The present invention relates to a radio base station, a user terminal, a radio communication method and a radio communication system in a next-generation mobile communication system.
Conventionally, various radio access schemes are used in radio communication systems. For example, in UMTS (Universal Mobile Telecommunication System), which is also referred to as “W-CDMA (Wideband Code Division Multiple Access),” code division multiple access (CDMA) is used. Also, in LTE (Long Term Evolution), orthogonal frequency division multiple access (OFDMA) is used (see, for example, non-patent literature 1).
Now, as shown in
In FRA, downlink signals for a plurality of user terminals are superposed over the same radio resource allocated by OFDMA, and transmitted with different transmission power depending on each user terminal's channel gain. On the receiving side, the downlink signal for a subject terminal is extracted adequately by cancelling the downlink signals for the other user terminals.
Also, as for link adaptation in each radio communication scheme, W-CDMA uses transmission power control (Fast TPC), and LTE uses adaptive modulation and coding (AMC), which adjusts the modulation scheme and coding rate adaptively. In FRA, the use of transmission power allocation and adaptive modulation and coding for multiple users (MUPA: Multi-User Power Allocation/AMC) is under study.
When NOMA is used, a user terminal, in order to adequately acquire the information for the subject terminal, can judge the order of decoding of received signals, whether or not to apply SIC, and so on, based on each user terminal's power allocation information. However, if the number of user terminals to be non-orthogonal-multiplexed over the same radio resource increases, the communication overhead pertaining to the reporting of power allocation information from the radio base station to the user terminals increases, and therefore the throughput decreases. Consequently, the method to realize non-orthogonal multiplexing while reducing the decrease of throughput is in demand.
The present invention has been made in view of the above, and it is therefore an object of the present invention to provide a user terminal, a radio base station, a radio communication method and a radio communication system that can realize non-orthogonal multiple access while reducing the decrease of throughput.
A radio base station, according to the present invention, has a selection section that selects a predetermined decoding pattern, based on channel state information of a user terminal, from among a plurality of decoding patterns in which information regarding the order of decoding of non-orthogonal multiple access signals and/or whether or not SIC (Successive Interference Cancellation) is applied is defined, and a transmission section that transmits information to represent the selected decoding pattern to the user terminal.
According to the present invention, it is possible to realize non-orthogonal multiple access while reducing the decrease of throughput.
In NOMA, a plurality of user terminals UE are non-orthogonal-multiplexed over the same radio resource by applying varying transmission power depending on channel gain (for example, the received SINR, the RSRP (Reference Signal Received Power), etc.), path loss and so on. For example, in
Also, in NOMA, the downlink signal for a subject terminal is extracted by cancelling interference signals from received signals, by means of SIC, which is a successive interference canceller-based signal separation method. For the downlink signal for the subject terminal, downlink signals for other terminals that are non-orthogonal-multiplexed over the same radio resource, and that use greater transmission power than the subject terminal become interference signals. Consequently, the downlink signal for the subject terminal is extracted by cancelling the downlink signals for the other user terminals UE with greater transmission power than the subject terminal.
For example, referring to
Meanwhile, the received SINR at the user terminal UE 1 is higher than the received SINR at the user terminal UE 2, so that the downlink signal for the user terminal UE 1 is transmitted with smaller transmission power than the downlink signal for the user terminal UE 2. Consequently, the user terminal UE 2 that is located far from the radio base station BS can ignore the interference by the downlink signal for the user terminal UE 1 that is non-orthogonal-multiplexed over same radio resource, and therefore extracts and adequately decodes the downlink signal for the subject terminal without carrying out interference cancellation by means of SIC.
In this way, when NOMA is applied to the downlink, a plurality of user terminals UE 1 and UE 2 with varying channel gains can be multiplexed over the same radio resource, so that it is possible to improve the spectral efficiency.
Now, the transmission process in NOMA will be described.
