JP2012124852A - Wpma transmission scheme - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allocate user resources that can provide both excellent BER (Bit Error Rate) characteristic and PAPR (Peak to Average Power Ratio) characteristic in an uplink of a digital mobile wireless communication.SOLUTION: In a WPMA (Wavelet Packet Multiple Access) transmission scheme in which wavelet modulation with time and frequency resolutions is user-multiplexed, the total sum of reception power in each spectrum of a wavelet is calculated for each user by a transmission side user resource allocation algorithm (a block 12). A corresponding wavelet is allocated to a user with the maximum total sum.

Description

  The present invention relates to a WPMA (Wavelet Packet Multiple Access) transmission system in which wavelet packet modulation (hereinafter referred to as WPM) having time and frequency resolution is multiplexed, and in particular, assigns user resources using wavelet packets. About things.

  As a conventional method, there is an OFDMA (Orthogonal Frequency Division Multiple Access) method (see Non-Patent Document 1). This method enables effective use of frequencies by limiting an ideal frequency band and multipath fading interference removal with a low amount of computation in the frequency domain. Moreover, there exists a thing of the nonpatent literature 2 as a WPMA transmission system.

S. Pietrzyk, “OFDMA for Broadband Wireless Access,” Artech House, 2006. Susumu Sugawara, “Wavelet Beginners Guide” Tokyo Denki University Press, 1995

  In these transmission methods, in the uplink from the mobile terminal to the base station, instantaneous power-to-average power ratio (PAPR) characteristics, which are one of the cost reduction indicators of the terminal, and bit errors It is necessary to achieve both bit error rate (BER) characteristics.

  An object of the present invention is to allocate user resources that can provide both excellent BER characteristics and PAPR characteristics in the uplink of digital mobile radio communications.

In order to achieve the above object, according to the first aspect of the present invention, in the WPMA (Wavelet Packet Multiple Access) transmission system in which wavelet modulation having time and frequency resolution is user-multiplexed, the sum of received power in each wavelet spectrum. Is calculated for each user, and a wavelet corresponding to the user with the largest sum is assigned to realize multiple access.
In the invention according to claim 2, in the WPMA transmission system according to claim 1, the MSM (Multi Scale Modulation) type having the property that the frequency width is wider as the time width is narrower and the frequency width is narrower as the time width is wider. A frame configuration is used.
The invention according to claim 3 is characterized in that in the WPMA transmission system according to claim 1 or 2, transmission frequency domain equalization is performed in the uplink.

It is a block diagram which shows the system configuration | structure of the WPMA transmission system in one Embodiment of this invention. It is a figure which shows the frame structure (a) of a WPMA transmission system, and the mode (b) of a reconstruction. It is a figure which shows the example of the fading characteristic of each user when the number of users is 3. It is a figure which shows a power spectrum when a signal is input only into each cell in a WPMA transmission system. It is a figure which shows the flowchart of the proposal algorithm in the user allocation method of a WPMA transmission system. It is a model figure which shows the outline of a multipath channel model. FIG. 6 is a diagram showing the results of simulation, and shows the BER characteristics when E _b / N ₀ = 0 to 20 [dB]. The figure which shows the result of simulation shows the PAPR characteristic.

  In the present embodiment, a new algorithm is proposed for an allocation method in which a user to which information is allocated is selected from WPMA power spectrum and user channel information for WPMA in which wavelet modulation WPM having time and frequency resolution is user-multiplexed. . In the present embodiment, the influence of fading on the communication path is reduced by performing precoding such as transmission equalization at the time of information transmission. As a result, according to the present embodiment, excellent PAPR and BER characteristics can be realized.

  Hereinafter, the present embodiment will be specifically described with reference to the drawings. First, the configuration of the WPMA transmission system in this embodiment will be described, and then the transmission characteristics will be described using simulation results of error rate characteristics in a noise environment and a multipath fading environment.

First, the WPMA system model will be described. The block diagram is shown in FIG. On the transmitter side, a QPSK (Quadrature Phase Shift Keying) signal is assigned to an MSM (Multi Scale Modulation, hereinafter MSM) type frame configuration (block 11). The frame configuration is shown in FIG. This frame configuration has the property that the frequency width is wider as the time width is narrower and the frequency width is narrower as the time width is wider, and is a frame configuration reflecting a signal. If the integers j and k are the frequency and symbol numbers in the time direction, respectively, and the discrete wavelet transform DWT (Discrete Wavelet Transform) of an arbitrary function f (x) is d _k ^(j) , then d _k ^(j) is Is defined as

  Here, ψ (x) is a Haar Wavelet function and is defined as follows.

