There are (at least) three considerations that come into play here:
1) Intended Application
W-CDMA is a Wide Area Network (WAN) technology, designed to deliver wireless service to mobile users anywhere within a large geographic coverage area. WiMAX is a Metropolitan Area Network (MAN) technology, originally designed to deliver “last mile” wireless service to fixed devices — for example, providing an office network with Internet access over a WiMAX uplink to an ISP’s Point of Presence. 802.16e mobile extensions to WiMAX are still being specified. So: are your VOIP users going to be fixed (use WiMAX) or mobile (use W-CDMA)?
2) Required Coverage Area
Commercial W-CDMA services are just beginning to be offered in major metropolitan areas; you’ll need to find a carrier with a W-CDMA network wherever your VOIP users will roam. WiMAX, also in the early stages of deployment, involves installing a “wireless uplink” between your WiMAX base station and a WISP’s POP within miles of your office. So: can you find a cellular operator (W-CDMA) or WISP (WiMAX) to meet your coverage needs? If not, WiMAX can also be used in unlicensed spectrum to support privately-deployed networks — for example, to create your own interoffice wireless link.
3) Quality of Service
Bandwidth and other characteristics of commercial W-CDMA and WiMAX data services vary widely, so you really need to evaluate specific carrier offerings and their ability to meet your VOIP needs. For example, UMTS is theoretically capable of delivering data at speeds up to 2 Mbps; WiMAX can deliver from 15 to 134 Mbps. But VOIP sessions don’t require high bandwidth; they require consistent delivery of small packets. For example, running a couple of VOIP sessions over a Wi-Fi network works fine, but as contention increases, VOIP can be degraded to the point that it becomes unusable — unless Quality of Service (QoS) is used to prioritize VOIP traffic. Carriers will deliver commercial voice services over W-CDMA, but do they have any incentive to make it easy for you to run VOIP over W-CDMA data services? For discussion, see this URL: www.tekrati.com/T2/Analyst_Research/ResearchAnnouncementsDetails.asp?Newsid=4751