Last Hop CommunicationLast Hop Communication

WIRE-LINE

NETWORK

Base Station (BS)Cell

Wireless LinksWired Links

Mobile Switching Center (MSC)

Mobile unit

ISDN/PSTN/Internet

Cellular FrameworkCellular Framework

MSC/VLR MSC/VLRBSC BSC

BTS

BTS

Local SwitchCellular Network

PSTN Network

Mobile Terminal

Air Link

HLR

Terms to remember MSC: Mobile Switching Center VLR: Visiting Location Register HLR: Home Location Register BSC: Base Station Controller BTS: Base Transmitter Station Mobile Terminal Air Link

Cell: geometric representation of areas. Geographic area is divided into cells, each serviced by an antenna called base station (BS)

Mobile Switching Center (MSC) controls several BSs and serves as gateway to the backbone network (PSTN, ISDN, Internet)

WHY CHANNEL REUSE? Limited number of frequency spectrum allocated by FCC and remarkable

growth of mobile (wireless) communication users

Frequency band allocated by FCC to the mobile telephone system is 824-849 MHz for transmission from mobiles (uplink) and 869-894 MHz for transmission from base stations (downlink)

With a channel spacing of 30 KHz, this frequency band can accommodate 832 duplex channels

Frequency Reuse: use same carrier frequency or channel at different areas (cells) avoiding co-channel interference

Number of simultaneous calls (capacity) greatly exceeds the total number of frequencies (channels) allocated

Channel assignment problemChannel assignment problem

Given a set of channels (F) and a set of base stations (B) in the coverage area

Goal: Determine an assignment of channel(s) to base stations such that frequency reuse is maximized

Frequency AssignmentFrequency Assignment

Determine the minimum distance at which a frequency can be reused with no interference, called the co-channel reuse distance (D)

Signal-to-interference ratio, (C / I), is an index of channel interference

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Shift Parameter: i = 3, j = 2Shift Parameter: i = 3, j = 2

Nearest Co-channel CellsNearest Co-channel Cells Determine two parameters i and j, called shift parameters

Starting from any cell, move i cells along any one of the six emanating chains of hexagons. Turn clock (or counter clock)-wise by 60 degree and then move j cells along the chain. The destination is the nearest co-channel cell

(For each cell, there are two sets of six nearest co-channel cells)

By repeating this pattern, clusters of cells are formed such that each cell within a cluster is assigned a different set of frequencies. Such a cluster is called a frequency reuse pattern

Number of cells in a reuse pattern is given by

This is the number of different channel sets required

Possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, …..for i, j = 0, 1, 2, …

22N = + +N = + +ii 22jji ji j

Nearest Co-channel Cells (Contd.)Nearest Co-channel Cells (Contd.) Let D = distance between two co-channel cells and R = cell radius Co-channel reuse ratio is D/R = 3N for hexagons Carrier signal-to-interference ratio

If minimal allowable C / I is known, D / R and hence the size of the compact pattern can be derived

C / I C / I (D / R) / 6 for hexagon (D / R) / 6 for hexagon44

ParameterParameter Cluster Cluster Size (N)Size (N)

Reuse Ratio Reuse Ratio (D/R)(D/R)

i = 1, j = 1i = 1, j = 1 33 33

i = 1, j = 2i = 1, j = 2 77 4.584.58

i = 2, j = 2i = 2, j = 2 1212 66

i = 1, j = 3i = 1, j = 3 1313 6.246.24

Fixed Channel Assignment (FCA)Fixed Channel Assignment (FCA) Permanently assign a fixed set of channels to each cell and

reuse them in the co-channel cells

A user is assigned an unoccupied channel on demand, which is relinquishes after the call is over

If the number of calls exceeds the channel set for a cell, the excess calls are blocked

To maximize reuse of the available frequency channels, various graph coloring techniques have been proposed

Static channel assignment schemes perform well under heavy traffic conditions

Does not solve “hot spot” problem

No cell has a proprietary set of channels. Channels are allocated to a user on demand from a central pool on the basis of a cost function

Random DCA- Randomly assign an available channel -- poor channel utilization

Channel Ordering- A cell can use any channel, but each has a different ordering. Select a channel with the highest priority for the cell

Dynamic Channel Assignment (DCA)Dynamic Channel Assignment (DCA)

A set of permanent channels is allocated to each cell. When there is a channel storage in a cell, channels are assigned from a set of flexible channels according to a dynamic assignment strategy

Flexible channels can be distributed among cells in a scheduled or predictive manner

Scheduled distribution assumes knowledge of future changes in traffic distribution, while predictive scheme continuously monitors the traffic in each cell so that flexible channel reallocation can be done at any time

Hybrid Channel Assignment (HCA)Hybrid Channel Assignment (HCA)

Hand-off is the process of switching from one frequency channel to another by the user in midst of a communication

Normally induced by the quality of the ongoing communication channel parameters: Received Signal Strength (RSS), Signal-to-Noise Ratio (SNR) and Bit Error Rate (BER)

RSS attenuates due to the distance from BS, slow fading (shadow or lognormal fading), and fast fading (Rayleigh fading)

