In cellular radio communication systems, it is desired to control uplink load in cells of the radio communication system in order to achieve desired coverage and stability of the cells. The uplink load is often measured in terms of power received by a radio base station of the cellular communication system.
In a known High Speed Packet Access (HSPA) system, uplink load control is managed by a Node B. In an exemplifying configuration, the Node B comprises one or more rake receivers. On the HSPA uplink, user equipments share the same time and frequency resource. Therefore, when the Node B detects a signal from a specific user equipment, the received power of other user equipments at the Node B is regarded as interference to the specific user equipment. In other words, the total received power at the Node B is regarded as a cell load. When the total received power is high, the cell load is high.
In practice, when the Node B performs uplink load control for coverage and stability, the Node B estimates a rise over thermal for coverage and a noise rise for stability to obtain measures of the cell load.
The rise over thermal (RoT) of the cell, which is the total received power over the thermal noise floor power is given by:
                    η        =                              I            tot                    N                                    (        A        )            
The total received power, Itot, in a cell consists of uplink power from users in the own cell, Iown, uplink Wideband Code Division Multiple Access (WCDMA) radio link power from users in the neighbour cells, Inei, as well as the thermal noise floor power N, thus,Itot=Iown+Inei+N  (B)
Considering a user equipment at the cell border attempting to connect to the cell, the total received power from all of the users at the Node B is interference to this user equipment. If the interference is too high, the limited power of the user equipment may not be able to ensure a successful connection to the Node B. This results in a coverage problem. Therefore, a purpose of load control of High Speed Uplink Packet Access (HSUPA) is to control the total received power at the Node B to be below a coverage limit such that the user equipment at the cell border is able to connect to the cell when desired. The limit depends on which size, it is desired that the cell has: a lower limit for a larger cell size, and vice versa.
The noise rise for stability, λ, is determined by subtracting the neighbour cell interference contribution from Itot. The following equation thus applies:
                    λ        =                                                            I                tot                            -                              I                nei                                      N                    .                                    (        C        )            
The noise rise for stability is compared to a stability limit. The reason is that if the load in the cell is too high the interference between users will cause power rushes in the system. The power rushes occurs when user equipments increase their transmit power in an uncontrolled manner. In more detail, consider a user equipment which increases its power, which then causes Signal-and-Interference-to-Noise-Ratio (SINR) of other user equipments to be reduced. These user equipments will then increase their transmit power in response to the reduced SINR. This causes SINR for all other user equipments to be further reduced. Again, all these other user equipments will increase their transmit power in response to the reduced SINR. As a result, the uncontrolled power rushes occur in situations where it is not feasible to maintain all scheduled communication resources.
Referring back to the cell load, a difference between the cell load and the limits for coverage and stability is referred to as a power headroom. See FIG. 1. The power headroom, or load headroom, is measured at an air interface of the receiver of the Node B. The Node B comprises a scheduler that aims at filling the load headroom of the air interface such that requests, from user equipments, for different bit rates are met. As stated above, the air-interface load in WCDMA is typically determined in terms of the rise over thermal for coverage and the noise rise for stability.
The scheduler performs scheduling decisions, e.g. determines uplink grants for each user equipment requesting a certain bit rate in order to perform uplink load control, referred to as an uplink load control procedure herein, as initially mentioned. In the uplink load control procedure, the scheduler distributes resources among the user equipments. When evaluating scheduling decisions, the scheduler predicts the load that results from uplink grants scheduled to the user equipments in the cell. Then, the scheduler assures that the scheduled load does not exceed the limits for coverage and stability (or load thresholds for coverage and stability).
Now consider a known Node B comprising multi-stage receiver for cancelling interference in multiple stages. A problem in relation to Node Bs comprising multi-stage receivers, such as an interference cancelling turbo receiver (Turbo-IC receiver), is then that uplink load control is not sufficiently efficient.