3 FHSSS with hopping controlled by data and
3 -FH-SSS with hopping controlled by data and PN code: The binary data and the PN code are first applied to a control logic that controls the frequency hopping and FSK data modulation. The frequency synthesizer will perform two jobs, hopping and modulation. Operation of the control logic depends on whether the system is fast or slow FH. LSB data MSB
mixer Received FH signal data mixer
As shown in the figure, the 2 nd LSB is used for FSK modulation. Hence, the separation between logic'1' and logic'0' is two times the channel spacing of the synthesizer (2∆f). Hence, the channel occupied by the transmitted bit is called 'transmission channel'. The channel could occupy if the data is reversed and this is called 'complementary channel'. Operation of such a system depends if the system is fast or slow FH: For Fast-FH with say time hopping Th=Tb=bit duration, then one bit is transmitted each Th time. If for example, the data are 101101…, then the output frequencies will be f 4 f 5 f 3 f 2 f 5…… Note that above figure is plotted assuming that the data controls the 2 nd LSB, i. e the difference between logic “ 1’ and “ 0’ frequencies is 2∆f. f 6 ∆f f 5 f 4 f 3 f 2 f 1 0 Th 2 Th 3 Th 4 Th 5 Th time
B-For slow-FH with say Th = 3 Tb , then 3 bits are transmitted at each frequency. If for example the data bits are 10001110110111…. , then the control logic will generate a single bit controlling the frequency synthesizer for a group of 3 bits of the data. If the 3 -bit data word is represented as decimal(0 -7), the control bit is set to '0' if the decimal ≤ 3. The control bit is set to '1' if the decimal ≥ 4. For the given data then: 1 0 0 0 1 1 1 0 0 0 1 =4 3 5 6 6 1 (decimal) Hence, the control bits will be=1 0 1 1 1 0. Note that the time duration of each control bits is 3 Tb=Th. Figure below shows the case of slow-FH: Where the logics Tb marked inside each f 6 1 box are the control f 5 1 1 ∆f bits. Hence for the 1 f 4 1 given data and control 0 0 1 f 3 bits, the output f 2 frequencies are: f 4 f 5 f 1 f 3 f 4 f 3…. . 0 Th 2 Th 3 Th 4 Th 5 Th time
4 -FH-MFSK-SSS: This is widely used for addressed mobile radio systems because this system can serve many users. Each user has his own address. Here, there are two parts of modulation process at the Tx, addressing and encoding. Addressing is performed by a PN code that generates a sequence of 'L' digits (i. e its period is L) each 'k' bit long: For the m th user, the PN generator will generate the sequence: Cm 1, Cm 2, Cm 3, ……Cm. L. , where each Cmi Analog consists of k bits. information Ym Binary data 1 k k Xm 1 k Cm
The analog information is transformed into binary data using any modulation (PCM, DM, …). Then this binary data is serially loaded into a shift register to generate k-bit words of the information Xm for the mth user. A PN generator generating k-bit words Cm of the code for the mth user. Then: Ym =[ Xm + Cm ] mod 2 k This Ym will control a frequency synthesizer to generate one of the 2 k tones according to Ym. The transmitted tones will carry both the data information Xm and the address Cm. At the Rx, a spectrum analyzer estimates the frequency of incoming signal to give the signal Ym that is directly proportional to the input frequency. From which: Xm = [Ym – Cm ] mod 2 k It is assumed that the local PN generator at the Rx is synchronized with that at the Tx.
Ym Xm Cm
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