PPDU: PHY frame format for S2-PSK & HS-PSK

This post reviews the design of the Physical-layer Packet Data Unit (PPDU) for S2-PSK and HS-PSK operating modes within PHY IV of IEEE 802.15.7m. The general structure of PPDU for all OCC Operating modes is illustrated in Figure 1.

Figure 1 – Structure of PPDU

• SHR: Synchronization Header
• PHR: Physical-Layer Header
• HCS: Header Check Sequence
• PSDU: PHY service data unit

“Use of over-the-air PHY frame configuration is forbidden for PHY types IV, V and VI. It is mandatory that PHY frame configuration be done via the PHY PIB. This is due to the fact that unlike traditional wireless LAN/PAN, the data rates associated with OCC are such that the configuration overhead cannot be tolerated. This means that there is no “base default” transmission mode. In addition, it is anticipated that configuration will be with application layer “APPS” that are specifically loaded to support a particular OCC PHY mode. The PHY PIB is not transmitted; rather, it is written by the Device Management Entity and is read by the PHY layer.” — IEEE 802.15.7m

So, Unlike VLC, it is not required to have a base operating-mode for all of OCC systems. This means that the SHR field is not the same for different OCC modes. Without over-the-air PHY frame configuration, the operation manner for configuring PPDU of OCC is done by an APP-based solution.

The details on the format of PHY IV operating modes are reviewed as follows.

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S2-PSK PPDU

 

Figure 2 – S2-PSK waveform and its bit mapping

 

S2-PSK preamble field

The preamble field for S2-PSK has four binary states that are equivalent to two data bit times long (see Table X-TBD). Each binary among the binary sequence “1 1 1 1” shall be mapped into waveforms (as seen in Figure 2).

Table 1- S2-PSK preamble field

Preamble duration	Two-bit interval
S2-PSK preamble	1 1 1 1

S2-PSK PHY Header

There is no need for S2-PSK PHY header to reduce transmission overhead. The entire information related to S2-PSK PPDU format is readable from PHY PIB attributes.

 

S2-PSK PSDU

The PHY data service packet unit can be transmitted right after the preamble. Thus, the transmission overhead is only caused by the preamble.

The length of S2-PSK PSDU is variable, and its range is also readable from PHY PIB attributes. So once again, no overhead is wasted.

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HS-PSK PPDU

 

 

Figure 3 – Illustration of HS-PSK waveforms

 

HS-PSK preamble field

The preamble field for HS-PSK has two S2-PSK data bit times long (see table 2). This means the binary sequence “1111” shall be transmitted by two S2-PSK bit times via the S2-PSK modulation, while the DS8-PSK modulation transmits no data (i.e., idle pattern).

Table 2- HS-PSK Preamble

Preamble duration	Two S2-PSK bit times
S2-PSK bit sequence	1 1 1 1
DS8-PSK bit sequence	No data (idle patterns)

 

HS-PSK PHY header

PHY PIB attributes shall mandatorily configure PHY header subfields. Besides, PHR field shall be optionally used to notice the change of the following PHY header subfields:

Table 3- HS-PSK PHR subfields

PHY header subfields	Bit-width	Explanation on usage
PSDU length	16	PSDU length in byte
HSC	16	Header check sequence

 

HS-PSK Header Check Sequence

CRC-16 shall be used as HSC. The generation of CRC-16 (with polynomial generator 0x1021) is described in Annex C.

 

HS-PSK PSDU field

The PSDU consists of multiple S2-PSK cycle times; each cycle is as a sub-frame that has a low dimming period and a high dimming period that each period also includes numerous DS8-PSK data symbols. Each symbol carries 3-bits data (see table 4).

The number of DS8-PSK symbols (N) that either the low dimming or the high dimming period of S2-PSK carry is equal to the optical clock rates ratio between the DS8-PSK and the S2-PSK.

Table 4– HS-PSK PSDU sub-frame format

S2-PSK Low dimming period				S2-PSK High dimming period
DS8-PSK symbol 1	DS8-PSK symbol 2	…	DS8-PSK symbol N	DS8-PSK symbol 1	DS8-PSK symbol 2	…	DS8-PSK symbol N

The presence of another HS-PSK preamble indicates the end of the PSDU of HS-PSK. The configuration of PSDU length is implemented via the PHY PIB phyHspskPsduLength and being noticed by sending the updated PSDU length via the PHY header subfield.

 

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Summary

• The PPDU formats for S2-PSK and HS-PSK are provided
• Note that all of OCC PPDU formats can be reconfigurable by passing the PHY PIB attributes managed by the Tx/Rx PHY. Reconfiguration ability in OCC PHY gives more freedom for implementers to customize the communication for their particular purpose. Please check the next post for an explanation of this issue.