Cellular traffic
Encyclopedia
This article discusses the mobile cellular network aspect of teletraffic measurements. Mobile radio
networks have traffic issues that do not arise in connection with the fixed line PSTN. Important aspects of cellular traffic include: quality of service targets, traffic capacity and cell size, spectral efficiency
and sectorization, traffic capacity versus coverage, and channel holding time
analysis.
Teletraffic engineering
in telecommunications network planning ensures that network costs are minimised without compromising the quality of service
(QoS) delivered to the user of the network. This field of engineering is based on probability theory
and can be used to analyse mobile radio networks, as well as other telecommunications network
s.
A mobile handset which is moving in a cell will record a signal strength
that varies. Signal strength is subject to slow fading, fast fading and interference from other signals, resulting in degradation of the carrier-to-interference
ratio (C/I). A high C/I ratio yields quality communication. A good C/I ratio is achieved in cellular systems by using optimum power levels through the power control of most links. When carrier power is too high, excessive interference is created, degrading the C/I ratio for other traffic and reducing the traffic capacity of the radio subsystem. When carrier power is too low, C/I is too low and QoS targets are not met.
(QoS) targets are set, for: traffic congestion and blocking, dominant coverage area, C/I, dropped call rate, handover failure rate, overall call success rate, ...
cells. The traffic engineer can achieve the goal of satisfying the increasing population of customers by increasing the number of
cells in the area concerned, so this will also increases the number of base stations. This method is called cell splitting (and combined with sectorization)
is the only way of providing services to a burgeoning population. This simply works by dividing the cells already present into smaller sizes
hence increasing the traffic capacity.
Reduction of the cell radius enables the cell to accommodate extra traffic.
The cost of equipment can also be cut down by reducing the number of base stations through setting up three neighbouring
cells, with the cells serving three 120° sectors with different channel groups.
Mobile radio networks are operated with finite, limited resources (the spectrum of frequencies available). These resources have to be
used effectively to ensure that all users receive service, that is, the quality of service is consistently maintained. This need to carefully use the
limited spectrum, brought about the development of cells in mobile networks, enabling frequency re-use by successive clusters of cells. Systems that efficiently use the available
spectrum have been developed e.g. the GSM
system. Walke defines spectral efficiency as the traffic
capacity unit divided by the product of bandwidth and surface area element, and is dependent on the number of radio channels
per cell and the cluster size (number of cells in a group of cells):
where Nc is the number of channels per cell, BW is the system bandwidth, and Ac is Area of cell.
Sectorization is briefly described in traffic load and cell size as a way to cut down equipment costs in a cellular
network. When applied to clusters of cells sectorization also reduces co-channel interference, according to Walke. This is
because the power radiated backward from a directional base station antenna is minimal and interfering with adjacent cells is reduced.
(The number of channels is directly proportional to the number of cells.) The maximum traffic capacity of sectored antennas (directional) is greater than that of omnidirectional antennas by a factor which is the number of sectors per cell (or cell cluster).
Multiple Access be considered for the relationship between traffic capacity and coverage (area covered by cells).
CDMA cellular systems can allow an increase in traffic capacity at the expense of the quality of service
.
In TDMA/FDMA cellular radio systems, Fixed Channel Allocation (FCA) is used to allocate channels to customers. In FCA the
number of channels in the cell remains constant irrespective of the number of customers in that cell. This results in
traffic congestion and some calls being lost when traffic gets heavy.
A better way of channel allocation in cellular
systems is Dynamic Channel Allocation (DCA) which is supported by the GSM
, DCS
and other systems. DCA is a
better way not only for handling bursty cell traffic but also in efficiently utilising the cellular radio resources.
DCA allows the number of channels in a cell to vary with the traffic load, hence increasing channel capacity with little
costs.
Since a cell is allocated a group of frequency carries (e.g. f1-f7) for each user, this range of frequencies is the
bandwidth of that cell, BW. If that cell covers an area Ac, and each user has bandwidth B then the number of channels will be
BW/B. The density of channels will be
. This formula shows that as the coverage area Ac is increased, the
channel density decreases.
and reuse distance determine the quality of service
of a cellular network. Channel Holding Time is another parameter that can affect the quality of service
in a cellular network, hence it is considered when planning the network. Calculating the channel holding time, however is not easy. (This is the time a Mobile Station (MS) remains in the same cell during
a call). Channel holding time is therefore less than call holding time if the MS travels more than one cell
as handover will take place and the MS relinquishes the channel. Practically, it is not possible to determine exactly the channel
holding time. As a result, different models exist for the channel holding time distribution. In industry, a good approximation
of the channel holding time is usually sufficient to determine the network traffic capability.
One of the papers in Key and Smith defines channel holding time as being equal to the average holding time divided by the average number of handovers per call plus one.
