4.3. RECEIVER DESIGN 151
interference(ISI). As discussed in Section 4.4, the receiver noise is proportional to the
receiver bandwidth and can be reduced by using a low-pass filter whose bandwidth
∆fis smaller than the bit rate. Since other components of the receiver are designed
to have a bandwidth larger than the filter bandwidth, the receiver bandwidth is deter-
mined by the low-pass filter used in the linear channel. For∆f<B, the electrical pulse
spreads beyond the allocated bit slot. Such a spreading can interfere with the detection
of neighboring bits, a phenomenon referred to as ISI.
It is possible to design a low-pass filter in such a way that ISI is minimized [1].
Since the combination of preamplifier, main amplifier, and the filter acts as a linear
system (hence the namelinear channel), the output voltage can be written as
Vout(t)=∫∞−∞zT(t−t′)Ip(t′)dt′, (4.3.1)whereIp(t)is the photocurrent generated in response to the incident optical power
(Ip=RPin). In the frequency domain,
V ̃out(ω)=ZT(ω)I ̃p(ω), (4.3.2)whereZTis the total impedance at the frequencyωand a tilde represents the Fourier
transform. Here,ZT(ω)is determined by the transfer functions associated with various
receiver components and can be written as [3]
ZT(ω)=Gp(ω)GA(ω)HF(ω)/Yin(ω), (4.3.3)whereYin(ω)is the input admittance andGp(ω),GA(ω), andHF(ω)are transfer func-
tions of the preamplifier, the main amplifier, and the filter. It is useful to isolate the
frequency dependence ofV ̃out(ω)andI ̃p(ω)through normalized spectral functions
Hout(ω)andHp(ω), which are related to the Fourier transform of the output and input
pulse shapes, respectively, and write Eq. (4.3.2) as
Hout(ω)=HT(ω)Hp(ω), (4.3.4)whereHT(ω)is the total transfer function of the linear channel and is related to the total
impedance asHT(ω)=ZT(ω)/ZT( 0 ). If the amplifiers have a much larger bandwidth
than the low-pass filter,HT(ω)can be approximated byHF(ω).
The ISI is minimized whenHout(ω)corresponds to the transfer function of araised-
cosine filterand is given by [3]
Hout(f)={ 1
2 [^1 +cos(πf/B)], f<B,
0 , f≥B,(4.3.5)
wheref=ω/ 2 πandBis the bit rate. The impulse response, obtained by taking the
Fourier transform ofHout(f), is given by
hout(t)=sin( 2 πBt)
2 πBt1
1 −( 2 Bt)^2. (4.3.6)
The functional form ofhout(t)corresponds to the shape of the voltage pulseVout(t)
received by the decision circuit. At the decision instantt=0,hout(t)=1, and the