A communication scheme succeeding the WCDMA scheme and the HSDPA scheme, namely, the LTE scheme is discussed by a standardization group 3GPP for the WCDMA scheme.
As radio access schemes for the LTE scheme, the OFDMA (Orthogonal Frequency Division Multiplexing Access) and the SC-FDMA (Single-Carrier Frequency Division Multiple Access) are used for downlink and uplink, respectively.
The OFDMA is a scheme where a certain frequency band is divided into narrower frequency bands (subcarriers) and these frequency bands carry data for transmission. According to the OFDMA, it is possible to achieve faster transmission and improve frequency utilization efficiency by arranging the subcarriers in the frequency band so densely that the subcarriers can partially overlap one another, without having mutual interference of the subcarriers.
Meanwhile, the SC-FDMA is a transmission scheme where a certain frequency band is divided and thus different frequency bands are used for transmission among a plurality of mobile stations, so that interference among the plurality of mobile stations can be reduced. According to the SC-FDMA, it is possible to achieve low power consumption and wide coverage for the mobile stations, since the SC-FDMA is characterized in that transmission power varies to a lesser extent.
The LTE scheme is a mobile communication system configured to perform communication while sharing one or more physical channels among the plurality of mobile stations for both of the uplink and the downlink. The channels shared by the plurality of mobile stations are generally referred to as shared channels.
In the LTE scheme, a shared channel (a physical channel) of the uplink is referred to as a “physical uplink shared channel (PUSCH)”, and a shared channel (a physical channel) of the downlink is referred to as a “physical downlink shared channel (PDSCH)”.
Meanwhile, a shared channel (a transport channel) of the uplink is referred to as an “uplink shared channel (UL-SCH)”, and a shared channel (a transport channel) of the downlink is referred to as a “downlink shared channel (DL-SCH)”.
Moreover, in the mobile communication system utilizing the above-described shared channels, it is necessary to perform signaling for each sub-frame (1 ms in the LTE scheme) to determine to which mobile station the above-described shared channels should be allocated.
In general, the above-mentioned sub-frame may also be referred to as a “TTI (transmission time interval)”.
In the LTE scheme, a control channel used for such signaling is referred to as a “physical downlink control channel (PDCCH)”.
Note that the above-mentioned PDCCH may also be referred to as a “downlink L1/L2 control channel (DL-L1/L2 control channel)” or as “downlink control information (DCI)”.
For example, information transmitted on the physical downlink control channel includes “downlink scheduling information”, an “uplink scheduling grant”, a “transmission power control command bit”, and the like.
Meanwhile, a HARQ indicator (ACK (acknowledgement)/NACK (negative acknowledgement)) concerning an uplink shared signal is transmitted on the downlink via a HARQ indicator channel.
As a physical channel, such a HARQ indicator channel is a “physical hybrid ARQ indicator channel (PHICH)”.
The downlink scheduling information and the uplink scheduling grant described above correspond to the information for signaling to determine to which mobile station the above-described shared channels should be allocated.
For example, the above-described downlink scheduling information includes “allocation information of a downlink resource block”, an “ID of a mobile station UE”, the “number of streams”, “information on a precoding vector”, a “data size”, a “modulation scheme”, “information on HARQ (hybrid automatic repeat request)”, and the like concerning the downlink shared channel.
Note that the above-mentioned downlink scheduling information may also be referred to as “downlink scheduling grant” or as “downlink assignment information”.
For example, the above-mentioned uplink scheduling grant includes “allocation information of an uplink resource”, an “ID of a mobile station UE”, a “data size”, a “modulation scheme”, “uplink transmission power information”, “information on a demodulation reference signal of uplink MIMO”, and the like concerning the uplink shared channel.
Incidentally, the uplink of the LTE scheme applies synchronous hybrid automatic repeat request (synchronous HARQ) as a scheme of HARQ.
That is to say, as shown in FIG. 1, the uplink shared signal is retransmitted at a predetermined timing starting from a timing of initial transmission, or more specifically, on a predetermined cycle through the uplink shared channel.
In FIG. 1, the uplink shared signal is retransmitted on a cycle of every eight sub-frames. Here, the uplink shared signal may be retransmitted on a cycle other than every eight sub-frames.
Meanwhile, retransmission of the uplink shared signal is instructed from a base station apparatus to the mobile station by using the HARQ indicator or the uplink scheduling grant.
When retransmission of the uplink shared signal is instructed by way of the HARQ indicator, the mobile station retransmits the uplink shared signal by using the resource block and the modulation scheme which are the same as those used in a previous transmission.
On the other hand, when retransmission of the uplink shared signal is instructed by way of the uplink scheduling grant, the mobile station retransmits the uplink shared signal by using the resource block and the modulation scheme which are specified by the uplink scheduling grant.
The HARQ control on the uplink in the LTE scheme will be described in more detail by use of FIG. 2. FIG. 2 shows an example of HARQ processing on the uplink.
As shown in FIG. 2, at reference numeral 202 (a sub-frame #i) (where i is an integer satisfying i>0), the base station apparatus uses the uplink scheduling grant on the physical downlink control channel and instructs the mobile station to perform communication using the uplink shared channel at a sub-frame #i+4.
At reference numeral 204 (the sub-frame #i+4), the mobile station transmits the uplink shared signal to the base station apparatus, and the base station apparatus receives the uplink shared signal and attempts decoding thereof.
At reference numeral 206 (a sub-frame #i+8), the base station apparatus transmits either the HARQ indicator or the uplink scheduling grant based on a decoding result.
