KR101209501B1

KR101209501B1 - Receiver

Info

Publication number: KR101209501B1
Application number: KR1020107023983A
Authority: KR
Inventors: 미쓰루 다나베; 유키오 오카다; 미쓰루 마에다
Original assignee: 파나소닉 주식회사
Priority date: 2008-03-26
Filing date: 2009-03-26
Publication date: 2012-12-07
Also published as: KR20100126568A; JP2009239545A; JP5215704B2; CN101981842A; HK1154131A1; TWI385990B; WO2009119736A1; TW201014291A; CN101981842B

Abstract

The receiver 10 includes a multi-level modulation method determination circuit 16, a phase correction method selection circuit 17, a phase correction circuit 14, and a determination circuit 15. The multi-level modulation scheme determination circuit 16 determines the multi-level modulation scheme used for the modulated signal according to the modulated signal received from the transmitter. The phase correction method selection circuit 17 predetermines the phase correction method used for phase correction of symbols of the modulated signal based on the multi-level class of the multi-level modulation method determined by the multi-level modulation method determination circuit 16. Select from a plurality of phase correction schemes. The phase correction circuit 14 corrects the phase of a symbol using the phase correction method selected by the phase correction method selection circuit 17. The determination circuit 15 determines the bit sequence of the phase-corrected symbol by the phase correction circuit 14 based on the multi-level modulation scheme determined by the multi-level modulation scheme determination circuit 16.

Description

Receiver used in communication system using adaptive modulation method {RECEIVER}

The present invention relates to a receiver, in particular a receiver for use in a communication system using an adaptive modulation scheme.

Conventionally, there is a communication system using an adaptive modulation scheme. This communication system has a plurality of multi-level modulation schemes having different multi-level classes (bit rates), and switches, for example, the multi-level modulation scheme to be used depending on the environment (line quality) surrounding the self. In this case, the best transmission efficiency can be obtained based on the line quality. Here, as the multi-level modulation method, there are, for example, Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (QAM), 64 QAM, etc. in order from a low bit rate method. (See, for example, Japanese Laid-Open Patent Publication 2007-150906, IEEE 802.11a-1999).

However, each of the transmitter and the receiver constituting the communication system has a reference signal source. The reference signal source uses a crystal transmitter. The oscillation frequency (hereinafter referred to as " reference frequency ") of each reference signal source of the transmitter and receiver includes an error due to the precision of the crystal transmitter. As a result, an error of about ppm occurs between the reference frequency of the transmitter side performing modulation processing and the reference frequency of the receiver side performing demodulation processing. This error in the reference frequency is caused by the phase rotation of the data received by the receiver. The occurrence of such phase rotation greatly affects the bit error rate (hereinafter, referred to as "BER") after demodulation. Therefore, at the time of demodulation, the receiver corrects phase rotation of the received data. In particular, in the case of multi-level modulation using a multicarrier modulation scheme such as orthogonal frequency division multiplexing (OFDM) modulation, the influence of phase rotation due to frequency error during demodulation is increased. This is because the occupancy time of a single symbol increases, and the interval of phase correction becomes long.

Conventionally, as a phase correction method for correcting the phase rotation of a symbol, a phase correction method using a pilot subcarrier (for example, see Japanese Laid-Open Patent Publication No. 2008-22339), or a phase correction method using a pilot symbol (example For example, see Japanese Laid-Open Patent Publication 2006-352746.

In the phase correction method using the pilot subcarrier, when large frequency selectivity exists in the propagation characteristic of a signal under the influence of multipath paging, a correction error becomes large.

Therefore, in a radio wave environment with strong frequency selectivity, a phase correction method using a pilot symbol is effective.

On the other hand, the phase correction method using a pilot symbol is not a sequential correction for each OFDM symbol, unlike the phase correction method using a pilot subcarrier. Therefore, the interval at which the pilot symbol is embedded in the modulated signal must be narrowed as the multilevel class of the multilevel modulation scheme increases. Therefore, if the adaptive modulation scheme can perform satisfactory phase correction for all the multilevel modulation schemes, the transmission efficiency is lowered, particularly in the low multilevel modulation scheme of the multilevel class.

In addition, phase correction using a pilot symbol cannot be phase corrected until a pilot symbol is obtained, so phase correction is performed and the interval is relatively long. Therefore, in the case of the multi-level modulation system of high multi level class, there is a fear that the error of phase rotation exceeds the allowable range during the demodulation process.

In this manner, the multilevel modulation method cannot be applied to the phase correction method. However, conventional receivers use the same phase correction scheme for modulated signals received from a transmitter. Therefore, the conventional receiver cannot perform optimal phase correction in accordance with the multi-level modulation scheme performed by the transmitter.

This invention is made | formed in view of the said reason. It is an object of the present invention to provide a receiver capable of performing optimal phase correction in accordance with a multi-level modulation scheme performed by a transmitter.

The receiver according to the present invention is used in an adaptive modulation communication system related to a transmitter for transmitting a modulation signal generated using a multi-level modulation scheme selected from a plurality of multi-level modulation schemes according to line quality. The modulated signal has a symbol sequence representing data to be transmitted to the receiver. The symbol has a corresponding relationship with a bit sequence by a multi-level modulation scheme selected by the transmitter. The receiver according to the present invention includes a multi-level modulation method determination unit, a phase correction method selection unit, a phase correction unit, and a determination unit. The multilevel modulation scheme determination unit is configured to determine the multilevel modulation scheme used for the modulation signal in accordance with the modulation signal received from the transmitter. The phase correction method selection unit includes a plurality of predetermined phase correction methods used for correcting a phase of a symbol of the modulated signal based on the multi-level class of the multi-level modulation method determined by the multi-level modulation method determination unit. It is configured to select from the phase correction scheme of. The phase correction unit is configured to correct the phase of the symbol by using the phase correction method selected by the phase correction method selection unit. The determination unit is configured to determine a bit sequence corresponding to the symbol whose phase has been corrected by the phase correction unit based on the multi-level modulation method determined by the modulation method determination unit.