  The assigned QPSK signal is modulated by being reconstructed by an IDWT (Inversed Discrete Wavelet Transform) in block 13. The reconstruction algorithm is expressed by the following equation. FIG. 2B shows the state of reconstruction.

  Thereafter, transmission equalization processing (block 14) is performed. Perform Fast Fourier Transform (FFT), add transmission equalization weights, and convert back to time signal by Inverse Fast Fourier Transform (IFFT), then GI (Guard Interval ) (Block 15) to transmit the signal. On the receiver side, the GI is removed (block 16), the subcarrier assigned to itself is selected, demodulated by the decomposition algorithm (block 17), and divided into the original time frequency components. The decomposition algorithm is expressed by the following equation.

  Then, by making the frame configuration known, QPSK demapping (block 18) is performed to obtain information.

  Next, the user resource allocation algorithm (block 12) shown in FIG. 1 will be described in detail. The present invention is directed to an uplink system. In a multi-user system, since the communication path is different for each terminal, the influence of frequency selective Rayleigh fading due to multipath is different. FIG. 3 shows an example of user characteristics of N = 16, the number of users, 3 and 8-path 1 dB attenuated quasi-static discrete Rayleigh fading, where T is the symbol length and N is the number of subcarriers. In FIG. 3, it is discretized by a discrete frequency interval Δf = 1 / T.

  In the OFDMA scheme, user allocation is performed by selecting a user with a high received signal level in the fading frequency domain. However, since the WPMA scheme has temporal resolution, it is possible to correspond to the subcarrier in FIG. 3 on a one-to-one basis. Can not.

Therefore, the WPMA method focuses on the discrete power spectrum. FIG. 4 shows a discrete power spectrum when the QPSK signal is inputted only to each cell in the figure and the discrete Fourier transform is performed in the MSM type frame configuration shown in FIG. Cells 1 to 16 correspond to c ₀ ⁽⁻⁴⁾ to d ₇ ⁽⁻¹⁾ in FIG. In this discrete power spectrum, for example, if a signal is assigned to cell 1, it can be understood that the information is almost concentrated on a frequency of 0 [Hz] (see reference numeral 41).

  Here, when considering together with FIG. 3 which is the fading frequency spectrum of each user, high quality transmission can be performed by allocating subcarriers to users with a high fading reception signal level at a frequency of 0 [Hz]. Can be expected.

Such an allocation method is similarly performed for all cells. A flowchart of the proposed algorithm in the allocation method is shown in FIG. The algorithm will be described assuming that the number of subcarriers is N, the total number of users is M, the discrete frequency interval is Δf, the discrete power spectrum of cell k is C _k (f), and the discrete fading characteristic of user j is U _j (f). The discrete fading characteristic of user j is obtained by feedback from the receiving side.

First, k = 1 is set as an initial value (step 51). Thereafter, j = 1 to M and j are changed by the number of all users, and A _{k, j} in the following equation is calculated for each of them (step 53).

Then, paying attention to the user j having the largest A _{k, j} , the subcarriers in cell k are assigned to the user (step 54). That is, the sum of received power in each wavelet spectrum is calculated for each user, and the wavelet corresponding to the user with the largest sum is assigned.

  Finally, if k <N, the algorithm is repeated with k = k + 1. When k reaches the number N of subcarriers (step 52: Yes), the algorithm is terminated.

  Next, the transmission equalization method of the present invention will be described. In the uplink, the OFDMA method can completely separate signals between frequencies, and therefore, it is possible to perform frequency domain equalization FDE (Frequency Domain Equalization), which is a fading removal operation, on the receiving side.

  On the other hand, a certain frequency in the uplink of WPMA is shared by a plurality of users, but the transmission signal of each user receives different fading signals, and cannot be completely separated between frequencies on the receiving side. Therefore, even if FDE is performed on the receiving side, orthogonality between users is only partially reproduced, resulting in large inter-user interference.

  This occurs because the channel transfer function is not flat but frequency selective. Therefore, transmission equalization is performed to equivalently flatten the transfer function of the channel. It is possible to cancel interference by performing transmission equalization in advance on the transmission side using frequency domain equalization weights that are inverse functions of the channel transfer function.