Usually an exponential model
is preferred to calculate the channel holding time for simplicity in simulations. This model gives the distribution
function of channel holding time and it is an approximation that can be used to obtain estimates channel holding time. The exponential model may not be correctly modelling the channel holding time
distribution as other papers may try to prove, but it gives an approximation. Channel holding time is not easily determined explicitly, call holding time and user's movements have to be
determined in order to implicitly give channel holding time. The mobility of the user and the cell shape and size
cause the channel holding time to have a different distribution function to that of call duration (call holding time). This difference is large for highly mobile users and small cell sizes. Since the channel holding time and
call duration relationships are affected by mobility and cell size, for a stationary MS and large cell sizes,
channel holding time and call duration are the same.
Mobile Radio
This article is about professional equipment. For mobile radios used in amateur radio, see amateur radio mobile operation. Mobile radio or mobiles refer to wireless communications systems and devices which are based on radio frequencies, and where the path of communications is movable on either...
networks have traffic issues that do not arise in connection with the fixed line PSTN. Important aspects of cellular traffic include: quality of service targets, traffic capacity and cell size, spectral efficiency
Spectral efficiency
Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system...
and sectorization, traffic capacity versus coverage, and channel holding time
Holding time
Holding time may refer to:*Holding , used to delay aircraft already in flight*Holding time , where a child is laid upon to produce a cathartic response...
analysis.
Teletraffic engineering
Teletraffic engineering
Telecommunications traffic engineering, teletraffic engineering, or traffic engineering is the application of traffic engineering theory to telecommunications...
in telecommunications network planning ensures that network costs are minimised without compromising the quality of service
Mobile QoS
Quality of service mechanism controls the performance, reliability and usability of a telecommunications service. Mobile cellular service providers may offer mobile QoS to customers just as the fixed line PSTN services providers and Internet Service Provides may offer QoS...
(QoS) delivered to the user of the network. This field of engineering is based on probability theory
Probability theory
Probability theory is the branch of mathematics concerned with analysis of random phenomena. The central objects of probability theory are random variables, stochastic processes, and events: mathematical abstractions of non-deterministic events or measured quantities that may either be single...
and can be used to analyse mobile radio networks, as well as other telecommunications network
Telecommunications network
A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...
s.
A mobile handset which is moving in a cell will record a signal strength
Mobile phone signal
A mobile phone signal is the strength of the connection to the mobile phone with its network. Depending on various factors, such as proximity to a tower, obstructions such as buildings or trees, etc, the signal may vary. Most mobile devices use a set of bars of varying heights to display the...
that varies. Signal strength is subject to slow fading, fast fading and interference from other signals, resulting in degradation of the carrier-to-interference
Signal-to-interference ratio
The signal-to-interference ratio , also known as the carrier-to-interference ratio , is the quotient between the average received modulated carrier power S or C and the average received co-channel interference power I, i.e...
ratio (C/I). A high C/I ratio yields quality communication. A good C/I ratio is achieved in cellular systems by using optimum power levels through the power control of most links. When carrier power is too high, excessive interference is created, degrading the C/I ratio for other traffic and reducing the traffic capacity of the radio subsystem. When carrier power is too low, C/I is too low and QoS targets are not met.
Quality of Service targets
At the time that the cells of a radio subsystem are designed, Quality of ServiceMobile QoS
Quality of service mechanism controls the performance, reliability and usability of a telecommunications service. Mobile cellular service providers may offer mobile QoS to customers just as the fixed line PSTN services providers and Internet Service Provides may offer QoS...
(QoS) targets are set, for: traffic congestion and blocking, dominant coverage area, C/I, dropped call rate, handover failure rate, overall call success rate, ...
Traffic load and cell size
The more traffic generated, the more base stations will be needed to service the customers. The number of base stations for a simple cellular network is equal to the number ofcells. The traffic engineer can achieve the goal of satisfying the increasing population of customers by increasing the number of
cells in the area concerned, so this will also increases the number of base stations. This method is called cell splitting (and combined with sectorization)
is the only way of providing services to a burgeoning population. This simply works by dividing the cells already present into smaller sizes
hence increasing the traffic capacity.
Reduction of the cell radius enables the cell to accommodate extra traffic.
The cost of equipment can also be cut down by reducing the number of base stations through setting up three neighbouring
cells, with the cells serving three 120° sectors with different channel groups.
Mobile radio networks are operated with finite, limited resources (the spectrum of frequencies available). These resources have to be
used effectively to ensure that all users receive service, that is, the quality of service is consistently maintained. This need to carefully use the
limited spectrum, brought about the development of cells in mobile networks, enabling frequency re-use by successive clusters of cells. Systems that efficiently use the available
spectrum have been developed e.g. the GSM
Global System for Mobile Communications
GSM , is a standard set developed by the European Telecommunications Standards Institute to describe technologies for second generation digital cellular networks...
system. Walke defines spectral efficiency as the traffic
capacity unit divided by the product of bandwidth and surface area element, and is dependent on the number of radio channels
per cell and the cluster size (number of cells in a group of cells):
where Nc is the number of channels per cell, BW is the system bandwidth, and Ac is Area of cell.