According to the present invention, a phase correction scheme is selected based on the multi-level class of the multi-level modulation scheme performed by the transmitter. Therefore, optimal phase correction can be performed according to the multi-level modulation scheme performed by the transmitter.

Preferably, the transmitter has a primary modulation scheme and a secondary modulation scheme. The primary modulation scheme is a multilevel modulation scheme selected by a predetermined criterion from a plurality of multilevel modulation schemes having different multilevel classes, and generates a primary modulation symbol representing the symbol. The secondary modulation scheme is a multicarrier modulation scheme. In the second modulation scheme, a second modulation symbol is generated by multiplexing a plurality of subcarriers having a complex amplitude based on the first modulation symbol. The secondary modulation scheme constitutes the modulated signal composed of a plurality of the secondary modulation symbols. The modulated signal includes a pilot symbol for each predetermined regular interval. The pilot symbol is an existing secondary modulation symbol specific to the receiver. The predetermined secondary modulation symbol is composed of subcarriers having a predetermined complex amplitude. The secondary modulation symbol includes a pilot subcarrier. The pilot subcarrier is a predetermined subcarrier specific to the receiver. The predetermined subcarrier has a predetermined complex amplitude. The phase correction method selection unit is configured to correct the phase of the symbol by using the pilot symbols when the multi level class of the multi level modulation method determined by the multi level modulation method determination unit is less than a predetermined value. Select a first phase correction scheme and select a second phase correction scheme to correct the phase of the symbol using the pilot subcarrier if the multi-level class is greater than or equal to a predetermined value.

In this case, when the multi-level class of the multi-level modulation scheme performed by the transmitter is less than a predetermined value, phase correction using the pilot symbol having a large phase correction effect is performed even in a radio wave environment having high frequency selectivity. On the other hand, when the multi-level class of the multi-level modulation scheme performed by the transmitter is equal to or greater than a predetermined value, phase correction using the pilot subcarrier that realizes high transmission efficiency and is cheap is performed. Therefore, optimal phase correction can be performed according to the multi-level modulation scheme performed by the transmitter.

More preferably, the phase correction scheme selection unit selects the report of the first phase correction scheme if the multi-level grade of the multi-level modulation scheme determined by the multi-level modulation scheme determination unit is less than a predetermined value. And select both of the first phase correction scheme and the second phase correction scheme if the multi-level grade is equal to or greater than a predetermined value.

In this case, the first phase correction using the pilot symbol is always performed. Therefore, the control of the phase correction unit is simplified. Further, in the first phase correction method, phase correction is performed for each subcarrier. Therefore, even in a radio wave environment with strong frequency selectivity, the effect of phase correction can always be increased.

More preferably, the phase correction unit is configured to perform the symbol according to the second phase correction method when both of the first phase correction method and the second phase correction method are selected by the phase correction method selection unit. And corrects the phase of the symbol according to the first phase correction scheme.

In this case, phase correction can be performed for each symbol by using a pilot subcarrier. In addition, an error caused by phase correction using a pilot subcarrier can be eliminated by phase correction using a pilot symbol. Therefore, optimal phase correction can be performed for a multilevel modulation method having a large multilevel class.

Preferably, the predetermined value is set based on the transmission efficiency when the phase correction unit corrects the phase of the symbol by using the phase correction method selected by the phase correction method selection unit.

In this case, the optimum phase correction can be performed according to the multi-level modulation scheme performed by the transmitter, and the transmission efficiency can be improved.

1 is a schematic diagram of a receiver of a first embodiment.
2 is a schematic diagram of a communication system having the receiver.
3 is an explanatory diagram of a well studio distribution of 16 QAM.
4 is an explanatory diagram of arrangement of subcarriers and pilot subcarriers.
5 is an explanatory diagram of a structure of an OFDM signal.
6 is an explanatory diagram of a buried structure of a pilot symbol.
7 is a schematic diagram of a receiver of the second embodiment.

(First embodiment)

As shown in FIG. 2, the receiver 10 of this embodiment constitutes an adaptive modulated communication system (hereinafter referred to as a "communication system") associated with the transmitter 20. As shown in FIG. The communication system performs packet communication with the transmitter 20 by an OFDM signal modulated (in this embodiment, OFDM modulated). The transmission path 30 for transmitting the OFDM modulated wave from the transmitter 20 to the receiver 10 may be either wired or wireless.

The transmitter 20 employs a multi-level modulation scheme as the primary modulation and employs OFDM modulation as the secondary modulation. The transmitter 20 performs error correction encoding on data (information bit sequence) to be transmitted to the receiver 10. The transmitter 20 also serial-to-parallel converts the error correction coded data. The transmitter 20 generates a complex symbol (primary modulation symbol) for modulating the subcarrier from the serialized / parallel converted data based on the correspondence between the symbol and the bit sequence determined by the multi-level modulation scheme ( Symbol mapping). The transmitter 20 generates a complex baseband OFDM signal (OFDM symbol or secondary modulation symbol) in digital format by performing inverse discrete Fourier transform (secondary modulation) on the complex symbols in order, and then usually performing serial conversion. The transmitter 20 digital-to-analog converts (DA converts) the complex baseband OFDM signal. The transmitter 20 passes the DA-converted OFDM signal through a filter for removing the image signal generated by the DA transform, multiplies the carrier wave (performs frequency conversion), and performs a predetermined signal amplification to perform OFDM modulation. Generate a wave. The transmitter 20 transmits the OFDM modulated wave generated in this way to the transmission path 30.

