Patent Application
20080095133
In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which
In the following, preferred embodiments of the invention will be described with reference to a third generation mobile communications system, such as the UMTS (Universal Mobile Communications System). This invention is not, however, meant to be restricted to these embodiments. Consequently, the invention may be applied in any cellular communications system that provides packet switched radio service capable of layer-1 common control signalling. Examples of other systems include the IMT-2000 and its evolution techniques (such as Beyond-3G including LTE (3.9G) and 4G). The specifications of mobile communications systems advance rapidly. This may require additional changes to the invention. For this reason, the terminology and the expressions used should be interpreted in their broadest sense since they are meant to illustrate the invention and not to restrict it. The relevant inventive aspect is the functionality concerned, not the network element or equipment where it is executed.
The present application relates to inter-cell interference (ICI) from layer-1 (L1) common control channels to layer-1 traffic channels. It discloses a method for avoiding interference in a traffic channel (or a shared data channel in an LTE system) from layer-1 common control channels of neighbour cells. Herein, the layer-1 common control channels refer to a L1 Synchronization (Sync) Channel, L1 Broadcast Channel (BCH), L1 Pilot Channel, and/or L1 Shared Control Channel.
In prior art systems, if the second user terminal UE2 is served by the third base station BS3, scheduled data transmission from BS3 to UE2 is severely interfered by common control channel signalling transmitted at maximum power from the second base station BS2. (Because the common control channels are usually modulated/coded in a robust fashion the interference caused by scheduled data transmission to common control channels is a minor issue). Inter-cell interference caused in prior art systems to scheduled data transmission from BS3 to UE2 by common control channel signalling from BS2 is further illustrated in
The present solution is intended for asynchronous systems, but it may also be utilized in synchronous systems. In the scope of long term evolution (LTE) discussions in the 3GPP, the common control channels usually have fixed positions in the radio frames. For example, the 1st symbol of the radio frame is reserved for an L1 pilot channel, and the centremost 1.25 MHz is reserved for an L1 synchronization channel. The synchronization channel appears at the end of the sub-frame in every 4 sub-frame in LTE. However, in the following, the present solution will be explained in terms of an L1 pilot channel and/or an L1 shared control channel that appear at the beginning of every radio frame. The present solution can be applied to an L1 synchronization channel and L1 broadcast channel (BCH) as well, as the user terminal obtains the timing position of the L1 synchronization channel of the neighbour cells when carrying out the handover measurements.
When a mobile user terminal UE2 is located within the coverage of both a serving cell C3 and a neighbour cell C2, the user terminal UE2 carries out handover measurements. When carrying out the handover measurements, UE2 has to synchronize itself to the neighbour cell C2 and measure levels of handover measurement quantities (e.g. PSSI (Pilot Signal Strength Indicator)). The synchronization enables the user terminal to detect the timing difference (and/or frame offset) between the neighbour cell C2 and the serving cell C3, as UE2 obtains a frame and/or symbol timing (i.e. the timing of a certain symbol, or the timing of a certain frame, or both) of both the serving cell C3 and the neighbour cell C2.
According to a conventional handover measurement procedure, the user terminal UE2 reports the measured levels of handover measurement quantities (e.g. PSSI) to the serving BS3. The reporting is carried out every 200 ms, for example. In the present solution, UE2 is further arranged to calculate a frame offset and report it to the serving BS3, for example, in an RRC message related to the handover measurement procedure. The obtained frame offset does not have to be very accurate; for example, a symbol-based accuracy may be enough. Therefore, assuming that a single radio frame (sub-frame in LTE systems) contains 7 symbols, 3 bits are enough for indicating relative positions. In order to carry out mobility measurements, the user terminal is arranged to identify and/or measure the signal strength and/or the timing of the neighbour cell and the serving cell.
On the basis of the frame offset information received from UE2, the serving BS3 stores and/or updates a relative frame offset between BS3 and BS2. Table 1 shows a frame offset table that can be maintained at the serving BS3 for each user terminal and/or each group of user terminals. Since L1 common control channels are transmitted at the beginning of a radio frame (usually as the 1st symbol of the frame), the serving BS3 obtains the timing positions of the common control channel of the neighbour cells by using the frame offsets. Table 1 is an example of a frame offset table (at symbol accuracy) maintained in the serving BS3 for individual user terminals or individual groups of user terminals. Here “0” implies that the timing of the serving BS3 and the neighbouring base station match each other (in full synchronization) for the user terminal (or group of user terminals). It is assumed that a radio frame consists of 7 symbols. The idea is that non-suitable time-slots can be detected, and they are marked with “X”.
The user terminals in the frame offset table may also be selected such that the user terminal is included in the frame offset table if the strongest neighbouring base station is within an x dB window relative to its serving base station. Here x dB can be a selected implementation parameter (which is not necessarily a handover window parameter).
When a power sequence operates at maximum power level in certain time-frequency resource blocks, the serving base station BS3 checks the frame offset table before scheduling the user terminals. The base station BS3 scheduler implements an “inverse muting” action according to the present solution, wherein BS3 avoids scheduling “collision” time-frequency resource blocks (or symbols) for the user terminal (or the group of terminals), in which blocks the user terminal would suffer from inter-cell interference caused by a common control channel of a neighbour cell. Instead, BS3 schedules these symbols for another user terminal (or another group of terminals).
According to an embodiment, the second symbol in the radio frame are not scheduled for UE2 (or UE group 2), but other symbols in the radio frame are scheduled for UE2 (or UE group 2) instead. In that case, it is not necessary to change the power sequence itself. For those time-frequency resource blocks that do not transmit at maximum power, the BS3 scheduler does not check its frame-offset table but schedules the users normally.
It should be noted that the present solution is also applicable to systems in which no power sequence is applied. In that case, the implementation is as follows: when the base station is ready for scheduling a new frame, the base station obtains a relative timing position of a common control channel of neighbour cells via user terminal measurements. Then, the base station avoids a “collision” timing with common control channels of the neighbour cells when scheduling the user terminals.
The present solution is primarily intended for operations on the physical layer (i.e. on the layer-1 packet scheduler) and for radio resource management (RRM). The present solution enables improving cell throughput by avoiding a “collision” timing with common control channels of neighbour cells when scheduling the user terminals. The present solution does not require changing the static soft-reuse IC schemes (such as PSEQ-IC). No new signalling is required between neighbouring base stations. Existing prior art signalling related to the handover measurement procedure can also be utilized between the base station and the user terminal. The present solution can also be implemented in systems that do not apply PSEQ-IC.
The items and steps shown in the figures are simplified and aim only at describing the idea of the invention. Other items may be used and/or other functions carried out between the steps. The items serve only as examples and they may contain only some of the information mentioned above. The items may also include other information, and the titles may deviate from those given above. Instead of or in addition to a base station, above described operations may be performed in any other element of a cellular communications system.
In addition to prior art means, a system or system network nodes that implement the functionality of the invention comprise means for processing information relating to reducing inter-cell interference as described above. Existing network nodes and user terminals comprise processors and memory that can be utilized in the operations of the invention. Any changes needed in implementing the invention may be carried out using supplements or updates of software routines and/or routines included in application specific integrated circuits (ASIC) and/or programmable circuits, such as EPLDs (Electrically Programmable Logic Device) or FPGAs (Field Programmable Gate Array).
It will be obvious to a person skilled in the art that as technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.
