The Cellular Concept and Its Implementations

The Cellular Concept The cellular concept was developed and introduced by the Bell Laboratories in the early 1970s. One of the most successful initial implementations of the cellular concept was the advanced mobile phone system (AMPS), which has been available in the United States since A cellular system is generally characterized as: a high capacity land mobile system in which available frequency spectrum is partitioned into discrete channels which are assigned in groups to geographic cells covering a cellular Geographic Service Area (GSA). The discrete channels are capable of being reused in different cells within the service area. Thus the principle of cellular systems is to divide a large geographic service area into cells with diameters from 2 to 50 km, each of which is allocated a number of radio frequency (RF) channels. Transmitters in each adjacent cell operate on different frequencies to avoid interference. Since, however, transmit power and antenna height in each cell are relatively low, cells that are sufficiently far apart can reuse the same set of frequencies without causing cochannel interference.

Multiple Access Technologies for Cellular System Generally a fixed amount of frequency spectrum is allocated to a cellular system by the national regulator (e.g., in the United States, the Federal Communications Commission). Multiple-access techniques are then deployed so that many users can share the available spectrum in an efficient manner. Multiple access systems specify how signals from different sources can be combined efficiently for transmission over a given radio frequency band and then separated at the destination without mutual interference. The three basic multiple access methods currently in use in cellular systems are: frequency division multiple access (FDMA) time division multiple access (TDMA) code division multiple access (CDMA) In case of FDMA, users share the available spectrum in the frequency domain, and a user is allocated a part of the frequency band called the traffic channel. The user's signal power is therefore concentrated in this relatively narrow band in the frequency domain,

and different users are assigned different traffic (frequency) channels on a demand basis. Interference from adjacent channels is limited by the use of guard bands and bandpass filters that maintain separation of signals associated with different users. In TDMA techniques that are utilized in many digital cellular systems, the available spectrum is partitioned into narrow frequency bands or frequency channels (as in FDMA), which in turn are divided into a number of time slots. An individual user is assigned a time slot that permits access to the frequency channel for the duration of the time slot. Thus, the traffic channel in case of TDMA consists of a time slot in a periodic train of time slots that make up a frame. In case of the North American digital cellular standard IS-136, each frequency channel (30 kHz) is divided into three time slots, whereas for the European digital cellular standard GSM, each frequency channel (200 kHz) is divided into eight time slots (full rate). In case of TDMA systems, guard bands are needed both between frequency channels and between time slots.

The CDMA systems utilizes the spread spectrum technique, whereby a spreading code (called a pseudo random noise or PN code) is used to allow multiple users to share a block of frequency spectrum. In CDMA cellular systems (e.g., IS-95 in the United States) that use direct sequence spread spectrum techniques, the (digital) information from an individual user is modulated by means of the unique PN code (spreading sequence) assigned to each user. All the PN code modulated signals from different users are then transmitted over the entire CDMA frequency channel (e.g., 1.23 MHz in case of IS-95). At the receiving end, the desired signal is recovered by despreading the signal with a copy of the spreading sequence (PN Code) for the individual user in the receiving correlator. All the other signals (belonging to other users), whose PN codes do not match that of the desired signal, are not despread and as a result, are perceived as noise by the correlator. Since the signals in the case of CDMA utilize the entire allocated block of spectrum, no guard bands of any kind are necessary within the allocated block.

Cellular System Operation and Planning: General Principles System Architecture As mentioned earlier, the coverage area of a cellular system is partitioned into a number of smaller areas or cells with each cell served by a base station(BS) for radio coverage. The base stations are connected through fixed links to a mobile switching center (MSC), which is a local switching exchange with additional features to handle mobility management requirements of a cellular system. The MSC interacts with some form of database that maintains subscriber data and location information. MSCs also interconnect with the PSTN, because the majority of calls in a cellular mobile system either originate from or terminate at fixed network terminals. Based on the frequency spectrum made available by the licensing authority and the cellular standard in use, the cellular system is able to define a number of radio channels for use across its serving area. The available radio channels are then partitioned into groups of channels, and these groups of channels are allocated to individual cells forming the

Entire serving area. Individual channels or a particular group of channels can be reused in cells that are located far enough apart. A key feature of radio system planning activity consists in designing the cell sizes, assigning their locations, and allocating radio channels to individual cells. Whereas definition of channels for assignment to individual cells within a cellular system is straightforward in the case of systems based on FDMA and TDMA methods, systems based on the spread spectrum CDMA technique require a different view of what constitutes a radio channel in this context. Location Updating and Call Setup To deliver an incoming call to a mobile station, the network (i.e., the MSC and the associated location database) must maintain information on the location of a mobile station as it moves through the coverage area. The mobile station monitors the overhead information broadcast by the network on the signaling channel and updates the operating parameters as necessary. It also checks the location information (e.g., location area identity) broadcast by the new cell and, if it differs from the previous cell, the mobile advises the network of its new location

Whereupon the network updates its location register(s). The information is then used to route incoming calls to the MSC currently serving the mobile and to determine the paging broadcast area from the mobile. The exact procedures for mobile organized and mobile terminated call setup depend on the technical standard deployed in a particular mobile system. The procedures described here, however, apply the most cases. A mobile user organized a call by keying in the called number and depressing the send key. Note that there is no equivalent of dial tone in a cellular system. The mobile transmits an access request on the uplink signaling channel. If the network can process the call, BS sends a speech channel allocation message, which enables the mobile to lock on the designated speech channel allocated to that cell while the network proceeds to setup the connection to the called party. A terminal validation procedure may also be invoked as part of the originating call setup to ensure that the terminal originating the call is a legitimate terminal.

Handoff and Power Control During a call, the serving BS monitors the signal quality/strength from the mobile. If the signal quality/strength falls below a predestinated threshold, the network requests the neighboring base stations to measure the signal quality from the mobile. If another BS indicates better signal quality/strength than the serving BS, a signaling message is sent to the mobile on the speech channel from the current BS asking the mobile to retune to a free channel in the neighboring cell. The mobile retunes to the new channel, and simultaneously the network switches the call to the new BS. Signal quality measurements and new cell selection generally take several seconds, but the change of speech channels (handoff) is essentially transparent to the user except for a very brief break in transmission in FDMA or TDMA based systems. Generally the size of cells within a given cellular system may vary from a radius of 1 km (inner city) to more than 20 km (rural). Thus it is not necessary for the mobile station to transmit at full power at all times to maintain satisfactory signal level at the BS.