Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
2000-02-28
2004-11-23
Chin, Vivian (Department: 2682)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S442000, C455S436000, C370S318000
Reexamination Certificate
active
06823193
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the control of power levels of transmitted signals in telecommunication systems, such as spread spectrum multiple access systems. In particular, it relates to control of transmit power levels of base stations involved in diversity communication with a mobile station.
BACKGROUND AND SUMMARY OF THE INVENTION
Transmit power control is important to communication systems having many simultaneous transmitters to reduce the mutual interference of such transmitters. It is particularly important in communication systems that use code division multiple access (CDMA) to obtain high system capacity. Power control is important for the uplink, i.e., for transmissions from a mobile station to the radio network including several base stations, and for the downlink, i.e., for transmissions from one or more base stations to a mobile station.
Uplink power control may be provided by a closed-loop method where a base station measures the strength of a signal received from a mobile station and then periodically transmits one transmit power control (TPC) bit to the mobile station every certain time period, e.g., 1.25 milliseconds. Based on the received power control bit, the mobile station increases or decreases its transmit (uplink) power by a predetermined amount. On the downlink, one power control approach is for the mobile terminal to measure a received downlink power level from a base station and report the measurement back to the base station, which might adjust its transmit power in predetermined circumstances.
Downlink power control in a system compliant with the IS-95-A standard is based on frame error rate (FER) measurements by the mobile station, which sends FER reports to the system. Sections 6.6.4.1.1 and 7.6.4.1.1 of the IS-95-A standard note that such FER reports can be sent when a threshold has been crossed and/or periodically. Typically, an FER report may be sent every 1-5 seconds. One problem with this method is that it can take a long time (i.e., several seconds) to accumulate the appropriate FER statistics. As a result, Rayleigh fading and shadow fading are not tracked. In fact, the IS-95-A method has proved to be so slow that it is usually does not provide much added benefit compared to not using downlink power control.
Some personal communications systems (PCS) using CDMA are similar to the cellular IS-95 standard in that the mobile station reports downlink frame errors whenever they occur. This puts the radio network in control of frame errors, but it still takes a long time to accumulate the appropriate statistics. In other communication system concepts like the UMTS Code Division Testbed (CODIT), the signal quality is determined by estimating the raw bit error rate (BER) instead of the FER. Hence, good statistics can be obtained faster, and a mobile station sends BER reports to the network more often (i.e., 1-10 times per second). System performance is improved in comparison to a system using downlink transmit power control according to the IS-95-A standard, but the CODIT method is still too slow to handle Rayleigh fading.
The mobile terminal may also measure the downlink signal- to- interference ratio (SIR) of the signal received from the base station and transmit an appropriate power control command on the uplink back to the base station. Each power control command may be an uncoded, single bit in order to minimize signalling overhead. Assuming a smaller frame length than that of IS-95-A, a bit rate of several hundred kilobits per second, a wider channel bandwidth of 5 MHZ, and a CDMA chip rate of four million chips per second, such a downlink power control approach tracks Rayleigh fading fairly well.
The situation is more complicated if the mobile terminal is in a “soft” diversity handover mode. Soft handover is also called macrodiversity and describes a situation where the mobile terminal communicates with two or more base stations simultaneously. Soft handover is described in U.S. Pat. No. 5,109,528 to Uddenfeldt and U.S. Pat. No. 5,327,577 to Uddenfeldt, both of which are incorporated here by reference.
When the mobile terminal is not in soft handover mode, the error rate of the uncoded power control commands should be small, e.g., about one percent, in normal transmission conditions. But the error rate of the uncoded downlink power control commands becomes a more serious problem when the mobile terminal is in soft handover. Because there are different paths or links between the mobile station and each of the handover base stations, and different conditions affect each of those paths, the errors impacting those commands vary across the radio links. These errors are typically independent so that different errors will likely differently impact the same command sent to each base station involved in the soft handover. A large number, e.g., 1500, of power control commands per second may be sent to two base stations involved in a soft handover in order to track rapid channels changes, e.g., Rayleigh fading. The result is that the average transmit powers of each of the base stations involved in the diversity handover may “drift” apart from where they should be. As a result, the commanded transmit power levels of the base stations drift apart to suboptimal levels from a system-capacity point of view.
The lost capacity occurs because at least one of the base stations in communication with a mobile station in soft handover mode will transmit at a power level that is too high. The magnitude of the difference between one downlink power level and another downlink power level affects the system's capacity because one base station's transmissions look like interference to other base stations. In addition, the rate that each downlink power level drifts is important because faster rates usually require the control commands to be issued more frequently. Higher frequency control commands generally increase the messaging load that must be carried by the control links between the base stations BS
1
, BS
2
and the RNC.
Several ways to combat base station power drift include reducing the probability of transmit power control command errors by sending fewer such commands, increasing bit redundancy of the transmit power commands, or reducing the size of the power adjustment made at the bast station in response to each command. Unfortunately, all of these approaches are not completely satisfactory because rapid changes in the radio channels are not tracked.
Alternatively, the power drift may be reduced by increasing control signaling between the base stations via one or more radio network control nodes to synchronize the downlink transmit powers of those base stations. Namely, the power transmission level of each base station in the diversity handover is compared to a power reference or threshold common to all the base stations in the diversity handover. The difference between the transmit power of each base station and the reference power is used to correct the transmit power level of that base station. Because the power correction depends on the difference between the actual transmit power at the base station and the common reference power, the various transmit powers of the different base stations in the diversity handover converge relatively quickly. Thus, even if the transmit power command from the mobile station is received in error in one or more of the base stations, the power correction based on the comparison to the common reference power compensates for such errors.
A disadvantage with this alternative is the increase in control signaling. This becomes even more of a problem the more frequently the reference power is updated. However, more frequent updates are desirable to track rapid channel changes.
Another problem with this alternative is that updating a single reference power for all the soft handover base stations does not permit the flexibility to set different diversity handover power references for different base stations. For example, it may be desirable to set the reference power of a dominant or favored base stat
Andersson Andreas
Corbett Eddie
Lieshout Gert-Jan van
Persson Magnus
Chin Vivian
Nixon & Vanderhye P.C.
Persino Raymond
Telefonaktiebolaget LM Ericsson (publ)
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