Hand-offs are triggered either by the BS or the mobile station itself

Hand-off ProblemHand-off Problem

BS-1BS-1 BS-2BS-2

Handoff TypesHandoff Types

Intra-Cell Inter-Cell

Soft Handoff Hard Handoff

The quality of the RSS from the mobile station is monitored by the BS. When the RSS is below a certain threshold. BS instructs the mobile station to collect signal strength measurements from neighboring BSs

Case 1: mobile station sends the collected information to the BS.BS conveys the signal information to its parent MSC (mobile switching center) which selects the most suitable next BS for the mobile stationBoth the selected BS and the mobile station are informed when new BS assigns an unoccupied channel to the mobile station

Case 2: mobile station itself selects the most suitable BS.The mobile station informs the current BS, who conveys information about the next BS to its MSCThe selected BS is informed by the MSC which assigns a new channel

Hand-off: Who Triggers?Hand-off: Who Triggers?

BS handles hand-off requests in the same manner as originating calls- Disadvantage: Ignores the fact an ongoing call has higher priority for a new channel than originating calls

- Solution: Prioritize hand-off channel assignment at the expense of tolerable increase in call blocking probability

Guard channel concepts (Prioritizing Handoffs) - Reserve some channels exclusively for hand-offs. Remaining channels shared equally between hand-offs and originating calls

- For fixed assignment. Each cell has a set of guard channels. While for dynamic assignment, channels are assigned during hand-off from a central pool

- Disadvantages: -- Penalty in reduction of total carried traffic. Since fewer channels are available for originating calls. Can be partially solved by queuing up blocked originating calls

-- Insufficient spectrum utilization – need to evaluate an optimum number of guard channels.

Hand-off PoliciesHand-off Policies

Capacity Improvement and Interference ReductionCapacity Improvement and Interference Reduction

There is a close correspondence between the network capacity (expressed by N) and the interference conditions (expressed by C/I)

Cell sectoring reduces the interference by reducing the number of co-channel interferers that each cell is exposed to. For example, for 60 degrees sectorization, only one interferer is present, compared to 6 in omnidirectional antennas. But, cell sectorization also splits the channel sets into smaller groups

Cell splitting allows to create more smaller cells. Thus, the same number of channels is used for smaller area. For the same probability of blocking, more users could be allocated

Cell Splitting: ExampleCell Splitting: Example

3322

11

4477

66

55

33

22

11

44

77

66

55

3322

11

4477

6655

Advantages: more capacity, only local redesign of the system Disadvantages: more hand-offs, increased interference levels, more

infrastructures

Cell SectoringCell Sectoring

Advantages and disadvantages of1. Cell splitting2. Cell sectoring

Cell sizing (Micro-cellular/Pico-cellular Systems)- To allow more capacity, the size of the cells are scaled down

- Since the quality of service (C/I) depends only on the ratio (D/R), the performance (i.e., interference level) is unaffected by the scaling

- However, the same number of channels can now be used in a smaller area (i.e., larger user density), increasing the total number of concurrent users.

- Smaller cells also imply less transmitted power – thus smaller and lighter handsets are possible. However, smaller cells also imply larger hand-off rate

- Cell size reduction leads to the micro cells (size of a shopping mall) and pico cells (size of a building floor)

- There is a trade-off between bandwidth and coverage area, transceivers with lower range can cover higher bandwidths

Capacity Improvement and Interference ReductionCapacity Improvement and Interference Reduction

Cell Sizing: ExampleCell Sizing: Example

Case 1:Cell radius = 1 mileNumber of cells = 32Number of channels = 336Reuse factor = 7 48 channels per cell 1536 concurrent calls

Case 2:Cell radius = 0.5 mile ( = 0.5)Number of cells = 128Number of channels = 336Reuse factor = 7 48 channels per cell 6144 concurrent calls

Multiple Access Control (MAC) ProtocolsMultiple Access Control (MAC) Protocols

MAC allows multiple users to share a common channel. MAC allows multiple users to share a common channel.

Conflict-free protocolsConflict-free protocols ensure successful transmission. Channel can be ensure successful transmission. Channel can be allocated to users statically or dynamically.allocated to users statically or dynamically.

Only static conflict-free protocols are used in cellular mobile communicationsOnly static conflict-free protocols are used in cellular mobile communications- - Frequency Division Multiple Access (FDMA): provides a fraction of the (FDMA): provides a fraction of the frequency range to each user for all the timefrequency range to each user for all the time

- - Time Division Multiple Access (TDMA)(TDMA) : : The entire frequency band is The entire frequency band is allocated to a single user for a fraction of timeallocated to a single user for a fraction of time- - Code Division Multiple Access (CDMA) : provides every user all the (CDMA) : provides every user all the bandwidth all the timebandwidth all the time

Frequency Division Multiple Access (FDMA)Frequency Division Multiple Access (FDMA)

Channels are assigned to the user for the duration of a call. No other Channels are assigned to the user for the duration of a call. No other user can access the channel during that time. When call terminates, user can access the channel during that time. When call terminates, the same channel can be re-assigned to another userthe same channel can be re-assigned to another user