Here, the transmitter 20 has a plurality of multi-level modulation schemes, for example, 16 QAM and 64 QAM, having different multi-level classes as primary modulation schemes. When performing symbol mapping, the transmitter 20 selects a multi-level modulation scheme from a plurality of multi-level modulation schemes having different multi-level classes as a predetermined reference (that is, the transmitter 20 performs adaptive modulation). For example, the transmitter 20 selects the multi-level modulation method with the highest multi-level class so that the data transmission speed is the fastest. In addition, the transmitter 20 may select a multi-level modulation method so that the transmission speed becomes a fixed value or more according to the transmission path 30 state (line quality) or data capacity.

The transmitter 20 also includes a reference signal source (not shown) with a crystal transmitter. The transmitter 20 performs inverse discrete Fourier transform (OFDM modulation), frequency conversion, or the like using the reference frequency emitted by the reference signal source. The reference signal source is also provided to the receiver 10.

In this way, the transmitter 20 transmits an OFDM modulated wave generated using a multi-level modulation method selected from a plurality of multi-level modulation methods 16QAM and 64QAM by predetermined criteria.

Here, in a multi-level modulation system of high multi level class such as 64 QAM, the tolerance angle when determining the bit sequence of a symbol using a complex symbol on a complex plane is small. Therefore, it is necessary to perform phase correction for each symbol. Hereinafter, the tolerance error angle in QAM will be described taking 16 QAM as an example.

FIG. 3 shows the symbol arrangement (signal point arrangement) on the complex plane for the bit sequences [0000)] to [1111] of 16 QAM, assuming gray codes. The dividing line L1 in FIG. 3 passes through the midpoint of the line segment connecting the bit sequence [1110] and each symbol point of the bit sequence [1010] in the direction of the quadrature-phase (Q) axis. The dividing line L2 passes through the midpoint of the line segment connecting between the bit sequence [1010] and each symbol point of the bit sequence [1011] in the I-in-phase axis direction. When the complex symbol received by the receiver 10 exists in the area A12 surrounded by the dividing lines L1 and L2 including the bit sequence [1010], the complex symbol is assumed to have a high probability of representing the bit sequence [1010]. . However, in practice, there is a fear that the complex symbol existing in the area A12 in the transmitter 20 may not exist in the area A12 in the receiver 10 due to the phase rotation caused by the error of the reference frequency. In this case, an error occurs in the determination of the bit sequence.

The complex symbol corresponding to the bit sequence [1010] received by the receiver 10 is Gaussian distributed around the symbol point representing the bit sequence [1010] if there is no error in the reference frequency. Therefore, there is no contradiction in setting the tolerance angle θ1 [degrees] of each symbol point provided that the symbol point does not leave the original area. For example, for 16 QAM, the tolerance angle θ1 [degree] is 16.88.

Table 1 shows the tolerance angle θ1 of typical QAM. As is apparent from Table 1, as the multi-level rating increases, the tolerance angle θ1 decreases.

Multi Level Modulation 16QAM 64QAM 256QAM 1024QAM Permissible error angle θ1 [degree] 16.88 10.55 7.69 6.06

Next, in OFDM modulation, the phase rotation per OFDM symbol due to the error of the reference frequency is examined. As a factor of the phase error by phase rotation of an OFDM symbol, the error (first error) at the carrier frequency synchronization (frequency conversion) required for the demodulation process of OFDM, and the error (second error) at the sampling frequency synchronization (fast Fourier transform) Two) are considered. The occupancy time Ta per OFDM symbol is expressed by the following equation (1) when the sample frequency of the fast Fourier transform is fs, the size (FFT size) of the fast Fourier transform is N point, and the time of the guard interval is Tgi. .

(Equation 1)

The phase error due to the first error and the second error is additive. Therefore, if the carrier frequency is fc, and the reference frequency error between the reference frequencies used in modulation and demodulation is e, the phase error angle θ2 per OFDM symbol is expressed by the following equation (2).

(Equation 2)

For example, according to the IEEE 802.11a-1999 specification (the IEEE 802.11a standard of the wireless LAN defined by the Institute of Electrical and Electronics Engineers of the IEEE), if the reference frequency error of each reference signal source is 20 ppm, the amount of modulation and demodulation is processed. This results in a reference frequency error e of 40 ppm. The error of the carrier frequency fc is generally converged to a frequency error of fs / 2 by the automatic frequency correction circuit of the receiver. Therefore, Formula (2) can be transformed into following Formula (3).

(Equation 3)

According to the IEEE 802.11a-1999 specification, the sample frequency fs of the fast Fourier transform is 20 MHz, the occupancy time Ta of the OFDM symbol is 4 μsec (where the guard interval time Tgi is 0.8 μsec), and the size of the fast Fourier transform is 64. Is the point.

If equation (3) is calculated according to the specifications of IEEE 802.11a-1999, the phase error angle θ2 is 2.88. Therefore, at 64 QAM, the tolerance exceeds θ1 with four symbols. According to the IEEE 802.11a-1999 specification, one packet is required to be a maximum of 1000 bytes. Therefore, without adding redundant bits due to error correction, an OFDM symbol that can be transmitted in one packet becomes about 27 symbols. Therefore, when four tolerances exceed the tolerance angle θ1, one packet cannot be demodulated correctly.

Therefore, in IEEE 802.11a-1999, as shown in Fig. 4, four out of all 52 subcarriers are pilot subcarriers PSC1 to PSC4 irrelevant to data transmission, and the remaining 48 subcarriers SC0 to are used for data transmission. It is prescribed to be SC47. Therefore, phase correction for each OFDM symbol can be performed using the pilot subcarriers PSC1 to PSC4.

