JP5633843B2 - OFDM transmission apparatus and method - Google Patents

Description

  The present invention relates to an OFDM transmission apparatus and method used for digital mobile communication using orthogonal frequency division multiplexing (OFDM).

  In recent years, digital mobile communication using an Orthogonal Frequency Division Multiplexing (OFDM) system has been put into practical use (for example, see Patent Document 1). This OFDM system is a communication system having various advantages such as high resistance to multipath transmission paths and high frequency utilization efficiency. In the OFDM system, data is mapped to a large number of subcarriers arranged so as to be orthogonal to each other, and a time series signal is derived by performing inverse Fourier transform on the data group, and a transmission symbol for radio transmission is created. On the receiving side, a transmitted data group is derived by Fourier transforming such symbols, and the transmission data is reproduced by restoring individual data associated with each subcarrier.

In a general OFDM transmitter, as shown in FIG. 9, the system band includes a large number of subcarriers. Data to be transmitted is mapped to each subcarrier. The pilot subcarrier is inserted into the data subcarrier for frequency offset detection and time synchronization shift detection. In order to prevent unnecessary signals from leaking out of the band, null subcarriers as zero symbols (which have a signal value of zero) that do not perform data transfer are assigned to both ends of the band. Incidentally, this null subcarrier corresponding to the center frequency f ₀ is also allocated in the DC subcarrier.

JP 2009-225111 A

  By the way, in the conventional OFDM system described above, it is assumed that the channel bandwidth allocated to each channel transmitted by OFDM is equivalent to the system bandwidth preset in the OFDM transmitter. Usually, the channel bandwidth allocated to each channel transmitted by OFDM is often 5 MHz, and the system bandwidth preset in the OFDM transmitter is 5 MHz so that it is equivalent to this. It is set to be.

  However, particularly in recent years, it is conceivable that the television broadcast is switched from the analog broadcast to the digital broadcast, and the vacant frequency band is increased. As a result, it has been studied to use this idle frequency more effectively. In particular, there is a possibility that this idle frequency can be used for OFDM communication. As a result, as shown in FIG. 10, the channel bandwidth allocated to each channel transmitted by OFDM may be allocated 6 MHz instead of the conventional 5 MHz. As a result, the channel bandwidth is set in a wide range while including the system bandwidth set in advance from the beginning.

  The system bandwidth in the conventional OFDM transmitter is based on the premise that null subcarriers are allocated to both ends of the band. For this reason, as shown in FIG. 10, although the channel bandwidth is set wide and there is room, leakage of unnecessary signals outside the band is less likely to occur. Null subcarriers are assigned to both ends. For this reason, the channel bandwidth allocated to each channel cannot be effectively utilized by providing such null subcarriers.

  For example, as shown in FIG. 11, in the system band, the numbers of the subcarriers are assumed to be −X−Y−1 to X + Y in order. Here, the total number of subcarriers is 2X + 2Y + 2. Here, X is a natural number, and the number of null subcarriers (guard subcarriers) allocated to both ends of the band is 2Y. As a result, the number of carriers into which data subcarriers or pilot subcarriers can be inserted is 2X + 2Y + 2-2Y-1 = 2X + 1. Here, it is the DC subcarrier that subtracts 1 at the end.

  Further, the ratio of the number of carriers into which data subcarriers or pilot subcarriers can be inserted to the entire system bandwidth is expressed by the following equation (1).

  (2X + 1) / (2X + 2Y + 2) × 100 (1)

  As can be seen from the equation (1), the ratio of the number of carriers into which data subcarriers or pilot subcarriers can be inserted to the entire system bandwidth is greatly reduced due to the presence of guard subcarriers.

  For example, in the IEEE 802.11 system, the total number of subcarriers in the system band is 64 (= 2X + 2Y + 2). The number of carriers into which data subcarriers or pilot subcarriers can be inserted is 52 (= 2X + 1). At this time, the DC subcarrier is 1, and the number of remaining subcarriers is 11 (= 2Y). In such a case, the ratio calculated by the equation (1) is 52/64 × 100≈81%.

  Consider a case where 17% of the entire system band is allocated as a guard band region. At this time, in the IEEE 802.11 system, 4.15 MHz is assigned to the system bandwidth 5 MHz, 8.3 MHz is assigned to the system bandwidth 10 MHz, and 16.6 MHz is assigned to the system bandwidth 20 MHz. become.

  In addition to the equation (1), if the channel bandwidth is further widened, and the channel bandwidth allocated to each channel transmitted by OFDM is widened, the denominator of the equation (1) is reduced accordingly. growing. As a result, the ratio of the number of carriers into which data subcarriers or pilot subcarriers can be inserted in the allocated channel bandwidth further decreases.

