Cellular traffic

Last updated

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.

Contents

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 networks.

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). [1] 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. [1]

Quality of Service targets

At the time that the cells of a radio subsystem are designed, Quality of Service (QoS) targets are set, for: traffic congestion and blocking, dominant coverage area, C/I, outage probability, handover failure rate, overall call success rate, data rate, delay, etc. [2]

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 of 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. [1] 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. [1] Systems that efficiently use the available spectrum have been developed e.g. the GSM system. Bernhard Walke [1] 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. [2] When applied to clusters of cells sectorization also reduces co-channel interference, according to Walke. [1] 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). [1]

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 Division 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. [3]

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. [4]

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 using 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. [1] 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 . [5] This formula shows that as the coverage area Ac is increased, the channel density decreases.

Channel holding time

Important parameters like the carrier-to-interference ratio (C/I), spectral efficiency 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). [3] 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 [3] 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. [3] 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. [3] 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. [3]

See also

Related Research Articles

<span class="mw-page-title-main">Code-division multiple access</span> Channel access method used by various radio communication technologies

Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication channel. This allows several users to share a band of frequencies. To permit this without undue interference between the users, CDMA employs spread spectrum technology and a special coding scheme.

<span class="mw-page-title-main">General Packet Radio Service</span> Packet oriented mobile data service on 2G and 3G

General Packet Radio Service (GPRS) is a packet oriented mobile data standard on the 2G and 3G cellular communication network's global system for mobile communications (GSM). GPRS was established by European Telecommunications Standards Institute (ETSI) in response to the earlier CDPD and i-mode packet-switched cellular technologies. It is now maintained by the 3rd Generation Partnership Project (3GPP).

Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitatively measure quality of service, several related aspects of the network service are often considered, such as packet loss, bit rate, throughput, transmission delay, availability, jitter, etc.

<span class="mw-page-title-main">Time-division multiple access</span> Channel access method for networks using a shared communications medium

Time-division multiple access (TDMA) is a channel access method for shared-medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission medium while using only a part of its channel capacity. Dynamic TDMA is a TDMA variant that dynamically reserves a variable number of time slots in each frame to variable bit-rate data streams, based on the traffic demand of each data stream.

<span class="mw-page-title-main">Communication channel</span> Physical or logical connection used for transmission of information

A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used for information transfer of, for example, a digital bit stream, from one or several senders to one or several receivers. A channel has a certain capacity for transmitting information, often measured by its bandwidth in Hz or its data rate in bits per second.

cdmaOne First CDMA-based digital cellular technology

Interim Standard 95 (IS-95) was the first ever CDMA-based digital cellular technology. It was developed by Qualcomm and later adopted as a standard by the Telecommunications Industry Association in TIA/EIA/IS-95 release published in 1995. The proprietary name for IS-95 is cdmaOne.

<span class="mw-page-title-main">Cell site</span> Communications equipment location

A cell site, cell phone tower, or cellular base station is a cellular-enabled mobile device site where antennas and electronic communications equipment are placed to create a cell, or adjacent cells, in a cellular network. The raised structure typically supports antenna and one or more sets of transmitter/receivers transceivers, digital signal processors, control electronics, a GPS receiver for timing, primary and backup electrical power sources, and sheltering.

<span class="mw-page-title-main">Cellular network</span> Communication network

A cellular network or mobile network is a communication network where the link to and from end nodes is wireless. The network is distributed over land areas called "cells", each served by at least one fixed-location transceiver. These base stations provide the cell with the network coverage which can be used for transmission of voice, data, and other types of content. A cell typically uses a different set of frequencies from neighboring cells, to avoid interference and provide guaranteed service quality within each cell.

<span class="mw-page-title-main">Evolution-Data Optimized</span> Telecommunications standard for the wireless transmission of data through radio signals

Evolution-Data Optimized is a telecommunications standard for the wireless transmission of data through radio signals, typically for broadband Internet access. EV-DO is an evolution of the CDMA2000 (IS-2000) standard which supports high data rates and can be deployed alongside a wireless carrier's voice services. It uses advanced multiplexing techniques including code-division multiple access (CDMA) as well as time-division multiplexing (TDM) to maximize throughput. It is a part of the CDMA2000 family of standards and has been adopted by many mobile phone service providers around the world particularly those previously employing CDMA networks. It is also used on the Globalstar satellite phone network.

