End of the Road for CDMA?

[706VZWUser]

By Elliott Drucker
WirelessWeek - June 01, 2008

With the coming age of LTE networks, might CDMA networks become legacy networks?

Last November, Verizon Wireless announced that it had selected Long Term Evolution (LTE) for eventual deployment of “fourth generation” technology. It’s been widely discussed in the industry that this move by Verizon casts some doubts over the future of competitive 4G technology Ultra Mobile Broadband (UMB).
Verizon’s move certainly seems to make LTE the winner, and what I find most interesting about it is that this appears to represent the end of the line for ongoing evolution of CDMA as the basis for new wireless air interface technologies.
Ever since the late 1980s when Qualcomm made a big splash with demonstrations of its use in cellular networks, CDMA generally has been recognized as the premier technology for mobile telephony. True, TDMA-based GSM has had bigger numbers worldwide, but when it came time for the keepers of GSM standards to look at the next generation (meaning 3G), they selected CDMA. Furthermore, CDMA has demonstrated superior spectrum efficiency compared with other digital channel schemes, and that has translated to lower costs for CDMA carriers like Verizon. CDMA has a number of other benefits in mobile networks, which I’ll touch on a bit later.
Apparently succeeding CDMA as the air interface technology of choice is orthogonal frequency division multiplex (OFDM). Both LTE and UMB rely on OFDM to provide the potential for very high data rates, particularly in the downlink. However, LTE does retain a CDMA channel structure for the uplink.
UMB, like the third 4G technology competitor WiMAX, uses OFDM exclusively in both the downlink and uplink. So with the future of LTE very much in doubt, it certainly appears that CDMA’s reign as the technology of choice for wireless evolution will end with the 3G networks – EV-DO and W-CDMA – that are currently reaching maturity.
LEGACY NETWORKS
It’s obviously premature to say that CDMA has reached the end of its life cycle. The reach of EV-DO networks is still expanding, with future revisions and enhancements being planned. W-CDMA got a major boost with adoption of HSDPA and HSUPA protocols, and as with EV-DO, ongoing enhancements of W-CDMA are still being developed. But at some point within the foreseeable future, it is likely that major carriers will stop investing in 3G and move on to the promise of much higher bandwidths with 4G technologies. At that point, it appears that CDMA systems will fall into the category of “legacy networks.”
Or will they? The relatively brief history of the wireless industry seems to offer conflicting clues. On the one hand, the American version of TDMA (or digital AMPS) entered “legacy” status almost immediately when its major users decided to migrate to other technologies. And while Sprint seems to run hot and cold about maintaining the Nextel iDEN network, it too appears to be a legacy technology. But then there is GSM/GPRS/EDGE, whose numbers continue to grow worldwide years after the superiority of CDMA technology became apparent.
But history aside, there are a number of reasons to expect that CDMA will be around for a while. First of all, OFDM technology remains largely untested in mobile applications, particularly those involving intensive usage in urban environments. There is no guarantee that the success demonstrated in Wi-Fi, and more recently in early deployments of fixed WiMAX systems, will translate to similar performance in the far less benign channel conditions commonly encountered in mobile networks.
Even if OFDM delivers on its lofty promises for 4G, it probably will not prove to be superior to CDMA for most voice applications. That’s because the main advantage to OFDM, being able to take advantage of good channel conditions (where they exist) to deliver high throughput bandwidth, is largely irrelevant for the relatively modest data rates required for voice service.
On the other hand, CDMA’s inherent ability to optimally adapt to widely and rapidly varying channel conditions by constant adjustment of transmit power means that interference is minimized and channel capacity is optimized.
Now, I’m not talking about the packet data-oriented CDMA schemes used in EV-DO and HSDPA/HSUPA, but rather the circuit-switched voice optimized ones found in 1X-RTT and W-CDMA. Why these? Because provision of full duplex (i.e. circuit-switched) voice service over packet data networks, typically using VoIP, entails a rather substantial penalty in data overheads.
Transport of overhead isn’t all that big of a deal given the massive bandwidth of fiber-optic networks that carry the Internet, which helps explain the current trend of migrating wire-borne voice telephony to IP. But spectrum efficiency is the holy grail of wireless network optimization. Certain voice applications, most notably push-to-talk, may operate advantageously using IP on the air interface. But for conventional circuit switched calls over widely varying mobile channel conditions, it’s questionable that VoIP, even with the advantages of OFDM, will deliver the spectrum efficiency of CDMA with dedicated channels.
A few years ago, it was widely predicted that CDMA carriers such as Verizon would abandon their 1X-RTT networks and migrate all voice traffic to VoIP on EV-DO. That certainly hasn’t happened, and there is no obvious reason to believe that migration to OFDM for wireless packet data air interfaces will make mobile VoIP any more compelling.
As long as plain old dial-up voice service constitutes a big part of what mobile networks do – and I don’t see that changing for a while – CDMA technology will continue to play a leading role.