Math  /  Algebra

QuestionA binary PAM wave is to be transmitted over a low-pass channel with bandwider of 75 kHz . The bit duration is 10μ s10 \mu \mathrm{~s}. Find a raised-cosine pulse spectrum that satisfies these requirements.

Studdy Solution

STEP 1

1. The channel is a low-pass channel with a bandwidth of 75 kHz.
2. The bit duration is 10μs10 \, \mu \mathrm{s}.
3. We need to find a raised-cosine pulse spectrum that fits these parameters.

STEP 2

1. Understand the relationship between bit duration and bandwidth.
2. Determine the symbol rate.
3. Calculate the Nyquist bandwidth.
4. Determine the roll-off factor.
5. Define the raised-cosine pulse spectrum.

STEP 3

The bit duration Tb T_b is given as 10μs10 \, \mu \mathrm{s}. The bit rate Rb R_b is the reciprocal of the bit duration:
Rb=1Tb=110×106=100,000bits per second (bps) R_b = \frac{1}{T_b} = \frac{1}{10 \times 10^{-6}} = 100,000 \, \text{bits per second (bps)}

STEP 4

The symbol rate Rs R_s for binary PAM is the same as the bit rate Rb R_b because each symbol represents one bit. Therefore:
Rs=100,000symbols per second (sps) R_s = 100,000 \, \text{symbols per second (sps)}

STEP 5

The Nyquist bandwidth BN B_N is half the symbol rate for a raised-cosine filter:
BN=Rs2=100,0002=50kHz B_N = \frac{R_s}{2} = \frac{100,000}{2} = 50 \, \text{kHz}

STEP 6

The channel bandwidth is given as 75 kHz. The excess bandwidth (roll-off factor β \beta ) can be calculated as:
β=Channel BandwidthBNBN=75,00050,00050,000=0.5 \beta = \frac{\text{Channel Bandwidth} - B_N}{B_N} = \frac{75,000 - 50,000}{50,000} = 0.5

STEP 7

The raised-cosine pulse spectrum H(f) H(f) is defined by the roll-off factor β \beta and the Nyquist bandwidth BN B_N . The spectrum is:
H(f)={1,fBN12[1+cos(π(fBN)βBN)],BN<f(1+β)BN0,f>(1+β)BN H(f) = \begin{cases} 1, & |f| \leq B_N \\ \frac{1}{2} \left[ 1 + \cos\left(\frac{\pi (|f| - B_N)}{\beta B_N}\right) \right], & B_N < |f| \leq (1 + \beta)B_N \\ 0, & |f| > (1 + \beta)B_N \end{cases}
For this problem, BN=50kHz B_N = 50 \, \text{kHz} and β=0.5 \beta = 0.5 .
The raised-cosine pulse spectrum that satisfies these requirements is defined by the above equations with the given parameters.

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