Other Spread Spectrum Systems

Chirp
The chirp spread spectrum method employs a carrier that is swept over a range of frequencies. This method is primarily used in ranging and radar systems and very rarely used in commercial spread spectrum systems.


Time Hopping
In a time-hopped signal, the carrier's on-off state is keyed by the PN sequence resulting in a very low duty cycle. The speed of keying determines the amount of signal spreading. Time hopped spread spectrum systems are currently not widely used in commercial industry. However, the arrival of cheap random access memory (RAM) and fast micro-controller chips make time hopping a viable alternative spread spectrum technique for the future.


Today, DSSS wireless LANs have been sold more than any other wireless LAN technology and many companies are developing DSSS and FHSS products on PCMCIA cards. The IEEE 802.11 has standards for DSSS and FHSS systems operating in Industrial, Scientific, and Medical (ISM) bands. Wireless LAN users need the entire capacity of the channel for quick transmission of data bursts generated by each terminal and CDMA is not suitable for that situation. As a result CDMA is not considered by IEEE 802.11 and it is not used in any of the major products in the market. However, some wireless LAN manufacturers assign several codes to an individual user to increase the data rate while still operating under ISM band restrictions. They refer to this technology as CDMA based wireless LAN technology. This is not the CDMA technology as used in the cellular telephone industry.