Telecommunications – Transmitter and receiver at separate stations – With control signal
Reexamination Certificate
1999-04-09
2001-12-25
Bost, Dwayne (Department: 2681)
Telecommunications
Transmitter and receiver at separate stations
With control signal
C455S436000
Reexamination Certificate
active
06334047
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates radio transmission power control in a code division multiple access cellular radio communications system.
BACKGROUND AND SUMMARY OF THE INVENTION
In a cellular communications system, a mobile radio station communicates over an assigned radio channel with a radio base station. Several base stations are connected to a switching node which is typically connected to a gateway that interfaces the cellular communications system with other communication systems. A call placed from an external network to a mobile station is directed to the gateway, and from the gateway through one or more switching nodes to a base station which serves the called mobile station. The base station pages the called mobile station and establishes a radio communications channel. A call originated by the mobile station follows a similar path in the opposite direction.
In a Code Division Multiple Access (CDMA) mobile communication system, spreading codes are used to distinguish information associated with different mobile stations or base stations transmitting over the same radio frequency band. In other words, individual radio “channels” correspond to and are discriminated on the basis of these codes. Various aspects of CDMA are set forth in one or more textbooks such as
Applications of CDMA and Wireless/Personal Communications
, Garg, Vijay K. et al., Prentice-Hall 1997.
Spread spectrum communications permit mobile transmissions to be received at two or more (“diverse”) base stations and processed simultaneously to generate one received signal. With these combined signal processing capabilities, it is possible to perform a handover from one base station to another, (or from one antenna sector to another antenna sector connected to the same base station), without any perceptible disturbance in the voice or data communications. This kind of handover is typically called diversity handover.
During diversity handover, the signaling and voice information from plural sources is combined in a common point with decisions made on the “quality” of the received data. In soft handover, as a mobile station involved in a call moves to the edge of a base station's cell, the adjacent cell's base station assigns a transceiver to the same call while a transceiver in the current base station continues to handle that call as well. As a result, the call is handed over on a make-before-break basis. Soft diversity handover is therefore a process where two or more base stations handle the call simultaneously until the mobile station moves sufficiently close to one of the base stations which then exclusively handles the call. “Softer” diversity handover occurs when the mobile station is in handover between two different antenna sectors connected to the same, multi-sectored base station using a similar make-before-break methodology.
Because all users of a CDMA communications system transmit information using the same frequency band at the same time, each user's communication interferes with the communications of the other users. In addition, signals received by a base station from a mobile station close to the base station are much stronger than signals received from other mobile stations located at the base station's cell boundary. As a result, distant mobile communications are overshadowed and dominated by close-in mobile stations which is why this condition is sometimes referred as the “near-far effect.”
The physical characteristics of a radio channel vary significantly for a number of reasons. For example, the signal propagation loss between a radio transmitter and receiver varies as a function of their respective locations, obstacles, weather, etc. As a result, large differences may arise in the strength of signals received at the base station from different mobiles. If the transmission power of a mobile station signal is too low, the receiving base station may not correctly decode a weak signal, and the signal will have to be corrected (if possible) or retransmitted. Accordingly, erroneous receipt of the signals adds to the delay associated with radio access procedures, increases data processing overhead, and reduces the available radio bandwidth because erroneously received signals must be retransmitted. On the other hand, if the mobile transmission power is too high, the signals transmitted by the mobile station create interference for the other mobile and base stations in the system. Ideally, all mobile-transmitted signals should arrive at the base station with about the same average power irrespective of their distance from the base station.
Interference is a particularly severe problem in CDMA systems because large numbers of radios transmit on the same frequency. If one mobile station transmits at a power output that is too large, the interference it creates degrades the signal-to-interference ratio (SIR) of signals received from other mobile radios to the point that a receiving base station cannot correctly demodulate transmissions from the other mobile radios. In fact, if a mobile station transmits a signal at twice the power level needed for the signal to be accurately received at the base station receiver, that mobile signal occupies roughly twice the system capacity as it would if the signal were transmit at the optimum power level. Unregulated, it is not uncommon for a strong mobile station to transmit signals that are received at the base station at many, many times the strength of other mobile transmissions. The loss of system capacity to such excessively “strong” mobile stations is unacceptable.
Additional problems are associated with transmitting with too much power. One is the so-called “party effect.” If a mobile transmits at too high of a power level, the other mobiles may increase their respective power levels so that they can “be heard” compounding the already serious interference problem.
Another problem is wasted battery power. It is very important to conserve the limited battery life in mobile radios. By far, the largest drain on a mobile's battery occurs during transmission. A significant objective for any power control approach, therefore, is to reduce transmit power where possible without increasing the number of retransmissions to an unacceptably high level as a consequence of that power reduction. Except for battery consumption, the above-described problems with setting transmission power also apply to downlink radio transmissions from base stations.
Transmit power control (TPC) is therefore important in any mobile radio communications system, and is a particularly significant factor in improving the performance and capacity of a CDMA system. In uplink TPC, the mobile station attempts to control its transmit power based on the power control messages sent to the mobile station from the base station with the goal of controlling the power level of signals received at the base station within a relatively small tolerance, e.g., 1 dB for all mobile station transmissions received at that base station.
More specifically, transmit power control strives to keep the received carrier-to-interference ratio (CIR) close to a target CIR. Alternate measures of signal quality may also be used such as received signal-to-interference ratio (SIR), received signal strength (RSSI), etc. The carrier-to-interference ratio actually received at a base station or mobile station depends on the received carrier power and the current interference level. Received carrier power corresponds to the transmit power level P
tx
minus the path loss L. The path loss L may also be represented as a negative gain. Such a gain factor includes two components for a radio channel: a slow fading gain G
s
, and a fast fading gain G
f
. The interference from other users in the CDMA system also depends on the spreading factor employed by other transmitters. Accordingly, the carrier-to-interference ratio may be roughly determined in accordance with the following:
CIR
=
P
i
G
i
∑
k
=
otherusers
P
k
G
k
SF
k
+
N
(
1
)
where P corresponds to the transmit
Andersson Christoffer
Ericson Mårten
Bost Dwayne
Contee Joy
Nixon & Vanderhye P.C.
Telefonaktiebolaget LM Ericsson (publ)
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