How Do Cell Phones Work? Not many people know it, but cell phones are actually two-way radios, much like the walkie-talkies of the past, albeit much more advanced. When you talk into your cell phone receiver, it registers your voice and converts the sound into radio waves. These waves travel through the air until they reach a receiver, which is usually found at a base station. This station will then send your call through a telephone network until it contacts the person you wish to speak with.
Similarly, when someone places a call to your cell phone, the signal travels through the telephone network until it reaches a station near you. The station sends the radio waves out into the neighboring areas. These radio waves are then picked up by your cell phone and converted into the sound of a human voice. Cell phones are a vast improvement over the telecommunications technology of the past, and are daily becoming a fixture of modern life. As always, communication is vital, and cell phones will help you to better communicate with the key people in your life.
Using a cell phone is one of the first steps you must take to participate effectively in the emerging global economy. Early Mobile Telephone System Architecture Traditional mobile service was structured in a fashion similar to television broadcasting: One very powerful transmitter located at the highest spot in an area would broadcast in a radius of up to 50 kilometres. 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 12 conversations (channels) each, the system capacity theoretically could be increased from 12 conversations or voice channels using one powerful transmitter to 1,200 conversations (channels) using one hundred low-power transmitters. Figure 2 shows a metropolitan area configured as a traditional mobile telephone network with one high-power transmitter. 2. 0 Mobile Telephone System Using the Cellular Concept
Interference problems caused by mobile units using the same channel in adjacent areas proved that all channels could not be reused in every cell. Areas had to be skipped before the same channel could be reused. Even though this affected the efficiency of the original concept, frequency reuse was still a viable solution to the problems of mobile telephony systems. Engineers discovered that the interference effects were not due to the distance between areas, but to the ratio of the distance between areas to the transmitter power (radius) of the areas.
By reducing the radius of an area by 50 per cent, service providers could increase the number of potential customers in an area fourfold. Systems based on areas with a one-kilometre radius would have one hundred times more channels than systems with areas 10 kilometres in radius. Speculation led to the conclusion that by reducing the radius of areas to a few hundred meters, millions of calls could be served. The cellular concept employs variable low-power levels, which allow cells to be sized according to the subscriber density and demand of a given area.
As the population grows, cells can be added to accommodate that growth. Frequencies used in one cell cluster can be reused in other cells. Conversations can be handed off from cell to cell to maintain constant phone service as the user moves between cells. 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. 0 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. 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. 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 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. The concept of frequency reuse is based on assigning to each cell a group of radio channels used within a small geographic area. Cells are assigned a group of channels that is completely different from neighbouring cells. The coverage area of cells is called the footprint. This footprint is limited by a boundary so that the same group of channels can be used in different cells that are far enough away from each other so that their frequencies do not interfere.
Cells with the same number have the same set of frequencies. Here, because the number of available frequencies is 7, the frequency reuse factor is 1/7. That is, each cell is using 1/7 of available cellular channels. Cell Splitting Unfortunately, economic considerations made the concept of creating full systems with many small areas impractical. To overcome this difficulty, system operators developed the idea of cell splitting. As a service area becomes full of users, this approach is used to split a single area into smaller ones.
In this way, urban centres can be split into as many areas as necessary to provide acceptable service levels in heavy-traffic regions, while larger, less expensive cells can be used to cover remote rural regions. Handoff The final obstacle in the development of the cellular network involved the problem created when a mobile subscriber travelled 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 mobile crosses 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. 0 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 the following basic topics: * frequency and channel assignments * type of radio modulation * maximum power levels * modulation parameters * messaging protocols * call-processing sequences The Advanced Mobile Phone Service (AMPS) AMPS was released in 1983 using the 800-MHz to 900-MHz frequency band and the 30-kHz bandwidth for each channel as a fully automated mobile telephone service.
It was the first standardized cellular service in the world and is currently the most widely used standard for cellular communications. Designed for use in cities, AMPS later expanded to rural areas. It maximized the cellular concept of frequency reuse by reducing radio power output. The AMPS telephones (or handsets) have the familiar telephone-style user interface and are compatible with any AMPS base station. This makes mobility between service providers (roaming) simpler for subscribers. Limitations associated with AMPS include the following: * low calling capacity * limited spectrum no room for spectrum growth * poor data communications * minimal privacy * inadequate fraud protection AMPS is used throughout the world and is particularly popular in the United States, South America, China, and Australia. AMPS uses frequency modulation (FM) for radio transmission. In the United States, transmissions from mobile to cell site use separate frequencies from the base station to the mobile subscriber. Narrowband Analog Mobile Phone Service (NAMPS) Since analog cellular was developed, systems have been implemented extensively throughout the world as first-generation cellular technology.
