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Mobile Telephone
Digital Cellular Systems 
(2nd Generation)

by: Lawrence Harte

Digital cellular is an industry term given to the new cellular technology that transmits voice information in digital form. This differs from Analog cellular in that the method of transmission for voice/data information is by means of digital signals. Digital mobile radio systems are often characterized by their type of access technology (TDMA or CDMA). The access technology determines how that digital information is transferred to and from the cellular system.


Digital cellular systems can usually serve several subscribers on a single radio channel at the same time. Depending on the type of system, this can range from 3 to over 20. To allow this, almost all digital cellular systems share the fundamental characteristics of digitizing and compressing voice information to accomplish this. This allows a single radio channel to be divided into several sub-channels (communication channels). Each communication channel can serve a single customer.


Because each subscriber typically uses the cellular system for only a few minutes a day, several subscribers can share each one of these communication channels during the day. As a rule, 20 - 32 subscribers can share each communication channel. So, if a digital radio channel has 8 communications channels (sub-channels), a cell site with 25 radio channels can support 4000 to 6400 subscribers (25 radio channels x 8 users per channel x 20 to 32 subscribers per communication channel).


Digital cellular systems use two key types of communication channels, control channels and voice channels. A control channel on a digital system is usually one of the sub-channels on the radio channel. This allows digital systems to combine a control channel and one or more voice channels on a single radio channel. The portion of the radio channel that is dedicated as a control channel carries only digital messages and signals that allow the mobile telephone to retrieve system control information and compete for access. The other sub-channels on the radio channel carry voice or data information.

This article is Part 6 of a 9 Part Series
Mobile Telephone List

Month
Mobile Technologies Oct 06
Mobile Devices Nov 06
Mobile Systems Dec 06
Mobile Systems Operation Jan 07
Analog Systems Feb 07
Digital Cellular Systems Mar 07
Packet Digital Cellular Systems Apr 07
Wideband Digital Cellular May 07
Mobile Services Jun 07

The basic operation of a digital cellular system involves initiation of the phone when it is powered on, listening for paging messages (idle), attempting access when required and conversation (or data) mode. 


When a digital mobile telephone is first powered on, it initializes itself by searching (scanning) a predetermined set of control channels and then tuning to the strongest one. During the initialization mode, it listens to messages on the control channel to retrieve system identification and setup information. Compared to analog systems, digital systems have more communication and control channels. This can result in the mobile phone taking more time to search for control channels. To quickly direct a mobile telephone to an available control channel, digital systems use several processes to help a mobile telephone to find an available control channel. These include having the phone memorize its last successful control channel location, a table of likely control channel locations and a mechanism for pointing to the location of a control channel on any of the operating channels.


After a digital mobile telephone has initialized, it enters an idle mode where it waits to be paged for an incoming call or for the user to initiate a call. When a call begins to be received or initiated, the mobile telephone enters system access mode to try to access the system via a control channel. When it gains access, the control channel sends a digital traffic channel designation message indicating an open communications channel. This channel may be on a different time slot on the same frequency or to a time slot on a different frequency. The digital mobile telephone then tunes to the designated communications channel and enters the conversation mode. As the mobile telephone operates on a digital voice channel, the digital system commonly uses some form of phase modulation (PM) to send and receive digital information. 

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A mobile telephone's attempt to obtain service from a cellular system is referred to as "access". Digital mobile telephones compete on the control channel to obtain access from a cellular system. Access is attempted when a command is received by the mobile telephone indicating the system needs to service that mobile telephone (such as a paging message indicating a call to be received) or as a result of a request from the user to place a call. Digital mobile telephones usually have the ability to validate their identities more securely during access than analog mobile telephones. This is made possible by a process called authentication. Authentication processes share secret data between the digital mobile phone and the cellular system. 


If the authentication is successful, the system sends out a channel assignment message commanding the mobile telephone to change to a new communication channel and conversation can begin.


After a mobile telephone has been commanded to tune to a radio voice channel, it sends digitized voice or other customer data. Periodically, control messages may be sent between the base station and the mobile telephone. Control messages may command the mobile telephone to adjust its power level, change frequencies, or request a special service (such as three way calling). To send control messages while the digital mobile phone is transferring digital voice, the voice information is either replaced by a short burst (called blank and burst or fast signaling), or else control messages can be sent along with the digitized voice signals (called slow signaling).


