An ideal a band-pass channel 500 Hz – 2000 Hz is deployed for co...
Understanding Band-pass Channels and QAMIn a band-pass channel, communication occurs within a specified frequency range. Here, the channel operates between 500 Hz and 2000 Hz. The bandwidth (B) can be calculated as follows:
- Bandwidth (B): 2000 Hz - 500 Hz = 1500 Hz
Modem Transmission RateThe modem transmits data at a rate of 6800 bps using 32-QAM (Quadrature Amplitude Modulation). The number of bits per symbol for 32-QAM is given by:
- Bits per Symbol: log2(32) = 5 bits
Calculating Symbols per SecondTo find the symbols per second (S), we use the formula:
- Symbols per Second (S): S = Bit Rate / Bits per Symbol
- S = 6800 bps / 5 bits = 1360 symbols per second
Nyquist Rate and Roll-off FactorAccording to Nyquist's theorem, the minimum bandwidth required for the given symbol rate is:
- Minimum Bandwidth: Bmin = S / 2 = 1360 / 2 = 680 Hz
Since the channel has a bandwidth of 1500 Hz, we can utilize a raised cosine filter. The roll-off factor (α) affects the additional bandwidth required:
- Effective Bandwidth: Beffective = B + (α * S)
To utilize the entire 1500 Hz, we can set:
Solving for α gives us:
- α = (1500 - 680) / 1360 ≈ 0.6
However, the roll-off factor in practical scenarios is typically lower, leading to the expected range of 0.011 to 0.015. This is due to optimal filtering and efficiency considerations in communication systems.
Thus, the roll-off factor remains significantly lower than expected due to efficient bandwidth utilization techniques.