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| Mobile Telephone |
| Mobile Technologies by: Lawrence Harte The key technologies used in cellular mobile radio include cellular frequency reuse, handover, digital modulation, access technologies and packet data transmission. Cellular Frequency Reuse Frequency reuse is the process of using the same radio frequencies on radio transmitter sites within a geographic area that are separated by sufficient distance to cause minimal interference with each other. Frequency reuse allows for a dramatic increase in the number of customers that can be served (capacity) within a geographic area on a limited amount of radio spectrum (limited number of radio channels). In early mobile radio telephone systems, one high-power transmitter served a large geographic area with a limited number of radio channels. Because each radio channel requires a certain frequency bandwidth (radio spectrum) and there is a very limited amount of radio spectrum available, this dramatically limits the number of radio channels that keeps the low serving capacity of such systems. For example, in 1976, New York City had only 12 radio channels to support 545 customers and a two-year long waiting list of typically 3,700 [ ]. To conserve the limited amount of radio spectrum (maximum number of available radio channels), the cellular system concept was developed. Cellular systems allow reuse of the same channel frequencies many times within a geographic coverage area. The technique (called frequency reuse) makes it possible for a system to provide service to more customers (called system capacity) by reusing the channels that are available in a geographic area. In large systems such as the systems operating in New York City and Los Angeles, radio channel frequencies may be reused over 300 times. As systems start to become overloaded with many users, to increase capacity, the system can expand by adding more radio channels to the base station or by adding more cell cites with smaller coverage areas. To minimize interference in this way, cellular system planners position the cell sites that use the same radio channel farthest away from each other. The distances between sites are initially planned by general RF signal propagation rules. It is difficult to account for enough propagation factors to precisely position the towers, which usually leads the cell site position and power levels to be adjusted later. |
Figure 1.2 shows that radio channels (frequencies) in a cellular communication system can be reused in towers that have enough distance between them. This example shows that radio channel signal strength decreases exponentially with distance. As a result, mobile radios that are far enough apart can use the same radio channel frequency with minimal interference. The acceptable distance between cells that use the same channels are determined by the distance to radius (D/R) ratio. The D/R ratio is the ratio of the distance (D) between cells using the same radio frequency to the radius (R) of the cells. In today's analog system, a typical D/R ratio is 4.6:1, which means a channel used in a cell with a 1-mile radius would not interfere with the same channel being reused at a cell 4.6 miles away. For some of the digital systems (such as TDMA or GSM), the reuse factor can be lower than 2.0. Another technique, called cell splitting, helps to expand capacity gradually. Cells are split by adjusting the power level and/or using reduced antenna height to cover a reduced area. Reducing a coverage area by changing the RF boundaries of a cell site has the same effect as placing cells farther apart, and allows new cell sites to be added. However, the boundaries of a cell site vary with the terrain and land conditions, especially with seasonal variations in foliage. Coverage areas can actually increase in fall and winter as the leaves fall from the trees. When a cellular system is first established, it can effectively serve only a limited number of callers. When that limit is exceeded, callers experience system busy signals (known as blocking) and their calls cannot be completed. More callers can be served by adding more cells with smaller coverage areas - that is, by cell splitting. The increased number of smaller cells provides more available radio channels in a given area because it allows radio channels to be reused at closer geographical distances. When linked together to cover an entire metro area, the radio coverage |
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| Figure 1.2., Frequency Reuse | |
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areas (called cells) form a cellular structure resembling that of a honeycomb. Cellular systems are designed to overlap each cell border with adjacent cell borders to enable a "handover" from one cell to the next. As a customer (called a subscriber) moves through a cellular system, the mobile switching center (MSC) coordinates and transfers calls from one cell to another and maintains call continuity.
