IR PHY

Overview

The Infrared (IR) Physical Layer (PHY) was one of the three physical layers defined in the original IEEE 802.11-1997 standard, alongside FHSS and DSSS. Unlike radio-based PHYs, the IR PHY used infrared light in the near-infrared band (850–950 nm) as its transmission medium.

Infrared PHY was designed for short-range indoor wireless LANs using diffuse (non-line-of-sight) reflection from walls and ceilings. It supported 1 Mbps and 2 Mbps data rates using Pulse-Position Modulation (PPM).

Although standardized, it was never commercialized and was later replaced by RF-based PHYs (DSSS → 802.11b, OFDM → 802.11a/g).

Key Characteristics

Parameter | Description |

|------------|————-| | Medium | Diffuse infrared light (850–950 nm) | | Transmission Range | 5–10 m (indoor only) | | Data Rates | 1 Mbps, 2 Mbps | | Modulation | 4-PPM (1 Mbps), 16-PPM (2 Mbps) | | Access Mechanism | CSMA/CA (DCF) | | Carrier Sense | Optical energy detection | | Multipath Handling | Diffuse reflection (non-directional) | | Frame Structure | Standard PLCP + PSDU | | Status | Obsolete (no commercial deployments) |

Operating Principle

Infrared PHY transmits digital data using short bursts of modulated infrared light. Instead of using a radio carrier, the information is encoded by the position of a light pulse in time within each symbol period — a technique known as Pulse-Position Modulation (PPM).

Each symbol period is divided into equal time slots: - The presence of a pulse in one slot indicates the data symbol. - The absence of a pulse** (other slots) indicates logical zeros.

This approach provides high energy efficiency and noise immunity under diffuse lighting.

Infrared Transmission and Reception

Transmitter: - Converts bits into pulse positions (4-PPM or 16-PPM). - Drives an infrared LED emitter (center wavelength ~880 nm). - Emits diffuse light across the room for non-line-of-sight coverage.

Receiver: - Uses a photodiode and optical band-pass filter to detect pulses. - Filters out ambient light and electrical noise. - Synchronizes to pulse timing using the PLCP preamble. - Decodes the time position of pulses to recover bits.

Modulation and Data Rates

Mode | Modulation | Bits per Symbol | Symbol Duration | Description |

|------|————-|-----------------|—————–|--------------| | 1 Mbps | 4-PPM | 2 | 4 µs | One of 4 slots contains a light pulse | | 2 Mbps | 16-PPM | 4 | 8 µs | One of 16 slots contains a light pulse |

Infrared PPM does not use a carrier frequency — transmission occurs by turning the LED emitter on and off according to the modulation timing.

PLCP Frame Structure

The Infrared PHY uses the same Physical Layer Convergence Procedure (PLCP) structure as other PHYs, ensuring a consistent interface to the MAC.

Field | Description |

|--------|————-| | Preamble | Timing and synchronization for receiver | | PLCP Header | Length, rate, and service fields | | PSDU | MAC frame payload |

  • Preamble enables symbol alignment and gain control.

  • PLCP Header communicates the data rate (1 or 2 Mbps).

  • PSDU contains the MAC Protocol Data Unit (MPDU).

MAC Interaction and DCF Operation

The MAC layer interacts with the IR PHY identically to radio PHYs.

  • CSMA/CA access: same DCF rules (DIFS, SIFS, backoff, NAV).

  • ACK, RTS/CTS, and retransmission: same as RF operation.

  • Physical carrier sense: detects infrared energy above threshold.

  • Virtual carrier sense (NAV): uses Duration field from MAC headers.

From the MAC’s perspective, the IR medium behaves like any other shared channel.

Carrier Sense and Clear Channel Assessment

Infrared carrier sense relies on optical energy detection:

  • The receiver measures incoming light intensity.

  • If optical power > threshold → “medium busy.”

  • If below threshold for a full DIFS → “medium idle.”

  • The backoff counter runs during idle slots, identical to RF-based DCF.

Because IR light cannot penetrate walls, the effective collision domain is limited to a single enclosed space.

Infrared PHY Timing Parameters

Parameter | 1 Mbps | 2 Mbps |

|------------|——–|--------| | Symbol Duration | 4 µs | 8 µs | | Slot Time (t_slot) | 20 µs | 20 µs | | SIFS | 10 µs | 10 µs | | DIFS | 50 µs | 50 µs | | Preamble Duration | 56 µs | 56 µs | | Maximum Range | ~10 m | ~5 m |

DCF timing parameters (SIFS, DIFS, EIFS) are defined in microseconds and are identical to DSSS values to maintain MAC uniformity.

Advantages

  • Immune to RF interference (since it uses light, not radio).

  • Constrained signal propagation (no wall penetration enhances security).

  • Suitable for RF-restricted environments (e.g., hospitals, aircraft).

  • Low implementation cost (uses LED and photodiode components).

Limitations

  • Line-of-sight or reflective path required.