However, when there is a large frequency selectivity in the propagation characteristics of the signal due to the multipath paging, the S / N ratio of the frequency in which the pilot subcarriers are embedded may be extremely deteriorated. In this case, the correction error of the phase correction method using the pilot subcarrier becomes large. Therefore, especially in a high level multilevel modulation system, BER may be worsened by performing phase correction. For example, in FIG. 4, since the frequency characteristic 1000 deteriorates near the pilot subcarrier PSC1, the precision of phase correction using pilot subcarrier PSC1 falls.

In a propagation environment with high frequency selectivity, a phase correction method using pilot symbols is effective. The pilot symbol consists of predetermined symbols on both the receiver 10 and the transmitter 20. In addition, the pilot symbols are embedded at regular time intervals in a modulation signal (packet) composed of OFDM symbols. Therefore, the phase correction for each subcarrier can be performed using a pilot symbol.

However, the phase correction scheme using pilot symbols is not a sequential correction for each OFDM symbol. Therefore, the interval at which the pilot symbol is embedded in the modulated signal must be narrowed as the multilevel class of the multilevel modulation scheme increases. For example, in 16 QAM, it is sufficient to embed pilot symbols in the modulated signal every 5 symbols. In contrast, in the 64 QAM, an interval at which pilot symbols are embedded in a modulated signal must be set every three symbols. Therefore, if the adaptive modulation system can perform satisfactory phase correction for all the multi-level modulation methods, the transmission efficiency is degraded, particularly in the low multi-level modulation method of the multi-level class.

In addition, since phase correction cannot be performed until the pilot symbol is obtained, the phase correction using the pilot symbol has a relatively long interval between phase corrections. Therefore, in the case of a multi-level modulation system of a high multi-level class, there is a fear of exceeding the tolerance angle θ1 during the demodulation process.

Thus, the transmitter 20 generates a modulated signal so that the receiver 10 can selectively perform phase correction using pilot symbols and phase correction using pilot subcarriers.

For example, a modulated signal (packet) is composed of a short preamble SP, a long preamble LP, and a data section D, as shown in FIG.

In order to establish symbol timing synchronization, the short preamble SP performs 10 times (X1 to X10) of the existing synchronization pattern (specific pattern) X for each basic period T1 (= 0.8 μsec) for the transmitter 20 and the receiver 10. It is configured repeatedly. In other words, the short preamble SP is composed of a repetitive signal of the basic period T1.

The long preamble LP is configured by repeating the existing synchronization pattern Y twice (Y1, Y2) every basic period T2 (= 3.2 μsec) for the transmitter 20 and the receiver 10 for channel estimation.

The data portion D is an area for data transmission in which data bits, modulation scheme information, and the like are stored.

In the modulated signal, the short preamble SP, the long preamble LP, and the data portion D are arranged in the order of the short preamble SP, the long preamble LP, and the data portion D.

At the beginning of each region of the long preamble LP and the data portion D, guard intervals GI1 and GI2 in which a part of the latter half of each region is copied are added. According to the card intervals GI1 and GI2, the influence of the multipath can be reduced.

As shown in FIG. 1, the receiver 10 includes an automatic frequency correction circuit (AFC) 11, a guard interval cancellation circuit 12, a fast Fourier transform circuit (FFT) 13, and a phase correction circuit (phase correction unit). 14, a determination circuit (judgment unit) 15, a multilevel modulation scheme determination circuit (multilevel modulation scheme determination unit) 16, and a phase correction scheme selection circuit (phase correction scheme selection unit) 17 are provided. do. In the figure, analog signal processing circuits such as signal amplification, frequency conversion (down conversion), interfering wave remove filter, and analog / digital conversion (AD conversion) in the analog unit are omitted.

After the analog-to-digital conversion (AD conversion) of the baseband signal, the automatic frequency correction circuit 11 corrects the phase rotation for each OFDM symbol by using the short preamble SP and the long preamble LP.

The automatic frequency correction circuit 11 first detects a relatively large frequency error between the reference frequency of the transmitter 20 and the reference frequency of the receiver 10 using the short preamble SP. The detection of the frequency error can be performed, for example, by multiplying the conjugate signal of the modulated signal delayed by the basic period T1 by the modulated signal after the basic period T1.

Next, the automatic frequency correction circuit 11 detects the frequency error using the long preamble LP. The detection of the frequency error using the long preamble LP can be performed in the same procedure as the detection of the frequency error using the short preamble SP. By using the long preamble LP, a relatively small frequency error of 1 / (2 * T2) {= fs / (2 * 64)} can be detected.

The automatic frequency correction circuit 11 multiplies the received modulated signal by the inverse phase of the frequency error detected using the short preamble SP and the long preamble LP. As a result, the automatic frequency correction circuit 11 performs phase correction (frequency correction).

The guard interval removal circuit 12 removes the guard intervals GI1 and GI2 added to the modulated signal by the transmitter 20.

The fast Fourier transform circuit 13 performs discrete Fourier transform on the OFDM symbol at a sample frequency corresponding to the reference frequency. As a result, the fast Fourier transform circuit 13 performs multicarrier demodulation that divides the plurality of subcarrier signals. Thus, the components of the complex symbols of each subcarrier are extracted.

The phase correction circuit 14 corrects the phase rotation of the primary modulation symbol due to the frequency error. The phase correction circuit 14 includes an estimation unit 141, an equalization unit 142, and a phase error removal unit 143.

