19-03-2012, 12:11 PM
Advanced Level Telecom Training Centre (ALTTC)
CHAPTER-1
INTRODUCTION
1.1 About The Organisation
Advanced Level Telecom Training Centre (ALTTC), Ghaziabad is the apex training institute of BSNL. An ISO 9001: 2008 accredited institute, ALTTC was set up as a joint venture of International Telecommunication Union, Geneva, UNDP and the Government of India in 1975. ALTTC functions on the frontiers of telecom technology, finance and management and imparts training to the leaders in the business. The strength of ALTTC lies in the state of art labs, massive infrastructure and trained, talented and qualified human resource pool.
The Centre's Mission statement is "To Deliver Excellence Through Training".
Academic Block, ALTTC Ghaziabad
The training areas cover vast spectrum of topics such as Digital Switching and IN, Mobile Communication: GSM, 3G CDMA Data communication: Broadband and Networking, Optical Networks: SDH, DWDM, NGN, Access Networks, Management, Telecom Finance, Information Technology, Building Science (Civil and Electrical) and Telecom Network Planning.
CHAPTER-2
INTRODUCTION TO CELLULAR MOBILE RADIO COMMUNICATION
A cellular radio system provides a wireless connection to the public telephone network for any user location within the radio range of the system. The term mobile has traditionally been used to classify a radio terminal that can be moved during communication. Cellular systems accommodate a large number of mobile units over a large area within a limited frequency spectrum. There are several types of radio transmission systems. We consider only full duplex systems. These are communication systems that allow simultaneous two-way communication. Transmission and reception for a full duplex system are typically on two different channels, so the user may constantly transmit while receiving signals from another user. Figure 1.1 shows a basic cellular system that consists of mobiles, base stations, and a switching center. Each mobile communicates via radio with one or more base stations. A call from a user can be transferred from one base station to another during the call. The process of transferring is called handoff.
Each mobile contains a transceiver (transmitter and receiver), an antenna, and control circuitry. The base stations consist of several transmitters and receivers, which simultaneously handle full duplex communications and generally have towers that support several transmitting and receiving antennas. The base station connects the simultaneous mobile calls via telephone lines, microwave links, or fiber-optic cables to the switching center. The switching center coordinates the activity of all of the base stations and connects the entire cellular system to the public telephone network.
2.1 CHANNELS
The channels used for transmission from the base station to the mobiles are called forward or downlink channels, and the channels used for transmission from the mobiles to the base station are called reverse or uplink channels. The two channels responsible for call initiation and service request are the forward control channel and reverse control channel.
Once a call is in progress, the switching center adjusts the transmitted power of the mobile (this process is called power control) and changes the channel of the mobile and base station (handoff) to maintain call quality as the mobile moves in and out of range of a given base station.
The cellular concept was a major breakthrough in solving the problem of spectral congestion. It offered high system capacity with a limited spectrum allocation.
In a modern conventional mobile radio communication system, each base station is allocated a portion of the total number of channels available to the entire system and nearby base stations are assigned different groups of channels so that all the available channels are assigned to a relatively small number of neighboring base stations. Neighboring base stations are assigned different groups of channels so that interference between the users in different cells is small.
The idealized allocation of cellular channels is illustrated in Figure 2, in which the cells are shown as contiguous hexagons. Cells labeled with the same number use the same group of channels. The same channels are never reused in contiguous cells but may be reused by noncontiguous cells. The κ cells that collectively use the complete set of available frequencies is called a cluster. In Figure 1.2, a cell cluster is outlined in bold and replicated over the coverage area. Two cells that employ the same allocation, and hence can interfere with each other, are
separated by more than one cell diameter. The factor κ is called the cluster size and is typically equal to 3, 4, 7, or 12.
To maximize the capacity over a given coverage area we have to choose the smallest possible value of κ. The factor 1/κ is called the frequency reuse factor of a cellular system. In Figure 1.2 the cluster size is equal to 7, and the frequency reuse factor is equal to 1/7.
The other-cell interference can be reduced by employing sectored antennas at the base station, with each sector using different frequency bands. However, using sectored antennas does not increase the number of slots and consequently the frequency reuse factor is not increased.
A multiple access system that is more tolerant to interference can be designed by using digital modulation techniques at the transmitter (including both source coding and channel error-correcting coding) and the corresponding signal processing techniques at the receiver.
2.2 FREQUENCY DIVISION AND TIME DIVISION MULTIPLE ACCESS
Multiple access schemes are used to allow many mobile users to share simultaneously a common bandwidth. As mentioned above, a full duplex communication system typically provides two distinct bands of frequencies (channels) for every user. The forward band provides traffic from the base station to the mobile, and the reverse band provides traffic from the mobile to the base station. Therefore, any duplex channel actually consists of two simplex channels.
Frequency division multiple access (FDMA) and time division multiple access (TDMA) are the two major access techniques used to share the available bandwidth in a conventional mobile radio communication systems.
2.2.1 Frequency Division Multiple Access (FDMA)
Frequency division multiple access assigns individual channels (frequency bands) to individual users. It can be seen from Figure 3 that each user is allocated a unique frequency band. These bands are assigned on demand to users who request service. During the period of the call, no other user can share the same frequency band. The bandwidths of FDMA channels are relatively narrow (25–30 kHz) as each channel supports only one call per carrier. That is, FDMA is usually implemented in narrowband systems. If an FDMA channel is not in use (for example, during pauses in telephone conversation) it sits idle and cannot be used by other users to increase the system capacity.
2.2.2 Time Division Multiple Access (TDMA)
Time division multiple access systems divide the transmission time into time slots, and in each slot only one user is allowed to either transmit or receive. It can be seen from Figure 4 that each user occupies cyclically repeating wording, so a channel may be thought of as a particular time slot that reoccurs at slot locations in every frame. Unlike in FDMA systems, which can accommodate analog frequency modulation (FM), digital data and digital modulation must be used with TDMA.
TDMA shares a single carrier frequency with several users, where each user makes use of non-overlapping time slots. Analogously to FDMA, if a channel is not in use, then the corresponding time slots sit idle and cannot be used by other users. Data transmission for users of a TDMA system is not continuous but occurs in bursts. Because of burst transmission, synchronization overhead is required in TDMA systems. In addition, guard slots are necessary to separate users. Generally, the complexity of TDMA mobile systems is higher compared with FDMA systems.