CDMA vs. GSM: Not a Factor in Dropped Calls, Finneran Says
April 2, 2007 (Vol. 28, No. 7)
Wireless expert Michael Finneran explains CDMA technology by comparing it to a cocktail reception at the United Nations. You’re speaking English, and you can’t understand any of the other conversations around you.
In CDMA, each phone’s transmission is multiplied or “spread” with a 64-bit “Walsh code.” As a result, the phone’s 19.2 Kbps bit rate is bumped up to 1.2288 Mcps (million chips per second) and sent on a 1.25 MHz carrier channel. CDMA allows up to 128 simultaneous calls on that 1.25 megahertz channel, Finneran explains. The CDMA base stations can then decipher the individual calls because they are each spread with a different Walsh code.
By comparison, GSM is like having a group of 16 people carrying on eight conversations, where each speaks one word at a time, Finneran says. That’s because in a GSM network, handsets take turns sending on a 200 kilohertz channel. Every call uses the full capacity of the channel one-eighth of the time. While it might sound like that would slow down information transfer, Finneran says the channel rate is 270 Kbps, and one pass is made around all eight users in a speedy 4.6 milliseconds.
Finneran says CDMA is a superior technology because it provides for more efficient use of the radio spectrum and is less susceptible to frequency selective fading. The handsets also incorporate a RAKE receiver that improves signal reception, he notes. But neither CDMA nor GSM networks should have a significant edge on dropped calls, Finneran concludes. Rather, dropped calls result from network design failures like not having enough calling capacity, good radio frequency coverage and some degree of cell overlap, he says. (