The estimation unit 141 estimates the impulse response of the frequency domain of the transmission path 30 for each subcarrier using the pilot symbols. The impulse response shows propagation characteristics for each subcarrier. The estimation unit 141 regards the existing data of the preamble (synchronization pattern X of the short preamble SP or the long preamble LP sync pattern Y) of the preamble as a pilot symbol, and the phase rotation and amplitude error of the existing data for the modulated signal following the preamble. (Impulse Response per Subcarrier) is estimated. For subsequent modulated signals, pilot symbols are taken at predetermined regular intervals, and phase rotation and amplitude errors are estimated from the existing data. The estimated phase rotation and amplitude errors may be valid until the correction is performed by the next pilot symbol, and the correction value of the next pilot symbol is weighted appropriately to the correction amount of the next pilot symbol and the correction amount of the current pilot symbol. You may use to make it. The pilot symbol may be the existing data of the complex amplitudes of all subcarriers, but the existing data may be embedded only in the subcarriers of the frequency domain with high frequency selectivity. In this case, the propagation characteristic of the subcarrier in which the existing data is not embedded may be derived from the propagation characteristic of the subcarrier in which the existing data is embedded.

The equalization unit 142 multiplies the complex symbol of each subcarrier following the preamble by the inverse characteristic of the impulse response for each subcarrier estimated by the estimation unit 141. Thereby, the equalization unit 142 corrects the nonuniformity of the frequency domain for each subcarrier, and corrects the phase rotation by the frequency error. And in the transmission path with a large amplitude fluctuation, the equalization unit 142 can also correct an amplitude error other than phase rotation.

In this manner, the estimation unit 141 and the equalization unit 142 perform a first phase correction method of correcting the phase of a symbol using a pilot symbol. The phase correction using the pilot symbol performs phase correction for each subcarrier, so that the phase correction effect is great even in a propagation environment with high frequency selectivity.

In the present embodiment, as shown in Fig. 4, four out of 52 subcarriers are pilot subcarriers PSC1 to PSC4 irrelevant to data transmission, and the remaining 48 are subcarriers SC0 to SC47 used for data transmission. The symbols on the pilot subcarriers PSC1 to PSC4 are existing data (predetermined symbols).

The phase error removal unit 143 performs phase correction for each OFDM symbol using four pilot subcarriers PSC1 to PSC4. The phase error removal unit 143 detects the frequency error in each pilot subcarrier from predetermined symbols of the pilot subcarriers PSC1 to PSC4. The phase error removal unit 143 calculates the phase error of each complex symbol which is discrete Fourier transformed from the same OFDM symbol using the detected frequency error. The phase error removal unit 143 then multiplies each complex symbol by an inverse phase of the calculated phase error. As a result, the phase error removing unit 143 corrects the phase rotation of the symbol due to the frequency error.

In this manner, the phase error removal unit 143 performs a second phase correction method of correcting the phase of a symbol using a pilot subcarrier.

The modulation method discrimination circuit 16 determines the multi-level modulation method used for the modulation signal in accordance with the modulation signal received from the transmitter 20. In this embodiment, based on the information of the modulation scheme included in the data portion D of the modulated signal received from the transmitter 20, multi-levels of each OFDM symbol (of each complex symbol transformed from the same OFDM symbol to discrete Fourier transforms) It is determined whether the modulation scheme is one of 16 QAM and 64 QAM.

The correction method selection circuit 17 is configured to determine a plurality of phase correction methods that are used for correcting the phase of the symbol of the modulated signal based on the multi-level class of the multi-level modulation method determined by the modulation method determination circuit 16. Select from phase correction method. In the present embodiment, the correction method selection circuit 17 selects the first phase correction method when the determination result of the modulation method determination circuit 16 is 16 QAM, and when the determination result is 64 QAM, Select the phase correction method.

The phase correction circuit 14 corrects the phase of a symbol by using the phase correction method selected by the correction method selection circuit 17.

The determination circuit 15 determines the bit sequence of the data based on the symbol whose phase has been corrected by the phase correction circuit 14 based on the multi-level modulation scheme determined by the multi-level modulation scheme determination circuit 16. . In more detail, the determination circuit 15 is based on the multi-level modulation scheme determined by the multi-level modulation scheme determination circuit 16, and each complex symbol whose phase has been corrected by the phase correction circuit 14 is corrected. Is converted into a soft decision value by a demapping process. As a result, the determination circuit 15 outputs the bit sequence of the data received from the transmitter 20 to the data processing circuit not shown in the receiver 10 or other than the receiver 10.

The phase error angle θ2 per OFDM symbol is 2.88 ° as described above. Table 2 shows the values of θ1 / θ2 in each of the multi-level modulation systems of QPSK, 16 QAM, and 64 QAM. In addition, Table 2 shows the minimum symbol interval M (maximum positive integer not exceeding [theta] 1 / [theta] 2) in which the pilot symbol is embedded in the modulated signal so as not to exceed the allowable error angle [theta] 1 during the demodulation process. In addition, Table 2 shows transmission efficiency P1 = M / (M + 1) when the pilot symbol is embedded in the modulated signal at the minimum symbol interval M. FIG.

Multi Level Modulation QPSK 16 QAM 64 QAM θ1 / θ2 15.63 5.86 3.66 M 15 5 3 P1 = M (M + 1) 0.94 0.83 0.75

As shown in Fig. 6, in each multi-level modulation method, the pilot symbol PS is embedded in the modulated signal for every M symbols. This prevents the phase error from exceeding the allowable error angle θ1 during the demodulation process. In addition, the transmission efficiency is the highest.

In the first phase correction scheme using the pilot symbol, the correction interval is longer than when using the pilot subcarrier. This is because each equalization parameter M symbols of the equalization unit 142 is updated. Therefore, in the case of 64 QAM, which is a multi-level modulation system of a high multi-level class (at least symbol interval M = 3, and pilot symbol interval is 5, there is a possibility that the phase error exceeds the tolerance angle θ1 during the demodulation process.

