Report On Cellular Communication Network Computer Science Essay

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Tracing a person who is in remote place is not very difficult nowadays. The use of cellular systems helps us to make connection with anybody having mobile phone. The condition of this connectivity is very different from the one we had in 1940s when concept of cellular phones was discovered. [1] Cellular phones are widely used nowadays and with each passage of day the subscribers are increased. Survey showed that in UK in end of 1992 there were 1.4 million subscribers [2] and in 1993 he number became 1.8million which shows a very high increase in number. [3]

A cellular communication system consists of large number of wireless transmitters which are of low power to make cells. These cells are the basic unit of such systems. The size of the cell can be varied by varying power levels according to the density and demand of users. When a mobile user move from one cell to the other, the cell 'hand offs' ongoing transmission to other cell to continue the transmission. [4]

Background

Development of cellular systems started in early 1920. It was first used by Detroit Michigan Police Dept. in their vehicles. [5] However in that system the biggest problem was that the channels were getting crowded very quickly. Then US implemented first public mobile communication system which had three channels at 150MHz [5]. The problem of interference was still present in that system though. In 1920, the mobile systems were using Morse coded on-off keying. [6] In early systems the mobile phones were huge, heavy and the communication was noisy.

In the period of 1950 to 1960 there were improvements in cellular systems because there was increase in bandwidth utilized and switching technology was improved. In around 1956, twelve channels were used around 450MHz. [4] In 1970 about 7gMHz was allocated for cellular communication. During 1974 to 1981 AT & T labs with the collaboration of other cellular vendors launched their cellular phones. On 13th October 1983, the first call was made on commercial cellular system [4]. After 1980s big companies like Nokia, Ericsson, Motorola etc adopted new technologies to maximize the quality of calls. Nowadays a cell phone can be connected to internet, it can be used to send and receive emails. WAP (Wireless Application Protocol) helps in accessing internet information on phone even one can do online shopping via WAP on cell phone. There are digital as well as analogue cellular technologies available. In analogue systems, AMPS (Advanced Mobile Phone System) is currently very important. It was first used in 1983 and it works in 800MHz band [7]. After 1980s the trend shifted towards digital systems. GSM was first digital system introduced.

Structure

The structure of a cellular network includes following components.

Mobile Station: This is a cell phone normally.

Base Station: Area of a cellular network is divided into small cells. A Base Station is present in each cell which is responsible for the communication with cell phones in its vicinity and also sends traffic to the Mobile Switching Centre.

Mobile Switching Centre: This ensures inter-cell communications and assigns channels for the BS and cell phones.

National Carrier Exchange: This is a gateway to the national fixed public switched telephone network (PSTN). This is normally part of MSC.

Figure : Structure of Cellular Network [12]

Standards

International Standard Bodies

ITU (International Telecommunication Union) developed standards for Modem over voice lines. It made regulations on international use of telephone, telegraph, and radio and satellite communications. IEEE-SA (The Standard Association of Institute of Electrical and Electronics Engineers) and ISO (International Standards Organization) developed LAN (Local Area Network). IEEE-SA has some leading developments in telecommunication. IEEE 802 works on local and metropolitan networks. IEEE 802.11 is wireless LAN group. This includes 802.11 a/b/g which are standards of wireless LAN. 802.11n is high speed data network. 802.15 is the standard of WPAN (Wireless Personal Area Network). 802.16 groups includes WiMax. 802.20 include mobile broadband wireless standards.

Standards in different areas

In US, AMPS system was developed in 1970. In North America TDMA was developed by TIA which become IS-54 standard. Cdmaone was developed by TIA and it become IS-95 standard. TIA is responsible for wireless communication standards. TR45 (Public Mobile) and TR46 (Personal Communication) are two main committees of ITA. TR45 includes six committees. TR45.1: For AMPS standardization

TR45.2: For IS-41 standards

TR45.3: For IS-136, US TDMA, UWC-136

TR45.4: For developing the interface between access and core networks.

TR45.5: For IS-95 and its evaluation to cdma2000.

TR45.6: Defines network architecture for IMT-2000

GSM was developed in Europe. There are twelve committees under one main committee which is responsible for each perspective of GSM standard.

In Japan, IMT-2000 standardization is done by two organizations ARIB and TTC [14].