  For this reason, when the newly given channel bandwidth becomes wider than the conventional system bandwidth, the channel bandwidth is increased by increasing the ratio of the number of carriers into which such data subcarriers can be inserted. It was necessary to utilize it more effectively.

  In addition to this, when the newly assigned channel bandwidth becomes wider than the conventional system bandwidth, the conventional mapping method of each subcarrier of OFDM can be significantly changed. For example, the modification cost and the cost for version upgrade accompanying this increase, and the burden of work labor associated with the modification also increases.

  Therefore, the present invention has been devised in view of the above-described problems, and the object of the present invention is to provide a new channel bandwidth that is wider than the system bandwidth. Another object of the present invention is to provide an OFDM transmission apparatus and method capable of increasing the proportion of the number of carriers into which data subcarriers and the like can be inserted at a lower cost and with less labor and making more effective use of the channel band.

  In order to solve the above-described problem, an OFDM transmission apparatus according to the present invention is an OFDM transmission apparatus that transmits an orthogonal frequency division multiplexing (OFDM) signal, and a channel bandwidth allocated to each channel transmitted by OFDM. Is provided with means for allocating data subcarriers or pilot subcarriers instead of allocating null subcarriers to both ends of the system band when the OFDM transmission apparatus covers a wider range of preset system bandwidths. Features.

  In order to solve the above-described problem, an OFDM transmission apparatus according to the present invention is a channel bandwidth allocated to each channel transmitted by OFDM in an OFDM transmission method for transmitting an orthogonal frequency division multiplexing (OFDM) signal. However, when the OFDM transmission apparatus broadly covers a preset system bandwidth, data subcarriers or pilot subcarriers are allocated instead of null subcarriers at both ends of the system band. .

  According to the present invention having the above-described configuration, when data subcarriers or pilot subcarriers are allocated to the edge of the system band, the tail of the carrier leaks from the system band. Even if the signal is leaked, the allocated channel bandwidth is wider than the system bandwidth, so that it is possible to prevent leakage of unnecessary signals outside the channel band.

  That is, according to the present invention, even when the newly given channel bandwidth becomes wider than the system bandwidth, the ratio of the number of carriers that can insert data subcarriers and the like at a lower cost and with less effort It is possible to provide an OFDM transmitter capable of increasing the channel bandwidth and more effectively utilizing the channel bandwidth.

It is a block block diagram which shows the structure of the OFDM transmitter which applied this invention. It is a figure which shows the process in the IFFT part of the OFDM transmitter to which this invention is applied. It is a block block diagram which shows the structure of the OFDM receiver to which this invention is applied. It is a figure for demonstrating the method to actually allocate a data subcarrier etc. by the OFDM transmitter to which this invention is applied. It is a figure for demonstrating the effect of the OFDM transmitter to which this invention is applied. It is a figure which shows the case where the number of each subcarrier is made into -X-1-X in order in a system band. It is a figure for demonstrating the state of the subcarrier allocated in this invention. It is a figure which shows the example of the new data subcarrier etc. for IEEE802.11 in TVWS. It is a figure which shows the arrangement | sequence state of the subcarrier of the system band in a general OFDM transmitter. It is a figure which shows the example by which the channel bandwidth is set rather than the system bandwidth. It is a figure for demonstrating the problem of a prior art.

  Hereinafter, embodiments of the present invention will be described in detail.

  FIG. 1 is a block diagram showing a configuration of an OFDM (Orthogonal Frequency Division Multiplexing) transmission apparatus 1 to which the present invention is applied.

  The OFDM transmitter 1 includes a subcarrier arrangement unit 11, a pilot insertion unit 12, a serial / parallel (S / P) unit 13, an IFFT unit 14, a parallel / serial (P / S) unit 15, a guard, An interval (GI) insertion unit 16, a D / A conversion unit 17, an LPF unit 18, a frequency conversion unit 19, a radio transmission unit 20, and an antenna 21 are sequentially connected.

  The subcarrier arrangement unit 11 arranges the input transmission signals and generates parallel complex signals of each series. Specifically, the subcarrier arrangement unit 11 includes a symbol before coordinate conversion and a symbol after coordinate conversion that are symmetrical with respect to the symmetry axis on the IQ plane used for coordinate conversion. The parallel complex signals of the respective sequences are generated so as to be arranged on subcarriers symmetrical on the frequency axis with respect to the center frequency of a predetermined communication band. Then, the subcarrier arrangement unit 11 outputs the generated parallel complex signal of each sequence to the pilot insertion unit.

