Telecommunications – Transmitter and receiver at separate stations – With control signal
Reexamination Certificate
1999-07-02
2002-12-10
Vo, Nguyen T. (Department: 2682)
Telecommunications
Transmitter and receiver at separate stations
With control signal
C455S522000, C375S346000
Reexamination Certificate
active
06493541
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to wireless communications, and more specifically, to radio transmit power control in a mobile radio communications system.
BACKGROUND AND SUMMARY OF THE INVENTION
In cellular communications systems, the mobile radio station communicates over an assigned radio channel with a radio base station. Several base stations are coupled 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.
Due to the rapid expansion of wireless mobile communications and the need for wideband multimedia services, there is a continuing need to better utilize the available frequency bandwidth. A common strategy in Frequency Division Multiple Access (FDMA)/Time Division Multiple Access (TDMA) systems is to reuse the frequencies in the network. The challenge with frequency reuse is to counteract or at least reduce the interference between transmitters in the system using the same frequency by controlling the transmit power levels of the radio signals and by separating to the extent practical the transmitters by a sufficient geographic distance. The radio transmit power levels of the mobile stations and base stations are ideally lowered so that only the minimum transmission power necessary to maintain satisfactory call quality is used. By reducing mobile and base station transmission power, the other radio communicators experience lower interference which means that the system capacity can be increased. The capacity of a transmission power regulated system can arguably be increased by approximately 70% compared to an unregulated system. Another reason to maintain lower transmit power levels, at least for battery-operated mobile stations, is to reduce the energy consumed by mobile stations during transmissions.
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-hence the term “spread spectrum.” In other words, individual radio “channels” are discriminated upon the basis of these codes. Various aspects of CDMA are set forth in textbooks such as
Applications of CDMA and Wireless/Personal Communications
, GARG, Vijay K. et al, Prentice-Hall 1997.
Spread spectrum communications permit mobile transmitted signals 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 communication. This kind of handover is typically called diversity handover and includes both soft and softer diversity handover.
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 to as the “near-far effect.”
Interference is therefore a particularly severe problem in CDMA systems. 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's 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. Such a loss of system capacity to excessively “strong” mobile stations is unacceptable.
Additional problems are associated with excessive transmit 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. The major drain on a mobile's battery occurs during transmission. Thus, a significant objective for any power control approach, therefore, is to reduce transmit power where possible. Except for battery consumption, the above-described problems with setting transmission power also apply to downlink radio transmissions from base stations to mobile stations.
In some mobile radio communications systems, power regulation is performed relatively infrequently being based on the unrealistic assumption that the disturbance level is more or less constant. The mobile's transmission power is only coarsely controlled, with the objective being to maintain the received desired signal level over the interference level. In practice, however, radio conditions and interference levels vary considerably with both time and place so that infrequent power regulation is not optimal.
One common approach to power control is to try and balance the transmit power level of signals on each radio channel so that all mobile stations or base stations receive signals with the same Signal-to-Inference Ratio (SIR). For every traffic scenario, there is a maximum SIR that can be obtained at all radio receivers at the same time. Ideally, if all information is available at one location, a global or centralized power control scheme may be employed to determine and assign the various necessary transmission powers so this maximum is achieved. But this approach requires extensive overhead signaling to keep the centralized intelligence entity fully informed and up to date. Another approach, less onerous at least with respect to control signaling, is to perform power control in a distributed fashion using only local SIR type measurements.
In both approaches, the appropriate target SIR must first be determined. If the SIR target value is set too high, the radio transmit powers might be increased to maximum levels determined by the physical limits of the system without achieving the specified SIR target value. Should that be the case, “graceful degradation” by uniformly reducing transmit power levels of all active radios can be employed to minimize the impact on the quality of service provided.
A significant goal for most if not all radio transmit power control procedures is to keep the signal strength and/or quality of the signal detected by a receiver above a threshold without using unnecessarily high transmit power. In contrast to infrequent power regulation schemes mentioned above, most CDMA-based systems employ a relatively high sampling rate for the power control algorithm, e.g., 1600 times per second. To minimize overhead control signaling, only one bit is used to communicate power control adjustments to the radio transmitter.
In the IS-95 CDMA standard and similar systems, the power is stepwise increased or decrea
Gunnarsson Fredrik
Gustafsson Fredrik
Wiberg Niclas
Ly Nghi H.
Nixon & Vanderhye P.C.
Telefonaktiebolaget LM Ericsson (publ)
Vo Nguyen T.
LandOfFree
Transmit power control time delay compensation in a wireless... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Transmit power control time delay compensation in a wireless..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Transmit power control time delay compensation in a wireless... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2944098