Quality of service (QoS) 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 providers may offer QoS. QoS mechanisms are always provided for circuit switched services, and are essential for non-elastic services, for example streaming multimedia. It is also essential in networks dominated by such services, which is the case in today's mobile communication networks.

The ideal telecommunication network has the following characteristics: broadband, multi-media, multi-point, multi-rate and economical implementation for a diversity of services (multi-services). The Broadband Integrated Services Digital Network (B-ISDN) was planned to provide these characteristics. Asynchronous Transfer Mode (ATM) was promoted as a target technology for meeting these requirements.

If a network service wishes to use a broadband network to transport a particular kind of traffic, it must first inform the network about what kind of traffic is to be transported, and the performance requirements of that traffic. The application presents this information to the network in the form of a traffic contract.

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. It is a measure of how efficiently a limited frequency spectrum is utilized by the physical layer protocol, and sometimes by the medium access control.

In radio resource management for wireless and cellular networks, channel allocation schemes allocate bandwidth and communication channels to base stations, access points and terminal equipment. The objective is to achieve maximum system spectral efficiency in bit/s/Hz/site by means of frequency reuse, but still assure a certain grade of service by avoiding co-channel interference and adjacent channel interference among nearby cells or networks that share the bandwidth.

Radio resource management (RRM) is the system level management of co-channel interference, radio resources, and other radio transmission characteristics in wireless communication systems, for example cellular networks, wireless local area networks, wireless sensor systems, and radio broadcasting networks. RRM involves strategies and algorithms for controlling parameters such as transmit power, user allocation, beamforming, data rates, handover criteria, modulation scheme, error coding scheme, etc. The objective is to utilize the limited radio-frequency spectrum resources and radio network infrastructure as efficiently as possible.

<span class="mw-page-title-main">Drive testing</span>

RF Drive testing is a method of measuring and assessing the coverage, capacity and Quality of Service (QoS) of a mobile radio network.

In radio, cooperative multiple-input multiple-output is a technology that can effectively exploit the spatial domain of mobile fading channels to bring significant performance improvements to wireless communication systems. It is also called network MIMO, distributed MIMO, virtual MIMO, and virtual antenna arrays.

CDMA spectral efficiency refers to the system spectral efficiency in bit/s/Hz/site or Erlang/MHz/site that can be achieved in a certain CDMA based wireless communication system. CDMA techniques are characterized by a very low link spectral efficiency in (bit/s)/Hz as compared to non-spread spectrum systems, but a comparable system spectral efficiency.

In cellular telecommunications, handover, or handoff, is the process of transferring an ongoing call or data session from one channel connected to the core network to another channel. In satellite communications it is the process of transferring satellite control responsibility from one earth station to another without loss or interruption of service.

In mathematics and telecommunications, stochastic geometry models of wireless networks refer to mathematical models based on stochastic geometry that are designed to represent aspects of wireless networks. The related research consists of analyzing these models with the aim of better understanding wireless communication networks in order to predict and control various network performance metrics. The models require using techniques from stochastic geometry and related fields including point processes, spatial statistics, geometric probability, percolation theory, as well as methods from more general mathematical disciplines such as geometry, probability theory, stochastic processes, queueing theory, information theory, and Fourier analysis.

References

  1. 1 2 3 4 5 6 7 8 Walke, Bernhard H. Mobile Radio Networks: Networking, protocols and traffic performance. West Sussex England: John Wiley, 2002. Chapter 2.
  2. 1 2 Guowang Miao; Jens Zander; Ki Won Sung; Ben Slimane (2016). Fundamentals of Mobile Data Networks. Cambridge University Press. ISBN   1107143217.
  3. 1 2 3 4 5 6 Key, P., and Smith, D. Teletraffic Engineering in a competitive world. Elsevier Science, Amsterdam Netherlands, 1999. Chapter 1 (Plenary) and 3 (Mobile).
  4. ↑ Teletraffic Issues Related to Channel Allocation in Digital Mobile Cellular Networks [ permanent dead link ]. Retrieved 15 March 2005.
  5. ↑ Chitamu, P. J., Telecommunications Access Networks. University of the Witwatersrand, Johannesburg, 2005.


This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.