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LG Electronics Inc.
VX9700
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.
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