Most digital telephones automatically conserve battery life as they transmit only for short periods of time (bursts). In addition to savings through digital burst transmission, digital phones ordinarily have the capability of discontinuous transmission that allows the inhibiting of the transmitter during periods of user silence. When the mobile telephone user begins to talk again, the transmitter is turned on again. The combination of the power savings allows some digital mobile telephones to have 2 to 5 times the battery life in the transmit mode.


Digital technology increases system efficiency by voice digitization, speech compression (coding), channel coding, and the use of spectrally efficient radio signal modulation. Standard voice digitization in the Public Switched Telephone Network (PSTN) produces a data rate of 64 kilobits per second (kbps). Because transmitting a digital signal via radio requires about 1 Hz of radio bandwidth for each bps, an uncompressed digital voice signal would require more than 64 kHz of radio bandwidth. Without compression, this bandwidth would make digital transmission less efficient than analog FM cellular, which uses only 25-30 kHz for a single voice channel. Therefore, digital systems compress speech information using a voice coder or Vocoder. Speech coding removes redundancy in the digital signal and attempts to ignore data patterns that are not characteristic of the human voice. The result is a digital signal that represents the voice audio frequency spectrum content, not a waveform.

A voice coder (vocoder) characterizes the input signal, looks up codes in a code book table that represents various digital patterns and chooses a pattern that comes closest to the input digitized signal. The amount of digitized speech compression used in digital cellular systems varies. For the IS-136 TDMA system, the compression is 8:1. For CDMA, the compression varies from 8:1 to 64:1 depending on speech activity. GSM systems compress the voice by 5:1.


As a general rule, with the same amount of speech coding analysis, the fewer bits used to characterize the waveform, the poorer the speech quality. If the complexity (signal processing) of the speech coder can be increased, it is possible to get improved voice quality with fewer bits. 


Voice digitization and speech coding take processing time. Typically, speech frames are digitized every 20 msec and inputted to the speech coder. The compression process, time alignment with the radio channel, and decompression at the receiving end all delay the voice signal. The combined delay can add up to 50-100 msec. Although such a delay is not usually noticeable in two-way conversation, it can cause an annoying echo when a speakerphone is used, or the side tone of the signal is high (so the users can hear themselves). However, an echo canceller can be used in the MSC to process the signal and remove the echo.


Once the digital speech information is compressed, control information bits must be added along with extra bits to protect from errors that will be introduced during radio transmission. The combined digital signal (compressed digitized voice and control information) is sent to the radio modulator where it is converted to a digitized RF signal. The efficient conversion to the RF signal constantly involves some form of phase shift modulation.


Figure 1.16 shows a basic digital cellular system. This diagram shows that there usually is only one type of digital radio channel called a digital traffic channel (DTC). The digital radio channel is ordinarily sub-divided into control channels and digital voice channels. Both the control channels and voice channels use the same type of digital modulation to send control and content data between the mobile phone and the base station. When used for voice, the digital signal is usually a compressed digital signal that is from a speech coder. When conversation is in progress, some of the digital bits are usually dedicated for control information (such as handoff). Similar to analog systems, digital base stations have two antennas that increase the ability to receive weak radio signals from mobile telephones. Base stations are connected to a mobile switching center (MSC) normally by a high speed telephone line or microwave radio system. This interconnection may allow compressed digital information (directly from the speech coder) to increase the number of voice channels that can be shared on a single connection line. The MSC is connected to the telephone network to allow mobile telephones to be connected to standard landline telephones.

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Figure 1.16., Digital Cellular System (2nd Generation) 
Second generation (2G) cellular is a term commonly used to describe digital cellular radio technology with advanced messaging and data transmission capabilities. The types of 2nd generation digital cellular systems include GSM, IS-136 TDMA and CDMA.


Global System for Mobile Communication (GSM)


The Global System for Mobile Communications (GSM) is a global digital radio system that uses Time Division Multiple Access (TDMA) technology. GSM is a digital cellular technology that was initially created to provide a single-standard pan-European cellular system. The GSM system is a digital-only system and was not designed to be backward compatible with the established analog systems. 


GSM began development in 1982, and the first commercial GSM digital cellular system was activated in 1991. GSM technology has evolved to be used in a variety of systems and frequencies (900 MHz, 1800 MHz and 1900 MHz) including Personal Communications Services (PCS) in North America and Personal Communications Network (PCN) systems throughout the world. 
When communicating in a GSM system, users can operate on the same radio channel simultaneously by sharing time slots. The GSM cellular system allows 8 mobile telephones to share a single 200 kHz 

bandwidth radio carrier waveform for voice or data communications. To allow duplex operation, GSM voice communication is conducted on two 200 kHz wide carrier frequency waveforms. 


