Image Processing Reference
In-Depth Information
are designed such that the autocorrelation between the sequence and a shifted version of it is low for
all lags of more than one chip duration. If a coherent matched-filter receiver is synchronized with the
direct signal
S
,thedelayedsignal
S
′
appears as white noise and produces only a minor distortion. In
the RAKE receiver approach, delayed signal copies are not treated as noise but as a useful source of
information [, section .]. Put briefly, a RAKE receiver tries to acquire the direct signal and the
strongest time-delayed copies and combines them coherently. However, RAKE receivers are much
more complex than simple matched-filter DSSS receivers.
In FHSS, the available spectrum is divided into a number of subchannels. The transmitter hops
through the subchannels according to a predetermined schedule, which is also known to the receiver.
The advantage of this scheme is that a subchannel currently subject to transmission errors is used only
for a short time before the transmitter hops to the next channel. he hopping frequency is an impor-
tant parameter of FHSS systems, since high frequencies require fast and accurate synchronization.
As an example, the FHSS version of IEEE . hops with . Hz and many packets can be trans-
mitted before the next hop. In Bluetooth the hopping frequency is . kHz and at most one packet
can be transmitted before the next hop. Packets are always transmitted without being interrupted by
hopping.
Recently, there has been considerable interest in orthogonal frequency division multiplexing
(OFDM) techniques []. OFDM is a multicarrier technique, where blocks of
N
different symbols
are transmitted in parallel over a number of
N
subcarriers. Hence, a single symbol has an increased
symbol duration
N
⋅
τ
as compared to full-rate transmission with symbol duration τ. The symbol
duration
N
τ
isusuallymuchlargerthanthedelayspreadofthechannel,thiswaycombatting
intersymbol-interference and increasing channel quality. The IEEE .a standard [] as well as
HIPERLAN/ [,] use an OFDM physical layer.
⋅
24.5 Problems and Solution Approaches on the
MAC and Link Layer
he MAC and the link layer are exposed most to the error behavior of wireless channels and should
do most of the work needed to improve the channel quality. Specifically for hard real-time com-
munications, the MAC layer is a key component: if the delays on the MAC layer are not bounded,
the upper layers cannot compensate this. In general, the operation of the MAC protocol is largely
influenced by the properties of the physical layer. Some of the unique problems of wireless media
are discussed in this section. For a general discussion of MAC and link layer protocols we refer the
reader to [,,,,].
24.5.1 Problems for Wireless MAC Protocols
Several problems arise due to path loss in conjunction with a threshold property—wireless receivers
require the signal to have a minimum strength to be recognized. For a given transmit power, this
requirement translates into an upper bound on the distance between two stations wishing to com-
municate;
∗
if the distance between two stations is larger, they cannot hear each others transmissions.
For MAC protocols based on carrier sensing (carrier sense multiple access [CSMA]), this property
creates the hidden terminal problem [] and the exposed terminal problem.
The hidden terminal problem is sketched in Figure .. Consider three stations A, B, and C with
transmission radii as indicated by the circles. Stations A and C are in range of B, but A is not in the
∗
To complicate things, wireless links are not necessarily bidirectional: it may well happen that station A can hear station
B but not vice versa.