Next, the radio base station selects a group of candidate user sets, on a per subband basis, from all the user terminals that belong in the coverage area (step ST02). A candidate user set refers to a combination of candidate user terminals that are non-orthogonal-multiplexed over a subband. The total number of candidate user sets per subband is represented by following equation 1, where the total number of user terminals that belong to the coverage area is M and the number of user terminals to be non-orthogonal-multiplexed is N. Note that the following operation process sequence (step ST03 to ST06) is carried out for all the candidate user sets (exhaustive search).
[1]
Next, the radio base station calculates the transmission power of the subband to allocate to each user terminal in the candidate user sets, based on the channel gain that is fed back from each user terminal (step ST03). Next, the radio base station calculates the SINR (the SINR for scheduling) of each user terminal's subband, anticipated under the application of non-orthogonal-multiplexing (step ST04), based on the transmission power that is calculated. Next, the radio base station determines the block error rate (BLER: Block Error Rate) of the MCS (Modulation and Coding Scheme) set from the SINR that is calculated, and calculates the scheduling throughput of each user terminal's subband (step ST05).
Next, from each user terminal's instantaneous throughput and average throughput, the radio base station calculates the scheduling metric of the candidate user set (step ST06). For the scheduling metric, for example, the PF (Proportional Fairness) scheduling metric may be calculated. The PF scheduling metric Msj,b is represented by following equation 2, where the average throughput is Tk and the instantaneous throughput is Rk,b. Note that the PF scheduling metric Msj,b represents the PF scheduling metric of the j-th candidate user set in the b-th subband. Also, k denotes the k-th user terminal in a candidate user set.
[2]
The radio base station selects the user set that maximizes the scheduling metric in a subband by carrying out steps ST03 to ST06 for all the candidate user sets (step ST07). Then, the radio base station carries out steps ST02 to ST07 on a per subband basis, and selects the user set to maximize the scheduling metric with respect to each subband.
Next, the radio base station calculates the average SINR of a subband that is allocated (step ST08), and selects an MCS that is common to each user terminal of the allocated subband (step ST09). Next, the radio base station allocates the downlink signals for the user terminals constituting a user set to the same subband, and non-orthogonal-multiplexes and transmits the downlink signals to each user terminal with transmission power that varies on a per subband basis (step ST10).
Next, each user terminal that is selected by the radio base station as being in a user set not only receives the downlink signal for the subject terminal, but also receives the downlink signals for other terminals that are non-orthogonal-multiplexed over the same radio resource (step ST11). Then, each user terminal cancels the downlink signals for other terminals with lower channel gain and greater transmission power than the subject terminal by means of SIC, and extracts (separates) the signal for the subject terminal. In this case, the downlink signals for other terminals with higher channel gain and lower transmission power than the subject terminal do not become interference signals, and are therefore ignored.
Now, in NOMA, each user terminal can measure channel gain, signal power and so on, from the reference signals included in received signals, and decides the decoding pattern of the received signals (the order of decoding and/or whether or not to apply SIC) based on these. However, if the decisions are made based on measurements, a failed measurement might lead to the use of the wrong decoding pattern, which gives a threat of deteriorated reception performance. Also, it is equally possible to report power allocation information (for example, the transmission power that is calculated in step ST03) of the signal for each user terminal from the radio base station to the user terminal, and decide the decoding pattern based on this power allocation information. For example, it is possible to determine whether or not to cancel the signal for each user terminal by means of SIC, depending on the magnitude of transmission power. However, if the number of user terminals to be non-orthogonal-multiplexed over the same radio resource increases, the communication overhead pertaining to the reporting of power allocation information also increases, and the problem arises that the throughput decreases.
In communication using NOMA, given the above problem that the overhead of communication increases when power allocation information is reported from the radio base station, the present inventors have thought that the configuration to report information representing a decoding pattern to each user terminal might realize non-orthogonal multiple access while reducing the decrease of throughput, and made the present invention. That is, the present invention prepares, in advance, a plurality of decoding patterns, in which information regarding the order of decoding of non-orthogonal multiple access signals and/or whether or not to apply SIC (Successive Interference Cancellation) is defined, selects suitable decoding patterns depending on each user terminal's communication environment, and transmits these decoding patterns to each user terminal.