Sectorization is briefly described in traffic load and cell size as a way to cut down equipment costs in a cellular
network. When applied to clusters of cells sectorization also reduces co-channel interference, according to Walke. This is
because the power radiated backward from a directional base station antenna is minimal and interfering with adjacent cells is reduced.
(The number of channels is directly proportional to the number of cells.) The maximum traffic capacity of sectored antennas (directional) is greater than that of omnidirectional antennas by a factor which is the number of sectors per cell (or cell cluster).
Traffic capacity versus coverage
Cellular systems use one or more of four different techniques of access (TDMA, FDMA, CDMA, SDMA). See Cellular concepts. Let a case of Code DivisionMultiple Access be considered for the relationship between traffic capacity and coverage (area covered by cells).
CDMA cellular systems can allow an increase in traffic capacity at the expense of the quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
.
In TDMA/FDMA cellular radio systems, Fixed Channel Allocation (FCA) is used to allocate channels to customers. In FCA the
number of channels in the cell remains constant irrespective of the number of customers in that cell. This results in
traffic congestion and some calls being lost when traffic gets heavy.
A better way of channel allocation in cellular
systems is Dynamic Channel Allocation (DCA) which is supported by the GSM
Global System for Mobile Communications
GSM , is a standard set developed by the European Telecommunications Standards Institute to describe technologies for second generation digital cellular networks...
, DCS
and other systems. DCA is a
better way not only for handling bursty cell traffic but also in efficiently utilising the cellular radio resources.
DCA allows the number of channels in a cell to vary with the traffic load, hence increasing channel capacity with little
costs.
Since a cell is allocated a group of frequency carries (e.g. f1-f7) for each user, this range of frequencies is the
bandwidth of that cell, BW. If that cell covers an area Ac, and each user has bandwidth B then the number of channels will be
BW/B. The density of channels will be
. This formula shows that as the coverage area Ac is increased, thechannel density decreases.
Channel holding time
Important parameters like the carrier-to-interference ratio (C/I), spectral efficiencySpectral efficiency
Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system...
and reuse distance determine the quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
of a cellular network. Channel Holding Time is another parameter that can affect the quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
in a cellular network, hence it is considered when planning the network. Calculating the channel holding time, however is not easy. (This is the time a Mobile Station (MS) remains in the same cell during
a call). Channel holding time is therefore less than call holding time if the MS travels more than one cell
as handover will take place and the MS relinquishes the channel. Practically, it is not possible to determine exactly the channel
holding time. As a result, different models exist for the channel holding time distribution. In industry, a good approximation
of the channel holding time is usually sufficient to determine the network traffic capability.
One of the papers in Key and Smith defines channel holding time as being equal to the average holding time divided by the average number of handovers per call plus one.
Usually an exponential model
Exponential distribution
In probability theory and statistics, the exponential distribution is a family of continuous probability distributions. It describes the time between events in a Poisson process, i.e...
is preferred to calculate the channel holding time for simplicity in simulations. This model gives the distribution
function of channel holding time and it is an approximation that can be used to obtain estimates channel holding time. The exponential model may not be correctly modelling the channel holding time
distribution as other papers may try to prove, but it gives an approximation. Channel holding time is not easily determined explicitly, call holding time and user's movements have to be
determined in order to implicitly give channel holding time. The mobility of the user and the cell shape and size
cause the channel holding time to have a different distribution function to that of call duration (call holding time). This difference is large for highly mobile users and small cell sizes. Since the channel holding time and
call duration relationships are affected by mobility and cell size, for a stationary MS and large cell sizes,
channel holding time and call duration are the same.
See also
|
Cellular repeater A cellular repeater, cell phone repeater, or wireless cellular signal booster, a type of bi-directional amplifier as commonly named in the wireless telecommunications industry, is a device used for boosting the cell phone reception to the local area by the usage of a reception antenna, a signal... Code division multiple access Code division multiple access is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000 and WCDMA , which are often referred to as simply CDMA, and use CDMA as an underlying channel access... (CDMA) Mobile phone A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator... MIMO In radio, multiple-input and multiple-output, or MIMO , is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology... (MIMO) Professional Mobile Radio Professional mobile radio are field radio communications systems which use portable, mobile, base station, and dispatch console radios... (PMR) Radio resource management Radio resource management is the system level control of co-channel interference and other radio transmission characteristics in wireless communication systems, for example cellular networks, wireless networks and broadcasting systems... (RRM) Signal strength In telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in... |
OpenBTS OpenBTS is a software-based GSM access point, allowing standard GSM-compatible mobile phones to make telephone calls without using existing telecommunication providers' networks... Cellular router Cellular routers are routers that provide shared Internet access by incorporating a cellular data modem and providing traditional interfaces like Ethernet and WiFi.... |