multiplexing and multiple access - hacettepe universitytoker/ele492/8. multipleaccess.pdf ·...

Multiplexing andMultiple Access

Spring 2017 ELE 492 – FUNDAMENTALS OF WIRELESS COMMUNICATIONS 1

Multiplexing and Multiple Access Until now we considered the communication from a single TX and

single RX.

Consider GSM: In a cell, there is a single BS but tens (hundreds) of users, how is that

possible?

Communication Resources: Time

Bandwidth

Polarization

Code

Space

etc.

Power

The idea is to share the resources among users to let multiple userscommunicate simultaneously.


Multiplexing and Multiple Access Frequency Division (FD): specified (sub)bands of frequency are allocated to different users,

Time Division (TD): Time slots are allocated to different users,

Code Division (CD): set of orthogonal or nearly orthogonal spread spectrum codes is allocated todifferent users,

Space Division (SD): Antenna beam patterns are used to differentiate different users,

Polarization Diversity (PD): Orthogonal polarizations are used to separate users.

Allocation can be fixed: subband, slot, etc. of each users is determined beforehand.

or dynamic: allocation can be determined according to channel conditions or user load.


FDM/A (Frequency Division Multiplexing/Multiple Access)

Spectrum is divided into several subbands The signal of each user is upconverted to its subband by a frequency mixer.

Each subband is separated by a guard band to mitigate interuser (intercarrier) interference A time limited signal cannot be band limited.


FDM/A Consider the telephony system. How can thousands of users can communicate simultaneously?

Typically speech is contained within 300-3000 Hz band.

Hierarchical grouping:

A single channel (conversation) 300-3400 Hz

First level group of 12 channels 60-108 kHz

Second level group 60 channels Supergroup

312-552 kHz

Supergroup can further bemodulated and transmittedover radio.


FDM/A


FDM/A Most of the communication satellites are GEO (geostationary/geosynchronous)

Most of the satellites contain transponders (nonregenerative repeaters) Received signal is amplified, frequency shifted and retransmitted on the downlink without any further

processing.

C-band, 6 GHz uplink carrier, 4 GHz downlink carrier,

Signal bandwidth 500 MHz,

12 transponders of 36 MHz bandwidth each,

FDM/FM/FDMA is used on each transponder

FDM: SSB signals are FDM’ed to form a multichannel composite signal similar to telephone signals,

FM: The composite signal is FM modulated onto a carrier and transmitted to the satellite,

FDMA: Subdivision of the 36 MHz transponder bandwidth may assigned to different users. Each userreceives a specific bandwidth allocation to access the transponder.


FDM/A Advantages and disadvantages of FDM: FDMA has a simple TX/RX structure, require little (digital) signal processing.

It does not require synchronization or central timing.

Each subband is almost independent of all other channels.

Frequency synchronization and stability are difficult: local oscillators must be very accurate.

Sensitivity to fading: If a subband goes into a (spectral) fade, there is no way to recover it. There is nofrequency diversity.

Sensitivity to random Frequency Modulation: due to multipath fading (phase of the received signal is time-varying)

Intermodulation: Non-linearity of the power amplifier at the TX causes third-order modulation products.

Lower spectral efficiency: due to guard-bands. No information is conveyed over them.

FDM(A) is mostly used in: Analog communication systems: FDMA is almost the only practical choice.

Combination of FDMA with other MA methods: like in GSM, FDMA is used in combination with TDMA.

High-data-rate systems: FDMA is disadvantageous when there are hundreds of users with narrow bands. Ifthere are few users with very large bandwidths (like in WLAN), disadvantages of FDMA gets less significant.


TDM/A (Time Division Multiplexing/Multiple Access) Time is divided into slots,

Against syncronization problems and ISI, guard times are places between slots.

Each slot is assigned to a user,

All users are allowed to occupy all the available bandwidth

Multiple slots may join to form a frame.


TDM/A Slots may be assigned in a fixed or dynamic way. If there is not enough traffic generated by the users, fixed assignment may waste resources.


FDMA + TDMA FDMA and TDMA can further be combined: GSM, DECT, LTE, WiMAX


Performance Comparison of FDMA and TDMA Bit Rate Provided by TDMA and FDMA

Consider M users.

Ignore any guard band or guard time.

Define a Communication Resource (CR) Bandwidth : W Hz

→ can be divided into M subchannel of bandwidth W/M Hz each.

Period (Frame length): T s → can be divided into M time slot of duration T/M s each.

Channel can support R bits/s/Hz bit rate (spectral efficiency).


User 1

time

freq

uen

cy User 2

User M

T s

W Hz

W/M Hz

Use

r 1

time

fre

qu

en

cy

Use

r 2

Use

r M

T s

W Hz

T/M s

FDMA: Each user has:

Duration T s,

Bandwidth W/M Hz,

Bit rate R xW/M bits/s,

Can transmit RxWxT/M bits per user

CR can convey RxWxT/M x M = RxWxT bits

TDMA: Each user has:

Duration T/M s,

Bandwidth W Hz,

Bit rate RxW bits/s,

Can transmit RxWxT/M bits over a CR

CR can convey RxWxT/M x M = RxWxT bits

FDM

ATD

MA

Performance Comparison of FDMA and TDMA Message Delays in FDMA and TDMA

Message delay:

w: average packet waiting time before transmission, τ: packet transmission time

FDMA: there is no waiting time, each packet is transmitted over T sec →

TDMA: average waiting time of a packet is

transmission time of each packet is


TDMA is superior to FDMA

Code Division Multiple Access Each user is assigned a unique spreading code.

