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Cellular Communications
Definition
A cellular mobile communications system uses a large number of low-power wireless transmitters to create cells—the basic geographic service area of a wireless communications system. Variable power levels allow cells to be sized according to the subscriber density and demand within a particular region. As mobile users travel from cell to cell, their conversations are "handed off" between cells in order to maintain seamless service. Channels (frequencies) used in one cell can be reused in another cell some distance away. Cells can be added to accommodate growth, creating new cells in unserved areas or overlaying cells in existing areas.
Overview
This tutorial discusses the basics of radio telephony systems, including both analog and digital systems. Upon completion of this tutorial, you should be able to accomplish the following:
- describe the basic components of a cellular system
- identify and describe digital wireless technologies
Topics
- Mobile Communicatio ns Principles
- Mobile Telephone System Using the Cellular Concept
- Cellular System Architecture
- North American Analog Cellular Systems
- Cellular System Components
- Digital Systems Self- Test Correct Answers
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Acronym Guide
1. Mobile Communications Principles
Each mobile uses a separate, temporary radio channel to talk to the cell site. The cell site talks to many mobiles at once, using one channel per mobile. Channels use a pair of frequencies for communication—one frequency, the forward link, for transmitting from the cell site, and one frequency, the reverse link, for the cell site to receive calls from the users. Radio energy dissipates over distance, so mobiles must stay near the base station to maintain communications. The basic structure of mobile networks include telephone systems and radio services. Where mobile radio service operates in a closed network and has no access to the telephone system, mobile telephone service allows interconnection to the telephone network (see Figure 1).
Figure 1: Basic Mobile Telephone Service Network
Early Mobile Telephone System Architecture
Traditional mobile service was structured similar to television broadcasting: One very powerful transmitter located at the highest spot in an area would broadcast in a radius of up to fifty kilometers. The cellular concept" structured the mobile telephone network in a different way. Instead of using one powerful transmitter, many low-power transmitters were placed throughout a coverage area. For example, by dividing a metropolitan region into one hundred different areas (cells) with low-power transmitters using twelve conversations (channels) each,
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off from cell to cell to maintain constant phone service as the user moves between cells (see Figure 3).
Figure 3: Mobile Telephone System Using a Cellular Architecture
The cellular radio equipment (base station) can communicate with mobiles as long as they are within range. Radio energy dissipates over distance, so the mobiles must be within the operating range of the base station. Like the early mobile radio system, the base station communicates with mobiles via a channel. The channel is made of two frequencies, one for transmitting to the base station and one to receive information from the base station.
3. Cellular System Architecture
Increases in demand and the poor quality of existing service led mobile service providers to research ways to improve the quality of service and to support more users in their systems. Because the amount of frequency spectrum available for mobile cellular use was limited, efficient use of the required frequencies was needed for mobile cellular coverage. In modern cellular telephony, rural and urban regions are divided into areas according to specific provisioning guidelines. Deployment parameters, such as amount of cell-splitting and cell sizes, are determined by engineers experienced in cellular system architecture.
Provisioning for each region is planned according to an engineering plan that includes cells, clusters, frequency reuse, and handovers.
Cells
A cell is the basic geographic unit of a cellular system.
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The term cellular comes from the honeycomb shape of the areas into which a coverage region is divided. Cells are base stations transmitting over small geographic areas that are represented as hexagons. Each cell size varies depending on the landscape. Because of constraints imposed by natural terrain and man-made structures, the true shape of cells is not a perfect hexagon.
Clusters
A cluster is a group of cells. No channels are reused within a cluster. Figure 4 illustrates a seven-cell cluster.
Figure 4: A Seven-Cell Cluster
Frequency Reuse
Because only a small number of radio channel frequencies were available for mobile systems, engineers had to find a way to reuse radio channels in order to carry more than one conversation at a time. The solution the industry adopted was called frequency planning or frequency reuse. Frequency reuse was implemented by restructuring the mobile telephone system architecture into the cellular concept.
