Digital Signal Processing Reference
In-Depth Information
the receiver antenna, the first task of the receiver is to place a window over
the pulses, the same way as is done at transmission side. The main purpose
of this
receive window
is to prevent the bulk of unwanted noise and in-band
interferer power from entering the front-end of the receiver. In fact, the receive
window acts as a coarse time-domain filter and relaxes the linearity require-
ments of the subsequent signal processing chain. Unfortunately, the benefits of
this receive window do not come for free: it is extremely difficult to maintain
a correct alignment with the received pulse stream, because the arrival time
of the pulses varies due to changes in the propagation path length. All pulse
energy that arrives during the off-state of the receive window is lost forever.
Synchronizing on a stream of pulses
To facilitate locking on the pulse stream, the receive window is chosen larger
than the transmit window. For example, if the period between two pulses is
divided into 10 equal (non-overlapping) receive slots, the search space of the
receiver becomes limited to only these 10 slots. The power of unwanted nar-
rowband interference is suppressed by 10 dB, and can be considered as the
processing gain of the pulse-based radio system. Increasing the window length
has some unintended consequences, though. In a multipath indoor channel, no
single pulse stream is being received. Due to multipath, several delayed ghost
copies of the same stream arrive at the antenna within a short period of time.
On average, an extended window length allows the receiver to capture more
pulse energy, which is a good thing. But pulses that arrive within a single re-
ceive slot will start to interfere with each other, the result of the reduced mul-
tipath resolvability of the extended receive window. There is in fact an equal
chance for positive or destructive interference, but this phenomenon results in
a reduced reliability (uptime) of the wireless link a static indoor channel.
To prevent this from happening, the main re-
ceiver is supported by several auxiliary receive
units. Each of the auxiliary receivers monitors
the channel at a different receive slot, however
not every possible receive slot has its own dedi-
cated receive unit. Instead, a receive unit can be
dynamically allocated to a slot. The first task of
the auxiliary receivers is to help the main receiver to bridge unexpected out-
ages of the main receiver. Data that is captured by the auxiliary receivers is
kept in standby and can be immediately injected in the main symbol stream in
case an insufficient signal quality has been detected. Each auxiliary unit has
its own dedicated equalizer, which aligns the alternate backup stream to the
main symbol stream. Finally, the information of the auxiliary receive unit is
INTERLEAVED
B
1
B
1
B
1
B
1
B
2
B
2
B
2
B
2
A
A
A
A
D
D
D
D
ISSR
