Essential Concepts in Communication Systems and Signal Processing

Fundamentals of Signal Transmission

Signal Characteristics

Signal

An electromagnetic field that, through its variations, conveys information from one place to another. Signals are classified as:

• Analog: Continuous.
• Digital: Sequence of discrete values.

Modulation

The process of varying a periodic signal (the carrier, which has constant amplitude and frequency) to convey information.

Types of Modulation:

• Amplitude Modulation (AM): The amplitude of the carrier is modified according to changes in the modulating signal.
• Frequency Modulation (FM): The frequency of the carrier is modified according to changes in the modulating signal.
• Phase Modulation (PM): The phase of the carrier is modified according to changes in the modulating signal.

The transmitted modulated signal uses a frequency band centered around the carrier frequency.

Reasons for Modulation

• Prevents signal mixing.
• Achieves a long communication range.
• Allows for reduced antenna size.

Impairments and Limitations

Attenuation

The decrease in a signal's amplitude (energy) as it propagates along a transmission medium.

Dispersion

Arises when the propagation delay of the transmission medium varies with frequency. This causes changes or distortion in the shape of the transmitted signal.

Noise

Unwanted signals originating from sources other than the transmitter.

Delay and Jitter

Delay: The time required for a signal to travel from one place to another.

• Propagation Delay: The time taken for the signal to propagate.
• Transmission Delay: The time taken to push all bits into the transmission medium.

Jitter: Variation in delay that can result in severe distortion of perceived quality in real-time services (e.g., VoIP).

Capacity and Efficiency

Bandwidth (BW)

The width of the range of frequencies that a user, signal, or channel occupies on a transmission medium. It is calculated as the difference between the highest and lowest frequencies.

Channel Capacity

The limitation on the highest data rate that a channel is able to support.

• Noiseless Channel: Governed by the Nyquist Theorem.
• Noisy Channel: Governed by the Shannon-Hartley Theorem.

Spectral Efficiency

The net bit rate divided by the bandwidth in Hertz used by the communication system (measured in bps/Hz).

Modulation Efficiency

The gross bit rate divided by the bandwidth. Sometimes there is no difference between Spectral and Modulation Efficiency, but if an error correction mechanism is introduced, a reliable link is achieved, though with a penalty on bandwidth.

System Management and Access

Multiplexing and Multiple Access

When a shared medium exists, a mechanism is needed for different users to utilize it.

If all signals are transmitted together at the same point, they can be Multiplexed using different:

• Frequency bands
• Time slots
• Orthogonal codes

The signals are then demultiplexed at the receiver.

If signals are transmitted directly to the same medium from different points, a Multiple Access mechanism is required. This mechanism defines which time slot, frequency range, or orthogonal code each transmitter should use.

Link Budget Analysis

The Link Budget is a fundamental parameter in every communication system. The receiver must obtain sufficient signal power to extract information. This required level is known as the receiver sensitivity.

By accounting for attenuation and gain, we calculate the level of power available at the receiver, which must be greater than the receiver sensitivity. Calculations are typically performed using logarithms (decibels).

Capacity Maintenance in Noise

To maintain maximum capacity when noise increases, two possibilities are to increase the bandwidth and increase the signal power.