## Example of frequency hopped CDMA

We have already classed GSM European Digital Cellular as a TDMA system, but it also has provision within the standard to change frequency on a frame-by-frame basis, making it a modest rate (1 / 4.165 = 240 hops/second) frequency hopping CDMA system. The motivation for adding the extra complexity of hopping to GSM is twofold. Firstly, the 200 kHz channel bandwidth of GSM is not sufficient to ensure that it will always be significantly wider than the coherence bandwidth of the multipath environment, and thus not corrupted by narrowband fading. Secondly, if there is a strong interference source on any given channel, the hopping process will ensure that frames are only corrupted on an occasional basis. |

A further example of an FH-CDMA system is the GTI Geonet radio, a high capacity public and private two-way radio solution. This product uses 25 kHz spaced channels transmitting at 36.9 kbps with p/4 QPSK modulation. The hop rate is faster than GSM at 152 hops/second, with a TDMA slot width of 2.2 ms and three slots per frame.