Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-11-07
2004-09-28
Chin, Vivian (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S422100, C455S423000, C455S424000, C455S434000, C455S436000, C455S442000, C455S522000, C455S063100, C455S067110, C455S561000, C370S310000
Reexamination Certificate
active
06799045
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to mobile radio communications, and more particularly, to reliable congestion control in a CDMA-based mobile radio communications system.
BACKGROUND AND SUMMARY
In a mobile radio communications system, a mobile radio station communicates over an assigned radio channel with a radio base station. Several base stations are usually connected to a switching node, which is typically connected to a gateway that interfaces the mobile radio communications system with other communications 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 nodes to a base stations 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; however, paging is not performed.
In a spread spectrum, Code Division Multiple Access (CDMA) mobile communications system, spreading codes are used to distinguish information associated with different mobile stations or base stations transmitting over the same frequency band. In other words, individual radio “channels” correspond to and are discriminated on the basis of these codes. Each coded signal overlaps all of the other coded signals as well as noise-related signals in both frequency and time. By correlating a composite signal with one of the distinguishing spreading codes, the corresponding information can be isolated and decoded.
Spread spectrum communications permit mobile station 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 without any perceptible disturbance in the voice or data communications. This type of handover is typically called diversity handover and may include a “soft” handover between two base stations and a “softer” diversity handover between two different antenna sectors connected to the same, multi-sectored base station.
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 other users. In addition, signals received by a base station from a mobile station that is close to the base station are much stronger than signals received from other mobile stations located at the base station cell boundary. As a result, close-in mobile stations may overshadow and dominate more distant mobile communications, which is why this condition is sometimes referred to as the “near-far effect.” Thus, control of mobile transmit power level is important in order to prevent such near-far effects. Power control is also needed to compensate for changing physical characteristics of a radio channel. Indeed, the signal propagation loss between a radio transmitter and receiver varies as a function of their respective locations, obstacles, weather, etc. Consequently, large differences may arise in the strength of signals received at the base station from different mobiles.
Ideally, all mobile-transmitted signals should arrive at the base station with about the same average power irrespective of the path loss to the base station. By regulating transmit power to the minimum necessary to maintain satisfactory call quality, capacity at the mobile radio communications system can be increased approximately seventy percent as compared with an unregulated system, (assuming that all the calls or connections have the same target signal-to-interference ratio). In addition, mobile stations consume less energy when transmit power levels are maintained at a lowest possible level, thereby reducing battery drain which results in mobile stations lighter in weight and smaller in size.
If the transmission power from a mobile signal is too low, (for whatever reason), the receiving base station may not correctly decode a weak signal, and the signal will have to be corrected (if possible) or retransmitted. Erroneous receipt of signals adds to the delay associated with radio access procedures, increases signal 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.
A significant problem in CDMA systems with transmitting too much power is the so-called “party effect.” If one mobile transmits at too high of a power level, (a person is talking too loudly at a party), the other mobiles may increase their power levels so that they can be “heard,” (over the loud talker), compounding the already serious interference problem. As each mobile increases its transmit power, (becomes a loud talker), the other mobiles react by raising their transmit powers. Soon all mobiles may be transmitting at maximum power with significantly degraded service and diminished capacity. Thus, while transmit power control is important in any mobile radio communications system, it is particularly important to the performance and capacity of a CDMA-based mobile radio communications system.
One parameter affecting the capacity of a CDMA-based system that can be measured by a base station is the total uplink (from mobile station-to-base station) interference level at the base station. The uplink interference includes the sum of all radio beams that reach a receiver in the base station for a specific radio frequency carrier, plus any received noise or interference from other sources. Because of the importance of interference level to the capacity of the CDMA-based radio network, a radio network controller normally receives measurement reports from radio base stations including periodic uplink interference and downlink power measurements. These measurement reports may be used by call admission and congestion control functions of the radio network controller. If the downlink and uplink interference levels are sufficiently low, the admission control function may “admit” a new call request and allocate the appropriate radio resources, assuming other conditions are met, e.g., there are sufficient radio resources currently available. However, if there are insufficient resources or the cell is at capacity or in an overload condition, the admission control function may restrict or reduce the amount of traffic and thereby interference. For example, new mobile connection requests may be rejected, data throughput may be reduced, data packets delayed, handovers to other frequencies/cells forced to occur, connections terminated, etc. Of course, these types of actions should be employed only where necessary; otherwise, the cellular network services and capacity are unnecessarily reduced.
Accordingly, it is an important goal in a cellular radio system to optimize the capacity of a particular cell without overloading that cell. Some type of metric is needed that provides an accurate measurement or other indicator of the current capacity, congestion level, or load in a cell. One possible metric is received signal strength as measured by the base station. Measurement of total received signal strength can be made using some sort of power sensor such as a diode or a resistor. For example, the total received voltage detected across the diode or the total heat generated by the resistor can be used to indicate the total received signal strength.
Unfortunately, a limitation with this measurement-based metric is accuracy. It is very difficult to accurately measure total uplink received power using these types of sensors because the outputs of such sensors change with temperature, aging, component tolerances, etc. Thus, while a desired measurement accuracy of the total uplink received power or interference level may be +/−1 dB (or less), the actual measurement acc
Chin Vivian
Persino Raymond B.
Telefonaktiebolaget LM Ericsson (publ)
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