Codes are expected to have good cross-correlation properties, ideally uncorrelated. Walsh-Hadamard codes:

Each row gives a code orthogonal to the others.

Example: Walsh-Hadamard codes for 4 users:


Space-Division and Polarization-Division Multiple Access

Multiple different antenna beampatterns can be used to servemultiple users at the same time,over the same bandwidth.

Similarly, vertical and horizontalpolarization can be used formultiplexing/multiple access.


Cellular Networks How can we increase the coverage area and the number of users to be served of a wireless system? We can increase coverage area by increasing TX power.

signal attenuates with n-2 at best. Waste of energy!

A TDMA or FDMA-only structure may not be adequate. They have limitations on the number of users.

Using a cellular network can be the solution: There are geographically separated BSs (base stations).

Can increase coverage without waste of energy.

Each BS has its own set of users. Can increase the number of users to be served.

If the BSs are close to each other (which IS the case), the two cells cause interference to each other. Consider the downlink of a MS (user) just at the boundary of two cells. Distance of the MS from both BSs are comparable.

Assume that MS1 is «registered» to BS1. Then it wants to demodulate the signal coming from BS1.

However, BS2 will transmit another signal to its registered MS2. This transmission will interfere the communication between BS1 and MS1.

Similar problem in the uplink.

The same frequency cannot be used at two neighbour cells!.


Cellular Networks Solution of the interference problem: Use different frequencies in neighbour cells.

Same frequency can be «re-used» only if two cells are separated at least by re-use distance, D (m). R: cell radius → re-use distance = D/R (cells)

Re-use distance is determined by link budget analysis.

Cluster: Group of cells that all use different frequencies.

Cluster size: Number of cells in a cluster. Lower cluster size, higher spectral efficiency.


Cellular Networks What is the shape of a cell: Circular cell: Circles (disks) can cover an area efficiently, but there remain gaps in between them.

Hexagon: is the best shape to cover an area without any gaps. Theoretically and ideally hexagon is used as the shape of a cell.

In practice, hexagon is hardly suitable to cover an area. Only if the terrain is completely flat and there are no obstacles around, hexagoncan represent the shape of a cell in practice.

Cell planning: Received power is measured over a geographical region and best BS places are chosenaccordingly.


Cellular Networks Cell planning with hexagonal cells: Cell radius:

Distance between the centres of neighbour cells:

Distance between the centres of two cells:

Find the values of i and k that make sure that the distance between the twocells is larger than the required reuse distance. At the same time, minimizing the cluster size (i.e. Minimizing waste of spectrum or equivalently

maximizing spectral efficiency).

Relation between cluster size N and the parameters i and k is:

N has to be integer → the only values it can take {1, 3, 4, 7, 9, 12, 13, 16 …}


i-cells up,k-cells upper left.

Cellular Networks Reuse distance (in terms of number cells in between)

Cluster size vs. re-use distance:


1G systems

2G systems

3G systems

Strong interference mitigationwith signal processing andmodulation techniques.

Interference mitigationby spatial separation.

Cellular Networks Cell planning: 1. From the specification for the minumum transmission quality, find the minimum distance between the desired

BS and interfering BS from link budget calculations.

2. Find the minimum cluster size (from slides 20 and 21)

3. Find the frequencies for each cell (from slide 20)

Example: For the sake of simplicity, consider the AMPS system. Each channel is 30 kHz wide, SIR = 18 dBfor satisfactory speech quality. Fading margin is 15 dB. Assume that power decreases by d-4.

→ at the cell boundary the mean values of the signal power must be 33 dB (2x103) stronger than theinterference power.

→ distance between the desired BS and the interfering BS is D/R=7.7.

→ from the table, the smallest «integer» cluster size >19.8 is 21

→ for a 5MHz spectrum, there are 5MHz/30kHz=167 possible freq. channels → 167/21=8 freq. chnl/cell


Methods for Increasing CapacityHow can we increase the number of users being served? or the data rate?

1. Increase the amount of spectrum used: Very expensive

2. More efficient, higher order modulation formats: Use modulation formats which require less bandwidth (with higher order modulation) and more resistant

to interference.

Typically, higher order modulation formats are more sensitive to noise and interference.

3. Advanced interference mitigation and coding: Possible, hot research topic.

4. Better source coding: Compression of speech, video and data streams allows more users to be served with the same resources.

5. Adaptive Modulation and coding, scheduling,


Methods for Increasing Capacity6. Use of sector cells: the hexagon is divided into several sectors (3 or 6). Each sector is served by one antenna. System capacity increases x3 or x6 times (theoretically).

7. Multiple antennas: i) can generate diversity, ii) MIMO

8. Partial frequency reuse: Same frequency is used at cell centres, conventional frequency re-useis applied to cell edges.


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