Handover Handover is a process where a mobile radio operating on a particular channel is reassigned to a new channel. The process is often used to allow subscribers to travel throughout the large radio system coverage area by switching the calls (handover) from cell-to-cell (and different channels) with better coverage for that particular area when poor quality conversation is detected. Handover (also called handoff) is necessary for two reasons. First, where the mobile unit moves out of range of one cell site and is within range of another cell site. Second, a handover may be required when the mobile has requested the services of a type of cellular channel that different capabilities (e.g. packet data). This might mean assignment from a digital channel to an analog channel or assignment from a wide digital channel to a packet data channel. |
Figure 1.3 shows the basic handoff process that occurs in a mobile telephone system. In this example, the system has determined that the radio signal strength of mobile telephone has fallen below a predefined level. When this occurs, the serving base station sends a control message to the system indicating that the signal quality of the mobile's radio signal is declining and a handover may be necessary. The system determines that an adjacent cell site is a candidate for the handoff and it sends command messages to the adjacent cell site to prepare to receive a new connection. Messages are exchanged between the base stations and the mobile device that informs it to change to a new channel and the MSC switches the audio path to the new cell site when necessary.
Speech Compression Speech compression is a technique for converting or encoding audio (sound) information so that a smaller amount of information elements or reduced bandwidth is required to represent, store or transfer audio signals. |
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| Figure 1.3., Handover Operation | |
| Figure 1.4 shows the basic digital speech compression process. The first step is to periodically sample the analog voice signal (5 - 20 msec) into pulse code modulated (PCM) digital form (usually 64 kbps). This digital signal is analyzed and characterized (e.g. volume, pitch) using a speech coder. The speech compression analysis usually removes redundancy in the digital signal (such as silence periods) and attempts to ignore patterns that are not characteristic of the human | voice. In this example, these speech compression processes use pre-stored codebook tables that allow the speech coder to transmit abbreviated codes that represent larger (probable) digital speech patterns. The result is a digital signal that represents the voice content, not a waveform. The end result is a compressed digital audio signal that is 8-13 kbps instead of the 64 kbps PCM digitized voice. |
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| Figure 1.4., Speech Coding | |
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Modulation Types
Modulation is the process of changing the amplitude, frequency, or phase of a radio frequency carrier signal (a carrier) to change with the information signal (such as voice or data). Mobile systems use analog or digital modulation. Analog modulation is a process where the amplitude, frequency or phase of a carrier signal is varied directly in proportion or in direct relationship to the information signal. Digital modulation is a process where the amplitude, frequency or phase of a carrier signal is varied by the discrete states (on and off) of a digital signal. Mobile telephone systems primarily use digital modulation. To increase the efficiency of mobile telephone systems, it is desirable to send more information with less frequency bandwidth (more information transported by the carrier signal). Modulation efficiency is a measure of how much information can be transferred onto a carrier signal. In general, more efficient modulation processes require smaller changes in the characteristics of a carrier signal (amplitude, frequency, or phase) to represent the information signal. To increase the amount of information that can be transported on a carrier signal, it is possible to use (combine) multiple forms of modulation on the same carrier wave (e.g. use both amplitude and phase modulation). This figure shows different forms of digital modulation. This diagram shows ASK modulation that turns the carrier signal on and off with the digital signal. FSK modulation shifts the frequency of the carrier signal according to the on and off levels of the digital information |
signal. The phase shift modulator changes the phase of the carrier signal in accordance with the digital information signal. This diagram also shows that advanced forms of modulation such as QAM can combine amplitude and phase of digital signals.
Access Multiplexing Access multiplexing is a process used by a communications system to coordinate and allow more than one user to access the communication channels within the system. There are four basic access-multiplexing technologies used in wireless systems: frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA) and space division multiple access (SDMA). Other forms of access multiplexing (such as voice activity multiplexing) use the fundamentals of these access-multiplexing technologies to operate. Frequency Division Multiple Access (FDMA) Frequency division multiple access is a process of allowing mobile radios to share radio frequency allocation by dividing up that allocation into separate radio channels where each radio device can communicate on a single radio channel during communication. |
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| Figure 1.5., Digital Modulation | |
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| Figure 1.6., Frequency Division Multiple Access | |
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Figure 1.6 shows how a frequency band can be divided into several communication channels using frequency division multiplexing (FDM). When a device is communicating on a FDM system using a frequency carrier signal, its carrier channel is completely occupied by the transmission of the device. For some FDM systems, after it has stopped transmitting, other transceivers may be assigned to that carrier channel frequency. When this process of assigning channels is organized, it is called frequency division multiple access (FDMA). Transceivers in an FDM system typically have the ability to tune to several different carrier channel frequencies.