On the other hand, the second phase correction method using the pilot subcarriers PSC1 to PSC4 is sequential correction for each OFDM symbol. Therefore, optimal phase correction can be performed for each OFDM symbol (for each complex symbol that is discrete Fourier transformed from the same OFDM symbol). Therefore, even in a multi-level modulation system of a high multi-level class, it is possible to prevent the phase error from exceeding the tolerance error angle θ1 during the demodulation process.

In the first phase correction scheme, it is necessary to shorten the interval between pilot symbols embedded in the modulated signal as the multi-level class increases. Therefore, transmission efficiency tends to be relatively low. As shown in Table 2, the transmission efficiency P1 becomes P1 = 0.83 at 16 QAM and P1 = 0.75 at 64 QAM. On the other hand, in the second phase correction method, four out of 52 subcarriers are used for the pilot subcarriers PSC1 to PSC4. Therefore, the transmission efficiency P2 when the pilot subcarrier is used is 0.92 = (48/52). Thus, the second phase correction scheme realizes higher transmission efficiency than the first phase correction scheme.

As described above, according to the receiver 10 of the present embodiment, the phase correction scheme is selected based on the multi-level class of the multi-level modulation scheme performed by the transmitter 20. Therefore, optimal phase correction can be performed according to the multi-level modulation scheme performed by the transmitter 20.

In particular, when the multi-level class of the multi-level modulation scheme performed by the transmitter 20 is less than a predetermined value (multi-level class corresponding to 64 QAM), the pilot having a large phase correction effect even in a propagation environment with high frequency selectivity Phase correction using symbols is performed. On the other hand, when the multi-level class of the multi-level modulation scheme performed by the transmitter 20 is equal to or greater than a predetermined value (multi-level class corresponding to 64 QAM), phase correction using a pilot subcarrier for realizing high transmission efficiency is performed. . Therefore, optimal phase correction can be performed according to the multi-level modulation scheme performed by the transmitter 20.

By the way, the phase correction system selection circuit 17 may be comprised as follows. In other words, the phase correction method selection circuit 17 determines whether the multi-level level of the multi-level modulation method determined by the multi-level modulation method determination circuit 16 is less than a predetermined value (multi-level level corresponding to 64 QAM). A report of one phase correction scheme is selected, and if the multi level grade is equal to or greater than a predetermined value (multi level grade corresponding to 64 QAM), both the first phase correction scheme and the second phase correction scheme are selected.

In this case, phase correction using pilot symbols is always performed. Therefore, the control of the phase correction circuit 14 is simplified. In the first phase correction method, phase correction is performed for each subcarrier. Therefore, even in a radio wave environment with strong frequency selectivity, the effect of phase correction can always be increased.

By the way, the estimation unit 141 estimates the impulse response of the transmission path 30 in the pilot symbols embedded in the predetermined symbols of the preamble and the modulation signal at regular intervals. In pilot symbols, existing data is embedded in all subcarriers. Therefore, with the use of pilot symbols, highly accurate phase correction can be performed for all subcarriers. However, until the pilot symbol is obtained, the impulse response of the transmission path 30 cannot be estimated. That is, in the phase correction using the pilot symbol, the correction interval becomes long. Therefore, in the case of a multilevel modulation system of a high multi-level class, there is a fear that the tolerance angle θ1 of the primary modulation symbol which is the complex amplitude of the subcarrier is exceeded during the demodulation process.

On the other hand, the phase error removal unit 143 estimates the impulse response of the transmission path 30 in the pilot subcarrier. Therefore, the impulse response of the transmission path 30 can be updated for each OFDM symbol. However, the estimated value of the impulse response for subcarriers other than the pilot subcarrier (that is, the subcarrier used for data transmission) is calculated by using external extrapolation or interpolation from the impulse response of the pilot subcarrier. Therefore, the estimated value of the impulse response of the subcarrier used for data transmission includes an error.

Therefore, when the impulse response is updated only by the phase error removing unit 143, the equalization unit 142 performs equalization with the inverse characteristic calculated from the impulse response after the update, so that an error accumulates for each update of the impulse response. Therefore, the accumulation amount of the error exceeds the allowable amount (permissible error angle θ1) during the predetermined process and there is no effect of phase correction. In particular, since the tolerance angle θ1 is as small as that of the multilevel modulation method having a large multilevel class, the effect of phase correction is likely to be lost.

Therefore, when the multi-level class of the multi-level modulation method determined by the multi-level modulation method determining circuit 16 is equal to or greater than a predetermined value (multi-level class corresponding to 64 QAM) (by the phase correction method selecting circuit 17) When both the first phase correction method and the second phase correction method are selected), the phase correction circuit 14 is preferably configured as follows. In other words, the phase correction circuit 14 corrects the phase of the symbol according to the second phase correction method, and then corrects the phase of the symbol according to the first phase correction method.

In this way, phase correction can be performed for each OFDM symbol using a pilot subcarrier. In addition, an error caused by phase correction using a pilot subcarrier can be eliminated by phase correction using a pilot symbol. Therefore, optimal phase correction can be performed for a multilevel modulation method having a large multilevel class.

By the way, in the above example, the predetermined value of the phase correction method selection circuit 17 is set as the multi level grade corresponding to 64 QAM. However, the predetermined value may be set based on the transmission efficiency when the phase correction circuit 14 corrects the phase of the symbol by using the phase correction method selected by the phase correction method selection circuit 17.

For example, consider the case where the transmitter 20 has QPSK, 16 QAM, and 64 QAM as a multi-level modulation scheme.

In this case, the multi-level modulation method determination circuit 16 determines that the multi-level modulation method used for the modulation signal is QPSK based on the information of the modulation method included in the data portion D of the modulation signal received from the transmitter 20. And one of 16 QAM and 64 QAM.

Here, in the case of the first phase correction scheme, the transmission efficiencies P1 of QPSK, 16 QAM, and 64 QAM are 0.97, 0.83, and 0.75 (see Table 2).

