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Comparison of GSM and CDMA
The objective of this white paper is to highlight the difference between GSM and CDMA technologies in wireless world.
This document gives an overview & brief technical information of GSM and CDMA technologies were evolved over the years.
Advantages and Disadvantages of GSM and CDMA are discussed, also commercial aspects of both wireless technologies are mentioned.
This document is useful for entire audience who would like to know basic differences of two major wireless technologies.
One of the most contentious battles being waged in the wireless infrastructure industry is the debate over the efficient use and allocation of finite airwaves.
For several years, the world's two main methods Code-Division Multiple Access (CDMA) and Global System for Mobile communications (GSM) have divided the wireless world into opposing camps. Ultimately, the emergence of a victorious technology may owe more to historical forces than the latest wireless innovation, or the merits of one standard over the other.
This document gives a brief comparison of GSM and CDMA technology.
CDMA : Code Division Multiple Access
CDMA (Code Division Multiple Access) is a second-generation digital mobile telephone standard which takes a different approach to the other competing standards. CDMA spreads the signal over a wide bandwidth, identifying each channel using unique digital codes.
Evolution of CDMA
Putting into an historical context, the roots of CDMA were originated during the period of pre world war II-America. CDMA is a recently patented technology that only became commercially available in the mid-1990s.
In 1940, Hollywood actress turned inventor, Hedy Lamarr, and co-inventor George Antheil, with World War II looming, co-patented a way for torpedoes to be controlled by sending signals over multiple radio frequencies using random patterns. Despite arduous efforts by the inventors to advance the technology from experiment to implementation, the U.S. Navy discarded their work as architecturally unfeasible.
The idea, which was known as frequency-hopping, and later as frequency-hopping spread-spectrum technology (FHSS), remained dormant until 1957. Engineers at the Sylvania Electronic Systems Division, in Buffalo, New York took up the idea, and after the Lamarr-Antheil patent expired, used it to secure communications for the U.S. during the 1962 Cuban Missile Crisis.
After becoming an integral part of government security technology, the U.S. military, in the mid-80s, declassified what has now become CDMA technology, a technique based on spread-spectrum technology.
Technical Details of CDMA
CDMA which incorporates spread-spectrum technology works by digitizing multiple conversations. It attaches a code known only to the sender and receiver, and then dicing the signals into bits and reassembling them. The military loved CDMA because coded signals with trillions of possible combinations resulted in extremely secure transmissions.
TDMA and GSM systems commonly start with a slice of spectrum referred to as one "carrier". Each carrier is then divided into time slots. Only one subscriber at a time is assigned to each time slot, or channel. No other conversations can access this channel until the subscriber's call is finished, or until that original call is handed off to a different channel by the system. For example, GSM systems create 8 time-division channels in 200 kHz (kilohertz) wide carriers.
With CDMA, all users share the same 1,250 kHz wide carrier, but unique digital codes are used to differentiate subscribers. The codes are shared by both the mobile station and the base station and are called "pseudo-random code sequences". Base stations in the system distinguish themselves from each other by transmitting different portions of the code at a given time. In other words, the base stations transmit time-offset versions of the same pseudo-random code.
In order to ensure that the time offsets used remain unique from each other, CDMA stations must remain synchronized to a common time reference. The global positioning system (GPS) provides this precise common time reference. GPS is a satellite-based radio navigation system capable of providing a practical and affordable means of determining continuous position, velocity, and time to an unlimited number of users.
One of the unique aspects of CDMA is that while there are certainly limits to the number of phone calls that can be handled by a carrier, this is not a fixed figure. The number of simultaneous connections any base station is able to handle is the result of a trade-off with the range of the base station and the quality of each connection. A standard CDMA connection has a digital transfer rate of 9.6Kbps (kilobits per second), the same as GSM. The voice data part of the connection is transmitted at a rate of 8Kbps. All connections are shared around the spread spectrum with a maximum transfer rate of 1.23Mbps (Megabits per second), theoretically allowing a maximum of 131 connections, compared to 48 GSM connections in a similar bandwidth.