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Figure 6: Cell Splitting
Handoff
The final obstacle in the development of the cellular network involved the problem created when a mobile subscriber traveled from one cell to another during a call. As adjacent areas do not use the same radio channels, a call must either be dropped or transferred from one radio channel to another when a user crosses the line between adjacent cells. Because dropping the call is unacceptable, the process of handoff was created. Handoff occurs when the mobile telephone network automatically transfers a call from radio channel to radio channel as a mobile crosses adjacent cells (see Figure 7).
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Figure 7: Handoff between Adjacent Cells
During a call, two parties are on one voice channel. When the mobile unit moves out of the coverage area of a given cell site, the reception becomes weak. At this point, the cell site in use requests a handoff. The system switches the call to a stronger-frequency channel in a new site without interrupting the call or alerting the user. The call continues as long as the user is talking, and the user does not notice the handoff at all.
4. North American Analog Cellular
Systems
Originally devised in the late 1970s to early 1980s, analog systems have been revised somewhat since that time and operate in the 800-MHz range. A group of government, telco, and equipment manufacturers worked together as a committee to develop a set of rules (protocols) that govern how cellular subscriber units (mobiles) communicate with the "cellular system." System development takes into consideration many different, and often opposing, requirements for the system, and often a compromise between conflicting requirements results. Cellular development involves some basic topics:
- frequency and channel assignments
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of low calling capacity. NAMPS is now operational in 35 U.S. and overseas markets and NAMPS was introduced as an interim solution to capacity problems. NAMPS is a U.S. cellular radio system that combines existing voice processing with digital signaling, tripling the capacity of today's AMPS systems. The NAMPS concept uses frequency division to get three channels in the AMPS 30-kHz single channel bandwidth. NAMPS provides three users in an AMPS channel by dividing the 30-kHz AMPS bandwidth into three 10-kHz channels. This increases the possibility of interference because channel bandwidth is reduced.
5. Cellular System Components
The cellular system offers mobile and portable telephone stations the same service provided fixed stations over conventional wired loops. It has the capacity to serve tens of thousands of subscribers in a major metropolitan area. The cellular communications system consists of the following four major components that work together to provide mobile service to subscribers (see Figure 8):
- public switched telephone network (PSTN)
- mobile telephone switching office (MTSO)
- cell site with antenna system
- mobile subscriber unit (MSU)
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Figure 8: Cellular System Components
PSTN
The PSTN is made up of local networks, the exchange area networks, and the long-haul network that interconnect telephones and other communication devices on a worldwide basis.
Mobile Telephone Switching Office (MTSO)
The MTSO is the central office for mobile switching. It houses the mobile switching center (MSC), field monitoring and relay stations for switching calls from cell sites to wireline central offices (PSTN). In analog cellular networks, the MSC controls the system operation. The MSC controls calls, tracks billing information, and locates cellular subscribers.
The Cell Site
The term cell site is used to refer to the physical location of radio equipment that provides coverage within a cell. A list of hardware located at a cell site includes power sources, interface equipment, radio frequency transmitters and receivers, and antenna systems.
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Figure 9: Digital Cellular System
The advantages of digital cellular technologies over analog cellular networks include increased capacity and security. Technology options such as TDMA and CDMA offer more channels in the same analog cellular bandwidth and encrypted voice and data. Because of the enormous amount of money that service providers have invested in AMPS hardware and software, providers look for a migration from AMPS to DAMPS by overlaying their existing networks with TDMA architectures.
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Table: AMPS/DAMPS Comparison Analog Digital
Standard EIA–553 (AMPS) IS–54 (TDMA + AMPS
Spectrum 824 MHz to 891 MHz 824 MHz to 891 MHz
Channel Bandwidth 30 kHz 30 kHz
Channels 21 CC / 395 VC 21 CC / 395 VC
Conversations per Channel
1 3 or 6
Subscriber Capacity 40 to 50 Conversations per cell
125 to 300 Conversations per cell
TX / RCV Type Continuous Time shared bursts
Carrier Type Constant phase Variable frequency
Constant frequency Variable phase
Mobile/Base Relationship
Mobile slaved to base Authority shared cooperatively
Privacy Poor Better—easily scrambled
Noise Immunity Poor High
Fraud Detection ESN plus optional password (PIN)
ESN plus optional password (PIN)
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rural application—that is, it reduces the cost of conventional wireline. FWA extends telephone service to rural areas by replacing a wireline local loop with radio communications. Other labels for wireless access include fixed loop, fixed radio access, wireless telephony, radio loop, fixed wireless, radio access, and Ionica. FWA systems employ TDMA or CDMA access technologies.
