Image Processing Reference
In-Depth Information
transmit powers and slower bitrates/modulation schemes increase the probability of successful packet
reception [].
A common way to protect data bits against bit errors is to use redundancy. Example approaches are
error detecting and correcting codes (also called FEC) [] and the transmission of multiple copies of
a packet []. [].The latter approach can also be classified as a time-diversity scheme []. It is beneficial
for the overall throughput to control the amount of redundancy according to the current channel
state such that none or only a little redundancy is added when the channel currently shows only few
errors [,].
Another standard way to deal with transmission errors are retransmissions and suitable ARQ
schemes. For channels with bursty errors, it is not clever to retransmit the same packet immedi-
ately on the same channel. Specifically, when the mean length of error bursts is of the same order or
larger than the packet length, both the original packet and its immediate retransmission are likely
to be hit by the same error burst. Hence, under these circumstances an immediate retransmission
wastes time and energy.
The transmitter can
postpone
the retransmission for a while and possibly transmit packets to other
stations/over other channels in the meantime. If the postponing delay is well chosen, the channel
has left the error burst and the retransmission is successful. Indeed, it has been demonstrated in
[-] that such an approach can reduce the number of wasted packets and increase the throughput
significantly. But, how to choose the postponing delay? One option is to adopt some fixed value which
could be based on measurements or on a priori knowledge about the channel. Another option is to
send occasionally small probing packets [] which the receiver has to acknowledge. If the transmitter
captures such an acknowledgment, it assumes the channel to be back in good state and continues data
transmission. For real-time systems, the postponing decision should not only consider the channel
state but also the deadline of a packet. The authors of [] describe a scheme which takes both the
estimated channel state (for postponing decisions) and the packet deadline into account to select one
coding scheme from a suite of available schemes.
Retransmissions do not necessarily need to use the same channel as the original packet. It is well
known that wireless channels are spatially diverse: a signal transmitted by station A can be in a deep
fade at geographical position
p
and at the same time good enough to be properly received at another
position
p
. This property is exploited by certain diversity techniques, for example, receiver diver-
sity []. The receiver has two antennas and can pick the stronger/better of the two signals it reads
from its antennas. If the spacing between the antennas is large enough,
∗
the signals appear to be
uncorrelated. The spatial diversity of wireless channels can also be explored at the protocol level.
Assume that station A transmits a packet to station B. he channel from A to B is currently in a deep
fade, but another station C successfully captures A's packet. If the channel from C to B is currently in
a good state, the packet can be successfully transmitted over this channel. herefore, station C helps
A with its retransmission. his idea has been applied in [] to the retransmission of data packets as
well as to poll-packets in a polling-based MAC protocol.
In general, ARQ schemes can be integrated with FEC schemes into hybrid error control schemes
[]. Ideally, for industrial applications, deadlines should be taken into account when designing
these schemes. In [,], retransmissions and FEC are combined with the concept of deadlines by
increasing the coding strength with each retransmitted packet as the packet deadline approaches.
This is called deadline-dependent coding. Another interesting hybrid error control technique is
packet combining [,,]. Put briefly, in these schemes the receiver tries to take advantage of
the partially useful information contained in already received erroneous copies of a packet. For
∗
∗The two antennas should at minimum have a mutual distance of % of the wavelength [, Chap. ]. If the system works
in the . GHz ISM band, this amounts to - cm.
