Image Processing Reference
In-Depth Information
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Specialized low energy protocols from building automation like Enocean [] are not
reliable and real-time suited as defined above.
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Passive electronic tagging systems (radio frequency identification tags [RFID]), as used
for electronic article surveillance, do not have sufficient range, speed, and flexibility.
Hence, it was necessary to design a new communication protocol for WISA, which, however, uses
much of the available standard hardware.
28.3 Communication Subsystem Design
The radio characteristics of the WISA system are those of the IEEE .. physical layer
specification used in Bluetooth []. The raw data rate is Mb/s and transmit power is mW. This
comparatively low transmit power is appropriate and sufficient for a range - m, as required for
the typical node and base station density.
Not so obvious is that the low transmit power and therefore limited range is also dictated by the
requirement of a sufficiently large average node density. A good comparison is human communica-
tion, where also a large number of people, e.g., at a cocktail party can communicate reliably and fast,
due to fairly low voice/range, which allows others to reuse the transmission medium in a certain short
distance. Just a few participants increasing their transmit power/range will disturb many others.
28.3.1 Medium Access and Retransmission
TDMA (one fixed time slot per device) with frequency division duplex (FDD) (device and base
station can send at the same time) and FH is used, as illustrated in Figure .. The downlink
transmission (from the base station) is always active, for the purpose of establishing frame and slot
synchronization of the S/As. Uplink transmissions from an S/A occur only when it has new data
or an acknowledgment to send (event triggered). WISA allows up to S/As per cell in different
time/frequency slots.
The TDMA frame length,
T
frame
, is µs. Each S/A may transmit in one out of slots
per frame. To support up to S/As per cell, each S/A is part of one of four uplink groups
{
UL
,UL
,UL
,UL
}
with separate frequencies. The five frequencies hop synchronously at each
frame boundary.
The slot structure is shown in Figure .. The downlink slot format is a compromise between the
efficiency of channel usage and synchronization latency of the downlink.
A device transmits its data packet in its assigned uplink time slot. An acknowledgment is expected
in the corresponding downlink slot, which is staggered by µs to allow for TX/RX-turnaround
time (see Figure .). If no acknowledgment is received, the S/A will retransmit the data packet in
the next frame. With frame-by-frame FH, the radio channel used for retransmission will largely be
independent of the previous transmission, thus increasing the probability of successful transmission.
28.3.2 Frequency Hopping
Figure . shows the measured frequency-depending attenuation in the . GHz ISM band. It can be
seen that the coherence bandwidth of the signal may be some tens of MHz (corresponding to propa-
gation path length differences of m). Wideband FH will thus improve transmission performance.
It also reduces the effect of other devices that operate in this band. he global system for mobile com-
munication (GSM) [] and Bluetooth [] systems are well-known wireless communication standards
using TDMA and FH. heir FH sequences are determined in a pseudorandom manner and therefore
do not control frequency separation between consecutive hops. Hence, consecutive transmissions
may have similar high packet error probability. It is for WISA of interest to employ FH sequences
