Installation Manual

2
APPLICANT: LARUS CO.
FCC ID: LYA 9000
3. Transmitter functional description.
16QAM Modulator. The purpose of this circuit (located in the Control Module) is to
quadrature-amplitude modulate the 140 MHz carrier with the serial bit stream, that contains
the information of the four multiplexed E1 carriers. Each E1 baseband carrier is 2.048 MHz,
thus the baseband width of the multiplexed serial bit stream entering the I-Q modulator is
8.192 MHz (+ some additional BW for overhead bits due to coding and control purposes).
The bit stream is converted to symbols here so that one symbol corresponds to four bits of
information. The baseband bandwidth is 2.048 MHz + 0.213 = 2.2613 MHz. The 140 MHz IF
bandwidth of the modulated carrier is 2x2.2613 = 4.522 MHz. This IF signal at 140 MHz is
fed to the Spread Spectrum Code Multiplier.
Spread Spectrum Code Multiplier. The function of this circuit (located in the RF
module) is to spread the spectrum to achieve a processing gain of at least 10 dB. Spreading
is accomplished by multiplying the modulated IF carrier with a pseudo-random (PN) pulse
sequence. Because of the randomness of baseband data stream and the spreading PN
pulse sequence, the spectrum is noise-like with
no distinct spectral lines
. The system is
designed to achieve a theoretical processing gain of about 10.45dB, which is obtained as
follows. The null-to-null IF bandwidth (or main lobe bandwidth) before spreading is 4.522
MHz. The chip width, Tc of the spreading pulse is 44.19 nsec, thus the null-to-null chip
bandwidth of the
spreading spectrum at IF is 2/Tc = 45.25 MHz. These two timing waveforms are multiplied,
hence, considering convolution in the frequency domain, the theoretical spectral null-to-null
bandwidth is the sum of the individual bandwidths:
Bw
(convolved)
= Bw
(modulation)
+ Bw
(chip)
= 49.77 MHz = Bw
(spread)
In practice, the spread bandwidth is narrower because of the sharp filtering in
subsequent stages. The sidelobes and nulls and band edges are effectively filtered out, and
based on measured data, the nominal bandwidth is taken here as 45 MHz.
The theoretical processing gain, Gp is the ratio of signal bandwidth after spreading to
the bandwidth before spreading. Hence,
Gp = 49.77/4.522 = 11 or 10.41 dB
The measured typical transmit output spread spectra of a TX low unit is shown in
Figure 2.
De-spreading is also taking place in the RF module. The de-spread 70 MHz receive
signal is then fed to the Control Module for the receiver demodulator functions through a
coaxial cable. In addition, this cable also carries the following signals: