband filter

Output of Correlator after

dispreading,

Input to Demodulator

*

CDMA Example

R

A

B

Receiver (a base station)

Transmitter (a mobile)

Transmitter

Codeword=010011

Codeword=101010

Data=1011

Data=0010

Data transmitted from A and B is multiplexed using CDMA and codeword.

The Receiver de-multiplexes the data using dispreading.

*

CDMA Example transmission from two sources

1 0 1 0 1 0

1 0 1 0 1 0

1 0 1 0 1 0

0 1 0 0 1 1

0 1 0 0 1 1

0 1 0 0 1 1

0 1 0 0 1 1

1 0 1 1 0 0

0 1 0 0 1 1

1 0 1 1 0 0

0 0 1 0

1 0 1 0 1 0

1 0 1 0 1 0

1 0 1 0 1 0

1 0 1 0 1 0

0 1 0 1 0 1

1 0 1 1

1 0 1 1 0 0

Transmitted

A+B

Signal

A Data

A

Codeword

B Data

B

Codeword

A Signal

B Signal

*

CDMA Example recovering signal A at the receiver

0 1 0 0

A+B

Signal

received

A

Codeword

at

receiver

Integrator

Output

Comparator

Output

Take the inverse of this to obtain A

*

CDMA Example recovering signal B at the receiver

1 1 0 1

A+B

Signal

received

B

Codeword

at

receiver

Integrator

Output

Comparator

Output

Take the inverse of this to obtain B

*

CDMA Example using wrong codeword at the receiver

X 0 1 1

Noise

A+B

Signal

received

Wrong

Codeword

Used at

receiver

Integrator

Output

Comparator

Output

Wrong codeword will not be able to decode the original data!

*

Hybrid Spread Spectrum Techniques
FDMA/CDMAAvailable wideband spectrum is frequency divided into number narrowband radio channels. CDMA is employed inside each channel. DS/FHMAThe signals are spread using spreading codes (direct sequence signals are obtained), but these signal are not transmitted over a constant carrier frequency; they are transmitted over a frequency hopping carrier frequency.
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Hybrid Spread Spectrum Techniques
Time Division CDMA (TCDMA)Each cell is using a different spreading code (CDMA employed between cells) that is conveyed to the mobiles in its range. Inside each cell (inside a CDMA channel), TDMA is employed to multiplex multiple users.Time Division Frequency HoppingAt each time slot, the user is hopped to a new frequency according to a pseudo-random hopping sequence. Employed in severe co-interference and multi-path environments. Bluetooth and GSM are using this technique.
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Capacity of CDMA Systems
Uplink Single-cell System Model
...

.

.

.

...

.

.

.

User 1

User 2

User k

User Ku

User n

Assumptions
Total active users Ku The intra-cell MAI can be
modeled as AWGN
Perfect power control is assumed Random sequences
*

BTS

Coarse estimate of the reverse link (uplink) capacityAssumptions:Single Cell.The interference caused by other users in the cell can be modeled as AWGN.Perfect power control is used, i.e. the received power of each user at the base station is the same.If the received power of each user is Ps watts, and the background noise can be ignored (ex: micro-cells), then the total interference power (MAI) at the output of the desired users detector is
where Ku is the total number of equal energy users in the cell. Suppose each user can operate against Gaussian noise at a bit-energy-to-noise density level of Eb/Io. Let W be the entire spread bandwidth, then the interference spectral density can be expressed as:

*

EE 552/452 Spring 2007


Interference

limited

Also, the bit energy Eb is

Thus,

Now, if we consider the factors of voice activity (Gv), sectorized

antenna gain (GA), and other-cell interface factor (f), where

Gv 1/v = 2.67

GA (three sectors) 2.4

f = (Interference form other cells)/(Interference from given cell) 0.6

*


In this case, Ku can be approximated by

Ex: If Gv 2.67, GA 2.4, f 0.6

If (Eb/Io) required is 6 dB (i.e. Eb/Io = 4)

which will be larger than the TDMA or FDMA systems in the cellular environment.

*

SDMA
Use spot beam antennas The different beam area can use TDMA, FDMA, CDMA Sectorized antenna can be thought of as a SDMA Adaptive antennas can be used in the future (simultaneously steer energy
in the direction of many users)

spot beam

antenna

*

Features:
A large number of independently steered high-gain beams can be formed without any resulting degradation in SNR ratio.Beams can be assigned to individual users, thereby assuring that all links operate with maximum gain.Adaptive beam forming can be easily implemented to improve the system capacity by suppressing co channel interference.
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Chapter 7 Multiple Access Techniques