The GSM system has several types of control channels that carry system and paging information, and coordinates access like the control channels on analog systems. The GSM digital control channels have many more capabilities than analog control channels such as broadcast message paging, extended sleep mode, and others. Because the GSM control channels use only a portion (one or more slots), they typically co-exist on a single radio channel with other time slots that are used for voice communication.


A GSM carrier transmits at a bit rate of 270 kbps. A single GSM digital radio channel or time slot is capable of transferring only 1/8th of that, which is about 33 kbps of information (actually less than that, due to the use of some bit time for non-information purposes such as synchronization bits). 


Time intervals on full rate GSM channels are divided into frames with 8 time slots on two different radio frequencies. One frequency is for transmitting from the mobile telephone; the other is for receiving to the mobile telephone. During a voice conversation at the mobile set, one time slot period is dedicated for transmitting, one for receiving, and six remain idle. The mobile telephone uses some of the idle time slots to measure the signal strength of surrounding cell carrier frequencies in preparation for handover. 

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On the 900 MHz band, GSM digital radio channels transmit on one frequency and receive on another frequency 45 MHz higher, but not at the same time. On the 1.9 GHz band, the difference between transmit and receive frequencies is 80 MHz. The mobile telephone receives a burst of data on one frequency, then transmits a burst on another frequency, and then measures the signal strength of at least one adjacent cell, before repeating the process. 


North American TDMA (IS-136 TDMA)


The North American TDMA system (IS-136) is a digital system that uses TDMA access technology. It evolved from the IS-54 specification that was developed in North America in the late 1980's to allow the gradual evolution of the AMPS system to digital service. The IS-136 system is sometimes referred to as Digital AMPS (DAMPS) or North American digital cellular (NADC). 


In 1988, the Cellular Telecommunications Industry Association created a development guideline for the next generation of cellular technology for North America. This guideline was called the User Performance Requirements (UPR) and the Telecommunications Industry Association (TIA) used this guideline to create a TDMA digital standard, called IS-54. This digital specification evolved from the original EIA-553 AMPS specification. The first revision of the IS-54 specification (Rev 0) identified the basic parameters (e.g. time slot structure, type of radio channel modulation, and message formats) needed to begin designing TDMA cellular equipment. There have been several enhancements to IS-54 since its introduction and in 1995; IS-54 was incorporated as part of the IS-136 specification. 


A primary feature of the IS-136 systems is their ease of adaptation to the existing AMPS system. Much of this adaptability is due to the fact that IS-136 radio channels retain the same 30 kHz bandwidth as AMPS system channels. Most base stations can therefore replace TDMA radio units in locations previously occupied by AMPS radio units. Another factor in favor of adaptability is that new dual mode mobile telephones were developed to operate on either IS-136 digital traffic (voice and data) channels or the existing AMPS radio channels 

as requested in the CTIA UPR document. This allows a single mobile telephone to operate on any AMPS system and use the IS-136 system whenever it is available.


The IS-136 specification concentrates on features that were not present in the earlier IS-54 TDMA system. These include longer standby time, short message service functions, and support for small private or residential systems that can coexist with the public systems. In addition, IS-136 defines a digital control channel to accompany the Digital Traffic Channel (DTC). The digital control channel allows a mobile telephone to operate in a single digital-only mode. Revision A of the IS-136 specification now supports operation in the 800 MHz range for the existing AMPS and DAMPS systems as well as the newly allocated 1900MHz bands for PCS systems. This permits dual band, dual mode phones (800 MHz and 1900 MHz for AMPS and DAMPS). The primary difference between the two bands is that mobile telephones cannot transmit using analog signals at 1900MHz.


The IS-136 cellular system allows for mobile telephones to use either 30 kHz analog (AMPS) or 30 kHz digital (TDMA) radio channels. The IS-136 TDMA radio channel allows multiple mobile telephones to share the same radio frequency channel by time-sharing. All IS-136 TDMA digital radio channels are divided into frames with 6 time slots. The time slots used for the correspondingly numbered forward and reverse channels are time-related so that the mobile telephone does not simultaneously transmit and receive. 


