8. MultipleAccess

Multiplexing and Multiple Access Spring 2015

ELE 492 – FUNDAMENTALS OF WIRELESS COMMUNICATIONS

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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 users communicate simultaneously. Spring 2015


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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 to different 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. Spring 2015


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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.

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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 be modulated and transmitted over radio.

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FDM/A

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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 user receives a specific bandwidth allocation to access the transponder.

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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 no frequency diversity.  Sensitivity to random Frequency Modulation: due to multipath fading (phase of the received signal is timevarying)  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. If there are few users with very large bandwidths (like in WLAN), disadvantages of FDMA gets less significant. Spring 2015


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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.

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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.

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

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

Ts

 Consider M users

 Define a Communication Resource (CR)  Bandwidth : W Hz

frequency

Ignore any guard band or guard time.

FDMA

User 1

W/M Hz

User 2

W Hz

User M

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

 Period (Frame length): T s

time

 → can be divided into M time slot of duration T/M s each. Ts

 CR can support R bits/s bit rate in total.

T/M s

 Bandwidth W/M Hz,

 Bandwidth W Hz,

 Bit rate R/M bits/s,

 Bit rate R/M bits/s,

 Can transmit RT/M bits over a CR

 Can transmit RT/M bits over a CR

 CR can convey RT/M x M = RT bits Spring 2015

 CR can convey RT/M x M = RT bits ELE 492 – FUNDAMENTALS OF WIRELESS COMMUNICATIONS

User M

 Duration T/M s,

User 2

 Duration T s,

User 1

 Each user has:

frequency

 Each user has:

TDMA

TDMA:

FDMA:

W Hz

time

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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 TDMA is superior to FDMA

transmission time of each packet is

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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:

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Space-Division and Polarization-Division Multiple Access  Multiple different antenna beam patterns can be used to serve multiple users at the same time, over the same bandwidth.

 Similarly, vertical and horizontal polarization can be used for multiplexing/multiple access.

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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!.

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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.

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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, hexagon can represent the shape of a cell in practice.

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

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Cellular Networks  Cell planning with hexagonal cells:  Cell radius:  Distance between the centres of neighbour cells:

i-cells up, k-cells upper left.

 Distance between the centres of two cells:

 Find the values of i and k that make sure that the distance between the two cells 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 …}

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Cellular Networks  Reuse distance (in terms of number cells in between)  Cluster size vs. re-use distance:

3G systems

Strong interference mitigation with signal processing and modulation techniques.

2G systems

1G systems

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Interference mitigation by spatial separation.

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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 dB for 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 the interference 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 Spring 2015


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Methods for Increasing Capacity How 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,

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Methods for Increasing Capacity 6. 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-use is applied to cell edges.

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8. MultipleAccess

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