PHY modes within IEEE 802.15.7m draft – a quick review

PHY operating modes 

The latest IEEE 802.15.7m draft standard (until Aug. 2018) is on the Sponsor Balloting recirculation. This version has updated a lot in compared to the previous version before LiFi modes are moved to new task group under IEEE 802.15. A list of operating modes between two drafts is a good start.

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Notation

• OOK: On-off keying
• VPPM: Variable pulse position modulation
• CSK: Color shift keying
• UFSOOK: Undersampled frequency shift on-off keying
• S2-PSK: Spatial 2-phase shift keying
• HS-PSK: Hybrid modulation of S2-PSK and dimmable SM-PSK
• Offset-VPWM: Offset variable pulse width modulation
• PWM/ PPM: Pulse width modulation/ pulse position modulation
• CM-FSK: Compatible multiple frequency shift keying
• C-OOK: Compatible On-Off Keying
• RS-FSK: Rolling shutter frequency shift keying
• VTASC: Variable transparent amplitude shape code
• DC-biased DMT : Direct current-biased Discrete Multitone
• OFDM: Orthogonal frequency-division multiplexing
• DCO-OFDM: Direct current-biased optical OFDM
• eU-OFDM: enhanced unipolar OFDM
• RPO-OFDM: Reverse polarity optical OFDM
• ACO-OFDM: Asymmetrically clipped optical OFDM

Image Sensor Communication is another name for Optical Camera Communication. These names have the same meaning.

Before splitting LiFi modes

Whereas the IEEE 802.15.7-2011 standard relied on three modulation schemes (OOK, VPPM, and CSK), the TG7m draft specification (before May 2017) introduced multiple modulation schemes that target a broad range of OCC applications and services and carrier modulation techniques for high-speed PD communication.

Table 1- IEEE 802.15.7m Initial Operating Modes

PHY	Description	Modulation schemes
I	Existing IEEE 802.15.7-2011	OOK; VPPM
II	Existing IEEE 802.15.7-2011	VPPM; OOK
III	Existing IEEE 802.15.7-2011	CSK
IV	Image sensor communication modes added to TG7m	UFSOOK; Twinkle VPPM; S2-PSK; HS-PSK; Offset-VPWM
V	Image sensor communication modes added to TG7m	PWM/ PPM; CM-FSK; C-OOK; RS-FSK
VI	Image sensor communication modes added to TG7m	A-QL; VTASC; Invisible data-embedding; HA-QL
VII	High-rate PD communication modes added to TG7m	DC-biased DMT
VIII	High-rate PD communication modes added to TG7m	DCO-OFDM; eU-OFDM; RPO-OFDM; ACO-OFDM

In comparison to previous sets of PHY modes (including PHYs I–III) and newly added high-rate PD communication modes (including PHYs VII to VIII), camera-based PHY modes operate at the lowest range of optical clock rate (i.e., lower than 100 kHz). Besides, all of these OCC modes are baseband, rectangular-shaped pulsed modulation. These two notable differences between OCC and VLC/LiFi modes are the cause of the technical consideration to deal with limitations in frame rates and shutter speeds of supported cameras.

Despite being designed explicitly for OCC systems, some TG7m technical contributors have claimed that PD can also be used to demodulate data from modulated waveforms of these three sets of operating modes (PHY IV to PHY VII). Moreover, the mechanism to support PD receiver using these waveforms are pending for TG7m committee; we solely consider these PHY sets for camera-based communication modes for later discussions.

After splitting LiFi modes

Recently in May 2017, PHYs VII and VIII (LiFi modes) have been moved to form a new Task Group under IEEE 802.15 due to the uniqueness between OCC/LED-ID and LiFi technologies and the market potential relevance. The new LiFi Task Group is known as TG13 Multi-Gigabit/s OWC.

Table 2- IEEE 802.15.7m Updated Operating Modes

PHY	Description	Modulation schemes
I	Existing IEEE 802.15.7-2011	OOK; VPPM
II	Existing IEEE 802.15.7-2011	VPPM; OOK
III	Existing IEEE 802.15.7-2011	CSK
IV	Image sensor communication modes added to TG7m	UFSOOK; Twinkle VPPM; S2-PSK; HS-PSK; Offset-VPWM
V	Image sensor communication modes added to TG7m	MPM; CM-FSK; C-OOK; RS-FSK
VI	Image sensor communication modes added to TG7m	A-QL; VTASC; HA-QL; IDE.

MPM: Mirror Pulse Modulation (another name of PWM/PPM of Panasonic mode)

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Description of PHY Operating Modes

This description is official in the current draft standard (till August 2018); however, I feel that we need a detailed comparison.

• PHY IV: This PHY is intended for use with discrete light sources with data rates up to 22 kbps using various modulations.
• PHY V: This PHY is intended for use with diffused surface light sources with data rates up to 5.71kbps using various modulations.
• PHY VI: This PHY is intended for use with video displays with data rates in the kbps using various modulations.

Thus, I make this table to compare between OCC PHY operating modes. Note that this is an unofficial comparison, but it might be helpful somehow. The expected performance, characteristics, and intended systems are included herein.

Table 3 – An unofficial comparison between OCC PHY Operating Modes

	PHY IV  (RoI signaling techniques)	PHY V (Rolling Shutter technique)	PHY VI (Screen codes)
Distance	Hundreds meter	Several meters	Several meters
Data rate	Tens of bps for RoI signaling stream Tens of kbps for the high-rate data stream	< 10 kbps	kbps–Mbps
Intended Systems (Tx-Rx)	Car light/traffic light/LED Signage Tx RoI-camera Rx	LED panel Tx Rolling shutter camera Rx	Screen Tx Typical cameras Rx
Characteristics	The integration of a RoI signaling stream and a high-speed data stream is implemented in a hybrid system The RoI signaling stream allows fast, reliable detection and tracking of multiple light sources simultaneously.	The resolution of the LED panel on the captured image limits the amount of data per image The trade-off between the distance and the data rate makes this the most suitable option for indoor application.	Multiple Input-Multiple Output (MIMO) is benefited from the spatial resolution of both Tx and Rx Well suited for short-range OWC system utilizing screen.

Conclusion

• A quick review of IEEE 802.15.7m PHY modes was made.
• A description (official vs. unofficial) was also given for a general view to OCC modes.