To be more precise, the base station apparatus transmits a HARQ indicator (ACK), when the decoding result of the uplink shared signal is OK.
Alternatively, the base station apparatus may newly transmit the uplink scheduling grant instructing transmission of the uplink shared signal, when data that are supposed to be newly transmitted exist in a transmission buffer of the mobile station.
On the other hand, when the decoding result of the uplink shared signal is NG, the base station apparatus either transmits a HARQ indicator (NACK) or transmits uplink scheduling grant instructing retransmission of the uplink shared signal.
At reference numeral 206 (the sub-frame #i+8), when NACK is transmitted via the HARQ indicator channel or the uplink scheduling grant instructing retransmission of the uplink shared signal is transmitted, the mobile station retransmits the uplink shared signal at a sub-frame #i+12 (reference numeral 208).
Here, at reference numeral 206 (the sub-frame #i+8), when ACK is transmitted via the HARQ indicator channel or when the uplink scheduling grant instructing transmission of a new uplink shared signal is transmitted, the uplink shared signal transmitted at reference numeral 204 is not retransmitted at the sub-frame #i+12.
Meanwhile, generally in the mobile communication system, a handover (HO) for changing the base station apparatuses to communicate with is executed, when the mobile station moves from a cell currently performing communication to an adjacent cell.
Moreover, prior to the handover, the mobile station measures communication quality in the adjacent cell being a candidate for a handover destination and reports a measurement result to the base station.
For example, such communication quality is assumed to include a reception level of a reference signal, a received SINR, and the like.
The report of the measurement to the base station apparatus is made in the form of a “measurement report”.
Based on the measurement report, the base station apparatus determines that the mobile station should perform the handover, and a message instructing the handover is sent to the mobile station as a “handover command”.
Here, the cell of the handover destination may be not only a cell at the same frequency in the same system, but also any of a cell at a different frequency in the same system and a cell using a different radio access technology (RAT).
The frequency of the cell using the different radio access technology is generally a different frequency from that of a handover source. Accordingly, the frequency of the cell at the handover destination is inevitably different from the frequency of the cell of the handover source.
FIG. 3 schematically shows how the handover is performed between the cells of the different frequencies. FIG. 3 shows a mobile communication system employing the LTE scheme including a mobile communication system using a first frequency f1 and a mobile communication system using a second frequency f2, and a mobile communication system employing the WCDMA scheme using a third frequency f3 which is different from the frequencies f1 and f2.
Incidentally, the mobile station generally includes only one radio signal processor unit and is therefore unable to perform transmission and reception of signals having different frequencies at the same time. For this reason, it is necessary to tune a reception frequency again when performing measurement in a cell using a different frequency (a different frequency cell) from a frequency of an active cell (a serving cell).
Accordingly, the base station apparatus notifies the mobile station of a gap period for the measurement, and thus the mobile station performs the measurement in the different frequency cell within the gap period.
To be more precise, the base station apparatus notifies the mobile station of a “length of the gap period”, a “cycle of coming of the gap period”, the “frequency of the different frequency cell”, and the like, in accordance with RRC measurement control, and the mobile station performs the measurement in the different frequency cell (including processing to change the frequency, to catch the synchronization channel, to measure quality and so forth) within the specified gap period, for example.
The above-mentioned gap period may also be referred to as a gap term, for example. FIG. 4 shows an image diagram of such a gap term. In FIG. 4, a gap term having a length of “6 ms” is set up on a 40-ms cycle.
It should be noted that “different frequency measurement” in this specification is a concept which includes not only actions to search a cell of a different frequency and to measure communication quality in the cell but also actions to search a cell of a different RAT and to measure communication quality in the cell.
As described previously, the mobile station performs the different frequency measurement in the gap period. As a consequence, the mobile station cannot perform communication with the base station apparatus in the active cell (the serving cell) within the above-described gap period.
In the following, the HARQ control on the uplink in the case of presence of the above-described gap period will be described by using FIG. 5.
In FIG. 5, the gap period is defined from a sub-frame #i+1 to a sub-frame #i+6. That is, a sub-frame (#i+4) for transmitting the uplink shared signal is included in the gap period.
In this case, even when the base station apparatus instructs the mobile station at reference numeral 502 (a sub-frame #i) by way of the uplink scheduling grant on the physical downlink control channel to perform communication by using the uplink shared channel at the sub-frame #i+4, the mobile station cannot perform transmission of the uplink shared signal at reference numeral 504.
That is, the mobile station skips transmission of the uplink shared signal at reference numeral 504. In this case, the mobile station is suggested to perform retransmission of the uplink shared signal at reference numeral 508.
Here, transmission of the uplink shared signal at reference numeral 508 is virtually a first transmission but is also a second transmission, in terms of the number of times of HARQ transmission.
However, the HARQ control method on the uplink when the above-described gap period is present involves the following problems.
First, as shown in FIG. 6, the control method when the above-described gap period includes a sub-frame (reference numeral 606) for transmitting either the HARQ indicator concerning the uplink shared signal or the uplink scheduling grant for instructing retransmission although the gap period does not include a sub-frame (reference numeral 604) for transmitting the uplink shared signal is yet to be clarified.
Second, as shown in FIG. 7, the control method when the above-described gap period includes both of the sub-frame (reference numeral 604) for transmitting the uplink shared signal and the sub-frame (reference numeral 606) for transmitting either the HARQ indicator concerning the uplink shared signal or the uplink scheduling grant for instructing retransmission is yet to be clarified.