AnyDATA DTS-800 CDK, Руководство по обслуживанию

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2. Features and Advantages of CDMA Module
2.1 Various Types of Diversities
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 more wider frequency band. The fading related to normal frequency can affect the
normal 200~300kHz among signal bands and accordingly, serious affect 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 and 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 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 office transmit signals in the cells are received by the cell site in the same strength.
Moreover, the backward closed loop power control used by the mobile station to control power with 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 1.25 msec
(800 times per second).
By doing so, the gain tolerance and the different radio propagation loss on the forward/backward link are complemented.