Transcript Infrared (IR) and Radio Frequency (RF) Communication

Infrared (IR) Communication
Light Spectrum
Microwaves
Radio (RF)
Visible
Infrared (IR)
X-Rays
Ultraviolet
Gamma
Rays
Freq.
(Hz)
109
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$
1012
1014 1015
1017
1020
FCC
Implementation costs rise significantly around 1-10 GHz. (But
one important exception is IR at around 500 THz ; very
inexpensive.)
Signals above 100 GHz cannot penetrate walls
Most signals below 300 GHz are regulated by the FCC
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How do you Transmit with IR
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No conductor
– Signal spreads from origin
– Signal strength decreases with distance
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Can’t use amplitude
– Too much noise from other IR sources (lights, people, etc.)
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Can’t use sign (no +/-)
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Solution: Modulation
– Send pulses of light to represent binary information
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Modulation Schemes
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SIR - Serial Infrared
– 2400bps to 115,200bps
– SIR modem is simple and low-cost
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SDLC-based MIR
– 576Kbps and 1.152Mbps
– guarantees a minimal occurrence rate of light pulses
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FIR - Fast Infrared
– 4Mbps
– power usage constant (always the same)
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SIR - Serial Infrared Modulation
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1.6us (or 3/16 bit interval) pulse of light for each
‘0’ in a standard asynchronous data stream
– Inverse of serial RS-232 where signal is held high, then pulled
down for zeros
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Stream consists of a start bit, N data bits, and 1
stop bit (serial packet)
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SDLC-based MIR
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A data stream ‘0’ is coded for by the presence of
an optical pulse 1/4 of the bit interval in duration
(217ns for 1.152Mbps)
The SDLC protocol, with its zero-insertion bitstuffing approach, guarantees a minimal
occurrence rate of zeroes (and therefore light
pulses) in the data stream
This also ensures
synchronization between a
transmitter and receiver
can be maintained
throughout a packet
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FIR - Fast Infrared
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More Ethernet-like in its framing
– a packet is made up of a preamble, start of frame delimiter
(SFD) and a data payload.
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Utilizes 1:4 Pulse Position Modulation (PPM)
– each pair of bits in the data stream are represented by a pulse
of light emitted in one of 4 available slot positions which
comprise a 4PPM symbol
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PPM always requires a pulse
for each slot, so power
usage is independent of the
data
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Introduction
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IR Communication Protocols
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To use IR hardware easily, we build up layers of
protocol that get us farther away from the
implementation each step.
Common Protocols:
– Serial IR
– IrDA
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Differences between Waves and Wire
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Distance/$$$
– Wave transmissions are much more expensive than wire
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Speed
– Wire is a very simple and highly controllable medium which
allows for much higher transfer rates
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Limitations
– IR:
• Line of Sight - IR can not travel through opaque objects, so transceivers
must be able to see each other. For many commercial products they must be
within 15º of each other.
• Signal Power - even though we can not see IR, making the signal too strong
can easily blind us
– RF:
• FCC Regulation
• Interference with objects and other RF waves
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