Advertisment

Wireless with CDMA

author-image
PCQ Bureau
New Update

Code Division Multiple Access or CDMA technology is relatively new compared to GSM. It’s a form of DSSS (Direct Sequence Spread Spectrum) and is very different from the TDM (Time Division Multiplexing) scheme used by GSM. It has many advantages over GSM, but also has its own set of limitations. Here, we’ll understand the difference between the two technologies.

Advertisment

To explain the difference between TDM and CDMA, imagine that the wireless subscriber base represents a room full of people trying to hold one-to-one conversations. In TDM, each person takes brief turns at speaking, first one then another. In CDMA, each couple talks at the same time but in different languages and is therefore not bothered by the background noise.

The key is to extract the desired signal while rejecting everything else as random noise.

SMS from a landline

MTNL is now not taking bookings for their CDMA version–Garuda–though, it might be started soon. Instead at Supercomm, they showcased another interesting service, which may be offered to consumers soon–SMS on PSTN. This service uses the Dolphin network to send SMS messages. An SMS device will be attached to the PSTN phone the users end, or there could be a separate SMS phone altogether. As of now the GSM subscribers will get a Dolphin number as the caller ID, instead of the sender’s address. But the service is still at the drawing board stage. This would be, hopefully, taken care of when it’s actually launched. 
Advertisment

Technical differences



In TDM users take turns sharing the frequency in a round-robin fashion, with each one periodically getting the entire bandwidth for a little burst. In effect each user is given a timeslot on which to send and receive data.

The core principle of spread spectrum on the other hand, is the use of noise-like carrier waves and wider bandwidth than that required for simple point-to-point communication at the same data rate. Spread Spectrum generally employs two techniques: FH (Frequency Hopping) or DS (Direct Sequence).

In FH, the transmitter hops from frequency to frequency in a regular pattern. This is popular for military communication because it makes transmission hard to detect and next to impossible to jam. The history for this goes back to the early days of World War II.

Advertisment

DS allows each station to transmit over the entire frequency spectrum all the time. Multiple simultaneous transmissions are separated using some sort of coding technique, that is, each user is assigned a chip sequence and the sender and receiver synchronize by the receiver locking into the chip sequence of the sender. All the other (unsynchronized) transmissions are then seen as random noise. So with CDMA each user uses the full frequency spectrum of 1.25 MHz.

The standards



CDMA is used in both 2G and 3G networks. 2G CDMA standards are called cdmaOne, and there are two in 3G called CDMA2000 and WCDMA. cdmaONE was based on two versions the EIA/TIA IS-95 standard, namely IS-95A and B.

IS-95A was the first CDMA cellular standard and in addition to voice, it also provided data connections at 14.4 kbps. It was first deployed in September 1996 by Hutchison (Hong Kong). IS-95B provided much faster data throughput at 115 kbps in addition to voice and was first deployed in September 1999 in Korea followed by operators in Japan and Peru. 

Advertisment

CDMA2000 includes two technologies–CDMA2000 1X and CDMA2000 1xEV. The former can double the voice capacity of cdmaOne networks and delivers peak packet data speeds of 307 kbps on a single (1.25 MHz) carrier. It supports advanced applications such as e-mail, SMS, MMS, games, GPS-based location services, picture and music downloads. The world’s first CDMA2000 1X commercial system was launched by SK Telecom of Korea in October 2000.

Since then, CDMA2000 1X has been deployed in Asia, North and South America and Europe. 

CDMA2000 1xEV-DO (Evolution- Data Only) can deliver peak data speeds of 2.4Mbps on a single (1.25 MHz) carrier and can therefore support heavy applications such as video conferencing and streaming. It leverages the existing suite of Internet Protocols or IP, and hence supports all popular operating systems and software applications. It provides for an “always on” user experience, so that users are free to send and receive information from the Internet and their corporate intranets, any time, anywhere. The first commercial CDMA2000 1xEV-DO network was also deployed by SK Telecom in January 2002. 

Advertisment

CDMA vs GSM



CDMA provides better capacity for voice and data communications, allowing more subscribers to connect at any given time.

Compared to GSM, it delivers better spectral efficiency and power management, leading to enhanced battery life and longer talk time. On the limitation front, it has restricted roaming, meaning it can’t offer international roaming in all parts of the world unlike GSM. The lack of Smart Cards is another problem. While GSM uses SIM (Subscriber Identification Module), these are not widely accepted in CDMA. So, if you want to change your number or switch to another operator, you’ll have to get your handset reprogrammed. In densely populated urban areas, cell sites are put up in close proximity. In such a situation, voice quality can deteriorate under CDMA when signals from too many base stations are present at the subscriber’s phone but none are dominant.

Currently, the technology is being used to provide limited mobility through the WLL (Wireless in Local Loop) service. It is a system that connects subscribers to the basic phone operator’s PSTN (public switched-telephone network), the normal phone system, using radio signals as a substitute for copper. This means that your WLL (CDMA) phones will work in a particular region, within a range of few kilometers of your subscription location, i.e., the local loop. But with players like Reliance providing this service nationwide, you might be able to use your phones across different

regions. 

Anoop Mangla

Advertisment