CN100521673C - Down link frequency division multiple access switching in method of frequency selecting block transmitting system - Google Patents
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Abstract
A method for switching in downlink TDMA of frequency selection unblocking transmission system includes initializing distribution of subchannel set, sending an auxiliary data to mobile table switched in current base station ( BS ) by BS, selecting flag information and sending it to BS by mobile table, carrying out orthogonal transform on modulated signal of mobile table and setting up downlink of communication from BS to mobile table by BS, transforming sampled signal to frequency domain and selecting out usable signal by mobile table, changing subchannel set by BS according to requirement and carrying out communication with BS by mobile table according to received information.
Description
(I) technical field
The invention relates to a broadband digital communication transmission method, belonging to the technical field of broadband wireless communication.
(II) background of the invention
Communication technology has been developed over the last decades, particularly the nineties of the twentieth century, with profound effects on the development of people's daily lives and national economy. In the future, communication technologies are developing towards high-speed broadband, so that many broadband digital transmission technologies are receiving wide attention, and orthogonal Frequency Division Multiplexing (hereinafter, referred to as "0 FDM: 0rthogonal Frequency Division Multiplexing") and single carrier with Frequency Domain Equalization (hereinafter, referred to as "SC-FDE") are two broadband digital transmission technologies that are regarded by people, and both of them belong to block transmission technologies, while OFDM is concerned to a far greater extent than SC-FDE at present, and is regarded as a support technology in various standards, for example: IEEE802.11a in a Wireless Local Area Network (WLAN); IEEE802.16 in Wireless Metropolitan Area Network (WMAN: Wireless Metropolisan Area Network); various high-speed Digital Subscriber lines (xDSL) in wired data transmission are standards based on OFDM technology. SC-FDE is not adopted by these standards, but is proposed as a physical layer transmission technique in IEEE802.16 in combination with OFDM.
The mathematical model of the conventional SC-FDE system is briefly described below.
In the SC-FDE system, a frame of discrete time domain signals transmitted by a transmitting end is s (N), (N is 0, 1, …, N-1), and through a multipath channel, where an impulse response of the channel is h (N), (N is 0, 1, … L-1), interference of Additive White Gaussian Noise (AWGN) is received during signal transmission, where w (N) is assumed to be Noise, (N is 0, 1, …, N-1), and after CP is removed, a received time domain signal r (N) is:
wherein,
representing a circular convolution operation.
At the receiving end, the signal is transformed to the frequency domain by Discrete Fourier Transform (DFT), and the obtained frequency domain signal is as follows according to the time domain convolution theorem of DFT:
R(k)＝S(k)H(k)+W(k)，(k＝0，1，…，N-1) (2)
wherein, r (k), s (k), h (k), w (k) are r (N), s (N), h (N), w (N) are frequency domain symbols obtained by N-point DFT, and h (k), (k ═ 0, 1, …, N-1) are frequency domain responses of the channel. After zero-forcing equalization, the frequency domain signals are:
and finally, performing Inverse Discrete Fourier Transform (IDFT) on the signal, converting the signal back to a time domain for judgment, and obtaining the data transmitted by the transmitting end.
OFDM and SC-FDE both belong to the block transmission technology and the system they form is called a block transmission system.
Channels of broadband mobile communication generally exhibit severe frequency selective fading, and the influence of the frequency selective channels on a block transmission system is mainly expressed in that: frequency selective fading is caused by multipath propagation or delay spread of a signal, some frequency spectrum components of the signal are attenuated very low when the signal propagates in a frequency selective fading channel, and under the condition that a deep attenuation point exists in the channel, the signal is influenced more greatly, so that the signal is distorted, intersymbol interference is caused, and system performance is influenced.
In many important applications of OFDM and SC-FDE (such as WLAN, WMAN, xDSL and future broadband mobile communication), there is a reverse channel, and then the transmitting end of the block transmission system can utilize the channel state information returned by the reverse channel and some adaptive techniques to improve the performance and efficiency of the whole system.
The chinese patent application No. 200410036439.6 provides a single carrier block transmission method in a frequency selection mode, which includes the following steps:
(1) after the two parties of the transceiver set up communication, the receiving end finds out M usable sub-channels from N pieces of estimated channel state information, mark usable channel and forbidden channel separately at the same time, form the mark information of sub-channel, send the mark information of sub-channel back to the sending end through the reverse channel;
(2) after receiving the sub-channel marking information sent back by the receiving end, the sending end changes the signal frequency spectrum according to the information and transmits signals by using available sub-channels;
(3) and after receiving the signals, the receiving end transforms the signals to a frequency domain, selects the signals on the available sub-channels according to the sub-channel marking information, and then balances and judges the selected signals to finally obtain the transmitted data.
For detailed steps, refer to "a frequency-selective single carrier block transmission method" (applied for chinese patent application No. 200410036439.6), and are not described herein again.
In mobile communication systems, efficient multiple access techniques must be employed. The basic types of Multiple access techniques are frequency Division Multiple access, fdma (frequency Division Multiple access), time Division Multiple access, tdma (time Division Multiple access), and code Division Multiple access, cdma (code Division Multiple access). FDMA and TDMA are simple to implement but require guard bands in the frequency and time domains, respectively, and are inefficient. CDMA is a multiple access technology with significantly higher user capacity than TDMA and FDMA, but is complex to implement; multi-carrier CDMA (MC-CDMA) also suffers from the same problems as normal CDMA. Carrier sense/collision avoidance techniques in wireless local area networks are inefficient.