The IS-136 system allows a standard time slot on a TDMA radio channel to be used as a digital control channel (DCC). The DCC carries the same system and paging information as the analog control channel (ACC). In addition to the control messages, the DCC has more capabilities than the ACC such as extended sleep mode, short message service (SMS), private and public control channels, and others. 


The total bit rate of the carrier frequency waveform is 48.6 kbps. This is time-shared and some of the transmitted bits are used for synchronization and other control purposes; this results in a user-available data rate of 13 kbps. Some of the 13 kbps are used for error detection and correction, so only 8 kbps of data are available for full rate digitally coded speech.

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The RF power levels for the mobile phones are almost exactly the same as for the AMPS telephones. The primary difference in the power levels is a reduction in minimum power level that mobile telephones can be instructed to reduce to. This allows for very small cell coverage areas, typically the size of cells that would be used for wireless office or home cordless systems.


Extended TDMA (E-TDMA)TM


Extended TDMATM was developed by Hughes Network Systems in 1990 as an extension to the existing IS-136 TDMA industry standard. ETDMA uses the existing TDMA radio channel bandwidth and channel structure. Its receivers are tri-mode as they can operate in AMPS, TDMA, or ETDMA modes. While a TDMA system assigns a mobile telephone fixed time slot numbers for each call, ETDMA dynamically assigned time slots on an as needed basis. The ETDMA system contains a half-rate speech coder (4 kb/s) that reduces the number of information bits that must be transmitted and received each second. This makes use of voice silence periods to inhibit slot transmission so other users may share the transmit slot. The overall benefit is that more users can share the same radio channel equipment and improved radio communications performance. The combination of a low bit rate speech coder, voice activity detection and interference averaging increases the radio channel efficiency to beyond 10 times the existing AMPS capacity. 


ETDMA radio channels are structured into the same frames and slots structures as the standard IS-54 radio channels. Some or all of the time slots on all of the radio channels are shared for ETDMA communication, which is similar to IS-54 and IS-136 radio channels, or else slots can be shared on different frequencies. When a Mobile telephone is operating in extended mode, the ETDMA system must continually coordinate time slot and frequency channel assignments. The ETDMA system performs this by using a time slot control system. On an ETDMA capable radio channel some of the time slots are dedicated as control slots on an as needed basis. ETDMA systems can assign an AMPS channel, a TDMA full-rate or half-rate channel, or an ETDMA channel. The existing 30 kHz AMPS control channels are used to assign analog voice and digital traffic channels


In an ETDMA system, some of the radio channels include a control slot that coordinates time slot allocation. This usually accounts for an estimated 15% of available time slots in a system. The control time slots assign an ETDMA subscriber to voice time slots on multiple radio channels. 


ETDMA uses the following process to allocate time slots from moment to moment as needed. The cellular radio maintains constant communications with the Base Station through the control time slot. When a conversation begins, the cellular radio uses the control slot to request a voice time slot from the Base Station. Through the control 

slot, the Base Station assigns a voice time slot and sets the cellular radio to transmit in that assigned voice time slot. During each momentary lull in phone conversation, the transmitting cellular radio gives up its voice time slot, which is then placed back into the Base Station's pool of available time slots.
When a cellular radio is ready to receive a voice conversation, the Base Station uses the control slot to tell it which voice time slot has the conversation. The cellular radio receiver then tunes to the appropriate slot. Through the control slot, the Base Station constantly monitors the cellular radio to determine whether it has given up a slot or needs a slot. In turn, the cellular radio constantly monitors the control slot to learn which time slot contains voice conversation being sent to it.


Integrated Dispatch Enhanced Network (iDEN)


Integrated Dispatch Enhanced Network (iDEN) a digital radio system that provides for voice, dispatch and data services. iDEN was formerly called Motorola Integrated Radio System (MIRS). iDEN was deployed in 1996 for enhanced specialized mobile radio (E-SMR) service. The iDEN system radio channel bandwidth is 25 kHz and it is divided into frames that have 6 times slots per frame. The iDEN system allows 6 mobile radios to simultaneously share a single radio channel for dispatch voice quality and up to 3 mobile radios can simultaneously share a radio channel for cellular like voice quality.


Code Division Multiple Access (IS-95 CDMA)


Code Division Multiple Access (CDMA) system (IS 95) is a digital cellular system that uses CDMA access technology. IS-95 technology was initially developed by Qualcomm in the late 1980's. CDMA cellular service began testing in the United States in San Diego, California during 1991. In 1995, IS-95 CDMA commercial service began in Hong Kong and now many CDMA systems are operating throughout the world, including a 1.9 GHz all-digital system in the USA that has been operating since November 1996.