On the other hand, in the case of the second phase correction method, the transmission efficiency P2 is 0.92.

In this case, the phase correction method selection circuit 17 selects phase correction using pilot symbols for the multi-level modulation method in which the transmission efficiency P1 is higher than the transmission efficiency P2 (= 0.92). That is, the phase correction method selection circuit 17 selects the first phase correction method in the case of QPSK having a transmission efficiency P1 of 0.97. On the other hand, phase correction using a pilot subcarrier is selected for the multi-level modulation scheme in which the transmission efficiency P1 is lower than the transmission efficiency P2. That is, the phase correction method selection circuit 17 selects the second phase correction method in the case of 16 QAM in which the transmission efficiency P1 is 0.83. Similarly, the phase correction method selection circuit 17 selects the second phase correction method in the case of 64 QAM in which the transmission efficiency P1 is 0.75.

That is, the phase correction scheme selection circuit 17, when the multi-level rating of the multi-level modulation scheme (QPSK or 16 QAM or 64 QAM) performed by the transmitter 20 is lower than the predetermined value according to the transmission efficiency P2, Phase correction using a pilot subcarrier is performed. On the other hand, the phase correction method selection circuit 17 performs phase correction using pilot symbols when the multi level class of the multi level modulation method performed by the transmitter 20 is equal to or greater than a predetermined value according to the transmission efficiency P2.

In this case, the optimum phase correction can be performed according to the multi-level modulation method performed by the transmitter 20, and the transmission efficiency can be improved.

(Second Embodiment)

The receiver 40 of this embodiment is used for a single carrier communication system.

The transmitter 20 used for the single carrier communication system performs error correction encoding on data transmitted to the receiver 40. In addition, the transmitter 20 generates a complex symbol from the error correction coded data based on the correspondence between the symbol determined by the multi-level modulation method and the bit sequence (symbol mapping). Transmitter 20 performs appropriate waveform shaping processing on complex symbols to perform frequency conversion by multiplying the baseband signal generated by using a symbol sequence passed through a filter that removes an image signal resulting from DA conversion, followed by a carrier wave. After shifting to the required frequency band, a predetermined signal amplification is performed to generate a modulated wave. The transmitter 20 transmits the generated modulated wave to the transmission path 30.

Here, the transmitter 20 has a plurality of multi-level modulation schemes having different multi-level classes, for example, QPSK and 16 QAM. When performing symbol mapping, the transmitter 20 selects a multi-level modulation method of which transmission speed is fastest according to the state of the transmission path 30 among a plurality of multi-level modulation methods having different multi-level classes (that is, the transmitter). 20 performs adaptive modulation).

In addition, the transmitter 20 includes a reference signal source (not shown) having a crystal transmitter. The transmitter 20 performs the frequency conversion or the like using the reference frequency generated by the reference signal source. The reference signal source is also provided to the receiver 40.

The transmitter 20 transmits a modulated wave generated from a plurality of multi-level modulation methods (QPSK and 64 QAM) using a multi-level modulation method selected according to the line quality. The modulated wave has a symbol sequence representing data to be transmitted to the receiver 40. This symbol is determined by a multi-level modulation scheme selected by the transmitter 20 and its correspondence with the bit sequence.

As shown in FIG. 7, the receiver 40 of the present embodiment includes an A / D conversion circuit 41, an FIR filter 42, a down sampling circuit 43, a phase correction circuit 44, and a determination circuit 45. And a multi level modulation method determination circuit (multi level modulation method determination unit) 46, and a phase correction method selection circuit (phase correction method selection unit) 47. In the figure, analog signal processing circuits such as signal amplification in the analog unit, jamming filter, and the like are omitted.

The A / D conversion circuit 41 generates a carrier at a reference frequency at which a reference signal source (not shown) of the receiver 40 occurs. The A / D conversion circuit 41 down-converts the modulated signal by generating the baseband signal by multiplying the modulated signal received through the transmission path 30 with the carrier wave. The A / D conversion circuit 41 converts the baseband signal to analog / digital output and outputs it to the FIR filter 42.

The down sampling circuit 43 downsamples the baseband signal received through the FIR filter 42. The down sampling circuit 43 outputs the down sampled baseband signal to the phase correction circuit 44.

The phase correction circuit 44 includes a phase error removal unit 441, a modulator 442, and a phase estimation unit 443. The phase correction circuit 44 selectively uses two phase correction methods, a phase correction method using remodulation and a phase correction method using pilot symbols, to correct phase rotation due to frequency error.

The phase correction method using the remodulation is performed by the phase error removal unit 441, the modulator 442, and the phase estimation unit 443. When performing the phase correction method using remodulation, the phase error removal unit 441 transmits the output of the down sampling circuit 43 to the determination circuit 45 as it is. The modulator 442 converts the bit sequence determined by the determination circuit 45 into an IQ signal on a complex plane and re-modulates the symbol into a complex number. The phase estimation unit 443 calculates the product of the remodulated signal output from the modulator 442 and the output of the down sampling circuit 43. Thereby, the phase estimation unit 443 calculates a phase error. The phase error removal unit 441 multiplies each complex symbol by an inverse phase (phase coefficient) of the phase error calculated by the phase estimation unit 443. Thereby, the phase error removal unit 441 corrects phase rotation by a frequency error. That is, the phase correction circuit 44 performs phase correction using remodulation. Phase correction using remodulation is performed regularly.

The phase correction method using the pilot symbol is performed by the phase error removal unit 441. Here, the pilot symbol in this embodiment is a predetermined symbol having an existing phase, but is not a multicarrier modulated pilot symbol as in the first embodiment. However, since the phase correction method using this predetermined symbol is the same as that of the first embodiment, description thereof is omitted.