Now, a first example of the present embodiment will be described below. With the first example, a radio base station transmits information that represents a common decoding pattern, to each user terminal whose signal is non-orthogonal-multiplexed over the same radio resource. Also, with the first example, a plurality of decoding patterns are configured so that the order of decoding of each user terminal is specified uniquely. Furthermore, each user terminal is configured to be able to identify the user terminals whose signals are non-orthogonal-multiplexed over the same radio resource.
It is equally possible to employ a configuration, in which the decoding patterns do not expressly include information regarding the order of decoding or whether or not SIC is applied, and in which a user terminal determines this information from information of the decoding patterns and/or from information besides the decoding patterns. With the present embodiment, the decoding patterns show the order of decoding of non-orthogonal multiple access signals, and whether or not to apply SIC is decided from the order of decoding. To be more specific, a user terminal does not apply SIC when the subject terminal alone is included in the order of decoding represented by a decoding pattern (for example, as in the patterns 1 and 2 in
Meanwhile, it is equally possible to employ a configuration, in which the decoding patterns only include information as to whether or not the user terminals apply SIC, and the user terminals judge the order of decoding. For example, if the radio base station transmits information to the effect that SIC is not applied, to the UE 1, while the UE 1 is performing interference cancellation and decoding of received signals based on the pattern 3 of
Note that, with the present embodiment, the radio base station and each user terminal are configured to be able to refer to the same decoding pattern. To be more specific, information regarding the same multiple decoding patterns may be held in advance in the respective storage fields of the radio base station and the user terminals. Also, the radio base station and the user terminals may be configured to be able to refer to the same decoding pattern as appropriate by changing the decoding patterns and reporting information regarding the changed decoding patterns to each other.
Next, information to represent the selected common decoding pattern is transmitted to each user terminal (step ST22). In the event of
Finally, each user terminal receives the information to represent a specific decoding pattern, transmitted from the radio base station (step ST23). Using this information, each user terminal performs interference cancellation and decoding of received signals, depending on the order of decoding and whether or not SIC is applied, as shown in the decoding pattern selected by the radio base station.
With the first example, the process according to the flowchart shown in
Note that, with the present embodiment, each user terminal identifies user terminals based on the DM-RS port that is assigned to each user terminal by the radio base station. The DM-RS (DeModulation Reference Signal) is a signal which the radio base station inserts upon transmitting the PDSCH, so that the user terminals can carry out channel estimation, which is required in demodulation. In particular, in MIMO (Multi Input Multi Output) transmission to use a plurality of antennas, DM-RSs may be transmitted using varying DM-RS ports on a per user terminal basis. For example, when two of DM-RS port 1 and port 2 are available for use as DM-RS ports, it may be possible to decide that the UE 1 is the terminal to use the DM-RS port 1 and that the UE 2 is the terminal to use the DM-RS port 2. However, the identification of the user terminals is by no means limited to this. For example, it is possible to report information regarding the transmission power from the radio base station to each user terminal (transmission power ratio, etc.) by using higher layer signaling (for example, RRC signaling), and identify the user terminals based on this information. Furthermore, the radio base station may expressly report, to each user terminal, which terminal in a decoding pattern the user terminal is.
Using the case of
As described above, the radio base station according to the first example of the present embodiment can judge the order of decoding of signals and whether or not SIC is applied, based on information to represent a decoding pattern that is common to each user terminal and require a small amount of communication, so that it is possible to realize non-orthogonal multiple access while reducing the decrease of throughput.
Now, a second example of the present embodiment will be described below. With the second example, the radio base station transmits information, in which decoding patterns that are defined individually, on a per user terminal basis, are represented, to each user terminal whose signal is non-orthogonal-multiplexed over the same radio resource. Although, with the above first example, the order of decoding of each user terminal is specified uniquely in the decoding patterns, with the second example, a plurality of decoding patterns are configured so that at least the order of decoding of the user terminals receiving these decoding patterns can be specified. Now, the second example will be described primarily with reference to the differences from the first example.