Spread spectrum radio technology has been used for many years in military applications. CDMA is a particular form of spread spectrum radio technology. In 1989, CDMA spread spectrum technology was presented to the industry standards committee but it did not meet with immediate approval. The standards committee had just resolved a two-year debate between TDMA and FDMA and was not eager to consider another access technology. 


The IS-95 CDMA system allows for voice or data communications on either a 30 kHz AMPS radio channel (when used on the 800 MHz cellular band) or a new 1.25 MHz CDMA radio channel. The IS-95 

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CDMA radio channel allows multiple mobile telephones to communicate on the same frequency at the same time by special coding of their radio signals. 


CDMA radio channels carry control, voice, and data signals simultaneously by dividing a single traffic channel (TCH) into different sub-channels. Each of these channels is identified by a unique code. When operating on a CDMA radio channel, each user is assigned to a code for transmission and reception. Some codes in the TCH transfer control channel information while others transfer voice channel information. 


The control channel that is part of a digital traffic channel on a CDMA system has new advanced features. This digital control channel (DCC) carries system and paging information, and coordinates access similar to the analog control channel (ACC). The DCC has many more capabilities than the ACC such as a precision synchronization signal, extended sleep mode and others. Because each CDMA radio channel has many codes, more than one control channel can exist on a single CDMA radio channel and the CDMA control channels co-exist with other coded channels that are used for voice. 


The IS-95 CDMA cellular system has several key attributes that are different from other cellular systems. The same CDMA radio carrier frequencies may be optionally used in adjacent cell sites, which eliminates the need for frequency planning. The wide-band radio channel provides less severe fading, which the inventors claim results in consistent quality voice transmission under varying radio signal conditions. The CDMA system is compatible with the established access technology and it allows analog (EIA-553) and dual mode (IS-95) subscribers to use the same analog control channels. Some of the voice channels are replaced by CDMA digital transmissions, allowing several users to be multiplexed (shared) on a single RF channel. As with other digital technologies, CDMA produces capacity expansion by allowing multiple users to share a single digital RF channel. 


The IS-95 CDMA radio channel divides the radio spectrum into wide 1.25 MHz digital radio channels. CDMA radio channels differ from those of other technologies in that CDMA multiplies (and therefore spreads the spectrum bandwidth of) each signal with a unique pseudo-random noise (PN) code that identifies each user within a radio channel. CDMA transmits digitized voice and control signals on the same frequency band. Each CDMA radio channel contains the signals of many ongoing calls (voice channels) together with pilot, synchronization, paging, and access (control) channels. Digital mobile telephones select the signal they are receiving by correlating (matching) the received signal with the proper PN sequence. The correlation enhances the power level of the selected signal and leaves others un-enhanced.


Each IS-95 CDMA radio channel is divided into 64 separate logical (PN coded) channels. A few of these channels are used for control, and the remainders carry voice information and data. Because CDMA transmits digital information combined with unique codes, each logical channel can transfer data at different rates (e.g. 4800 b/s, 9600 b/s).

 

CDMA systems use a maximum of 64 coded (logical) traffic channels, but they cannot always use all of these. A CDMA radio channel of 64 traffic channels can transmit at a maximum information throughput rate of approximately 192 kbps [ ], so the combined data throughput for all users cannot exceed 192 kbps. To obtain a maximum of 64 communication channels for each CDMA radio channel, the average data rate for each user should approximate 3 kbps. If the average data rate is higher, less than 64 traffic channels can be used. CDMA systems can vary the data rate for each user dependent on voice activity (variable rate speech coding), thereby decreasing the average number of bits per user to about 3.8 kbps [ ]). Varying the data rate according to user requirement allows more users to share the radio channel but with slightly reduced voice quality. This is called soft capacity limit.


In 1997 the CDMA Development Group (CDG) registered the trademark cdmaOneTM as a label to identify second-generation digital systems based on the IS-95 standard and related technologies.


Japanese Personal Digital Cellular (PDC)


The PDC system is a TDMA technology with a radio interface that is very similar to IS-136, in that it has six timeslots and an almost identical data rate, and a core network architecture that is very similar to GSM. PDC operates in both the 900 MHz and 1,400 MHz regions of the radio spectrum.

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