Frequency Hopping

Frequency hopping is the repeated switching of frequencies during radio transmission so as to minimize the effectiveness of unauthorized interception or jamming of telecommunications, also known as, the frequency-hopping code division multiple accesses (FH-CDMA) (Scholtz, 1982). A transmitter in a spread spectrum facilitates a signal to be transmitted across a frequency band, which is wider than the minimum bandwidth needed by the information signal. Its advantages include improved privacy, increased signal capacity and decreased narrowband interference. The FH-CDMA system works in such a way that, a transmitter moves between available frequencies according to a random or preplanned algorithm, which then ensures that the transmitter and the receiver are in tune with the same centre frequency (Scholtz, 1982).

As The transmitter transmits a short burst of data on a narrow band, it tunes into another frequency and transmits again, while the receiver being in tune with the transmitter also moves its frequency over a given bandwidth several times a second, thereby transmitting on one frequency for a certain period of time before hoping to another frequency and transmitting again as the cycle continues (Scholtz, 1982). The advantage of this system is that, it is cheaper, highly resistant to narrowband interference, uses less energy, and difficult to intercept as there is coexistence of several access points in the same area. An alternative to FH-CDMA is the direct sequence code division multiple accesses (DS-CDMA), which instead of hopping from frequency to frequency, as the FH-CDMA does, it chops the data into small pieces and spreads them across the frequency domain (Price, 1983).

There are basically two types of frequency hopping namely; the base Band Frequency Hoping (BBH) and Synthesizer Frequency Hopping (SFH) with their main difference being the way they establish the Base to Mobile Station Link (downlink) (Price, 1983). The Base Band Frequency Hopping is prepared  by routing the traffic channel data through fixed frequency DRCUs, by the use of the TDM highway, on a timeslot basis through a fixed tuned  transmitters combined either in low loss tuned combines or hybrid  combiners. The result is that, information for every call is moved among the available DRCUs on a per burst basis (Burst of 577$), while call hops are between same timeslots of all DRCUS, for example, if a network running on a fixed frequency plan is switched over to BBH (BCCH included in MA list) without any frequency changes there will be an improvement in the network quality (Price, 1983). The second type of frequency hopping is the Synthesizer Frequency hopping, which makes use of high speed switching of the transmitter and the receiver frequency synthesizers of the individual DRCUs. Here, DRCUs changes transmitting frequency at every burst while calls stay on the same DRCU where it started, that is, transmission and reception are done on the same timeslot and DRCU (Price, 1983).

A polish engineer, Leonard Danilewicz, came up with the idea in 1929, however, the most well known inventor was Hedy Lamarr, a movie star, and George Antheil, a pianist, who in 1942 were issued with a patent for the technique during world war 2 after they discovered how to control the frequency hops of the piano and envisioned it as a way of providing secure communications during the war (Kozaczuk, 1984).. This early version used a piano-roll to change between 88 frequencies and was projected to make radio-guided torpedoes harder for enemies to detect or jam (Popovski, Yomo & Prasad, 2006).  Although it was Hedy who first discovered the technique, Sylvania, in 1950s, introduced a related concept and called it spread spectrum, which can be used by the military or civilians.

The military’s communications radios utilize cryptographic procedures to produce the channel sequence that is under a secret Transmission Security Key TRANSEC that the sender and the receiver have in common (Popovski, Yomo & Prasad, 2006). Improved military frequency hopping radios, including the US military, use separate encryption devices like KY-57 which provides superior protection against eavesdropping, jamming or the discovery of the sequence of frequencies using algorithms (Popovski, Yomo & Prasad, 2006). After the amendment of rules that permit frequency hopping stretched spectrum systems in the 2.4 GHZ band (unregulated) by the Federal Communications Commission (FCC), the frequency hopping technique has been used for civilian items like walkie-talkies being marketed as eXtreme Radio Services (eXRS).

However, the main challenge facing systems that frequency hop lies on how to harmonize the transmitter and the receiver such that, there is a warranty that the transmitter will utilize all the channels after a specified time. The receiver will recognize the correct transmitter through the selection of a random channel; listening for legitimate data on the specific channel by identifying a particular sequence of that data that will not occur over the section of data for the channel (Popovski, Yomo & Prasad, 2006). A fixed table of channel sequence can be used so that, when they have been synchronized, the receiver and the transmitter are able to maintain communication by following the table with a checksum segment for integrity and further identification.

There are many variations of the FHSS, including the AFH, which is used to improve conflict with interference with radio frequency by staying away from the use of crowded frequencies in the hopping channel. The  AFH will only use the good quality frequencies and avoid the bad ones because, it may be going through frequency selective fading, it is being jammed or an authorized entity  is trying to use the bands thus, it should be accompanied by a means for identifying good and bad channels (Popovski, Yomo & Prasad, 2006). Another form of variation is associated with cognitive radio, this is where the networks as well as devices display frequency-agile operations, and is a variety of frequency hopping, which goes through the existing frequencies consecutively.

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