In the second generation of analog cellular systems, NAMPS was designed to solve the problem 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 3 channels in the AMPS 30-kHz single channel bandwidth. NAMPS provides 3 users in an AMPS channel by dividing the 30-kHz AMPS bandwidth into 3 10-kHz channels.
This increases the possibility of interference because channel bandwidth is reduced. 5. 0 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. * public switched telephone network (PSTN) * mobile telephone switching office (MTSO) cell site with antenna system * mobile subscriber unit (MSU) 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. Mobile Subscriber Units (MSUs) The mobile subscriber unit consists of a control unit and a transceiver that transmits and receives radio transmissions to and from a cell site.
The following three types of MSUs are available: * the mobile telephone (typical transmit power is 4. 0 watts) * the portable (typical transmit power is 0. 6 watts) * the transportable (typical transmit power is 1. 6 watts) * The mobile telephone is installed in the trunk of a car, and the handset is installed in a convenient location to the driver. Portable and transportable telephones are hand-held and can be used anywhere. The use of portable and transportable telephones is limited to the charge life of the internal battery. 6. 0 Digital Systems
As demand for mobile telephone service has increased, service providers found that basic engineering assumptions borrowed from wire line (landline) networks did not hold true in mobile systems. While the average landline phone call lasts at least 10 minutes, mobile calls usually run 90 seconds. Engineers who expected to assign 50 or more mobile phones to the same radio channel found that by doing so they increased the probability that a user would not get dial tone this is known as call-blocking probability. As a consequence, the early systems quickly became saturated, and the quality of service decreased rapidly.
The critical problem was capacity. The general characteristics of time division multiple access (TDMA), Global System for Mobile Communications (GSM), personal communications service (PCS) 1900, and code division multiple access (CDMA) promise to significantly increase the efficiency of cellular telephone systems to allow a greater number of simultaneous conversations. Figure 8 shows the components of a typical 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 digital analog mobile phone service (DAMPS) by overlaying their existing networks with TDMA architectures. Time Division Multiple Access (TDMA) North American digital cellular (NADC) is called DAMPS and TDMA. Because AMPS preceded digital cellular systems, DAMPS uses the same setup protocols as analog AMPS.
TDMA has the following characteristics: * IS? 54 standard specifies traffic on digital voice channels * initial implementation triples the calling capacity of AMPS systems * capacity improvements of 6 to 15 times that of AMPS are possible * many blocks of spectrum in 800 MHz and 1900 MHz are used * all transmissions are digital * TDMA/FDMA application 7. 3 callers per radio carrier (6 callers on half rate later), providing 3 times the AMPS capacity TDMA is one of several technologies used in wireless communications. TDMA provides each call with time slots so that several calls can occupy one bandwidth.
Each caller is assigned a specific time slot. In some cellular systems, digital packets of information are sent during each time slot and reassembled by the receiving equipment into the original voice components. TDMA uses the same frequency band and channel allocations as AMPS. Like NAMPS, TDMA provides three to six time channels in the same bandwidth as a single AMPS channel. Unlike NAMPS, digital systems have the means to compress the spectrum used to transmit voice information by compressing idle time and redundancy of normal speech. TDMA is the digital standard and has 30-kHz bandwidth.
Using digital voice encoders, TDMA is able to use up to six channels in the same bandwidth where AMPS uses one channel. Extended Time Division Multiple Access (E? TDMA) The E? TDMA standard claims a capacity of fifteen times that of analogue cellular systems. This capacity is achieved by compressing quiet time during conversations. E? TDMA divides the finite number of cellular frequencies into more time slots than TDMA. This allows the system to support more simultaneous cellular calls. Fixed Wireless Access (FWA) FWA is a radio-based local exchange service in which telephone service is provided by common carriers (see Figure 9).
It is primarily a rural application that is, it reduces the cost of conventional wire line. FWA extends telephone service to rural areas by replacing a wire line 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 Ionic. FWA systems employ TDMA or CDMA access technologies. Personal Communications Service (PCS) The future of telecommunications includes PCS. PCS at 1900 MHz (PCS 1900) is the North American implementation of digital cellular system (DCS) 1800 (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 that restarts with each nth 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. -MHz uplink paired with 1930. 2-MHz downlink). Code Division Multiple Access (CDMA) CDMA is a digital air interface standard, claiming 8 to 15 times the capacity of analogue. It employs a commercial adaptation of military, spread-spectrum, single-sideband technology. Based on spread spectrum theory, it is essentially the same as wire line 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). 7. 0 REFERENCES 1. Basic Communication Engineering by Rusnani Arifin, Juliana Md Sharif, Nor Hidayah Saad and Mohd Aminuddin Murad. 2. http://www. gsmfavorites. com/documents/introduction/gsm/ 3. en. wikipedia. org/wiki/History_of_mobile_phones