Telecommunications – Transmitter and receiver at separate stations – With control signal
Reexamination Certificate
2000-02-14
2002-05-07
Hunter, Daniel (Department: 2684)
Telecommunications
Transmitter and receiver at separate stations
With control signal
C455S522000
Reexamination Certificate
active
06385437
ABSTRACT:
PRIORITY
This application claims priority to an application entitled “Power Control Apparatus and Method for Inter-Frequency Handoff in CDMA Communication System” filed in the Korean Industrial Property Office on Feb. 13, 1999 and assigned Ser. No. 99-5263, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a power control apparatus and method for a mobile communication system, and in particular, to an apparatus and method for performing power control during inter-frequency hard handoff.
2. Description of the Related Art
In general, many mobile communication systems use a Code Division Multiple Access (CDMA) technique. For power control in the reverse link of a CDMA system, the base station measures the strength of the pilot signal transmitted from a mobile station and compares the measured value with a predetermined power control threshold. When the measured pilot signal strength is lower than the threshold, the base station generates a power-up command, and otherwise, when the measured pilot signal strength is higher than the threshold, the base station generates a power-down command. The base station transmits the power control command back to the mobile station in order to control transmission power of the mobile station transmitter. Generally, the power control threshold is set to a predetermined step value according to whether or not there exists a frame error after channel decoding.
In a system where the mobile station and the base station exchange data at certain frequencies, there are times when the frequency being used needs to be changed. The process for changing the frequency is referred to as inter-frequency handoff. Cases of inter-frequency handoff include (1) when the mobile station changes to a frequency of a base station other than the present base station in service, (2) when the mobile station changes from the present frequency to another frequency because of a lack of system capacity for the present frequency or the present frequency has a bad channel environment, and (3) when the mobile station changes from one communication technique to another communication technique.
As one of the initial steps in performing inter-frequency handoff, the base station sends a message commanding the mobile station to measure the channel condition of the new frequency. Upon receipt of the message, the mobile station changes (or shifts) the transmission/reception (Tx/Rx) frequency to the other frequency, measures the strength of the received signal at the changed frequency (or measures E
c
/I
or
, the energy to interference ratio of the received signal), and then returns to the original frequency. This may occur within a specific frame or over two frames. When measuring the channel condition of the different frequency, transmission of the signal on the original frequency will be interrupted for a moment due to the change of the frequency. This is because the mobile station, using a single frequency synthesizer, simultaneously changes the Tx frequency and the Rx frequency. In order to compensate for the energy loss due to the interruption in signal transmission at one portion within a frame, the mobile station increases transmission power at the other portion within the frame. Here, losing a certain duration within the frame in order to measure the channel condition of the different frequency is referred to as slotted mode (or compressed mode). In slotted mode, the data rate is increased by controlling the spreading factor (SF) at the remaining data transmission duration within the frame. This allows the transmission of data, which would have been transmitted during the non-transmission duration or interruption, during the remaining data transmission duration. In this case, the transmission power at the transmission duration is increased to compensate for the energy loss at the non-transmission duration.
Thus, because the transmission power temporarily increases at the portions of the frame other than the duration where the mobile station measures the channel condition of a target frequency, the power control performed between the base station and the mobile station needs to be changed. The power control process will become more apparent from the following description. Herein, the first frequency refers to the frequency at which the mobile station transmits presently, and the second frequency refers to the target frequency to which the mobile station will move when it performs the inter-frequency handoff.
The base station instructs the mobile station to measure the channel condition of the second frequency, and the mobile station interrupts transmission during a specific part of the frame in order to measure the channel condition of the second frequency according to the base station's instruction. However, the base station cannot know the exact starting point and duration of the interrupted transmission. That is, because the mobile station measures the channel condition of the second frequency unilaterally without forewarning the base station as to the starting point and duration of the interrupted transmission, the base station does not know the exact transmission-off or interruption duration. The base station will receive all the frame data, including the pure interference signal received during the transmission-off duration, and transmit the entire frame to the channel decoder. Therefore, a receiver of the base station will decode the pure interference signal, mistaking the interference signal for traffic data. In this case, the error rate of the decoded traffic data increases as compared with the case where the portion corresponding to the interference signal is set to ‘0’ indicating a undecided value. The undecided value to is set to ‘0’ on the assumption that the transmitted values are +1 and −1 (‘0’ having been binary-mapped to ‘+1 ’ and ‘1’ to ‘−1’ before transmission). To prevent this, the base station and the mobile station exchange information about the exact starting point and duration (t
search
[ms]) of the transmission-off duration through signaling (hereinafter, the information exchanged between the base station and the mobile station will be referred to as inter-frequency hard handoff signaling). The mobile station changes from the first frequency to the second frequency according to the inter-frequency hard handoff signaling, and measures the channel condition of the second frequency.
FIG. 1
shows the variation in mobile station transmission power during the initial stages of an inter-frequency handoff according to the prior art. The term “regular power control” as used herein means that the mobile (or base station) increases or decreases the transmission power by a predetermined step size according to a power control command transmitted from the base station (or mobile station). For example, the predetermined step size can be set to ±0.25, 0.5 1.0 or 2.0 dB. Power control is performed on a power control group (PCG) unit basis, wherein the frame length is a multiple of the power control group.
Referring to
FIG. 1
, the mobile station transmits a signal by performing regular power control until time A, when it increases the transmission power by &Dgr;
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[dB ], in order to compensate for the loss of the transmission signal during the transmission-off duration t
search
[ms] when the channel condition of the second frequency is measured. During frame durations A-D, the base station maintains the previous threshold for the received pilot signal power even though there is a transmission-off duration during this time period. Therefore, when the mobile station increases the transmission power at time A, the pilot signal power received at the base station after time A is higher than the threshold, so that the base station continuously sends a power-down command to the mobile station. If the mobile station obeyed the power-down command, it would be meaningless to increase the transmission po
Ahn Jae-Min
Kang Hee-Won
Kim Jae-Yoel
Park Chang-Soo
Dilworth & Barrese LLP
Hunter Daniel
Iqbal Khawar
Samsung Electronics Co,. Ltd.
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