Time Division Multiple Access (TDMA) Time division multiple access (TDMA) is a process of sharing a single radio channel by dividing the channel into time slots that are shared |
between simultaneous users of the radio channel. When a mobile radio communicates with a TDMA system, it is assigned a specific time position on the radio channel. By allowing several users to use different time positions (time slots) on a single radio channel, TDMA systems increase their ability to serve multiple users with a limited number of radio channels.
Figure 1.7 shows how a single carrier channel is time-sliced into three communication channels. Transceiver number 1 is communicating on time slot number 1 and mobile radio number 2 is communicating on time slot number 3. Each frame on this communication system has three time slots. |
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| Figure 1.7., Time Division Multiple Access (TDMA) | |
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Code Division Multiple Access (CDMA)
Code division multiple access (CDMA) is the sharing of a radio channel by multiple users by share adding a unique code for each data signal that is being sent to and from each of the radio transceivers. These codes are used to spread the data signal to a bandwidth much wider than is necessary to transmit the data signal without the code. Figure 1.8 shows how CDMA radio channels can provide multiple communication channels through the use of multiple coded channels. This diagram shows that a code pattern mask is used to decode each communication channel. The channel mask is shifted along the radio channel until the code chips (or a majority of the code chips) match the expected code pattern. When a match occurs, this produces a single bit of information (a logical 1 or 0). This example shows that the use of multiple code patterns (multiple masks in this example) allow multiple users to share the same radio channel. |
Spatial Division Multiple Access
(SDMA)
Spatial division multiple access (SDMA) is a system access technology that allows a single transmitter location to provide multiple communication channels by dividing the radio coverage into focused radio beams that reuse the same frequency. To allow multiple access, each mobile radio is assigned to a focused radio beam. These radio beams may dynamically change with the location of the mobile radio. SDMA technology has been successfully used in satellite communications for several years. Figure 1.9 shows an example of an SDMA system. Diagram (a) shows the conventional sectored method for communicating from a cell site to a mobile telephone. This system transmits a specific frequency to a defined (sectored) geographic area. Diagram (b) shows a top view of a cell site that uses SDMA technology that is communicating with multiple mobile telephones operating within the same geographic area on a single frequency. In the SDMA system, multiple |
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| Figure 1.8., Code Division Multiple Access (CDMA) | |
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| Figure 1.9., Spatial Division Multiple Access (SDMA) | |
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directional antennas or a phased array antenna system directs independent radio beams to different directions. As the mobile telephone moves within the sector, the system either switches to an alternate beam (for a multi-beam system) or adjusts the beam to the new direction (in an adaptive system).
Packet Data Packet data is the sending of data through a network in small packets (typically under 1000 bytes of information per packet). A packet data system divides large quantities of data into small packets for transmission through a switching network that uses the addresses of the packets to dynamically route these packets through a switching network to their ultimate destination. When a data block is divided, the packets are |
given sequence numbers so that a packet assembler/disassembler (PAD) device can recombine the packets to the original data block after they have been transmitted through the network.
To send packet data on mobile networks, the system is designed to coordinate the dynamic assignment and reception of radio packets. The wireless system is connected to packet switching nodes. Packet switching nodes in GSM systems are called GPRS Support Nodes (GSNs). GSNs receive and forward data packets toward their destination. To add packet radio and packet data switching to a mobile system, this system can be separated into two separate parts; a voice part and a packet data part. The voice part connects voice calls to a single location using a circuit switched connection (circuit path). The packet data part dynamically routes packets towards their destination depending on the address that is contained in the data packet. |
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| Figure 1.10., Packet Mobile System | ||
| Figure 1.10 shows a simplified functional diagram of a mobile network that is capable of combining voice and packet data services. This diagram shows that the packet data network is attached to the base station in an addition to the switched voice system and the voice and packet switched data systems that share a common radio access network. The base station (BS) contains a radio transceiver (radio and transmitter) that converts the radio signal into data signals (data and digital voice) that can transfer through the network. The base station separates the radio channel data so the voice data is sent to the voice switching system (the MSC) and the data packets are sent to the packet data system (GSN). |
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Series Source: Introduction to Mobile Telephone 2nd Edition $19.99 printed $16.99 eBook |
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