CDMA supports variable bandwidth connections, with enhanced standards at 13Kbps and 64Kbps for superior quality speech and faster data connections. This 'bandwidth-on-demand' limits the number of simultaneous connections to a particular base station.
Traditional uses of spread spectrum are in military operations. Because of the wide bandwidth of a spread spectrum signal, it is very difficult to jam, difficult to interfere with, and difficult to identify. This is in contrast to technologies using a narrower bandwidth of frequencies. Since a wideband spread spectrum signal is very hard to detect, it appears as nothing more than a slight rise in the "noise floor" or interference level. With other technologies, the power of the signal is concentrated in a narrower band, which makes it easier to detect. The narrow band is also more prone to interference than the spread-spectrum of CDMA.
Increased privacy is inherent in CDMA technology. CDMA phone calls will be secure from the casual eavesdropper since, unlike an analog conversation, a simple radio receiver will not be able to pick individual digital conversations out of the overall RF radiation in a frequency band. TDMA (time division multiple access) and GSM systems have to add an extra stage of encryption to the signal to ensure privacy, whereas encryption is inherent to the CDMA system.
Advantages and Disadvantages of CDMA
• Increased cellular communications security.
• Simultaneous conversations.
• Increased efficiency, meaning that the carrier can serve more subscribers.
• Smaller phones.
• Low power requirements and little cell-to-cell coordination needed by operators.
• Extended reach - beneficial to rural users situated far from cells.
• Due to its proprietary nature, all of CDMA's flaws are not known to the engineering community.
• CDMA is relatively new, and the network is not as mature as GSM.
• CDMA cannot offer international roaming, a large GSM advantage.
Commercialization of CDMA
Qualcomm, which patented CDMA and other telecommunications companies were attracted to the technology. The reason behind this is it enabled many simultaneous conversations, rather than the limited stop-and-go transmissions of analog and the previous digital option.
CDMA was not field tested for commercial use until 1991, and was launched commercially in Hong Kong in 1995. CDMA technology is currently used by major cellular carriers in the United States and is the backbone of Sprint's Personal Communications System (PCS). Along with Sprint, major users of CDMA technology are Verizon and few other telecom companies
Aspects of CDMA technology have been incorporated into the third-generation IMT-2000 standard. This standard is designed to allow interoperability between the different networks and integration with satellite technology. This third-generation technology will allow broadband data access that can be used for voice, video and data communication at speeds of up to 2Mbps.
MOTOROLA & LUCENT are dominant cell phone manufacturers using CDMA technology.
GSM : Global System for Mobile Communication
GSM (Global System for Mobile Communication) is another second-generation digital mobile telephone standard which uses Time Division Multiple Access method. GSM is major competitive technology for CDMA.
Evolution of GSM
During the early 1980s, analog cellular telephone systems were experiencing rapid growth in Europe, particularly in Scandinavia, France, Germany and the United Kingdom. Each country developed its own system, which was incompatible with everyone else's in equipment and operation. This was an undesirable situation because not only the mobile equipment was limited to operation within national boundaries but there was a very limited market for each type of equipment. So economies of scale and the subsequent savings could not be realized.
The Europeans realized this early on and in 1982 the Conference of European Posts and Telegraphs (CEPT) formed a study group called the Groupe Spécial Mobile (GSM). This group was formed to study and develop a pan¬European public land mobile system. The proposed system had to meet certain criteria:
• good subjective speech quality
• low terminal and service cost
• support for international roaming
• ability to support handheld terminals
• support for range of new services and facilities
• spectral efficiency
• ISDN compatibility
In 1989, GSM responsibility was transferred to the European Telecommunication Standards Institute (ETSI). Phase-I of the GSM specifications were published in 1990. Commercial service was started in mid¬ 1991 which is later accepted and used by most of the countries.