The multi-level modulation method determination circuit 46 determines whether the multi-level modulation method for each packet is one of QPSK and 16 QAM based on the information of the modulation method included in the modulation signal received from the transmitter 20. .

The phase correction method selection circuit 47 selects the phase correction method to be executed by the phase correction circuit 44 based on the multi-level class of the multi-level modulation method determined by the multi-level modulation method determination circuit 46. .

Here, in the case of 16 QAM, the distance between symbols on the complex plane is short, so that the tolerance angle θ1 of each symbol point is smaller than that of QPSK. Therefore, when the multi-level modulation scheme is 16 QAM, if the same phase correction scheme as that of the QPSK is used, there is a possibility that a large number of errors are included in the bit sequence after the demodulation, and thus the complex symbol after the remodulation cannot be said to be accurate.

Therefore, the phase correction method selection circuit 47 selects the phase correction method using remodulation when the determination result of the multi-level modulation method determination circuit 46 is QPSK. The phase correction method selection circuit 47 selects a phase correction method using a pilot symbol when the determination result of the multi-level modulation method determination circuit 46 is 16 QAM.

The determination circuit 45 determines a bit sequence corresponding to a symbol whose phase has been corrected by the phase correction circuit 44 based on the multi-level modulation method determined by the multi-level modulation method determination circuit 46. In more detail, the determination circuit 45 is based on the multi-level modulation method determined by the multi-level modulation method determination circuit 46, and each complex symbol whose phase is corrected by the phase correction circuit 44 is corrected. Is converted into a soft determination value by a demapping process. As a result, the determination circuit 45 outputs the bit sequence of the data received from the transmitter 20 to the data processing circuit not shown in the receiver 40 or other than the receiver 40.

As described above, the receiver 40 of the present embodiment has a multi-level class of the multi-level modulation scheme (QPSK or 16 QAM) performed by the transmitter 20 that is less than the multi-level class corresponding to the predetermined value 16 QAM. In this case, phase correction using remodulation is performed. In addition, the receiver 40 performs phase correction using pilot symbols when the multi-level class of the multi-level modulation scheme performed by the transmitter 20 is equal to or greater than a predetermined value (multi-level class corresponding to 16 QAM). .

As described above, according to the receiver 40, the phase correction scheme is selected based on the multi-level class of the multi-level modulation scheme performed by the transmitter 20. Therefore, optimal phase correction can be performed according to the multi-level modulation scheme performed by the transmitter 20.

Claims

A receiver for use in an adaptive modulation communication system associated with a transmitter configured to transmit a modulated signal generated using a multilevel modulation scheme selected by a predetermined criterion from a plurality of multilevel modulation schemes.
The modulated signal comprises a sequence of symbols representing data transmitted to the receiver,
The symbol has a corresponding relationship with a bit sequence determined by a multi-level modulation scheme selected by the transmitter,
The receiver includes a multi-level modulation method determination unit, a phase correction method selection unit, a phase correction unit, and a determination unit,
The multi-level modulation scheme determination unit is configured to determine, according to the modulation signal received from the transmitter, the multi-level modulation scheme used to generate the received modulation signal,
The phase correction method selection unit is a phase correction used to correct a phase of a symbol of the modulated signal based on a multi level grade of the multi level modulation method determined by the multi level modulation method determination unit. Configured to select a method from a plurality of predetermined phase correction methods,
The phase correction unit is configured to correct the phase of the symbol by using the phase correction method selected by the phase correction method selection unit,
The determination unit is configured to determine a bit sequence corresponding to the symbol whose phase has been corrected by the phase correction unit based on the multi-level modulation method determined by the modulation method determination unit,
The transmitter is configured to perform a primary modulation scheme and a secondary modulation scheme,
The primary modulation scheme is configured as a multilevel modulation scheme that generates a primary modulation symbol representing the symbol and is selected by a predetermined criterion from a plurality of multilevel modulation schemes having different multilevel grades,
The secondary modulation scheme generates a secondary modulation symbol by multiplexing a plurality of subcarriers having a complex amplitude based on the primary modulation symbol, and generates the modulation signal composed of the plurality of secondary modulation symbols. It is configured as a multicarrier modulation scheme,
The modulated signal has a pilot symbol for each predetermined regular interval,
The pilot symbol is configured as a predetermined secondary modulation symbol specific to the receiver,
The predetermined second modulation symbol is provided as a subcarrier having a predetermined complex amplitude specific to the receiver,
The secondary modulation symbol comprises a pilot subcarrier,
The pilot subcarrier is configured as a predetermined subcarrier specific to the receiver,
The predetermined subcarrier has a predetermined complex amplitude specific to the receiver,
The phase correction method selection unit is configured to correct the phase of the symbol by using the pilot symbols when the multi level class of the multi level modulation method determined by the multi level modulation method determination unit is less than a predetermined value. Select a first phase correction scheme and select a second phase correction scheme to correct the phase of the symbol using the pilot subcarrier if the multi-level class is greater than or equal to a predetermined value.

The method of claim 1,
The phase correction method selection unit selects the report of the first phase correction method if the multi level class of the multi level modulation method determined by the multi level modulation method determination unit is less than a predetermined value, and the multi level And select both of the first phase correction method and the second phase correction method if the multi-level class of the multi-level modulation method determined by the modulation method determination unit is equal to or greater than a predetermined value.

The method of claim 2,
The phase correction unit corrects the phase of the symbol according to the second phase correction method when both of the first phase correction method and the second phase correction method are selected by the phase correction method selection unit. And then correct the phase of the symbol in accordance with the first phase correction scheme.

The method of claim 1,
And the predetermined value is selected based on a transmission efficiency when the phase correction unit corrects the phase of the symbol by using the phase correction method selected by the phase correction method selection unit.

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