  The pilot insertion unit 12 inserts a known signal for channel estimation by time multiplexing into a transmission signal composed of parallel complex signals transmitted from the subcarrier arrangement unit 11. Then, the pilot insertion unit 12 outputs the transmission signal of each sequence into which the transmission path estimation known signal is inserted to the S / P unit.

  The S / P unit 13 outputs a complex baseband signal of an I component signal and a Q component signal generated by performing primary modulation using a modulation method such as QPSK or 16QAM by a modulator (not shown). Convert to parallel signal of (natural number) series. Then, the S / P unit 13 outputs the m-sequence transmission signal to the IFFT unit 14.

  The IFFT unit 14 performs an IFFT operation on the parallel complex signal output from the S / P unit 13. Then, IFFT unit 14 outputs the transmission signal after the IFFT calculation to P / S unit 15. FIG. 2 illustrates the processing in the IFFT unit 14. In FIG. 2 (a), Y = 0 and IFFT calculation is performed on data subcarriers or pilot subcarriers having subcarrier numbers # 0 to #X and # -X-1 to # -1. Is an example in which is a time-axis signal. At this time, the DC subcarrier is subcarrier number # 0. In FIG. 2B, Y = 0.5 and IFFT calculation is performed on data subcarriers or pilot subcarriers having subcarrier numbers # 0 to #X and # -X-1 to # -1. This shows an example in which this is a time axis signal. At this time, the DC subcarrier is subcarrier number # 0, and the subcarrier number # -X-1 is a null subcarrier in the center.

  The P / S unit 15 converts the multicarrier signal output from the IFFT unit 14 into a time-series signal and outputs it to the GI insertion unit 16.

  The GI insertion unit 16 inserts a GI into the time series signal output from the P / S unit 15 and outputs it to the D / A conversion unit 17.

  The LPF 18 removes the aliasing component generated along with the D / A conversion from the analog signal output from the D / A unit 109. Then, the LPF 18 outputs the analog signal from which the aliasing component is removed to the frequency converter 19.

  The frequency conversion unit 19 up-converts the analog signal output from the LPF 18 from the baseband frequency to the intermediate frequency and outputs it to the radio transmission unit 20.

  The radio transmission unit 20 up-converts the intermediate frequency analog signal output from the frequency conversion unit 19 from the intermediate frequency frequency to the radio frequency, amplifies the analog signal, and outputs the amplified signal to the antenna 21.

  The antenna 21 transmits an analog signal of a radio frequency output from the radio transmission unit 20 as an OFDM signal.

  FIG. 3 is a block diagram showing a configuration of the OFDM receiver 2 to which the present invention is applied. This OFDM receiver 2 includes an antenna 31, a radio reception unit 32, an orthogonal detection unit 33, an LPF 34, an A / D conversion unit 35, a GI removal unit 36, an S / P unit 37, and an FFT unit 38. The P / S unit 39, the subcarrier conversion unit 40, and the decoding processing unit 41 are sequentially connected.

  The antenna 31 receives the OFDM signal transmitted from the above-described OFDM transmission device 31 and outputs it to the wireless reception unit 32.

  The radio reception unit 32 down-converts the reception signal output from the antenna 31 from the radio frequency to the IF frequency, and outputs it to the quadrature detection unit 33.

  The quadrature detection unit 33 performs analog quadrature detection on the reception signal of the IF frequency output from the radio reception unit 32, and outputs the I component and the Q component to the LPF 34, respectively.

  The LPF 34 outputs to the A / D unit 35 the baseband in-phase component obtained by limiting the band by passing only the low frequency bands of the I component and Q component output from the quadrature detection unit 33.

  The A / D conversion unit 35 converts the I component and Q component output from the LPF 34 from an analog signal to a digital signal and outputs the converted signal to the GI removal unit 36.

  The GI removal unit 36 removes the GI and outputs it to the S / P unit 37.

  The S / P unit 37 converts the I component and Q component output from the GI removal unit 36 into N series parallel signals in units of OFDM symbols. Then, the S / P unit 37 outputs the converted N-sequence parallel signal to the FFT unit 38.

  The FFT unit 38 performs an N-point FFT operation on the N series of parallel signals output from the S / P unit 37. The FFT unit 38 converts the N series parallel signals from the frequency domain to the time domain by performing FFT. Then, the FFT unit 38 outputs a parallel signal that is a complex number after the FFT calculation to the P / S unit 39.