FDMA is used in nearly all first generation mobile communication FDMA is used in nearly all first generation mobile communication systems, like AMPS (30 KHz channels), NMT, Japanese TACS/NTTsystems, like AMPS (30 KHz channels), NMT, Japanese TACS/NTT

Channel 1Channel 1

Channel 2Channel 2

Channel 3Channel 3

Channel 4Channel 4

Band

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Time Division Multiple Access (TDMA)Time Division Multiple Access (TDMA) The whole channel is assigned to each user, but the users are The whole channel is assigned to each user, but the users are

multiplexed in time during communication. Each communicating user multiplexed in time during communication. Each communicating user is assigned a particular time slot, during which it communicates using is assigned a particular time slot, during which it communicates using the entire frequency spectrumthe entire frequency spectrum

The data rate of the channel is the sum of the data rates of all the The data rate of the channel is the sum of the data rates of all the multiplexed transmissionsmultiplexed transmissions

There is always channel interference between transmission in two There is always channel interference between transmission in two adjacent slots because transmissions tend to overlap in time. This adjacent slots because transmissions tend to overlap in time. This interference limits the number of users that can share the channelinterference limits the number of users that can share the channel

Channel 1Channel 1

Band

wid

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Channel 2Channel 2

Channel 3Channel 3

Channel 4Channel 4

Channel 1Channel 1

Channel 2Channel 2

Channel 3Channel 3

Time Division Multiple Access (TDMA)Time Division Multiple Access (TDMA)

Synchronization of transmission is very important in TDMA systems. Synchronization of transmission is very important in TDMA systems. Imperfect synchronizations also lead to channel interferenceImperfect synchronizations also lead to channel interference

TDMA is used in many second generation systems like GSM, EIA/TIA, IS-54 TDMA is used in many second generation systems like GSM, EIA/TIA, IS-54 (digital cellular system)(digital cellular system)

In IS-54, each 30 KHz channel is divided into 6 timeslots. Data rate in each In IS-54, each 30 KHz channel is divided into 6 timeslots. Data rate in each slot is 8 Kbps, i.e., 48Kbps per channelslot is 8 Kbps, i.e., 48Kbps per channel

Two slots for upstream traffic and two slots for downstream trafficTwo slots for upstream traffic and two slots for downstream traffic(8 Kbps coded speech + overhead in 16 Kbps)(8 Kbps coded speech + overhead in 16 Kbps)

Capacity of IS-54 is three times that of AMPS (i.e. three calls can be Capacity of IS-54 is three times that of AMPS (i.e. three calls can be

accommodated in 30 KHz bandwidth)accommodated in 30 KHz bandwidth)

Will be extended to six calls in the future (4 Kbps coded speech + overhead)Will be extended to six calls in the future (4 Kbps coded speech + overhead)

Code Division Multiple Access (CDMA)Code Division Multiple Access (CDMA) CDMA, a type of a CDMA, a type of a spread-spectrumspread-spectrum technique, allows multiple users technique, allows multiple users

to share the same channel by multiplexing their transmissions in code to share the same channel by multiplexing their transmissions in code space. Different signals from different users are encoded by different space. Different signals from different users are encoded by different codes (keys) and coexist both in time and frequency domainscodes (keys) and coexist both in time and frequency domains

A code is represented by a wideband pseudo noise (PN) signalA code is represented by a wideband pseudo noise (PN) signal

When decoding a transmitted signal at the receiver, because of low When decoding a transmitted signal at the receiver, because of low cross-correlation of different codes, other transmissions appear as cross-correlation of different codes, other transmissions appear as noise. This property enables the multiplexing of a number of noise. This property enables the multiplexing of a number of transmissions on the same channel with minimal interferencetransmissions on the same channel with minimal interference

The maximum allowable interference (from other transmissions) limits The maximum allowable interference (from other transmissions) limits the number of simultaneous transmissions on the same channelthe number of simultaneous transmissions on the same channel

All channels share bandwidthAll channels share bandwidth

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Code Division Multiple Access (CDMA)Code Division Multiple Access (CDMA)

Spreading of the signal bandwidth can be performed usingSpreading of the signal bandwidth can be performed using

- - Direct SequenceDirect Sequence (DS): the narrow band signal representing digital data (DS): the narrow band signal representing digital data is multiplied by a wideband pseudo noise (PN) signal representing the is multiplied by a wideband pseudo noise (PN) signal representing the code. Multiplication in the time domain translates to convolution in the code. Multiplication in the time domain translates to convolution in the spectral domain. Thus the resulting signal is widebandspectral domain. Thus the resulting signal is wideband

- - Frequency HoppingFrequency Hopping (FH): carrier frequency rapidly hops among a large (FH): carrier frequency rapidly hops among a large set of possible frequencies according to some pseudo random sequence set of possible frequencies according to some pseudo random sequence (the code). The set of frequencies spans a large bandwidth. Thus the (the code). The set of frequencies spans a large bandwidth. Thus the bandwidth of the transmitted signal appears as largely spreadbandwidth of the transmitted signal appears as largely spread