Without transmission equalization, the number of users in the uplink is N, the transmission signal of user k is s _k , the channel matrix is C _k , the wavelet inverse transform is H ^H , the fast Fourier transform is F, and the inverse transform is performed When F ^H and Λ _k are diagonal matrices and the noise is n, the received signal r at the base station is expressed by the following equation.

  If the signal after FDE is performed to extract the information of the user k on the receiving side is represented by r ′, the signal is expressed as follows.

Thus, even if FDE is performed on the receiving side, orthogonality is only partially reproduced, and inter-user interference occurs. On the other hand, when transmission equalization is performed and equalization weight F ^H Λ′F is added at the time of transmission, the received signal r is expressed by the following equation.

    If the wavelet transform H is performed on the receiving side by this operation, it is possible to take out the transmission signal of each user.

  Computer simulations of the system of the present invention are performed and their characteristics are compared. The communication channel is a multipath number 8 power 1 dB exponentially attenuated quasi-static Rayleigh fading channel. The delay profile of this communication path is shown in FIG.

Each parameter is assumed to be gray mapping QPSK modulation, the number of subcarriers N = 64, one symbol time T _s = 0.001 [sec], and a guard interval length of 8.

FIG. 7 shows the BER characteristics when E _b / N ₀ = 0 to 20 [dB] by computer simulation. Further, FIG. 8 shows the result of PAPR characteristics when the number of users is two. In FIG. 8, a complementary cumulative probability distribution function (CDDF) is used for evaluation on the vertical axis. As comparison targets, BER characteristics of OFDMA that performs reception ZF (Zero Forcing) equalization and OFDMA that performs transmission ZF transmission equalization, which are conventional methods, are shown. Since this time is an uncoded system, the receiving side does not change the characteristics even if MMSE equalization is performed. Also, this time, all users are assumed to be received in complete synchronization, and receiver timing recovery and channel estimation are ideal. As shown in FIG. 7, when the number of users is 1 or 2, the BER characteristics of the present invention and the transmission equalization OFDMA are superior to the conventional reception equalization OFDMA. When the number of users is 4, Eb / N0 is up to about 6.5 [dB]. The reception equalization OFDMA, which is a conventional technique, has the best characteristics (see reference numeral 71), but it can be seen that the BER characteristics of the present invention and the transmission equalization OFDMA are excellent thereafter. In addition, as shown in FIG. 8, the PAPR characteristics are substantially the same between the conventional equalization OFDMA and the transmission equalization OFDMA, and the present invention has a characteristic that the PAPR is reduced by about 1 dB with respect to 0.01 CCDF compared to the transmission equalization OFDMA. (See reference numeral 81).

  As described above, according to the present embodiment, each of the wavelet in the multi-access WPMA (Wavelet Packet Multiple Access) transmission scheme of the WPM (Wavelet Packet Modulation) transmission scheme among the digital mobile radio communication schemes. It is possible to provide a multi-access WPMA transmission system that calculates a sum of received power in a spectrum for each user and assigns a wavelet corresponding to the user having the largest sum to realize multiple access. With such a user resource allocation method of the multi-access transmission method, excellent BER characteristics and PAPR characteristics can be realized. In the present embodiment, the above-described WPMA transmission scheme uses an MSM frame configuration that is one of the configurations of the time / frequency division region. Furthermore, by using transmission frequency domain equalization in the uplink in the above-described WPMA transmission method, transmission without inter-user interference at the receiving base station can be realized.

  In the above-described embodiment, each block shown in FIG. 1 and each step shown in FIG. 5 are grasped as means for realizing each function, and are appropriately configured using hardware and software. obtain.

Claims (3)

  In the WPMA (Wavelet Packet Multiple Access) transmission method, in which wavelet modulation with time and frequency resolution is user-multiplexed, the sum of received power in each wavelet spectrum is calculated for each user, and the sum corresponds to the user with the largest sum. A WPMA transmission system characterized by assigning wavelets to realize multiple access.   2. The WPMA transmission system according to claim 1, wherein an MSM (Multi Scale Modulation) type frame configuration having a property that the frequency width is wider as the time width is narrower and the frequency width is narrower as the time width is wider is used. WPMA transmission method.   The WPMA transmission system according to claim 1 or 2, wherein transmission frequency domain equalization is performed in the uplink.