  The P / S unit 39 converts the 2m-sequence parallel signal output from the FFT unit 38 into a serial signal and outputs it as a received signal. The reception signal output from the P / S unit 39 is a complex baseband signal of I component and Q component subjected to primary modulation.

  The coordinate conversion unit 40 performs the coordinate conversion again so as to return to the state before the coordinate conversion. Then, the coordinate conversion unit 40 outputs the coordinate-converted signal to the decoding processing unit 41.

  The decoding processing unit 41 performs a decoding process on the signal output from the coordinate conversion unit 40.

  Next, a method for actually allocating data subcarriers or pilot subcarriers by the OFDM transmitter 1 having the above-described configuration will be described.

  For example, as shown in FIG. 4, in the present invention, a case is considered where the channel bandwidth allocated to each channel transmitted by OFDM includes a wider system bandwidth preset in the OFDM transmitter 1. Try. More specifically, consider the case where the allocated channel bandwidth is not 5 MHz but 6 MHz. In the example of FIG. 4, a non-use band is formed in which no carrier is allocated at the outer edge of the system band within the channel band.

  In such a case, in the present invention, data subcarriers or pilot subcarriers are allocated instead of null subcarriers at both ends of the system band. As shown in FIG. 4, the both ends of the system band are frequency regions near the end of the band in the system band, and a certain amount of bandwidth may be provided. That is, both ends of the system band may be a frequency region in which null subcarriers are intentionally provided in order to prevent leakage of unnecessary signals outside the band. In FIG. 4, the carrier at the end of the system band is a data subcarrier, but it is needless to say that it may be a pilot subcarrier.

  Actually, the process of assigning data subcarriers or pilot subcarriers to both ends of the system band in such a frequency axis is executed through the processes of the above-described subcarrier arrangement unit 11, pilot insertion unit 12, and the like, for example. It will be. Further, the OFDM receiver 2 receives the ODFM signal having such a configuration and performs a work of reading the signal.

  According to the present invention, if a data subcarrier or a pilot subcarrier is assigned to the edge of the system band as shown in FIG. 5, the tail of the carrier leaks from the system band. Even if a carrier leaks from this system band, the allocated channel bandwidth extends over a wider band than the system band, so that unnecessary signal leakage outside the channel band is prevented. Is possible.

  FIG. 6 shows a case where the numbers of the subcarriers are set to −X-1 to X in order in the system band. Here, the total number of subcarriers is 2X + 2. Here, X is a natural number. The total number of DC subcarriers is 1, which are assigned as null subcarriers. Therefore, the number of carriers that can be inserted as data subcarriers or pilot subcarriers is 2X + 1. Here, it is the DC subcarrier that subtracts 1 at the end. In the example of the present invention, the guard subcarriers conventionally assigned in the system band are set to zero.

  As a result, the ratio of the number of carriers into which data subcarriers or pilot subcarriers can be inserted to the entire system bandwidth is expressed by the following equation (A).

  (2X + 1) / (2X + 2) × 100 (A)

  When the system bandwidth is SBW and the channel bandwidth is CBW, the ratio of the number of carriers into which data subcarriers or pilot subcarriers can be inserted to the entire channel bandwidth is expressed by the following equation (B).

  (2X + 1) / (2X + 2) x (SBW / CBW) x 100 (B)

  In the present invention, it is possible to insert data subcarriers or pilot subcarriers over the entire system bandwidth. It is assumed that when the system bandwidth is 5 MHz, 10 MHz, and 20 MHz, channel bandwidths of 6 MHz, 12 MHz, and 24 MHz are allocated, respectively. At this time, in the present invention, an unused band is formed in which no carrier is allocated at the outer edge of the system band. The ratio of the unused area to the entire channel band is 16.6%, respectively. It becomes. In other words, this non-use area is a band outside the system band and to which no carrier can be allocated, so it can be considered as a so-called guard band area.

  Consider the case where it is desired to use 48 data subcarriers in IEEE 802.11. If all subcarriers in the system band can be used, 59 subcarriers can be allocated as data subcarriers. This means an increase in data output of about 23%. Here, the number of carriers in the entire channel bandwidth in the equations (A) and (B) is 64 (= 2X + 2). In the conventional IEEE802.11 system, when 48 subcarriers are allocated in the system bandwidth of 5 MHz, 48/64 = 75%. On the other hand, according to the present invention, a case is considered in which a channel bandwidth of 6 MHz is allocated to a system bandwidth of 5 MHz in IEEE 802.11. At this time, in the system band, it is possible to allocate data subcarriers or pilot subcarriers at 59/64 × 100 = 92.19% according to equation (A). In addition, as a ratio to the entire channel band, it is possible to allocate data subcarriers or pilot subcarriers at 59/64 × 5/6 × 100 = 76.82% according to equation (B).

