Inter Symbol Interference in Baseband Binary Data Transmission

In this article, we are going to discuss the baseband transmission of binary data. We know that in baseband transmission, there is no requirement for a carrier signal for transmission, and the channel is referred to as a low-pass channel. In baseband transmission of binary data, discrete pulse amplitude modulation is the most suitable technique. This technique is highly efficient in terms of power and bandwidth usage. In this method, the amplitude of the transmitted signal is varied in discrete form in accordance with the given digital data.

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Block Diagram of Baseband Transmission System

The typical block diagram of a binary baseband transmission system includes:

1. Information Source: Generates the binary data.
2. Line Encoder: Encodes the data into a suitable format.
3. Transmit Filter: Has a transfer function $G(f)$.
4. Channel: Represented by $H(f)$.
5. Receive Filter: Has a transfer function $Q(f)$.
6. Sampler and Decision Making Device: Samples the output at $T = iT_B$ and decides the binary output.

The purpose of these blocks is to transmit the binary data through a channel and recover it at the receiver end.

Process of Baseband Transmission

Initially, the source generates an input binary data sequence $V_k$ at the sampling instant $T = kT_B$, where $T_B$ is the bit duration and $k$ is an integer. The element $V_k$ represents a binary symbol (1 or 0). This binary data $V_k$ is applied to the line encoder, which operates with a clock pulse and produces a level encoded signal based on line encoding (e.g., non-return-to-zero).

The encoded output $E_k$ has positive and negative amplitude levels: +1 for binary 1 and -1 for binary 0. This signal, represented by $x_k$, acts as a modulating signal and is applied to the transmit filter $G(f)$. The filter produces a discrete pulse amplitude modulated signal, represented by $s(t)$.

Signal Transmission and Reception

The signal $s(t)$ is transmitted using a linear channel with a transfer function $H(f)$. At the output of the channel, we get $x(t)$, which is the convolution of $s(t)$ and $h(t)$. At the receiver side, the received signal passes through the receive filter $Q(f)$, resulting in the output $y(t)$.

The overall system can be characterized by its frequency response $\mu P(f)$ or in time domain $\mu P(t)$. Here, $\mu$ is a scaling factor, and $P(t)$ is the overall impulse response. The sampled output $Y(iT_B)$ is used to reconstruct the original modulating signal $a_k$ using a decision-making device with a threshold value λ.

Inter Symbol Interference (ISI) and Noise

A major problem in baseband transmission is inter symbol interference (ISI), caused by the dispersive nature of the low-pass channel. The frequency response of the low-pass channel deviates from the ideal low-pass filter, introducing ISI. Additionally, additive channel noise further affects the signal.

In the simplified baseband transmission system, the input signal $x(t)$ consists of the line encoded signal $a_k$ and Gaussian white additive noise $w(t)$. The output $y(t)$ is the modified pulse amplitude modulated signal, which is sampled at $T = iT_B$ to get $Y(iT_B)$.

Under ideal conditions, without ISI and noise, the sampled output $Y_i$ equals $\mu a_i$, indicating correct decoding of the transmitted bit. The decision-making device then determines the final output based on the threshold $\lambda$, outputting 1 if $Y_i > \lambda$ and 0 if $Y_i \leq \lambda$.

Conclusion

In summary, the baseband transmission of binary data involves several key components working together to ensure accurate data transmission and reception. By understanding and addressing issues such as ISI and noise, the performance and reliability of communication systems can be significantly improved.