Technical Details of GSM
The International Telecommunication Union (ITU), which manages the international allocation of radio spectrum (among other functions) allocated the bands 890-915 MHz for the uplink (mobile station to base station) and 935-960 MHz for the downlink (base station to mobile station) for mobile networks in Europe.
Since this range was already being used in the early 1980s by the analog systems of the day, the CEPT had the foresight to reserve the top 10 MHz of each band for the GSM network that was still being developed. Eventually, GSM will be allocated the entire 2x25 MHz bandwidth.
Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible.
The method chosen by GSM is a combination of Time¬ and Frequency ¬Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the total 25 MHz bandwidth into 124 carrier frequencies of 200 kHz bandwidth. One or more carrier frequencies are then assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme, into eight time slots. One time slot is used for transmission by the mobile and one for reception. They are separated in time so that the mobile unit does not receive and transmit at the same time, a fact that simplifies the electronics.
A GSM network is composed of several functional entities whose functions and interfaces are defined.
The GSM network can be divided into three broad parts.
• The Mobile Station is carried by the subscriber
• The Base Station Subsystem controls the radio link with the Mobile Station.
• The Network Subsystem, the main part which performs the switching of calls between the mobile and other fixed or mobile network users as well as management of mobile services, such as authentication.
The Mobile Station and the Base Station Subsystem communicate across the Um interface, also known as the air interface or radio link. The Base Station Subsystem communicates with the Mobile service Switching Center across another interface.
Advantages of GSM
• GSM is already used worldwide with over 450 million subscribers.
• International roaming permits subscribers to use one phone throughout Western Europe. CDMA will work in Asia, but not France, Germany, the U.K. and other popular European destinations.
• GSM is mature, having started in the mid-80s. This maturity means a more stable network with robust features. CDMA is still building its network.
• GSM's maturity means engineers cut their teeth on the technology, creating an unconscious preference.
• The availability of Subscriber Identity Modules, which are SMART CARDS that provide secure data encryption give GSM m-commerce advantages.
Major disadvantage of GSM is lack of access to American market.
Commercialization of GSM
The commercial services were started in 1991 and by 1993 there were 36 GSM networks in 22 countries, with 25 additional countries having already selected or considering GSM.
This is not only a European standard - South Africa, Australia, and many Middle and Far East countries have chosen GSM. By the beginning of 1994, there were 1.3 million subscribers worldwide. The acronym GSM now (aptly) stands for Global System for Mobile telecommunications.
GSM became more popular as AT&T Wireless' decided to overlay GSM atop its TDMA network. This means the European technology (GSM) gains instant access to North America's number two network.
Qualcomm's recently announced that Wideband-CDMA (WCDMA) won't be ready in Europe until 2005. This comes amidst the fact that the GSM's successor, General Packet Radio Services (GPRS) remains on target for deployment in 2001-2002.
NOKIA & ERICSSON are dominant cell phone manufacturers using GSM technology.
This white paper was written to give a brief comparison of two major technologies in wireless world.
In today’s wireless world, the battle between CDMA and GSM is muddled. At one point Europe clearly favored GSM and North America favored CDMA. The distinct advantage of one over the other has blurred as major carriers like AT&T Wireless begin to support GSM, and there are efforts to try out the compatibility between the two technologies.
GSM still holds the upper hand however with more number of users when compared to CDMA users.
For all of the historical and technological reasons it appears that GSM or some combination of GSM and CDMA will become the long sought global wireless standard. A universalization of wireless technologies can only stand to benefit the compatibility and development costs and demands on all wireless commerce participants.
CDMA – Code Division Multiple Access
GSM - Global System for Mobile Communication
WCDMA - Wideband-CDMA
GPRS - General Packet Radio Services
CEPT - Conference of European Posts and Telegraph
TDMA - Time Division Multiple Access
FDMA - Frequency Division Multiple Access
BPS - Bits per Second