WSN Physical Layer & Transceiver

Cheatsheet Content

Physical Layer & Transceiver Design Considerations in WSNs Energy Usage Profile Transmit Mode: Highest power consumption, due to RF power amplifier. Receive Mode: Significant power, as receiver circuitry (LNA, mixer, ADC) is active. Idle/Listen Mode: Moderate power, transceiver is on, listening for activity. Sleep Mode: Lowest power, most components powered down, but wakeup time is a factor. Startup/Shutdown Transients: Brief power spikes during mode transitions. Duty Cycling: Key strategy to minimize average power by cycling between active and sleep modes. Choice of Modulation Scheme Energy Efficiency: Modulations that require less transmit power for a given SNR are preferred. Spectral Efficiency: How much data can be transmitted per Hz of bandwidth. Less critical for WSNs with low data rates. Robustness to Noise/Fading: Ability to maintain data integrity under poor channel conditions. Complexity: Simpler modulation schemes (e.g., OOK, FSK) require less complex transceivers, leading to lower power consumption and cost. Common Schemes: On-Off Keying (OOK): Simple, low power, but susceptible to noise. Frequency Shift Keying (FSK): More robust than OOK, moderate complexity. Gaussian Minimum Shift Keying (GMSK): Constant envelope, good spectral efficiency, used in Zigbee/Bluetooth. Quadrature Phase Shift Keying (QPSK): Higher data rates, more complex, higher power. Dynamic Modulation Scaling (DMS) Concept: Adaptively change modulation scheme based on channel conditions, data rate requirements, and available energy. Benefit: Optimize energy consumption by using a simpler, more energy-efficient modulation when channel conditions are good or data rate is low. Mechanism: Monitor channel quality (e.g., RSSI, SNR). If channel is good, switch to a more energy-efficient (but lower data rate) modulation. If channel degrades, switch to a more robust (but higher power/complexity) modulation. Antenna Considerations Size & Form Factor: Small, low-profile antennas suitable for compact sensor nodes. Efficiency: High radiation efficiency to maximize transmitted power and received signal strength. Radiation Pattern: Typically omnidirectional for wide coverage, but directional antennas can be used for point-to-point links or energy harvesting. Matching: Proper impedance matching to the transceiver circuit is crucial for power transfer. Types: Monopole/Dipole: Simple, common. Patch Antennas: Low profile, can be integrated into PCBs. Inverted F-Antenna (IFA): Compact, common for ISM bands. MAC Protocols for Wireless Sensor Networks Energy Problems on the MAC Layer Idle Listening: Node listens to an idle channel, consuming energy unnecessarily. Collisions: Multiple nodes transmit simultaneously, leading to retransmissions and wasted energy. Overhearing: Node receives packets not intended for it, consuming energy. Control Packet Overhead: Sending/receiving control messages (RTS/CTS, ACKs) consumes energy. Packet Retransmissions: Due to collisions or poor channel, wasting energy. Low Duty Cycle Protocols and Wakeup Concepts Concept: Nodes spend most of their time in a low-power sleep state and periodically wake up to check for activity. Scheduled Rendezvous: Nodes agree on a common schedule for wake-up times. Sender wakes up, transmits, receiver wakes up at scheduled time to receive. Examples: S-MAC, T-MAC. Asynchronous Wake-up (Staggered Wake-up): Nodes wake up independently, often using a low-power listening (LPL) or preamble sampling approach. Sender transmits a long preamble; receiver wakes up, detects preamble, stays awake to receive data. Examples: B-MAC, X-MAC. Wake-up Radio: A secondary, ultra-low-power radio that is always active. It listens for a specific wake-up signal. When triggered, it activates the main, high-power radio. Minimizes idle listening of the main radio. Sparse Topology and Energy Management Concept: Nodes are deployed with low density, leading to sparse connectivity. Implications: Longer communication paths, potentially requiring multi-hop routing. Increased reliance on routing protocols to find paths through sparse networks. Energy-aware routing: choosing paths that minimize overall energy consumption or balance load. Topology control: dynamically adjusting transmit power or turning off radios to maintain desired connectivity with minimal energy. RTS/CTS Handshake (Request-To-Send/Clear-To-Send) Purpose: To mitigate the hidden terminal problem and reduce collisions in shared wireless medium. Mechanism: Sender (A) sends an RTS packet to Receiver (B) , indicating its intention to transmit and the duration of the transmission. Receiver (B) , if ready, replies with a CTS packet, granting permission and broadcasting the duration of the data transmission to neighboring nodes. All nodes hearing either RTS or CTS set a Network Allocation Vector (NAV) , indicating they should defer transmission for the specified duration. After receiving CTS, Sender (A) transmits data. Benefit: Reduces collisions for long data packets, but introduces overhead for short packets. Energy Consideration: The overhead of RTS/CTS can be significant for WSNs with small data packets, leading to energy waste. Sensor-MAC (S-MAC) Type: Low duty-cycle, scheduled rendezvous MAC protocol. Key Features: Periodic Listen/Sleep Cycles: Nodes synchronize their listen/sleep schedules. Synchronization: Nodes exchange SYNC packets to establish common schedule. Virtual Clusters: Nodes within a virtual cluster share the same schedule. RTS/CTS for Data Exchange: Used to reserve channel for data transmission. Adaptive Listening: Temporarily extending listen time if activity is detected. Energy Saving: Reduces idle listening by putting nodes to sleep. Drawbacks: Fixed duty cycle can be inefficient under varying traffic, latency can be high. Mediation Device Protocol (MDP) Concept: Involves a central "mediation device" or gateway that coordinates communication. Role of Mediation Device: Collects data from sensor nodes. Relays commands to sensor nodes. Can manage schedules, resource allocation, and routing paths. Often has more power/computational capability than individual sensor nodes. Benefits for WSNs: Simplifies sensor node design and reduces their power consumption (offloads complexity). Centralized control can optimize network performance and energy usage. Facilitates integration with external networks. Drawbacks: Single point of failure, potential bottleneck, increased infrastructure cost.