- LUCAS A. EPPARD
HISTORY OF GSM
At the 1982 Conference of European Post and Telecommunications (CEPT), the standardization body, Groupe Speciale Mobile, was created to start work on a single European standard. The name of this standard was later changed to Global System for Mobile Communications (GSM). GSM is a wide area wireless communications system that uses digital radio transmission to provide voice, data, and multimedia communication services. A GSM system manages the communication between mobile telephones, radio towers, and interconnecting switching systems. The development of the GSM specification began later that year, with the first commercial GSM system later being deployed in 1991. By 2004, there were over a billion GSM subscribers in more than 205 countries and territories across the globe.
Before the GSM system was implemented, most countries used cellular systems that were often in conflict with each other. Most mobile telephones could only function on their specific cellular network, and as a result many customers could not travel to neighboring countries and still expect service. With various types of systems only serving exclusive groups of people, the large-scale production necessary to manufacture low-cost subscriber equipment was not practical. This resulted in high equipment costs and systems were not very successful in the open market.
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In 1990, the first phase of GSM specifications was finalized. This included basic voice and data services. Around the same time, the initial efforts were made to modify the GSM specification to offer service at the 1800 MHz frequency range. DCS 1800, which the standard is now known as, is used for the Personal Communications Network (PCN). Since that time, Phase 2 of the GSM and DCS 1800 specifications has been completed. In this phase, enhanced data transfer capabilities and advanced short messages services were added. 
“GSM radio is a wireless communication system that divides geographic areas into small radio areas (cells) that are interconnected with each other.” Each of the coverage areas has one or more transmitters and receivers that converse with cellular handsets in its designated area. GSM radio systems function in a particular frequency band or bands that have been assigned to the system. Depending on the frequency plan, system administrators may choose to recycle specific radio channels at different cell sites. Through a combination of either TDMA or FDMA, subscribers share each radio channel. A GSM radio channel is 200 kHz wide and is further divided into frames that are made up of eight time slots. Each cell site may contain several channels which are shared by as many as 8 to 16 voice users per radio channel.
Time division multiple access (TDMA) is a method of sharing one radio channel through the division of time slots which are then subsequently allocated among multiple users of that radio channel. A mobile radio is assigned a particular time and position on a channel when it communicates with a TDMA system. Through this process, it is possible for a TDMA system to allow multiple users to operate on a single radio channel through the use of separate time slots. TDMA systems therefore amplify their capacity to serve several users with a limited number of radio channels. GSM employs time division multiplexing (TDM) to distribute one modulated carrier frequency radio waveform between 16 half rate or 8 full rate phone calls. Because of this, a clear distinction is often made between a communication channel and a radio carrier in many publications pertaining to GSM.
Duplex communication is the transmission of voice and/or data signals that allows simultaneous 2-way communication. In order for duplex communication to occur on analog systems, a separate voice path must be assigned to both a transmitter and frequency. This method of using two frequencies for duplex communication is called frequency division duplex (FDD). Time division duplex (TDD) is another technique that can be used for duplex communication. Time sharing allows two devices to achieve two way communications through TDD. Similar to how a walkie-talkie works; one device transmits while another receives. Once the original transmission is complete, the second device becomes the transmitter while the first becomes the receiver. This process then constantly repeats itself so that the data seems to flow in both directions instantaneously.
The GSM system utilizes both FDD and TDD communication. One frequency is used while the handset communicates with the cell tower, while the reverse communication from the tower to the handset uses the other frequency. However, TDD is also used in the GSM system while the transmitter and receiver talk at different times. This time offset between the transmission and reception also helps to streamline the design of the handset. The radio frequency separation between the forward (downlink) and reverse (uplink) frequencies differs between the frequency bands. As a general rule, the separation must be greater between the forward and reverse channels at the higher frequencies. For instance, the frequency separation in the GSM 900MHz system is 45 MHz while the separation is 95 MHz for PCN. Finally, the GSM PCS 1900 MHz system has a frequency separation of 80 MHz.