Next, information to represent the individual decoding pattern that is selected, is transmitted to each user terminal (step ST32). Finally, each user terminal receives the information to represent the specific decoding pattern, transmitted from the radio base station (step ST33).
With the case of
As described above, with the radio base station according to the second example of the present embodiment, it is possible to use even less reporting information, so that it is possible to realize non-orthogonal multiple access while more adequately reducing the decrease of throughput.
(Example Structure of Radio Communication System)
Now, the structure of the radio communication system according to the present embodiment will be described below. In this radio communication system, the above-described decoding pattern reporting methods for non-orthogonal multiple access are employed.
The radio communication system 1 shown in
The radio base stations 10 may be eNodeBs (eNBs) that form macro cells, or may be any of RRHs (Remote Radio Heads), femto base stations, pico base stations and so on, that form small cells. Also, the radio base stations 10 may be referred to as “transmitting/receiving points” and so on. The user terminals 20 are terminals to support various communication schemes such as LTE, LTE-A and so on, and may include both mobile communication terminals and fixed communication terminals.
In the radio communication system 1, as radio access schemes, OFDMA (Orthogonal Frequency Division Multiple Access) and NOMA (Non-Orthogonal Multiple Access) are applied to the downlink, and SC-FDMA (Single-Carrier Frequency Division Multiple Access) is applied to the uplink. OFDMA is a multi-carrier transmission scheme to divide the transmission band into subbands and orthogonal-multiplex user terminals 20, and NOMA is a multi-carrier transmission scheme to non-orthogonal-multiplex user terminals 20 with varying transmission power on a per subband basis. SC-FDMA is a single-carrier transmission scheme to allocate user terminals 20 to radio resources that are continuous in the frequency direction.
Also, in the radio communication system 1, as downlink communication channels, a downlink shared data channel (PDSCH (Physical Downlink Shared Channel)), which is used by each user terminal 20 on a shared basis, downlink L1/L2 control channels (PDCCH (Physical Downlink Control Channel), PCFICH (Physical Control Format Indicator Channel), PHICH (Physical Hybrid-ARQ Indicator Channel), EPDCCH (Enhanced Physical Downlink Control Channel)), a broadcast channel (PBCH (Physical Broadcast Channel)) and so on are used. User data and higher control information are transmitted by the PDSCH. Scheduling information for the PDSCH and the PUSCH is transmitted by the PDCCH and the EPDCCH. The number of OFDM symbols to use for the PDCCH is transmitted by the PCFICH. HARQ ACKs/NACKs in response to the PUSCH are transmitted by the PHICH.
Also, in the radio communication system 1, as uplink communication channels, an uplink shared channel (PUSCH (Physical Uplink Shared Channel)), which is used by each user terminal 20 on a shared basis, an uplink control channel (PUCCH (Physical Uplink Control Channel)), a random access channel (PRACH (Physical Random Access Channel)) and so on are used. User data and higher control information are transmitted by the PUSCH. Also, by the PUCCH or the PUSCH, downlink channel state information (CSI (Channel State Information)), ACKs/NACKs and so on are transmitted.
User data to be transmitted from the radio base station 10 to the user terminal 20 on the downlink is input from the higher station apparatus 30, into the baseband signal processing section 104, via the transmission path interface 106.
In the baseband signal processing section 104, the input user data is subjected to a PDCP (Packet Data Convergence Protocol) layer process, division and coupling of the user data, RLC (Radio Link Control) layer transmission processes such as an RLC retransmission control transmission process, MAC (Medium Access Control) retransmission control (for example, an HARQ transmission process), scheduling, transport format selection, channel coding, an IFFT (Inverse Fast Fourier Transform) process and a pre-coding process, and the result is transferred to each transmitting/receiving section 103. Also, downlink control data is subjected to transmission process such as channel coding and an inverse fast Fourier transform, and transferred to each transmitting/receiving section 103.
Each transmitting/receiving section 103 converts the baseband signals, which are pre-coded and output from the baseband signal processing section 104 on a per antenna basis, into a radio frequency band. The amplifying sections 102 amplify the radio frequency signals having been subjected to frequency conversion, and transmit the results through the transmitting/receiving antennas 101.
