Frequency Shit Keying Modulation
Frequency Shit Keying (FSK) modulation is a popular form of digital modulation used in low-cost application for transmitting data at moderator or low rate over wired as well as wireless channels. In general, an M-ary FSK modulation are becoming efficiency modulation scheme and several forms of M-ary FSK modulation are becoming popular for spread spectrum communications and other wireless applications. In this lesson, our discussion will be limited to binary frequency shift keying(BFSK).
Two carrier frequencies are used for binary frequency shift keying modulation. One frequency is called the ‘mark’ frequency (f2) and the other as the frequency (f1). By convention, the ‘mark’ frequency indicates the higher of the two carriers used. If Tb indicates the duration of one information but, the two time-limited signals can be expressed as:
The binary scheme uses two carriers and for special relationship between the two frequencies one can also define two orthonormal basis functions as shown below.
A binary FSK Transmitter is as shown, the incoming binary data sequence is applied to on-off level encoder. The output of encoder is √ volts for symbol 1 and 0 volts for symbol “0‟. When we have symbol 1 the upper channel is switched on with oscillator frequency f1, for symbol “0‟, because of inverter the lower channel is switched on with oscillator frequency f2. These two frequencies are combined using an adder circuit and then transmitted. The transmitted signal is nothing but required BFSK signal. The detector consists of two correlators. The incoming noisy BFSK signal x(t) is common to both correlator. The Coherent reference signal ᶲ1(t)&ᶲ2(t) are supplied to upper and lower correlators respectively.
The correlator outputs are then subtracted one from the other and resulting a random vector “l” (l=x1 - x2). The output “l” is compared with threshold of zero volts.
l > 0, the receiver decides in favour of symbol 1.
l< 0, the receiver decides in favour of symbol 0.
• Limiting factor: Physical capabilities of the carrier
• Not susceptible to noise as much as ASK
– On voice-grade lines, used up to 1200bps
– Used for high-frequency (3 to 30 MHz) radio transmission
– used at higher frequencies on LANs that use coaxial cable.
Therefore Binary FSK system has 2 dimensional signal space with two messages S1(t) and S2(t), [N=2 , m=2] they are represented,