Digital Signal Processing Reference
In-Depth Information
Conclusion and indicative numbers
As a conclusion of this overview of issr-supported pulse-based wideband ra-
dio system, some indicative numbers are given, on which the hardware imple-
mentation of the receiver in the next section will be based. The main goal is to
secure a reliable wireless link over a link distance up to 10 m. Calculations
26
for a channel with only thermal noise predict a theoretical capacity of about
220 Mbit/s. Based on these figures, a qpsk symbol rate of 100 Msymb/s was
chosen, which results in a raw data rate of 200 Mbit/s transported over the
channel. Note that there is a limitation on the energy spectral density (esd)
of the system, so increasing the symbol rate even more decreases the energy
per pulse. A higher symbol rate would also decrease the processing gain of the
receive window. At receiver side, the period between two transmitted pulses
is divided into 10 equivalent receive slots, which gives the receiver a multi-
path resolution in the order 1 ns. The in-band interferer suppression ratio of the
gated receiver is thus 10 dB.
The 3 most-significant bits of the analog-
to-digital converter are reserved to cope
with the residual in-band interferer power
and a suboptimal loading factor of the ad-
converter due to the incorrect gain settings
of the baseband amplifier. The extra word
length of the ad-converter in combination
with the suppression factor of the receive
window makes that a signal-to-interferer
Receive ADC - loading factor and headroom
MSB
headroom (18dB):
3 bits to cope with
interference.
5
4
3
loading ADC:
2 bits to represent
single phase of the
2
1
QPSK signal.
(SNR = 12dB)
receive ADC
ratio of
18 dB can be tolerated before the sensitivity of the receiver starts
to deteriorate, either due to clipping or due to an increase of the quantiza-
tion noise. If a 5-bit converter is being used (one for the i- and one for the
q-chain), the two lowest significant bits are used to represent the baseband
qpsk waveform. This way, the quantization noise is below the thermal noise
generated by the channel itself, limiting the implementation loss induced by
the ad-converter. The following chapter will discuss the implementation de-
tails of the analog signal chain from a single receive unit, starting from the
antenna input terminal up to the input of the ad-converter (not included).
−
26
Constant-gain, omni-directional antennas with 1
/f
2
aperture,
−
14
.
3 dBm transmit power, 5 GHz center
frequency and a 1 GHz-wide los channel.
