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Fast-frequency hopping

Fast-frequency hopping. In a fast FH/MFSK system, each hop is a chip. Thus,

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Fast-frequency hopping

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  1. Fast-frequency hopping In a fast FH/MFSK system, each hop is a chip. Thus, In general, fast-frequency hopping is used to defeat a smart jammer’s tactic that involves two functions: measurements of the spectral content of the transmitted signal, and retuning of the interfering signal to that portion of the frequency band. Clearly, to overcome the jammer, the transmitted signal must be hopped to a new carrier frequency before the jammer is able to complete the processing of these two functions. 2.4.4

  2. Fast-frequency hopping 2.4.4

  3. Fast-frequency hopping For data recovery at the receiver, noncoherent detection is used and two procedures may be consider: 1. For each FH/MFSK symbol, separate decisions are made on the frequency-hop chips received, and a simple rule based on majority vote is used to make an estimate of the dehopped MFSK symbol, where . 2. For each FH/MFSK symbol, likelihood functions are computed as functions of the total signal received over chips, and the largest one is selected. A receiver based on the second procedure is optimum in the sense that it minimizes the average probability of symbol error for a given . 2.4.4

  4. Fast-frequency hopping By similar derivation, we obtain Comparing to the traditional MFSK signal, the processing gain of fast-frequency hopping is Note that, with a contrast to slow-FH, the processing gain is times . The reason is that the chip rate of fast-FH is times to the chip rate of slow-FH. 2.4.4

  5. Example 2.4-6 Fast-frequency hopping (1/2) • Consider a fast FH/MFSK signal with the following parameters: Period of the PN sequence : 15 Number of MFSK tone : M=4 Length of PN segment per hop : k=2 Total number of frequency hops : Data transmission rate : Frequency hopping rate : (a) Find the chip rate, (b) find the required total FH bandwidth, (c) find the processing gain, and (d) determine the jamming margin for . 2.4.2

  6. Example 2.4-6 Fast-frequency hopping (2/2) 【Sol.】 In this example, so a symbol is transmitting twice by different carriers. (a) (b) To hold the orthogonal property, we select the frequency deviation as , so we have the total bandwidth as (c) (d) 2.4.2

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