Editor's Note: The author works for Sprint. This article includes descriptions of Sprint's networks and technologies.
What is CDMA?
CDMA, which is used in Sprint's nationwide wireless network, stands for Code Division Multiple Access. CDMA is different from other multiple access technologies such as Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA) because users are uniquely identified by codes rather than being identified by a time slot or frequency allocation. CDMA supports both circuit-switched voice and data as well as packet-switched data.
A second technology falls under the CDMA umbrella. This technology is called High Rate Packet Data or HRPD. Most often the solution is referred to as 1xEVDO or 1x EvolutionData Optimized. 1xEVDO supports very high speed packet data in the wireless network on the downlink (Network to handset). This high data rate is achieved by using a Time Division Multiplex forward link. However, 1xEVDO is still very much a CDMA system, but instead of using separate codes to serve each user, multiple codes are multiplexed together in time. In doing so, each user that is receiving data gets the whole CDMA forward link dedicated to them for a brief period in time. Sprint has started launching 1xEVDO service in a number of markets working towards coverage of the top 60 markets by the end of 2005/early 2006.
Because all users are sharing the same spectrum, and each user is identified by a unique code, CDMA becomes an interference-limited technology. This means that as the number of users increase, interference begins to dominate and the user code can no longer be identified, which would cause the call to drop. Because CDMA is interference-limited, the technology is managed by a very strict power-control algorithm that keeps the amount of power transmitted from both the base station and the handset at the lowest possible level. In order to keep the system transmitting at the lowest possible level, but still maintain a good RF link, the CDMA system changes the power on both the uplink and the downlink 800 times per second. This ensures that as the dynamic RF environment changes (e.g. the handset suddenly gets blocked by a large building which shadows the RF) an active call will not be dropped.
1xEVDO is very similar to IS-2000 Release 0 CDMA in the reverse link (handset to the base station). However, in the downlink, as previously mentioned, the 1xEVDO technology allocates the entire CDMA bandwidth to a single user for a period of time.
If a customer with a new handset supporting a new CDMA based technology roams to an area in which the network does not support the newer air interface technology, the handset will default to the older technology. For example, a 3G handset can operate on both a 3G network and a 2G network. The only difference is that when the handset is in the 2G Network, it will only have basic voice and data support, not the advanced features enabled by 3G services. When a user initiates a data session in 1xEVDO coverage, but then migrates out of 1xEVDO coverage, the data session is not lost, instead, the data session transitions to the IS-2000 air interface. This transition is easily supported because the supporting network for both the 1xEVDO air interface and the 1xRTT air interface is the same.
How do CDMA and GSM differ? Can we ever expect a single cellular communication standard?
With CDMA, users are separated by codes, whereas with GSM, users are separated in time and frequency. Both GSM and CDMA are multiple access technologies, they just achieve multiple access using different domains (code versus time and frequency).
The evolution of the GSM/GPRS networks is towards CDMA via the wCDMA or UMTS standard. However, this is not the same CDMA standard used currently. Harmonization attempts between the two standards bodies has only achieved minimal agreement: it remains a goal.
One standard that has an opportunity to achieve global adoption would be IEEE 802.16e (WiMAX mobile). This standard, still in development, could provide a migration that allows a common standard to coalesce.
What data and wireless CDMA networks are available from Sprint?
