LG PX8700, Service Manual, Page: 6 / 159

PX8700
LG Electronics Inc.
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2. Features and Advantages of CDMA Mobile Phone
2.1 Various Types of Diversities
When employing the narrow band modulation (30kHz band) that is the same as the analog FM modulation
system used in the existing Cellular system, the multi-paths of radio waves create a serious fading. However, in
the CDMA broadband modulation (1.25MHz band), three types of diversities (time, frequency, and space) are
used to reduce serious fading problems generated from radio channels in order to obtain high-quality calls.
Time diversity can be obtained through the use of code interleaving and error correction code whereas frequency
diversity can be obtained by spreading signal energy to wider frequency band. The fading related to normal
frequency can affect the normal 200~300KHz among signal bands and accordingly, serious effect can be
avoided. Moreover, space diversity (also called path diversity) can be realized with the following three types of
methods.
First, it can be obtained by the duplication of cell site receive antenna. Second, it can be obtained through the
use of multi-signal processing device that receives a transmit signal having each different transmission delay
time and then, combines them. Third, it can be obtained through the multiple cell site connection (Soft Handoff)
that connects the mobile station with more than two cell sites at the same time.
2.2 Power Control
The CDMA system utilizes the forward (from a base station to mobile stations) and backward (from the mobile
station to the base station) power control in order to increase the call processing capacity and obtain high-quality
calls. In case the originating signals of mobile stations are received by the cell site in the minimum call quality
level (signal to interference) through the use of transmit power control on all the mobile stations, the system
capacity can be maximized.If the signal power of mobile station is received too strong, the performance of that
mobile station is improved. However, because of this, the interference on other mobile stations using the same
channel is increased and accordingly, the call quality of other subscribers is reduced unless the maximum
accommodation capacity is reduced.
In the CDMA system, forward power control, backward open loop power control, and closed loop power control
methods are used. The forward power control is carried out in the cell site to reduce the transmit power on
mobile stations less affected by the multi-path fading and shadow phenomenon and the interference of other cell
sites when the mobile station is not engaged in the call or is relatively nearer to the corresponding cell site. This
is also used to provide additional power to mobile stations having high call error rates, located in bad reception
areas or far away from the cell site.
The backward open loop power control is carried out in a corresponding mobile station; the mobile station
measures power received from the cell site and then, reversely increases/decreases transmit power in order to
compensate channel changes caused by the forward link path loss and terrain characteristics in relation to the
mobile station in the cell site. By doing so, all the mobile transmit signals received by the base station have same
strength.
Moreover, the backward closed loop power control used by the mobile station is performed to control power
using the commands issued out by the cell site. The cell site receives the signal of each corresponding mobile
station and compares this with the pre-set threshold value and then, issues out power increase/decrease
commands to the corresponding mobile station every 1.25msec (800 times per second). By doing so, the gain
tolerance and the different radio propagation loss on the forward/backward link are complemented.