Mobile Computing

Cellular Networks

Cellular Systems- Coverage Area

Cellular Systems—Transmission Capacity

Cellular Systems- Hierarchy

Establishing a Call

Call setup-From MS to BS

Cont.. From BS to MS

Recall

Cell Cluster Re-use etc..

Cellular Systems--Cellular Concepts The cellular concept was a major breakthrough in solving

the problem of spectral congestion and user capacity. It offered very high capacity in a limited spectrum allocation without any major technological changes.

The cellular concept has the following system level ideas Replacing a single, high power transmitter with many low power

transmitters, each providing coverage to only a small area. Neighboring cells are assigned different groups of channels in

order to minimize interference. The same set of channels is then reused at different

geographical locations.

Cellular Concepts

When designing a cellular mobile communication system, it is important to provide good coverage and services in a high user-density area.

Reuse can be done once the total interference from all users in the cells using the same frequency (co-channel cell) for transmission suffers from sufficient attenuation. Factors need to be considered include: Geographical separation (path loss) Shadowing effect

Cell Footprint

The actual radio coverage of a cell is known as the cell footprint. Irregular cell structure and irregular placing of the

transmitter may be acceptable in the initial system design. However as traffic grows, where new cells and channels need to be added, it may lead to inability to reuse frequencies because of co-channel interference.

For systematic cell planning, a regular shape is assumed for the footprint.

Cell Footprint

Coverage contour should be circular. However it is impractical because it provides ambiguous areas with either multiple or no coverage.

Due to economic reasons, the hexagon has been chosen due to its maximum area coverage.

Hence, a conventional cellular layout is often defined by a uniform grid of regular hexagons.

Cell Footprint

Frequency reuse

A cellular system which has a total of S duplex channels.

S channels are divided among N cells, with each cell uses unique and disjoint channels.

If each cell is allocated a group of k channels, then

S = k N .

Terminology

Cluster size : The N cells which collectively use the complete set of available frequency is called the cluster size.

Co-channel cell : The set of cells using the same set of frequencies as the target cell.

Interference tier : A set of co-channel cells at the same distance from the reference cell is called an interference tier. The set of closest co-channel cells is call the first tier. There is always 6 co-channel cells in the first tier.

Co-ordinates for hexagonal cellular geometry With these co-

ordinates, an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer co-ordinates.

Co-ordinates for hexagonal cellular geometry

Designing a cellular system

N=19 (i=3, j=2)

Designing a cellular system

The cluster size must satisfy: N = i2 + ij + j2 where i, j are non-negative integers.

Designing a cellular system

Designing a cellular system

Can also verify that

where Q is the co-channel reuse ratio

Handover / Handoff

Occurs as a mobile moves into a different cell during an existing call, or when going from one cellular system into another. It must be user transparent, successful and not

too frequent. Not only involves identifying a new BS, but also

requires that the voice and control signals be allocated to channels associated with the new BS.

Handover / Handoff

Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver, a slightly stronger signal level PHO is used as a threshold at which a handover is made.

Handover / Handoff =handoff threshold -Minimum acceptablesignal to maintain the call too small:

Insufficient time

to complete handoff

before call is lost More call losses

too large: Too many handoffs Burden for MSC

Dwell Time

The time over which a user remains within one cell is called the dwell time.

The statistics of the dwell time are important for the practical design of handover algorithms.

The statistics of the dwell time vary greatly, depending on the speed of the user and the type of radio coverage.

Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS. This information is forwarded to the MSC who makes decisions regarding handover.

Mobile assisted handover (MAHO) : The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS.

Prioritizing Handover Dropped call is considered a more serious event

than call blocking. Channel assignment schemes therefore must give priority to handover requests.

A fraction of the total available channels in a cell is reserved only for handover requests. However, this reduces the total carried traffic. Dynamic allocation can improve this.

Queuing of handover requests is another method to decrease the probability of forced termination of a call due to a lack of available channel. The time span over which a handover is usually required leaves room for queuing handover request.

Practical handover

High speed users and low speed users have vastly different dwell times which might cause a high number of handover requests for high speed users. This will result in interference and traffic management problem.

Practical handover

The Umbrella Cell approach will help to solve this problems. High speed users are serviced by large (macro) cells, while low speed users are handled by small (micro) cells.