  In such a case, since two additional pilot subcarriers are necessary from the viewpoint of improving communication quality, the number of carriers that can be allocated to data subcarriers is 57. The ratio to the entire system band that can allocate data subcarriers is 57/64 × 100 = 89.06% based on the equation (A). Further, the ratio to the entire channel band where data subcarriers can be allocated is 57/64 × 5/6 × 100 = 74.22% based on the equation (B).

  That is, according to the present invention, the area used as the guard subcarrier in the conventional IEEE 802.11 system can be used for other purposes and pilot signals. Further, on the OFDM receiving apparatus 2 side, it is possible to use the surplus carrier region to reduce the peak ratio (PAPR: Peak to Average Power Ratio) with respect to the output of the entire system without particularly modifying the system. Become. Although this PAPR is an important problem in the OFDM communication system, it is possible to suitably use such surplus carriers for this PAPR reduction.

FIG. 7 (a) is a conventional model showing a configuration example of subcarriers when the FFT size is 64. FIG. In the conventional model shown in FIG. 7 (a), data subcarriers and pilot subcarriers are assigned to the entire system band. However, as an exception, null subcarriers are assigned to DC subcarriers. The subcarrier numbers are # -26 to # 26. Of each subcarrier, data subcarriers are assigned to d ₀ , d ₁ , d ₂ ,..., D ₄₆ , d ₄₇ in total 47 pieces. Also, among the subcarriers, pilot subcarriers, P _-21, P _-7, 4 pieces of P _7, P ₂₁ are allocated.

FIG. 7 (b) shows the state of subcarriers assigned in the present invention. Consider a case where P _-27 and P ₂₈ are newly assigned as two pilot signals. At this time, the total number of pilot subcarriers is 6, which is increased compared to the conventional four. For this reason, null subcarriers are only DC subcarriers. The remaining subcarriers can be used for data subcarriers. Therefore, in the example shown in FIG. 7B, it is possible to allocate 57 data subcarriers from d ₀ to d ₅₆ .

  Incidentally, FIG. 7B is only an example, and is not limited to the above-described subcarrier configuration, and may be replaced with any other subcarrier configuration.

  As an advantage of the carrier configuration described above, it is possible to make the basic design in IEEE802.11 composed of subcarrier numbers # -26 to # 26 unchanged. Further, as shown in FIG. 7B, it is possible to provide new subcarrier designs up to # -32 to # 31. In addition, for # -26 to # 26 in the old design, it is possible to include information in the subcarrier as well.

FIG. 8 shows an example of new data subcarriers and pilot subcarriers for IEEE 802.11 in TVWS (TV White Spaces). In the example of FIG. 8, six pilot subcarriers (P _-28 , P _-17 , P _-6 , P ₅ , P ₁₆ , P ₂₇ ) are used. That is, the number of pilot carriers of 6 means that the number of pilot carriers is 2 more than that of the conventional number of pilot carriers of 4. Therefore, there are 56 data subcarriers and one DC subcarrier. The proportion of pilot subcarriers in the entire subcarriers can be increased, and communication performance can be improved.

DESCRIPTION OF SYMBOLS 1 OFDM transmitter 2 OFDM receiver 11 Subcarrier arrangement | sequence part 12 Pilot insertion part 13 Serial / parallel (S / P) part 14 IFFT part 15 Parallel / serial (P / S) part 16 GI insertion part 17 D / A conversion part 18 LPF unit 19 Frequency conversion unit 20 Radio transmission unit 21, 31 Antenna 32 Radio reception unit 33 Quadrature detection unit 34 LPF
35 A / D conversion section 36 GI removal section 37 S / P section 38 FFT section 39 P / S section 40 Subcarrier conversion section 41 Decoding processing section

Claims (2)

In an OFDM transmitter that transmits orthogonal frequency division multiplexing (OFDM) signals,
Null subcarriers are allocated to both ends of the system band when the channel bandwidth allocated to each channel transmitted by OFDM encompasses a wider system bandwidth preset in the OFDM transmitter An OFDM transmitter characterized by comprising means for allocating data subcarriers or pilot subcarriers instead. In an OFDM transmission method for transmitting orthogonal frequency division multiplexing (OFDM) signals,
Null subcarriers are allocated to both ends of the system band when the channel bandwidth allocated to each channel transmitted by OFDM encompasses a wider system bandwidth preset in the OFDM transmitter An OFDM transmission method characterized by assigning data subcarriers or pilot subcarriers instead.

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