Voice service is a form of communication service in which multiple users can transmit information over the voice frequency band via a communication network. Voice service consists of the initiation of sessions between at least two users which allows for the real time, or near real time, transmission of voice signals between those users. A GSM network delivers numerous types of digital voice services. A number of factors can affect the quality of voice service on a GSM system. However, through the use of several distinct forms of speech compression a GSM system can dynamically adjust the voice quality. Each service provider has the ability to choose and manage which speed compression process (voice coding) their network utilizes. Service providers can increase the amount of users that they provide service to through the use of voice coders that possess higher speech compression rates. However, this comes at the cost of delivering a more degraded signal to each user. Besides basic voice services, the GSM system can also provide both group and broadcast voice services.
A data service is a communication service that transmits information between two or more devices. By means of a communication network, data services can be delivered either inside or outside of the audio frequency band. Data service consists of the creation of physical and logical communication sessions between two or more users. This allows for the non-real time or near-real time transfer of data type signals between users.
However, a data modem has to be used in the event that a signal is broadcast on a non-digital channel. The data modem is needed to translate the signal into tones that can be sent in the audio frequency band. GSM voice traffic channels cannot send or receive analog modem data because the speech coder utilized by the system is only capable of compressing voice signals. Medium speed packet data and low-speed circuit switched data are the two services provided in a GSM system. Circuit switched data is a communication process that maintains a committed communications path between two devices. This can be accomplished no matter the amount of data being sent between the devices. This provides sole use of the circuit to the communications equipment employing it regardless of whether or not the circuit is idle. In order to create a circuit-switched data connection, the address must be sent before a connection path is established. Once this occurs, data is continually transmitted using this path until it is disconnected by request from either the sender or receiver.
Packet switched data service is the other utilized by a GSM system. Through the division of data into small packets, information can be transferred between two points. In order to reconstruct the original data, the packets are routed through the network and recombined at the receiving end. This can be achieved because the destination address is contained within each individual packet. So no matter which route the packets take through the network they will end up at the same destination. The GSM system uses general packet radio service (GPRS) to provide its packet data service. In using the GPRS system, new gateways and packet control channels are added to a GSM system. It is important to note that GPRS is a type of packet-switched data service that is known as “always-on.” This means that when a device is turned on it acquires an IP address that it needs in order to communicate with the network.
Code Division Multiple Access (CDMA) is a digital cellular radio system currently in use in more than 35 countries around the world. It is a form of spread spectrum communications in which a radio signal is transmitted over a dedicated channel as a much wider signal than is traditionally necessary. By using a signal with a wider than normal bandwidth, interference from other users on the same section of bandwidth is minimized. This permits several users to share the radio channel at the same time. Come in two basic forms: frequency hopping and code division. Frequency hopping multiple access (FHMA) is an access technology in which mobile radios or cellular handsets share radio channels by breaking their transmissions down into short bursts over a number of frequencies. A device transmits for a short period of time on one frequency, and then hops to another radio frequency to continue its transmission.
CDMA allows a number of users to share a single radio channel frequency simultaneously by designating a unique code sequence to each mobile radio. By assigning a particular hop pattern to each mobile device collisions can be minimized. Even if collisions do occur randomly only a small amount of data may be lost. Furthermore, error detection and correction methods can be implemented in order to fix the data that was lost. Compared to previous narrowband wireless systems, CDMA technology is a wideband spread spectrum system. It is important to note that the features, services and requirements for CDMA were developed by standards organizations using many pre-existing technologies. For this reason, CDMA is considered a second generation (2G) cellular system.
HISTORY OF CDMA
The growth of CDMA technology began in the United States in 1989 as a result of the CTIA next cellular generation technology requirements. In September of the previous year the Cellular Telecommunications Industry Association (CTIA) presented the User Performance Requirements (UPR) for the next generation of wireless service. In 1989 the Telecommunications Industry Association (TIA) designated TDMA as the radio interface standard. Later that year QUALCOMM was able to build a CDMA system that met the CTIA requirements. This was achieved largely through the support of mobile telephone manufacturers and carriers, as well as new infrastructure equipment. QUALCOMM and its associates presented the results of their field trials at the end of 1991. This pushed the CTIA Board of Directors to implement a resolution which requested TIA to structurally organize new developments from wideband systems. By mid-1993 TIA had voted on and accepted IS-95 as the CDMA air interface standard radio specifications. The systems which were based on the IS-95 standard became known as cdmaOne systems. In 1995 the first commercial CDMA network was implemented in Hong Kong.