Generations

Generations of cell phones started from 0G and went till 5G. 0G is basically pre-cell phone technology for example radio and mobile rings used in cabs, ambulances and police cars. The first commercial mobile phone was launched by NTT Japan in 1979. The second launch of 1G mobile was simultaneous in 1981 in Finland, Norway, Sweden and Denmark [8]. The very first mobile launched to US market was Motorola DynaTAC 8000X which is also 1G mobile phone. The first digital mobile was launched in 1991 and it was GSM based mobile. SMS service was started in 1993. These 2G services are known as PCS (Personal Communication Service). In 3G mobiles high speed downlink packet access techniques were implemented and they were launched in 2001 by NTT Japan. The 4G is based on WiMax technology which will provide users the facilities like IP telephony, broadband internet access, gaming on mobile phone, other streamed multimedia, real time audio, mobile TV etc. 5G technology is still not launched however the main idea is that the global roaming is enabled on a single network.

Design Objectives

Congestion Control

Normally in a cellular network the traffic load is around or most of the times less the maximum load point defined but sometimes the load can cross the maximum limit due to the dynamicity of the network. Congestion occurs when the user's demands are more than the network capacity. The causes of this congestion on the network can be maximum network utilization during specific time frames, voice and data traffic's dynamic patterns, inconsistent behavior of the users calls etc. For example if too many users from a cell are moved away from the base station and station is trying to provide them the connection via the power control technique there is great chance that either BS will not have much power to facilitate them all because of the distance and the interference by other cells. In this scenario there is a great chance that the congestion in the network will took place [9]. The congestion will lead to decrease in quality of service, increase in reception delays and increase in error rate. Packet will be discarded in the result of all these and there will be retransmission of packets and overall throughput of the network will fall down.

Congestion control technique is required to prevent the congestion and if it happened to recover from it. The CC technique is very important for the reliability and stability of the network. It ensures both the fairness and efficiency to the users. CC technique needs to know the network traffic loads all the time so that it can ask users to reduce their traffic if demand is getting close to capacity of the network and vice versa. To avoid false alarms of the congestion and failure in congestion detection CC technique works on the average values.

Efficient spectrum utilization

Efficient utilization of spectrum means that to which extent information is transmitted over a given bandwidth of a system [10]. Handling of spectrum utilization is the biggest drawback of third generation systems. The channel allocation problem means that what is the best assignment of channels to the cells, keeping in mind the load in the cell and the structure of the cell. Same channels can be assigned to cells present at specific distance to ensure less interference. Different channel assignment techniques are used to efficiently handle frequencies according to the load [11].

Different techniques are used for efficient utilization of spectrum. These include frequency reuse, Microcells and multiple access system [12]. Frequencies re-use means that assign multiple channels to BS but control the interference via the power control for different channels. This ensures increased capacity to the users because more number of channels is available in a cell. Neighboring cells cannot work on the same frequency because of the interference. One option for frequency re-use is that create small size cells so that more number of cells are created in the system and channels are used in different cells. But small cells increased interference and handover. Increased handovers means that the chance of a mobile user to change the cell during the conversation is very high if cell size is small and this will introduce lots of delays in communications.

To overcome the interference and handovers problem concept of microcells is used. This means that each cell is divided into small units called microcells. Microcells contain a microcell site and cell contains BS for communication. Important is that there is same frequency used for each microcell within a cell so there is no handover delay. The procedure is that mobile phone sends signal to each microcell which sends signal to BS. Depending on the microcell having the strongest signal, BS allows cell to communicate to that microcell. The advantages of microcells include low interference and handovers and increased system capacity. However transmitted power to a mobile phone should be precise else it will create interference between the microcells [12].

In digital cellular systems a concept of multiple access systems is used for sharing one channel by simultaneous users within a cell. Time division multiple access (TDMA) and frequency division multiple access (FDMA) schemes are used for this purpose.

Fading

Fading reduces the power of the signals in the area. Two main reasons of fading include multipath fading and shielding. Multipath fading occurs when signal takes different paths when these signals reach at the destination they either cancel or attenuate each other. Shielding is terminology used for absence of signal at a particular region due to hills, buildings etc. To solve these problems BS should have an Equalizer [13] to take care of fading. Equalizer helps retrieving the original signal which gets infected by multipath fading or/and shielding.

Figure : Fading Effect [12]

Minimization of co-channel interference

Co-channel interference is created when same frequency is used by different cells. To avoid these cells having same frequency should have maximum distance. But the problem is that by doing this channel capacity will be compromised. To solve this interference concept of microcells can be used as discussed earlier. Another way of solving this problem is use of equalizer.

Conclusion

Cellular Communication Systems are very important for communication to remote and mobile users. These systems made it possible to get track of persons even if they are not near wired network. Cellular devices have passed through several generations and there are lots of standards devised for such devices. However there are still many issues which need to be addressed or which needs to have appropriate solution to them. The report shows that while designing a cellular communication system what are different design perspectives one needs to take care off. How to resolve the issues one can face while setting up cellular system.

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