On the other hand, data to be transmitted from the user terminal 20 to the radio base station 10 on the uplink is received in each transmitting/receiving antenna 101 and input in the amplifying sections 102. The amplifying sections 102 amplify the radio frequency signals input from each transmitting/receiving antennas 101, and send the results to the transmitting/receiving sections 103. The amplified radio frequency signals are subjected to frequency conversion in each transmitting/receiving section 103, and input in the baseband signal processing section 104.
In the baseband signal processing section 104, the user data that is included in the input baseband signals is subjected to an FFT (Fast Fourier Transform) process, an IDFT (Inverse Discrete Fourier Transform) process, error correction decoding, a MAC retransmission control receiving process, and RLC layer and PDCP layer receiving processes, and the result is transferred to the higher station apparatus 30 via the transmission path interface 106. The call processing section 105 performs call processing such as setting up and releasing communication channels, manages the state of the radio base station 10 and manages the radio resources.
Downlink data is received by a plurality of transmitting/receiving antennas 201 and input in the amplifying sections 202. The amplifying sections 202 amplify the radio frequency signals that are input in the transmitting/receiving antennas 201, and sent to each transmitting/receiving section 203. The radio frequency signals are converted into baseband signals in each transmitting/receiving section 203, and input in the baseband signal processing section 204. The baseband signal processing section 204 applies receiving process such as an FFT process, error correction decoding, a retransmission control receiving process and so on, to the baseband signals. The user data that is included in the downlink data is transferred to the application section 205. The application section 205 performs process related to higher layers above the physical layer and the MAC layer, and so on. In addition, in the downlink data, broadcast information is also transferred to the application section 205.
Meanwhile, uplink user data is input from the application section 205 into the baseband signal processing section 204. The baseband signal processing section 204 applies a retransmission control (HARQ (Hybrid ARQ)) transmission process, channel coding, pre-coding, a DFT process, an IFFT process and so on to the input user data, and transfers the result to each transmitting/receiving section 203. The baseband signals that are output from the baseband signal processing section 204 are converted into a radio frequency band in each transmitting/receiving section 203. After that, the amplifying sections 202 amplify the radio frequency signals having been subjected to frequency conversion, and transmit the results from the transmitting/receiving antennas 201.
As shown in
The scheduling section 301 determines the user sets to non-orthogonal-multiplex on given radio resources, depending on the channel gain of each user terminal 20. As for the user sets, for example, in each subband, the user set to maximize the PF (Proportional Fairness) scheduling metric is selected. The channel state information that is fed back from the user terminal 20 is received in the transmitting/receiving section 103 (see
Also, the scheduling section 301 selects a suitable decoding pattern for each user terminal 20 that is selected as being in the same user set, based on the channel state information. In the first example, a decoding pattern that is common to each user terminal 20 is selected, from a plurality of decoding patterns that are configured so that the order of decoding of each user terminal 20 is specified uniquely. By carrying out scheduling so that the DM-RS ports are allocated on a fixed basis depending on the location and channel gain of each user terminals 20, it is possible to specify each user terminal 20. Also, in the second example, a dedicated decoding pattern is selected for each user terminal 20, from a plurality of decoding patterns that are configured so that at least the order of decoding of the user terminals 20 to receive the decoding patterns can be specified.
The downlink control information generating section 302 generates user terminal-specific downlink control information (DCI), which is transmitted in the PDCCH or the EPDCCH. The downlink control information is output to the downlink control information coding/modulation section 303. The downlink control information coding/modulation section 303 carries out channel coding and modulation of the downlink control information. The modulated downlink control information is output to the downlink channel multiplexing section 307.