CDMA is the fasted growing technology in wireless communications. A CDMA network consists of the same basic components as other wireless systems. This includes a mobile station, base station, a controller, and finally a switching network. In CDMA, a carrier frequency is separated into 64 individual channels through the use of codes. Each channel transmits the data associated with a separate and unique conversation in digitally coded form. In some cases an independently coded channel transmits signals related to the start of a connection. In spread spectrum systems like CDMA, one carrier holds multiple channels. This can sometimes lead to confusion when discussing CDMA with someone who is familiar with the older analog frequency division multiplex (FDM) systems. With older FDM systems a channel is synonymous with a carrier. Therefore each pair of carrier frequencies was able to carry only one conversation.
All CDMA networks use a specific radio frequency band for signals from the base transmitter to the mobile receiver and a second distinct radio frequency band for the signals from the mobile transmitter to the base receiver. CDMA technology can be used in either the existing 800 MHz cellular frequency band or the 1900 MHz personal communications service (PCS) band. CDMA has the ability to function in the same radio spectrum allocation as older cellular systems when it is being used on those networks. The mobile station transmit frequency band is between 824-849 MHz, while the base station transmit frequency ranges from 869-894 MHz. When in use in the 800 MHz band, CDMA maintains a 45 MHz separation between the forward and reverse channels. Some radio carrier frequencies are defined for CDMA use in its cellular network. However, not all of these frequencies are used for CDMA transmission. This is because the FCC requires that analog radio transmission (AMPS) continue to operate.
CDMA operation is also compliant with the frequency structure of the PCS band. The separation of the forward and reverse channels is 80 MHz while operating on this band. The mobile station transmit frequency band is between 1850-1909 MHz; while the base station transmit frequency band ranges from 1930-1989 MHz. Some CDMA systems operate in the Personal Communications System (PCS) frequency bands. PCS is mostly found in North America on the 1900 MHz frequency band, where it is known as PCS-1900. Due to the commonality of the base band signals, some manufacturers make dual band CDMA handsets that are capable of operating on both the 800 MHz and 1900 MHz bands. If a subscriber’s handset is manufactured to allow dual frequency band operation it is possible to obtain service on either or both systems. However, this can only be achieved if the CDMA networks in the region are properly linked. As CDMA moved further towards a third generation technology, the CDMA2000 arose as the next level of the IS-95 standard.
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The term first generation system (1G) is most commonly used in regards to older analog cellular systems. These analog systems eventually gave way to the newer digital second generation systems (2G) that are popular today. 2G systems in use today include the GSM and cdmaOne systems previously mentioned in this paper as well as the US-TDMA and PDC systems. These systems allowed voice communications to go wireless in most of the industrialized world and have drastically increased mobile handset functions. Subscribers have come to expect their cellular networks to provide them with text messaging and data access capabilities. In the United States it is almost unheard of now for a network provider to not offer data services in excess of 500 MB if not at least 1GB per month. The abundance of 2G technologies spurred the demand for even greater services.
At the 1992 World Administrative Radio Conference (WARC) of the ITU (International Telecommunications Union), member nation delegates began to investigate the frequencies that would be available for future expansion of 3G systems. The International Mobile Telecommunications System 2000 (IMT-2000) was created as a result of this meeting. It is important to note that IMT-2000 is often used interchangeably with Universal Mobile Telecommunications System (UMTS). UMTS defined a wireless communication system that operates in the 2GHz frequency band. The primary objective of IMT-2000 standard was the development of a single universal air interface that was built upon pre-existing infrastructure. This would help to keep start-up costs low and potentially allow international roaming. The Third Generation Partnership Program (3GPP) was created to coordinate this process.
It became evident early on that a single air interface would be nearly impossible to attain so a second committee was formed to oversee the process of bringing other air interfaces up to the IMT-2000 standard. A second partnership program was established to ensure this would occur and the 3GPP2 was born. The two partnership programs are essentially drawn along the fault lines of GSM and CDMA. UMTS, which is often used synonymously with Wideband CDMA or WCDMA, has become the standard for GSM 3G technologies and falls under the umbrella of 3GPP1. The 3GPP2 on the other hand focuses on the CDMA2000 standard.
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