Practical handover

A hard handover does “break before make”, ie. The old channel connection is broken before the new allocated channel connection is setup. This obviously can cause call dropping.

In soft handover, we do “make before break”, ie. The new channel connection is established before the old channel connection is released. This is realized in CDMA where also BS diversity is used to improve boundary condition.

Interference and System Capacity In a given coverage area, there are several cells

that use the same set of frequencies. These cells are called co-channel cells. The interference between signals from these cells is called co-channel interference.

If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area, the transmit power of each BS is almost equal. We can show that worse case signal to co-channel interference is independent of the transmitted power. It becomes a function of the cell radius R, and the distance to the nearest co-channel cell D’.

Interference and System Capacity

Received power at a distance d from the transmitting antenna is approximated by

Useful signal at the cell boundary is the weakest, given by Pr (R). Interference signal from the co-channel cell is given to be Pr (D′) .

Interference and System Capacity

D’ is normally approximated by the base station separation between the two cells D, unless when accuracy is needed. Hence

Interference and System Capacity For the forward link, a very general case,

where Di is the distance of the ith interfering cell from the mobile, i0 is the total number of co-channel cells exist.

Interference and System Capacity

If only first tier co-channel cells are considered, then i0 = 6.

Unless otherwise stated, normally assuming Di ≈ D for all i.

Outage probability

The probability that a mobile station does not receive a usable signal.

For GSM, this is 12 dB and for AMPS, this is 18 dB. If there is 6 co-channel cells, then

Exercise : please verify this For n=4, a minimum cluster size of N=7 is needed to meet

the SIR requirements for AMPS. For n=4, a minimum cluster size of N=4 is required to meet

the SIR requirements for GSM

Outage probability

Outage probability

Approximation in distance has been made on the 2nd tier onwards.

Outage probability

More accurate SIR can be obtained by computing the actual distance.

Our computation of outage only based on path loss. For more accurate modeling, shadowing and fast fading need to be taken into consideration.

Coverage Problems Revision:

Recall that the mean measured value,

Measurement shows that at any value of d, the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value.

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB).

Cell coverage --Cellular Traffic The basic consideration in the design of a

cellular system is the sizing of the system. Sizing has two components to be considered. Coverage area Traffic handling capability

After the system is sized, channels are assigned to cells using the assignment schemes mentioned before.

Cell coverage --Terminology in traffic theory Trunking : exploits the statistical characteristics of the

users calling behaviour. Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels.

Grade of service (GoS) : A user is allocated a channel on a per call basis. GoS is a measure of the ability of a user to access a trunked system during the busiest hour. It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before).

Trunking theory : is used to determine the number of channels required to service a certain offered traffic at a specific GoS.

Call holding time (H) : the average duration of a call. Request rate (λ) : average number of call requests

perunit time.

Cell coverage --Traffic flow or intensity A

Measured in Erlang, which is defined as the call minute per minute.

The erlang (symbol E) is a dimensionless unit that is used in telephony as a measure of offered load or carried load on service-providing elements such as telephone circuits or telephone switching equipment. For example, a single cord circuit has the capacity to be used for 60 minutes in one hour. If one hundred six-minute calls are received then the total traffic in that hour is six hundred minutes or 10 Erlangs.

In 1946 the CCITT named the international unit of telephone traffic Erlangs in honor of Agner Krarup Erlang

Cell coverage --Traffic flow or intensity A

Total offered traffic for such a system is given as

A = λ ⋅H

Exercise : There are 3000 calls per hour in a cell, each lasting an average of 1.76 min.

answer

Offered traffic A = (3000/60)(1.76) = 88 Erlangs

Cell coverage

If the offered traffic exceeds the maximum possible carried traffic, blocking occurs. There are two different strategies to be used. Blocked calls cleared Blocked calls delayed

Trunking efficiency : is defined as the carried traffic intensity in Erlangs per channel, which is a value between zero and one. It is a function of the number of channels per cell and the specific GoS parameters.

Call arrival process: it is widely accepted that calls have a Poisson arrival.