  • Severely limited range (~5–10 m).

  • Susceptible to ambient light (sunlight, lamps).

  • No mobility support (receiver must remain within reflective coverage).

  • Low throughput compared to DSSS (11 Mbps) or OFDM (54 Mbps).

  • No interoperability with RF PHYs.

Coexistence and Compatibility

  • The IR PHY cannot coexist with DSSS or FHSS PHYs within the same BSS.

  • Devices operate on only one PHY type at a time.

  • The MAC layer abstracts PHY differences, but physical signals are incompatible.

  • No hybrid IR–RF bridging was standardized.

DCF Integration and Access Example

Even though it uses light instead of radio, IR follows the same contention rules.

Medium idle → wait DIFS → start backoff → transmit
    |
    └─ if medium busy: freeze backoff, wait until idle again

ACK and retransmission: - Receiver sends ACK after SIFS (in IR form). - Sender retries using exponential backoff if ACK is missed.

Sample Transmission Diagram

Time →
+-------------------------------------------------------------+
|<-- DIFS -->|<- backoff ->|--- IR Preamble ---|-- PSDU --| SIFS | ACK |
+-------------------------------------------------------------+
Medium Idle      Contention    Data Transmission  Interframe   Control

Typical Parameter Values

Parameter | Typical Value |

|------------|—————-| | IR wavelength | 850–950 nm | | Optical power | ~10 mW (diffuse LED) | | Photodiode sensitivity | ~–40 dBm optical equivalent | | Bit error rate target | <10⁻⁵ | | Indoor coverage | Single room |

Security and Environmental Aspects

  • Infrared signals are confined to a room, enhancing physical security.

  • Immune to RF-based eavesdropping and interference.

  • However, ambient light sources (e.g., sunlight, fluorescent lamps) can saturate the photodiode and increase bit errors.

  • Line-of-sight blocking (people, furniture) may cause temporary link loss.

Historical Context

Year | Event |

|------|——–| | 1997 | IR PHY introduced in original IEEE 802.11 | | 1999 | 802.11b DSSS supersedes IR | | 2000+ | No IR WLAN devices in production | | 2001 | IR officially deprecated in later revisions |

The IR PHY was inspired by IrDA (Infrared Data Association) concepts but added contention-based medium access (CSMA/CA) instead of point-to-point link control.

Summary

Concept | Description |

|----------|————-| | Medium | Diffuse infrared light (850–950 nm) | | Modulation | 4-PPM (1 Mbps), 16-PPM (2 Mbps) | | Access | CSMA/CA (DCF) identical to RF PHYs | | Carrier Sense | Optical energy detection | | Range | 5–10 m indoor only | | Advantages | No RF interference, secure, low cost | | Disadvantages | Short range, ambient-light sensitivity | | Successor | DSSS in 802.11b (RF, 11 Mbps) |

References

  • IEEE Std 802.11-1997, Clause 14 — Infrared PHY Specification

  • IEEE Std 802.11b-1999, Annex (PHY evolution overview)

  • Gast, M. 802.11 Wireless Networks: The Definitive Guide, O’Reilly

  • Kavehrad & Stuckel, “Indoor Broadband Optical Wireless Communications”

  • IEEE 802.11 Working Group historical drafts and archives

Figures

Infrared PHY principle

Infrared PHY transmission concept showing 4-PPM pulses, diffuse reflections, and photodiode reception.

802.11 MCS

spreading/coding

Modulation

BW

Total-Sub-Carriers

FSP

Tdata=1/FSP

GI

symbol

Bits/symbol

Code rate

Usable

Rate

Formula (Usable Rate = (Bits/Symbol ÷ Symbol Duration) × (1 / Code Rate))

IR

4PPM (optical pulse)

4-PPM

1

N/A

1 MHz

1 µs

N/A

1 µs

1

1

1

1 Mbps

(1 / 1 µs) × 1 = 1 Mbps

IR

16PPM (optical pulse)

16-PPM

1

N/A

2 MHz

0.5 µs

N/A

0.5 µs

2

1

2

2 Mbps

(2 / 0.5 µs) × 1 = 4 Mbps (but data coded as 2 Mbps)

Channel Number

Center Frequency (MHz)

Frequency Range

DFS Required

1

2412

2401 – 2423

No

2

2417

2406 – 2428

No

3

2422

2411 – 2433

No

4

2427

2416 – 2438

No

5

2432

2421 – 2443

No

6

2437

2426 – 2448

No

7

2442

2431 – 2453

No

8

2447

2436 – 2458

No

9

2452

2441 – 2463

No

10

2457

2446 – 2468

No

11

2462

2451 – 2473

No

12

2467

2456 – 2478

No

13

2472

2461 – 2483

No

14

2484

2473 – 2495

No

Band Name

Frequency Range (GHz)

Frequency Range (MHz)

Channels

ISM Band (Global)

2.400 – 2.4835

2400 – 2483.5

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 (12, 13, 14 vary by region)