The user terminal-specific downlink control information includes a DL assignment, which is PDSCH allocation information, a UL grant, which is PUSCH allocation information, and so on. Also, the downlink control information includes control information for requesting a CSI feedback to each user terminals 20, information that is required in the receiving process of signals that are non-orthogonal-multiplexed, and so on. For example, the downlink control information may include information regarding the decoding patterns that are common and specific to each user terminals 20, or may include information regarding the transmission power to each user terminals 20 (transmission power ratio, etc.). However, information regarding decoding patterns and transmission power may be included in higher control information as well, which is reported through higher layer signaling (for example, RRC signaling).
The downlink transmission data generating section 304 generates downlink user data on a per user terminals 20 basis. The downlink user data that is generated in the downlink transmission data generating section 304 is output, with the higher control information, as downlink transmission data to be transmitted in the PDSCH, to the downlink transmission data coding/modulation section 305. The downlink transmission data coding/modulation section 305 carries out channel coding and modulation of the downlink transmission data for each user terminals 20. The downlink transmission data is output to the downlink channel multiplexing section 307.
The downlink reference signal generating section 306 generates downlink reference signals (the CRS, the CSI-RS, the DM-RS, etc.). The downlink reference signals are output to the downlink channel multiplexing section 307.
The downlink channel multiplexing section 307 combines the downlink control information, the downlink reference signals and the downlink transmission data (including the higher control information), and generates a downlink signal. To be more specific, in accordance with scheduling information that is reported from the scheduling section 301, the downlink channel multiplexing section 307 carries out non-orthogonal-multiplexing so that the downlink signals for a plurality of user terminals 20, selected in the scheduling section 301, are transmitted with predetermined transmission power. The downlink signal that is generated in the downlink channel multiplexing section 307 is transmitted towards the user terminals 20 via various transmission processes.
Meanwhile, the user terminal 20 has a downlink control information receiving section 401, a channel estimation section 402, a feedback section 403, an interference cancellation section 404 and a downlink transmission data receiving section 405. A downlink signal that is transmitted from the radio base station 10 is separated into the downlink control information, the downlink transmission data (including the higher control information) and the downlink reference signal, via various receiving processes. The downlink control information is input in the downlink control information receiving section 401, the downlink transmission data is input in the downlink transmission data receiving section 405 via the interference cancellation section 404, and the downlink reference signal is input in the channel estimation section 402. The downlink control information receiving section 401 demodulates the downlink control information and outputs the result to the channel estimation section 402, the feedback section 403, the interference cancellation section 404 and so on.
The channel estimation section 402 performs channel estimation based on the downlink reference signal and acquires channel gain. The channel gain that is acquired by channel estimation is included in channel state information and fed back to the radio base station 10 via the feedback section 403. As described earlier, in the radio base station 10, a suitable decoding pattern is selected for each user terminal 20, based on the channel state information. According to the first example, a decoding pattern that is common to each user terminal 20 is selected from a plurality of decoding patterns that are configured so that the order of decoding of each user terminal 20 is specified uniquely. Also, UEs in the decoding patterns used in the interference cancellation section 404 may be specified based on the allocation of DM-RS ports.
The interference cancellation section 404 decides the order of decoding of signals and whether or not to apply SIC, based on information that represents the decoding pattern and that is transmitted from the radio base station, and, when SIC is to be carried out, cancels the interference by downlink signals allocated to other terminals, in accordance with the order of decoding. Also, when information regarding the transmission power and/or the transmission power ratio of the radio base station 10 to each user terminal 20 is received, this information can be used in interference cancellation.
As described above, the radio communication system 1 according to the present embodiment can realize non-orthogonal multiple access while reducing the decrease of throughput by the configuration to report information that represents decoding patterns to each user terminal.
The present invention is by no means limited to the above embodiment and can be implemented in various modifications. For example, it is possible to adequately change the number of carriers, the carrier bandwidth, the signaling method, the number of processing sections, the order of processes and so on in the above description, without departing from the scope of the present invention, and implement the present invention. Besides, the present invention can be implemented with various changes, without departing from the scope of the present invention.
The disclosure of Japanese Patent Application No. 2013-135757, filed on Jun. 28, 2013, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
Number | Date | Country | Kind |
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2013-135757 | Jun 2013 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP2014/058264 | 3/25/2014 | WO | 00 |