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5.3: Cellular Communications

  • Page ID
    41217
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    Cellular communications, as the name implies, are based on the concept of cells in which a terminal unit communicates with a basestation at the center of a cell. Each cell can be relatively small and a terminal unit travels smoothly from cell to cell with a connection transferring using what is called a hand-off process. For communication in closely spaced cells to work, interference from other radios must be managed. This is facilitated using the ability to recover from errors available with error correction schemes and the use of antenna beam-forming technology.

    5.3.1 Cellular Concept

    The cellular concept was outlined in a 1947 Bell Laboratories technical memorandum [4]. It described a system of frequency reuse with small geographical cells, and this remains the key concept of cellular radio. This was elaborated on in two articles published in 1957 and 1960 [5, 6]. The first widespread cellular radio system was the Advanced Mobile Phone System (AMPS), one of many 1G cellular radio systems, and was fully described by Bell Laboratories in a submission to the U.S. Federal Communications Commission (FCC) and in a patent filed on December 21, 1970 [7]. Bell Labs petitioned the FCC in 1958 for a frequency band around \(800\text{ MHz}\) for a cellular system. The FCC, believing that it was better to allocate spectrum for the public good (such as radio, television, and emergency services) was reluctant to act on the petition. In 1968, pressure on the FCC became too great and an agreement was reached in principle to make frequencies available. Thus began the research and development of cellular systems in the United States. In 1961, Ericsson reorganized to address mobile radio, including cellular radio systems. Nokia did not begin developing 1G cellular systems until the 1970s. NTT was working away as well and began developing cellular radio systems in 1967 [8]. Meanwhile, in January 1969, the Bell System launched an experimental cellular radio system employing frequency reuse for the first time to achieve optimum use of a limited number of RF channels. The first commercial cellular system was launched by the Bahrain Telephone Company in May 1978 using Matsushita equipment. This was followed by the launch of AMPS by Illinois Bell and AT&T in the United States in July 1978.

    In 1979 the World Administrative Radio Conference (WARC) allocated the \(862–960\text{ MHz}\) band for mobile radio, leading to the FCC releasing, in 1981, \(40\text{ MHz}\) in the \(800–900\text{ MHz}\) band for “cellular land-mobile phone service.” The service, as defined in the original documents, is (and this is still the best definition of cellular radio)

    a high capacity land mobile system in which assigned spectrum is divided into discrete channels which are assigned in groups to geographic cells covering a cellular geographic area. The discrete channels are capable of being reused within the service area.

    The key attributes here are

    clipboard_eeaeabf3b229173d934e2a85be8168811.png

    Figure \(\PageIndex{1}\): Cells arranged in clusters.

    • High capacity. Prior to he cellular system, mobile radio users were not always able to gain access to the radio network and frequently access required multiple attempts.
    • The concept of cells. The idea is to divide a large geographic area into cells, shown as the hexagons in Figure \(\PageIndex{1}\). The actual shape of the cells is influenced by obstructions such as hills and buildings, but the hexagonal shape is used to convey the concept of cells. The cells are arranged in clusters and the total number of channels available is divided among the cells in a cluster and the full set is repeated in each cluster. In Figure \(\PageIndex{1}\), \(3-,\: 7-,\) and \(12\)-cell clusters are shown. As will be explained later, the number of cells in a cluster affects both capacity (the fewer cells the better) and interference (the more cells per cluster, the further apart cells operating at the same frequency are, and so interference is less).
    • Frequency reuse. Frequencies used in one cell are reused in the corresponding cell in another cluster. As the cells are relatively close, it is important to dynamically control the power radiated by each radio, as radios in one cell will produce interference in other clusters.

    The shape of a cell depends on many factors. In a flat desert the coverage area of each basestation would be circular, so that with a cluster of cells there would be overlapping circles of coverage. (Power levels are adjusted to minimize the overlap of these circles.) Buildings, hills, lakes, etc., affect cell size. In a city, what is called the urban canyon effect, or urban waveguide effect greatly distorts cells and creates havoc in managing cellular systems [9, 10, 11]. In the urban canyon effect, good coverage extends for large distances down a street (see the inside front cover).

