Wireless MAC Protocols for WSNs

Wireless Communication Networks

• Wireless MAC protocols are used in wireless communication networks to regulate access to the common medium.
• Multiple nodes share a communication medium for transmitting their data packets.
• The choice of MAC protocol has a direct bearing on the reliability and efficiency of network transmissions.

Characteristics of MAC Protocols

• Energy Efficiency
• Scalability
• Adaptability
• Reliability

Carrier Sense Multiple Access (CSMA)

• Nodes sense the medium before transmission to reduce collisions.
• Two types of CSMA protocols:
  • Non-persistent CSMA: Node transmits data immediately if medium is idle, otherwise waits for a certain amount of time before attempting to transmit again.
  • Persistent CSMA: Node continuously senses the medium and transmits data immediately if medium is idle, otherwise waits for a random period of time before attempting to transmit again.
  • p-persistent CSMA: Node transmits data with a probability p once the medium becomes idle, and delays transmission with a probability 1 − p.

Physical Layer

• Desirable aspects of WSNs: ability to communicate over a wireless link, mobile applications, flexible deployment of nodes.
• Challenges of wireless communication: limited bandwidth, limited transmission range, poor packet delivery performance due to interference, attenuation, and multipath scattering.

Basic Components of a Digital Communication System

• Transmitter: represents one or more sensors that produce a message signal.
• Channel: the medium through which the signal is transmitted.
• Receiver: receives the transmitted signal and decodes it to retrieve the original message.

Source Encoding

• Analog signal is sampled and quantized by an analog-to-digital converter (ADC).
• Quantized elements are mapped into corresponding binary symbols of length (r) from a codebook (C).
• A codebook can be uniquely decoded if each sequence of symbols can be mapped back to a corresponding value in S = (s, s,..., s[n]).

Channel Encoding

• Produces a sequence of data that is robust to noise.
• Provides error detection and forward error correction mechanisms.

Channel Capacity

• The channel capacity can be estimated according to the Shannon-Hartley theorem.
• The capacity of a channel to transmit a message without an error is given by the equation: C = B log2 (1 + S/N).

Modulation

• The baseband signal is transformed into a bandpass signal to transmit and receive signals with short antennas.

Signal Propagation

• Wireless sensor networks operate in the license-free ISM spectrum.
• A simple channel model ignores the effect of interference and considers the surrounding noise as the predominant factor that affects the transmitted signal.
• Noise can be modeled as an additive white Gaussian noise (AWGN) that has a constant spectral density over the entire operating spectrum and a normal amplitude distribution.