Poisson process

In probability theory, a Poisson process is a stochastic process which counts the number of events and the time that these events occur in a given time interval. The time between each pair of consecutive events has an exponential distribution with parameter λ and each of these inter-arrival times is assumed to be independent of other inter-arrival times. The process is named after the French mathematician Siméon-Denis Poisson and is a good model of radioactive decay, telephone calls and requests for a particular document on a web server,

among many other phenomena

Channel Assignment Strategies Channel allocation schemes can affect the

performance of the system. Fixed Channel Allocation (FCA) :

Channels are divided in sets. A set of channels is permanently allocated to each cell

in the network. Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference.

Any call attempt within the cell can only be served by the unused channels in that particular cell. The service is blocked if all channels have used up.

Channel Assignment Strategies

(FCA) Most easiest to implement but least flexibility. An modification to this is ‘borrowing scheme’. Cell

(acceptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls.

Borrowing can be done in a few ways: borrowing from the adjacent cell which has largest number of free channels, select the first free channel found, etc.

To be available for borrowing, the channel must not interfere with existing calls. The borrowed channel should be returned once the channel becomes free.

Channel Assignment Strategies (DCA) Dynamic Channel Allocation (DCA) :

Voice channels are not allocated to any cell permanently. All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system.

Each time a call request is made, the serving BS requests a channel from the MSC. It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell, the reuse distance of the channel and other cost functions increase in complexity⇒

Centralized DCA scheme involves a single controller selecting a channel for each cell. Distributed DCA scheme involves a number of controllers scattered across the network.

For a new call, a free channel from central pool is selected based on either the co-channel distance, signal strength or signal to noise interference ratio.

Channel Assignment Strategies Flexible channel assignment

Divide the total number of channels into two groups, one of which is used for fixed allocation to the cells, while the other is kept as a central poor to be shared by all users.

Mix the advantages the FCA and DCA, available schemes are scheduled and predictive.

Channels need to be assigned to users to accommodate new calls handovers

with the objective of increasing capacity and minimizing

probability of a blocked call.

System Expansion Techniques As demand for wireless services increases, the

number of channels assigned to a cell eventually becomes insufficient to support the required number of users. More channels must therefore be made available per unit area. This can be accomplished by dividing each initial cell area

into a number of smaller cells, a technique known as cell-splitting.

It can also be accomplished by having more channels per cell, i.e. by having a smaller reuse factor. However, to have a smaller reuse factor, the co-channel interference must be reduced. This can be done by using antenna sectorization.

System Expansion Techniques--Cell splitting Cell splitting increases the number of BSs in order

to increase capacity. There will be a corresponding reduction in antenna height and transmitter power.

Cell splitting accommodates a modular growth capability. This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning.

Small cells lead to more cells/area which in turn leads to increased traffic capacity.

System Expansion Techniques--Cell splitting

System Expansion Techniques--Cell splitting For new cells to be smaller in size, the transmit

power must be reduced. If n=4, then with a reduction of cell radius by a factor of 2, the transmit power should be reduced by a factor of 24 (why?)

In theory, cell splitting could be repeated indefinitely. In practice it is limited

By the cost of base stations Handover (fast and low speed traffic) Not all cells are split at the same time : practical problems

of BS sites, such as co-channel interference exist Innovative channel assignment schemes must be

developed to address this problem for practical systems.

System Expansion Techniques--Cell splitting

System Expansion Techniques --Sectorization Keep the cell radius but decrease the D/R

ratio. In order to do this, we must reduce the relative interference without increasing the transmit power.

Sectorization relies on antenna placement and directivity to reduce co-channel interference. Beams are kept within either a 60° or a 120° sector.

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization If we partition a cell into three 120° sectors, the

number of co-channel cells are reduced from 6 to 2 in the first tier.

Using six sectors of 60°, we have only one co-channel cell in the first tier.

Each sector is limited to only using 1/3 or 1/6 of the available channels. We therefore have a decrease in trunking efficiency and an increase in the number of required antennas.

But how can the increase in system capacity be achieved?

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Sectorization

System Expansion Techniques --Micro cells Micro cells can be introduced to alleviate

capacity problems caused by “hotspots”. By clever channel assignment, the reuse

factor is unchanged. As for cell splitting, there will occur interference problems when macro and micro cells must co-exist.

System Expansion Techniques --Micro cells

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