    Achieving maximum frequency reuse is essential in achieving high capacity. In a conventional wireless system, be it broadcast or the mobile telephone service, basestations are separated by sufficient distance such that the signal levels fall below a noise threshold before the same frequencies are reused, as shown in Figure \(\PageIndex{2}\). There is clearly poor geographical use of the

    clipboard_e0bc64bcaa5d649fb6edd712f116537cc.png

    Figure \(\PageIndex{2}\): Interference in a conventional radio system. The two transmitters, \(\mathsf{1}\), are at the centers of the coverage circles defined by the background noise threshold.

    clipboard_e4317ac647b39b7917c2ad03ca9d8a4ce.png

    Figure \(\PageIndex{3}\): Interference in a cellular radio system.

    spectrum here. The geographic areas could be pushed closer to each other at the expense of introducing what is called cochannel interference—a receiver could pick up transmissions from two or more basestations operating at the same frequency. This is strictly avoided so that interference in conventional radio systems results solely from background noise. In a cellular system, there is a radical departure in concept from this. Consider the interference in a cellular system as shown in Figure \(\PageIndex{3}\). The signals in corresponding cells in different clusters interfere with each other and the interference is much larger than that of the background noise. Generally only in rural areas and when mobile units are near the boundaries of cells will the background noise level be significant. Interference can also be controlled by dynamically adjusting the basestation and mobile transmit powers to the minimum acceptable level. Tolerating interference from neighboring clusters is a key concept in cellular radio.

    The analog 1G AMPS system, which uses frequency modulation, has a qualitative minimum SIR of \(17\text{ dB}\) (about a factor of \(50\)) that was determined via subjective tests with a criterion that \(75\%\) of listeners ranked the voice quality as good or excellent. The seven-cell clustering shown in Figure \(\PageIndex{3}\)

    clipboard_e961a7ddfccc5df8a0f3e97d548ee61c9.png

    Figure \(\PageIndex{4}\): Process of handoff: (a) movement of a mobile unit through cells; (b) received signal strength indicator (RSSI) during movement of a mobile unit through cells; and (c) RSSI of a mobile unit showing the handoff triggering event.

    does not yield this required minimum SIR. So either a \(12\)-cell cluster is required or directed antennas are used, as these provide enough SIR.

    The 2G and 3G digital cellular systems use error correction coding and can tolerate high levels of interference and can reuse frequency channels more efficiently. Indeed, in the 3G CDMA system the tolerance to interference is so high that the concept of clustering is not required and every frequency channel is available in each cell.

    In 4G and 5G cellular radio interference must be low to enable high-order modulation to be used. The high modulation efficiencies more than makes up for the reduced frequency reuse.

    5.3.2 Personal Communication Services

    The personal communication services (PCS) concept was implemented in the early 1990s and was a development in the thinking of cellular communications. In PCS, the concept is that communication is from person to person, whereas in cellular radio communication as originally conceived, it was from terminal-to-terminal. The idea is that when a call is placed, an individual is being contacted rather than a piece of hardware. One way this is achieved is by using a card, a subscriber identification module (SIM card), to identify the user. A user can insert his or her SIM card into any (appropriate) handset and the handset becomes personalized. The term PCS is not commonly used now, as the concept has been incorporated in all evolved cellular systems.

    5.3.3 Call Flow and Handoff

    Mobile users can be expected to move frequently between cells and thus handoff procedures for transferring connections from one cell to the next are necessary. The triggering events that initiate handoff are shown in Figure \(\PageIndex{4}\). The main aspect is monitoring of the signal strength, the received signal strength indicator (RSSI), both in the handset for the signal received from a basestation and in the basestation for the signal received from a handset. If either of these falls below a threshold, computers in the basestation initiate a handoff procedure by polling nearby basestations for the RSSI they have for the user. If a suitable RSSI is found handoff proceeds and the other basestation takes control of the RF link to the mobile terminal.

    5.3.4 Cochannel Interference

    The minimum signal detectable in conventional wireless systems is determined by the received SIR. In cellular wireless systems the dominant interference is due to other transmitters in the cell and adjacent cells. The noise that is produced in the signal band from other transmitters operating at the same frequency is called cochannel interference. The level of cochannel interference is dependent on cell placement and the frequency reuse pattern. The degree to which cochannel interference can be controlled has a large effect on system capacity. Control of cochannel interference is largely achieved by controlling the power levels at the basestation and at the mobile units.


    This page titled 5.3: Cellular Communications is shared under a CC BY-NC license and was authored, remixed, and/or curated by Michael Steer.

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