Wireless

May 27, 2004
CDMA - The technology with a difference

Parag Kar

MUMBAI -- The technology has always played a prominent role in steering human lives. Today, mobile communication impacts the lives significantly, and the total number of mobile phones users worldwide has surpassed the land line subscribers. Various competing mobile technologies have emanated, and have been around for a while. However, the emergence of CDMA is revolutionising the way people communicate. It is one of the fastest growing technologies, with the worldwide subscriber base of over 190 million. It is taking the mobile communication on the path of 3G migration. The finer technological aspects and capabilities are driving this technology.

Basic differences
The basic communication technology has always been limited by factors such as noise and signal fading. Overcoming these are the basic objectives for enabling meaningful communication between any two points. The capacity and the quality of conversation depends on how effective we are in overcoming these limitations, and obviously there is a hard physical limit upto beyond which one can't go. CDMA adopted a philosophy, which has been the basic idea for all major technological breakthroughs, "If you can't beat them join them."

CDMA, as a technology, leverages the bottlenecks of conventional communication to improve the robustness of the overall signal over the air, thus in turn enhancing capacity and quality of the air interface. This capability comes from the ability of the CDMA base station to listen to multiple legs of signal received at any time, that otherwise would have caused interference for a conventional receiver, making the signal totally unintelligent. Another fundamental difference is capability to spread basic communication channel over the total carrier bandwidth of 1.25MHz, and transmitting all the channels over the total carrier bandwidth, decode them uniquely at the receiver, and very effective power control of users, by time test reliable power control algorithms. This enables efficient spectrum utilisation than any other conventional mobile technologies, which end up locking up of the air interface resources permanently during the call setup time, and has serious limitation of being able to dynamically release resources to other subscribers in case of voice and data inactivity. This results in very inefficient utilisation of the scarce air interface resources. Thus, CDMA technology is capable of supporting more Erlangs per Hertz of voice and data, than other comparable TDM technologies.

Data-enabled air interface
CDMA enables the air interface to support data applications easily than any other comparable technologies. This is because of the capability of the technology to create supplementary channels of various data rate granularity, and statistically multiplexing various data users on these channels. Other comparable technologies such as GPRS and EDGE allocate physical TDM channels as data channels. However, major differences are listed as under.

Firstly, in case of CDMA, the supplementary data channels are considerable fatter (153.6Kbps) than GPRS (14Kbps) and EDGE (59Kbps). This enables efficient and optimum utilization of the channel in case of CDMA, compared to GPRS and EDGE, where the pipes are considerable thinner. Secondly, in case GPRS and EDGE, data transmission requires allocation of time slots, which otherwise were supposed to be used for voice, thus physically taking away revenue generating voice resources for enabling data transmission. Thirdly, the TDM technologies hold on to the allocated nailed up time slots, even during voice inactivity, and is not flexible enough to trade off this inactivity period for more data on the data channels. This trade off is possible in case of cdma technology due to ubiquitous nature of the air interface channels within the carrier, and the fundamental design of the technology as such.

Easy evolution
The CDMA2000/1x has a smooth evolution path compared to any other comparable technologies, both on the network and handset compatibility. CDMA 2000/1x can be easily evolved to data optimised technology 1xEV-DO, that can support theoretically 2.4Mbps download on the air Interface. The 1xEV-DO has a total RF compatibility with the RF signal of CDMA 2000/1x, i.e. it can be coterminous along with the CDMA 2000/1x signal and occupies the same bandwidth as CDMA 2000/1x. This enables efficient utilization of spectrum, and the possibility of 1xEV-DO carrier to coexist along with CDMA 2000/1x in the same base station, enabling vital investment protection to the service provider. Other comparable technologies have limitations on RF and handset backward compatibility. GSM's data evolution comparable to 1xEV-DO is on a totally different RF band, and requires considerable large initial RF bandwidth.

Robust security
CDMA-based technologies are more secured compared with any other comparable technologies. CDMA is time tested 3G technology that has been developed in a manner to provide multilayer protection. However, 2G technologies are characterized by unco-ordinated import/export regulation, no verifiable security practices between carriers, carrier-centric charge model (customer is financially liable for fraudulent charges), and delegated fraud prevention model. On the other hand, 3G-CDMA-IS/41 has a uniform customer base with consistence import/export regulation for cryptography, known and verifiable security practices between carriers, customer-centric charge models and shared fraud prevention models between carriers. Till date, there are no known commercially available IS95-CDMA, CDMA2000/1x, or W-CDMA interception devices. Also for corporate individual use requires end-to-end data security, and shall require one to run IPSEC, directly from the user terminal device. IPSEC is a security protocol, which run over IP, and consumes overheads. CDMA's data capability helps it to deal with this extra overhead very easily.

Next-gen apps
There are certain key characteristics that distinguish conventional applications from next-generation applications. In order for enabling next-generation requires, we more bandwidth, faster signaling and bearer QoS over the air interface. Most of the widely acceptable third generation mobile technologies have CDMA as the baseline technology. CDMA enables higher average data speed required to support NGA, than any other comparable technologies. CDMA also enables finer granularity of data control, which enables efficient utilization of spectrum.

Easy RF planning
RF planning, in case of CDMA-based technologies, is comparatively much simpler than other conventional TDM technologies like GSM, as there is virtually no or little frequency planning required in CDMA. In CDMA channels are completely spread across in the carrier bandwidth, where as in case of GSM, channels are both time and frequency separated over the air interface, and hence requires complex RF planning.

Conclusion
Still there are issues that need to be resolved. The most important of them is international voice roaming among various subscribers. When evaluating this, one has to understand that CDMA as a technology is only half a decade old, and the roaming agreements still needs to fall in line for unrestricted international mobility, there are no technological constraint as such that restrict mobility. International roaming for CDMA has been activated even today in some countries, though in a limited scale. Qualcomm has developed chipset (GSM-1x), which even technologically permits roaming across diverse CDMA and GSM networks. With continuous growth of subscriber base, and eventually getting the benefit of economy of scale these limitation shall also wither away.

(The author is general manager, technology and solutions, Lucent Technologies India Ltd.)