LG RD3500, Service Manual

Page: 10 / 73

LG- K D3500
LGE
LG Electronics Inc.
8/21
3. Structure and Functions of CDMA Mobile Phone
The mobile station of CDMA system is made up of a radio frequency part and logic/control (digital)
part. The mobile station antenna is connected with the transmitter/receiver via a SAW duplexer filter
so that it can carry out the transmit/receive function at the same time.
The transmit frequency is the 25MHz band of 824~849MHz, whereas the receive frequency is the
25MHz band of 869~894MHz. The transmit/receive frequency is separated by 45MHz. The RF signal
from the antenna passes the LNA , bandpass SAW filter having the 1.25MHz band and then, is
directly converted into baseband signal by the frequency synthesizer and frequency down converter.
Baseband output signals that have been filtered from spurious signal are converted into digital signals
via an analog-to-digital converters(Rx ADC) and then, sent out respectively to 5 correlators in each
CDMA de-modulator. Of these, one is called a searcher whereas the remaining 4 are called data
receiver(finger). Rx signals include a great number of call signals that have been sent out by the
adjacent cells. These signals are detected with pseudo-noise sequence (PN Sequence). Signal to
interference ratio (C/I) on signals that match the desired PN sequence are increased through this type
of correlation detection process. Then, other signals obtain processing gain by not increasing the ratio.
The carrier wave of pilot channel from the cell site most adjacently located is demodulated in order to
obtain the sequence of encoded data symbols.
During the operation with one cell site, the searcher searches out multi-paths in accordance with
terrain and building reflections. On three data receivers, the most powerful four paths are allocated for
the parallel tracing and receiving. Fading resistance can be improved a great deal by obtaining the
diversity combined output for de-modulation. Moreover, the searcher can be used to determine the
most powerful path from the cell sites even during the soft handoff during the two cell sites. Moreover,
four data receivers are allocated in order to carry out the de-modulation of these paths. Data output
that has been demodulated change the data string in the combined data row as in the case of original
signals(deinterleaving), and then, are de-modulated by the forward error correction decoder which
uses the Viterbi algorithm.
On the other hand, mobile station user information sent out from the mobile station to the cell site pass
through the digital voice encoder via a mike. Then, they are encoded and forward errors are corrected
through the use of convolution encoder. Then, the order of code rows is changed in accordance with a
certain regulation in order to remove any errors in the interleaver. Symbols made through the above
process are spread after being loaded onto PN carrier waves. At this time, PN sequence is selected
by each address designated in each call.
Signals that have been code spread as above are digital modulated (OQPSK) and then, power
controlled at the automatic gain control amplifier (AGC Amp). Then, they are converted into RF band
by the frequency synthesizer synchronizing these signals to proper output frequencies. Transmit
signals obtained pass through the duplexer filter and then, are sent out to the cell site via the antenna.