IEEE 802.15.4 PHY

The PHY provides two services: the PHY data service and PHY management service interfacing to the physical layer management entity (PLME). The PHY data service enables the transmission and reception of PHY protocol data units (PPDU) across the physical radio channel.

The features of the PHY are activation and deactivation of the radio transceiver, energy detection (ED), link quality indication (LQI), channel selection, clear channel assessment (CCA) and transmitting as well as receiving packets across the physical medium.

The standard offers two PHY options based on the frequency band. Both are based on direct sequence spread spectrum (DSSS). The data rate is 250kbps at 2.4GHz, 40kbps at 915MHz and 20kbps at 868MHz. The higher data rate at 2.4GHz is attributed to a higher-order modulation scheme. Lower frequency provide longer range due to lower propagation losses. Low rate can be translated into better sensitivity and larger coverage area. Higher rate means higher throughput, lower latency or lower duty cycle.

There is a single channel between 868 and 868.6MHz, 10 channels between 902.0 and 928.0MHz, and 16 channels between 2.4 and 2.4835GHz as shown in Figure 3.2. Several channels in different frequency bands enables the ability to relocate within spectrum. The standard also allows dynamic channel selection, a scan function that steps through a list of supported channels in search of beacon, receiver energy detection, link quality indication, channel switching

Receiver sensitivities are -85dBm for 2.4GHz and -92dBm for 868/915MHz. The advantage of 6-8dB comes from the advantage of lower rate. THE ACHIEVABLE RANGE IS A FUNCTION OF RECEIVER SENSITIVITY AND TRANSMIT POWER.

The maximum transmit power shall conform with local regulations. A compliant device shall have its nominal transmit power level indicated by the PHY parameter, phyTransmitPower.

Figure: Operating frequency bands.

1. Receiver Energy Detection (ED)
   The receiver energy detection (ED) measurement is intended for use by a network layer as part of channel selection algorithm. It is an estimate of the received signal power within the bandwidth of an IEEE 802.15.4 channel. No attempt is made to identify or decode signals on the channel. The ED time should be equal to 8 symbol periods.
   The ED result shall be reported as an 8-bit integer ranging from 0x00 to 0xff. The minimum ED value (0) shall indicate received power less than 10dB above the specified receiver sensitivity. The range of received power spanned by the ED values shall be at least 40dB. Within this range, the mapping from the received power in decibels to ED values shall be linear with an accuracy of + − 6dB.
2. Link Quality Indication (LQI)
   Upon reception of a packet, the PHY sends the PSDU length, PSDU itself and link quality (LQ) in the PD-DATA.indication primitive. The LQI measurement is a characterization of the strength and/or quality of a received packet. The measurement may be implemented using receiver ED, a signal-to-noise estimation or a combination of these methods. The use of LQI result is up to the network or application layers.
   The LQI result should be reported as an integer ranging from 0x00 to 0xff. The minimum and maximum LQI values should be associated with the lowest and highest quality IEEE 802.15.4 signals detectable by the receiver and LQ values should be uniformly distributed between these two limits.
3. Clear Channel Assessment(CCA)
   The clear channel assessment (CCA) is performed according to at least one of the following three methods:
   +Energy above threshold. CCA shall report a busy medium upon detecting any energy above the ED threshold
   + Carrier sense only. CCA shall report a busy medium only upon the detection of a signal with the modulation and spreading characteristics of IEEE 802.15.4. This signal may be above or below the ED threshold.
   +Carrier sense with energy above threshold. CCA shall report a busy medium only upon the detection of a signal with the modulation and spreading characteristics of IEEE 802.15.4 with energy above the ED threshold.
4. PPDU Format
   The PPDU packet structure is illustrated in Figure . Each PPDU packet consists of the following basic components:
   +SHR, which allows a receiving device to synchronize and lock into the bit stream
   +PHR, which contains frame length information
   +a variable length payload, which carries the MAC sublayer frame.