25-09-2012, 10:56 AM
THROUGHPUT IMPROVEMENT IN CELLULAR NETWORK
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INTRODUCTION
The basic premise of a cellular network is that you can have a communication device that you can take with you, starting with car phones which have evolved into hand-held phones. Over time, hand-held phones have been getting smaller, gaining longer battery life, and getting new features like paging. Network coverage and capacity have also increased to the point where cellular service is available almost anywhere and at any time to those that want to use it. Cellular service has seen tremendous acceptance, especially in the last few years, with millions of new subscribers each year and the new subscriber rate growing. Conservative estimates predict that half a cellular phones will be in service by the end of the next decade, although the number may be well over a billion if recent estimates by European cellular providers are correct.
The cellular concept is a system level idea in which a single, high power transmitter (large cell) is replaced with many low power transmitters (small cells). The area serviced by a transmitter is called a cell. A cell is a relatively small area that is serviced by a single transmitter/receiver unit (often called a cell site). Mobile phones operating within this area use that cell site to communicate with the rest of the cellular network (and with the public phone network).
History of Cellular Service
Cellular service was invented by Bell Laboratories and introduced around 1980, based on radio-telephone systems that dated back to the 1940’s. The Bell Labs offering became the basis for the Advanced Mobile Phone System (AMPS), which is the current standard for U.S. cellular service. It is the least common denominator of U.S. cellular service, and the FCC has mandated that all U.S. cellular phones must fall back to AMPS service at least until the year 2002.
Many other nations adopted variants of AMPS service, such as Nordic Mobile Telephone (NMT), first introduced in Scandinavia, Total Access Communication System (TACS), first introduced in Britain, and other systems introduced in France, Italy, and Germany. The protocols and communications standards used by each of these varied slightly, so that the various European analogy systems were not compatible with each other.
The Evolution of Cellular Service
Most U.S. cellular service still uses the same AMPS analog technology that was used in the earliest mobile telephones, although digital cellular service is rapidly gaining popularity. The key motivator for this is that digital cellular networks can offer more subscriber channels over the same radio bandwidth, although digital networks can offer additional services as well. Unlike ISDN, ATM, and other innovations in regular telephony, cellular systems have fewer infrastructures to be replaced when new ideas are developed. Coupled with the rapid growth of cellular service, it is possible to make sweeping changes in the basic nature of cellular service. In fact, it is perhaps too easy to change, causing much debate over what the cellular network of the future should look like. A large division is over whether to use time division multiple access (TDMA) methods over existing analog frequencies or to use spread-spectrum code division multiple access (CDMA) methods. One thing that is certain, however, is that cellular service is quickly going from analog systems to digital systems.
Digital Cellular Service
Europe was the first to embrace digital service with the Global System for Mobile communication (GSM). The incompatible existing analog system in the 1980’s made it impossible to use a single mobile phone in several European countries. With the European Union and increased trade and commerce throughout Europe, a need was seen for a single European standard. To choose one of the existing standards would have given an unfair advantage to those that already provided that service, so it made sense to create an entirely new service that could take advantage of technological advances since the advent of cellular service. It established the GSM standard in the 1980’s; GSM was first implemented in 1992-1993. This all-digital standard became the least common denominator of service in Europe, and is quickly replacing the analog systems currently in place. U.S. digital systems have also recently emerged, with IS-54 (also called D-AMPS or U.S. TDMA) systems already in place, soon to be replaced by IS-136 systems (the successor to IS-54). There is also one major cellular service provider that is putting a competing system in place called IS-95 (or U.S. CDMA). There are also two important Japanese digital standards, the Personal Handyphone System (PHS) and Personal Digital Cellular (PDC), which both use TDMA like IS-54/136 and GSM.
Basic Network Operations
Traditional mobile phone service has used only terrestrial radio. In other words, it relies on ground-based cell sites, which are usually small towers with three antennas arranged in a triangle. Satellite implementations are possible, although they cannot use radio bandwidth as efficiently. Thus, they are used commercially primarily for pager, broadcast, and some specific site-to-site links.A cellular network is designed to connect to the existing phone system (also called the Public Switched Telephone Network or PSTN) or potentially to a data network (called a Public Data Network or PDN). The connection to the PSTN is not much different than the connection of other telephone switching equipment such as a Public Branch Exchange (PBX).
Cellular networks are comprised of terminals and base stations. Terminals are the end-user equipment, usually phones, and are often called Mobile Stations. Everything else in a cellular network is considered to be base station equipment.
Base Stations
There are three components of base stations. The cell sites, which are often referred to as base transceiver stations or BTSs, communicate directly with the end-user terminals. Base station controllers or BSCs control the base transceiver stations either over land links (typically) or over radio links. Mobile switching centers or MSCs, often called mobile telephone switching offices, control the base station controllers, usually over land links. There is no fixed ratio of BTS to BSC to MSC, although there are typically about five to ten BTSs per BSC and anywhere from one to ten BSCs per MSC, depending on the capacity needs and geographic distribution of an area. In fact, base station functions may be combined into a single site, especially in the more remote areas where a single site might serve as BTS, BSC, and MSC.
The following figure demonstrates six cell sites communicating over radio links to two base station controllers, which in turn communicate with a single mobile switching center that is connected to the public switched telephone network (the “regular” phone company) usually over land links. There is no fixed ratio of BTS to BSC to MSC, although there are typically about five to ten BTSs per BSC and anywhere from one to ten BSCs per MSC, depending on the capacity needs and geographic distribution of an area. In fact, base station functions may be combined into a single site, especially in the more remote areas where a single site might serve as BTS, BSC, and MSC. The following figure demonstrates six cell sites communicating over radio links to two base station controllers, which in turn communicate with a single mobile switching center that is connected to the public switched telephone network (the “regular” phone company). Cell sites have one antenna up for transmitting to the terminals and two down for receiving from the terminals: the two down antennas allow it to work like a bigger antenna and help counter multipath effects (which are described later). They generally operate around 900MHz, though other frequencies are also used (especially for PCS).
Cell Organization
Cellular communication got its name because of the “cell” structure of the base transceiver station service areas. For convenience, a service area is often subdivided into an array of hexagonal cells, each containing a single BTS. As the industry is maturing, the service areas of individual BTSs are being calculated more precisely (including such considerations as reflections off buildings) and they are being placed where they can be the most effective rather than in traditional “cells”, although the term “cellular” is still used. The arrangement of the cells is basically a form of space division multiple access (SDMA). Since frequencies can be reused by other cells that are far enough away not to interfere with the current cell. The degree of reuse (determined by how far apart cells must be to reuse the same frequency) is dependent upon the actual implementation of the radio link.
Organization of Project
The dissertation is organized in the following manner:
In Chapter 2 we discuss about the problem definition and objective of the project. The extensive literature review is done for this project is covered in chapter 3. In chapter 4 we discuss about challenges in cellular network and MIMO. Channel capacity, spatial capacity and other important concepts of MIMO are described. In chapter 5 cooperative communications in wireless network, various signaling methods used at present time and also related issues is being discussed. Chapter 6 is composed of system model, signal to interference noise ratio (SINR) and user throughput with and without cooperation. Also describes the SINR for different modes of Co-operation. Chapter 7 presents the proposed cooperation selection scheme and explains the algorithm implemented for the proposed method. Chapter 8 presents simulation results with GUI (graphics user interface) main window. Finally chapter 9 presents Conclusions with a brief mention of the future scope are presented followed by references.