Figure 10: Fixed Wireless Access
Personal Communications Services (PCS)
The future of telecommunications includes personal communications services. PCS at 1900 MHz (PCS1900) is the North American implementation of DCS (Global System for Mobile communications, or GSM). Trial networks were operational in the United States by 1993, and in 1994 the Federal Communications Commission (FCC) began spectrum auctions. As of 1995, the FCC auctioned commercial licenses. In the PCS frequency spectrum the operator's authorized frequency block contains a definite number of channels. The frequency plan assigns specific channels to specific cells, following a reuse pattern which restarts with each n th cell. The uplink and downlink bands are paired mirror images. As with AMPS, a channel number implies one uplink and one downlink frequency: e.g., Channel 512 = 1850.2 MHz uplink paired with 1930.2 MHz downlink.
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Code Division Multiple Access (CDMA)
Code division multiple access (CDMA) is a digital air interface standard, claiming eight to fifteen times the capacity of analog. It employs a commercial adaptation of military spread-spectrum single-sideband technology. Based on spread spectrum theory, it is essentially the same as wireline service—the primary difference is that access to the local exchange carrier (LEC) is provided via wireless phone. Because users are isolated by code, they can share the same carrier frequency, eliminating the frequency reuse problem encountered in AMPS and DAMPS. Every CDMA cell site can use the same 1.25 MHz band, so with respect to clusters, n = 1. This greatly simplifies frequency planning in a fully CDMA environment.
CDMA is an interference limited system. Unlike AMPS/TDMA, CDMA has a soft capacity limit; however, each user is a noise source on the shared channel and the noise contributed by users accumulates. This creates a practical limit to how many users a system will sustain. Mobiles that transmit excessive power increase interference to other mobiles. For CDMA, precise power control of mobiles is critical in maximizing the system's capacity and increasing battery life of the mobiles. The goal is to keep each mobile at the absolute minimum power level that is necessary to ensure acceptable service quality. Ideally, the power received at the base station from each mobile should be the same (minimum signal to interference).
Self-Test
- Interference effects in cellular systems are a result of ___________.
a. the distance between areas
b. the power of the transmitters
c. the ratio of the distance between areas to the transmitter power of the areas
d. the height of the antennas
- Larger cells are more useful in __________________.
a. densely populated urban areas
b. rural areas
c. lightly populated urban areas
d. mountainous areas
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- Frequency reuse was maximized by increasing the size of cells.
a. true
b. false
- Fixed wireless access is primarily a rural application.
a. true
b. false
Correct Answers
- Interference effects in cellular systems are a result of _________.
a. the distance between areas
b. the power of the transmitters
c. the ratio of the distance between areas to the transmitter power of the areas
d. the height of the antennas
See Topic 2
- Larger cells are more useful in _____________.
a. densely populated urban areas
b. rural areas
c. lightly populated urban areas
d. mountainous areas
See Topic 3
- The most widely used standard for cellular communications is _______.
a. the advanced mobile phone service (AMPS)
b. the mobile subscriber unit (MSU)
c. the mobile telephone switching office
d. code division multiple access (CDMA)
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See Topic 4
- How many conversations per channel can TDMA digital cellular carry at once?
a. 1
b. 2
c. 3
d. 10
See Topic 6
- Which of the following is not a limitation of AMPS?
a. low calling capacity
b. poor privacy protection
c. limited spectrum
d. wide coverage area
See Topic 4
- Digital cellular technologies offer increased capacity and security.
a. true
b. false
See Topic 6
- TDMA, a digital air interface standard, has twice the capacity of analog.
a. true
b. false
See Topic 6
- Cells are always hexagonal in shape.
a. true
b. false
