What is the difference between CDMA and GSM?

For people in the telecom industry, concepts like FDMA, TDMA and CDMA are fairly well-understood as these are the techniques used by telecom networks for enabling multiple access. Multiple access is the ability of a network to allow multiple people to access the radio frequency spectrum simultaneously. Therefore, the whole CDMA vs GSM discussion is a bit misleading because then the comparison is not between two multiple-access techniques but between two mobile communications technologies namely GSM and IS-95 (cdmaOne). Let’s take a closer look at these technologies to understand the difference between GSM and CDMA mobile networks.

GSM (Global System for Mobile Communications) is a second-generation (2G) technology standard that uses a combination of FDMA and TDMA for mobile communications. In this context, the term CDMA is used to collectively refer to IS-95 (cdmaOne) and CDMA2000 standards that enabled the second and third generations of mobile networks respectively. IS-95 and CDMA2000 employed CDMA technology (Code Division Multiple Access) for the air interface.

Background

The first generation of mobile networks (1G) was analogue and used FDMA technology to provide mobile services. FDMA or Frequency Division Multiple Access was employed by all the key first-generation technologies including Advanced Mobile Phone System (AMPS), Nordisk MobilTelefoni (NMT, English name: Nordic Mobile Telephone), Funktelefonnetz-C (C-Netz, English name: Radio Telephone Network C) and Total Access Communications System (TACS).

As soon as the digital era of mobile communications started from 2G (second-generation) onwards, new technology standards started to make an entry. The second generation of mobile networks were the first to introduce TDMA and CDMA into the mix. TDMA stands for Time Division Multiple Access and CDMA stands for Code Division Multiple Access. The most popular 2G standard GSM (Global System for Mobile) used a combination of TDMA and FDMA to offer mobile services in a digital and secure environment. D-AMPS or Digital AMPS was the other 2G standard that also used a combination of FDMA and TDMA. The first CDMA technology used in mobile communications was Interim Standard 1995 or IS-95 which was part of the 2G era.

IS-95 is commercially known as cdmaOne and was followed by CDMA2000 which was the 3G upgrade path for IS-95. The GSM networks, on the other hand, used a different path for the 3G upgrade and employed the UMTS technology.

CDMA-based 2G networks

IS-95 was the first standard in mobile communications that utilised the CDMA technology. The standard IS-95 has two variants; IS-95A and IS-95B. The frequency band used by IS-95A can either be 824 to 894 MHz or 1850 to 1990 MHz with separate frequency bands for the uplink and the downlink. The carrier frequencies used in IS-95 have a bandwidth of 1.25 MHz. The peak data rates of up to 14.4 kbps can be achieved by IS-95A, while the technology enhancements in IS-95B can increase the data rates to up to 115 kbps. IS-95 provided capacity advantages for its ability to accommodate more users per MHz of bandwidth.

The power consumption in these CDMA networks is low which allows mobile users to be able to make phone calls in decent quality even when the signal quality is not at its best. Low power consumption also extends the cell coverage which in turn increases the size of the cell. Due to soft handovers (also known as handoffs), the calls are less likely to be dropped.

GSM-based 2G networks

GSM was the most widely deployed 2G standard and it used a combination of TDMA and FDMA to provide voice, SMS and mobile data services. The initial GSM networks provided limited data services using the circuit-switched technology which wasn’t very efficient. Therefore, some enhancements were made to the GSM networks in the later years in the form of GPRS (General Packet Radio Service) and EDGE (Enhanced Data for Global Evolution). These enhancements added the packet-switched part to the existing GSM networks in order to offer data services in an efficient manner. GPRS can offer peak downlink speeds of up to 171.2 kbps while EDGE can offer peak downlink speeds of up to 384 kbps. GPRS and EDGE are often referred to as 2.5G and 2.75G respectively.

The original frequency band for GSM networks was from 890 MHz to 915 MHz for the uplink and 935 MHz to 960 MHz for the downlink. This frequency band is known as Primary GSM band or P-GSM. The primary GSM band was later extended in order to add 10 MHz to both the uplink as well as the downlink. The extended band is known as Extended GSM or E-GSM and it ranges from 880 MHz to 915 MHz for the uplink and 925 MHz to 960 MHz for the downlink.

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