The present invention relates generally to radio communication, and especially to improved measurement principles for cell selection and handover in MIMO (Multiple Input Multiple Output) radio communication systems.
In current radio communication systems, such as WCDMA (Wideband Code Division Multiple Access), cell selection and handover reporting are based on pilot measurements, such as SIR (Signal to Interference Ratio). The terminal is measuring the quality of e.g. the common pilot of a radio cell and this value is regularly reported to the RNC (Radio Network Controller), which then can take decisions on actions, such as adding or removing handover legs. In HSDPA (High Speed Downlink Packet Access) this type of measurement is also used for e.g. fast cell selection, i.e., the terminal can instantaneously connect to the base station that provides the “best” channel.
If and when multi-antenna systems such as MIMO (Multiple Input Multiple Output) are applied to e.g. HSDPA a single measurement as described above does not provide enough information to the system for optimal cell selection or handover. If several antennas are used at the base station and the common pilot is transmitted from a specific antenna, all quality measurements will reflect the potential transmission quality of that antenna only.
An object of the present invention is a method and apparatus for determining a more accurate measure representing transfer rate from a radio cell to a terminal in a MIMO system.
Another object is a cell selection support method and apparatus based on such measures.
Still another object is a handover support method and apparatus based on such measures.
These objects are achieved in accordance with the attached claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Briefly, the present invention is based on the idea of determining and combining, in a radio terminal, signal quality measures of pilot signals received from and associated with several antennas in a radio cell. Such combined measures from several radio cells are compared, and the radio cells associated with the best measures are used as candidates for cell selection or handover.
The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:
FIG. 1 illustrates a typical prior art radio communication system;
FIG. 2 is similar to FIG. 1, but illustrates the basic principles of the present invention;
FIG. 3 is a simplified block diagram of a radio terminal including an embodiment of a support apparatus in accordance with the present invention; and
FIG. 4 is a flow chart illustrating an embodiment of the support method in accordance with the present invention.
FIG. 1 illustrates a typical prior art radio communication system. A radio terminal MS is performing measurements for cell selection or handover on two different base stations BS1, BS2. These measurements are performed on one radio terminal antenna as indicated by the solid lines from the base stations to the mobile.
However, for a MIMO system the combined quality of all, or at least several, of the transmit antennas will contribute to the overall transmission capacity of a particular link. Thus, for such systems the prior art measurements are unreliable. For example, in a system with 4 transmit antennas at the base station side with the traditional common pilot transmitted from a pre-selected antenna, only the quality on this link will be used for cell selection or handover. However, it might happen that this link is in a deep fade towards base station BS1, and thus this leg will not be included in the handover set regardless of the quality towards all other antennas at base station BS1. The common pilot from the pre-selected antenna of base station BS2 might show a very high quality, and thus this link is added to the handover set even if the signal quality to the other antennas of base station BS2 is very poor.
FIG. 2 is similar to FIG. 1, but illustrates the basic principles of the present invention. Terminal MS now estimates the maximal supported data rate from all, or at least several, transmit antennas (as indicated by the solid lines), thereby reflecting the true capacity of the combined channel. Thus, the basic idea of the invention is that the radio terminal should use the pilots transmitted from all (or at least several) antennas to estimate, for example, the total transmission rate that can be decoded with a certain probability. This quantity is estimated for all cells that are within reach for the specific terminal and reported to the system. The system, e.g. the base station or RNC (Radio Network Controller) can then make the appropriate decision on cell selection and handover that maximizes the rate for a given terminal.
FIG. 3 is a simplified block diagram of a radio terminal including an embodiment of an apparatus in accordance with the present invention. In order to avoid cluttering of the figure, only elements that are needed to explain the present invention are included. Each radio terminal antenna is associated with a unit SINR-1, . . . , SINR-4 measuring the corresponding SINR (Signal to Interference plus Noise Ratio). These measurements are combined into a combined transfer rate measure in a summation unit SUM. The measurement procedure is repeated for all monitored radio cells RC-1, RC-2, . . . , and the resulting combined measures SUM-1, SUM-2, . . . together with the cell identifiers are stored in a memory MEM (for example implemented as a queue). A selection unit SEL selects the measures that are sufficiently high to support a predetermined target transfer rate by comparing the combined measures in memory MEM to a predetermined threshold TH. These radio cells and their associated combined measures are reported to the RNC for cell selection/handover.
The various units in the described embodiment of the present invention are typically implemented by a micro processor, a digital signal processor or a signal/micro processor combination and corresponding software, However an ASIC (Application Specific Integrated Circuit) is also feasible.
In the described embodiment the sum of measured SINR values from different antennas was used as a combined measure of transfer rate. Other possible measures are, for example, max SINR, sum SIR, max SIR, channel capacity, signal strength, etc. Another example is CQI (Channel Quality Indicator). This technique may report an MCS (Modulation Coding Scheme), which, if used, enables decoding of data with a predetermined block error rate, for example 10%.
Furthermore, in the described embodiment it was assumed that the radio terminal had the same number of antennas as the base stations. However, this is not a requirement of the present invention. For example, if the terminal has fewer antennas than a base station, the terminal may determine combined measures of different combinations of antennas.
In the embodiment illustrated in FIG. 3 there was a unit measuring SINR associated with each terminal antenna. However, it is also possible to use a single unit and switch this unit between the antennas.
In order to simplify the above description, it has been assumed that each base station (radio cell) is associated with a unique pilot signal that is the same for all antennas of the base station. However, although this is convenient for describing the basic concepts, it is not strictly necessary for practicing the invention. What is necessary is that the used pilot signals are known to the terminal and that the terminal is also aware of which pilot is associated with which base station. It would, for example, also be possible to associate each antenna in a base station with a unique pilot. Each of these pilots would, however, still be associated with the same radio cell.
FIG. 4 is a flow chart illustrating an embodiment of the method in accordance with the present invention. Step S1 determines SINR (or another measure, as described above) for different antennas of a radio cell. Step S2 combines these SINR values into a measure representing the supported total transfer rate characterizing the cell. Step S3 determines whether this measure exceeds a predetermined threshold TH. If the measure exceeds the threshold, step S4 reports the measure and a corresponding cell identifier to the RNC. The procedure then continues to step S5. If the measure does not exceed the threshold, the procedure continues directly from step S3 to step S5. Step S5 determines whether this is the last radio cell to monitor. If this is the case, the procedure ends (the procedure may of course be repeated at a later time). If this was not the last radio cell, the procedure selects the next cell in step S6 and wraps back to step S1.
An advantage of the described invention is that the system can base e.g. handover and/or cell selection on the actual total quality of the link and not only the quality between one antenna pair as in the prior art. Another advantage is that the invention is adaptable to receiver performance. Since e.g. the sum rate is used as a parameter, this means that terminals with different receiver implementations (for example a different number of antennas) can have different active sets even if they are located at the same geographical spot.
It will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departure from the scope thereof, which is defined by the appended claims.