Orthogonal Frequency Division Multiple access (ofdma), which is a Multiple access technique based on OFDM, is a new broadband Multiple access technique that has attracted attention in recent years. OFDMA divides the overall bandwidth into a large number of narrowband subchannels, with one user allocating one or several subbands (groups of subchannels), each containing a certain number of subchannels. OFDMA is simple to realize and has high spectrum utilization rate. In the downlink, there is no multi-user interference. The OFDMA scheme for establishing subbands generally has two schemes, one is that a certain number of adjacent subchannels form a subband, and the second schemeAll sub-channels, which are sub-bands, are scattered randomly or at certain intervals throughout the bandwidth. The second scheme has advantages over the first scheme, especially in frequency selective fading channels. In order to fully utilize the channel state information, a number of adaptive OFDMA techniques have been proposed, but the complexity is too high. Although the second scheme works well in frequency selective fading channel, it still suffers from the great influence of frequency selective fading, and usually must be combined with error correcting code with strong error correcting capability to control the error rate of the system to a relatively low level (e.g. 10)-4Below) and the code rate of such error correcting codes is generally low, e.g. generally 1/2, 1/3 or even lower, which greatly reduces the efficiency of the whole system.
Disclosure of the invention
The invention provides a downlink frequency division multiple access method of a frequency-selecting block transmission system aiming at the problems in the prior art, which can greatly improve the error code performance of the system under the condition of little complexity increase, thereby obviously improving the frequency spectrum efficiency of the system.
Since the base station accesses each mobile station in the same manner, the following description will be made of downlink communication of a certain mobile station U for the sake of description.
The method comprises the following steps:
(1) initial sub-channel group allocation, wherein the base station allocates a group of sub-channels for each mobile station accessed to the current base station and informs each mobile station of the sub-channel group condition to which the mobile station is allocated;
(2) the base station sends a frame of auxiliary data for channel estimation to a mobile station accessed to the current base station, the mobile stations respectively estimate the channel state information of a subchannel group used by the mobile station, the mobile station selects the first M subchannels with high gains as available subchannels in the subchannel group of the mobile station according to the gain of the subchannels to form subchannel marking information, and sends the subchannel marking information of the subchannel group of the mobile station to the base station;
(3) the base station carries out orthogonal transformation on the modulated signals of each mobile station according to the received subchannel marking information of each subchannel group, expands the orthogonal transformation into N-dimensional vectors, converts the N-dimensional vectors back to time domains for transmission, and establishes a downlink communication link from the base station to the mobile station;
(4) the mobile station transforms the received sampling signal to a frequency domain, performs frequency domain equalization on the received signal according to the subchannel marking information of the subchannel group of the mobile station U, selects a useful signal on an available subchannel, transforms the useful signal back to a time domain and completes judgment to obtain information data;
(5) according to the requirement, the base station can change the sub-channel group u to other sub-channel groups in the communication process, or the mobile station keeps the sub-channel group u unchanged and only changes the sub-channel mark information in the sub-channel group u; after the change, the base station sends the identification information of the changed sub-channel group to the mobile station or the mobile station sends the sub-channel marking information of the changed sub-channel group u to the base station, and the mobile station always communicates with the mobile station according to the recently received sub-channel group identification information or the base station always communicates with the mobile station according to the recently received sub-channel marking information of the sub-channel group u.
The detailed steps are as follows:
first, the following description will be made of the symbols involved:
k: the overall label of the sub-channel is that k is more than or equal to 0 and less than or equal to N-1. In a block transmission system, both communicating parties divide the entire available frequency band into N subchannels, and in a block transmission system implemented based on FFT, k is also the index of the frequency domain variable.
m: the local index of a subchannel is the index of the mth subchannel in the same subchannel group, and m is called the local index in the subchannel group.
Iu: identification information of subchannel group u, according to IuBoth the base station and the mobile station may obtain an overall index for all subchannels in subchannel set u.
DB(k) The method comprises the following steps Vector of N dimension, N is the number of all available sub-channels of the base station, if DB(k)＝u，1≤u≤UmaxIndicates that the kth sub-channel belongs to the sub-channel group U, U of the Uth mobile stationmaxIndicating the maximum number of users the base station is allowed to access.
Du(m): the subchannel label array for subchannel set u. DuWhere (m) ═ k denotes the overall index of the subchannel of subchannel group u, which is partially index m, is k. This array is stored both at the base station and at the mobile station U, the base station being based on DB(k) Can obtain Du(m), the mobile station U is based on the identification information I of the sub-channel group U transmitted from the base stationuTo obtain Du(m)。
Au(m): subchannel flag information for subchannel group u. A. theu(m) ═ 0, indicating that the subchannel in subchannel group u, which is locally labeled m, is an unavailable subchannel; a. theu(m) ═ 1, indicating that the subchannel in subchannel group u, which is locally labeled m, is an available subchannel.
Step (1), initial subchannel group allocation is carried out, a base station allocates a group of subchannels for each mobile station accessed to the current base station, and the situation of the subchannel group allocated to each mobile station is informed;
in a block transmission system, a base station divides the whole available frequency band into a plurality of sub-channels, and since the block transmission system needs to transform the discrete time domain signal without the CP to the discrete frequency domain by using DFT, in the discrete frequency domain, the total number of sub-channels of the base station is equal to the number of points of DFT, and each sub-channel corresponds to one point or component of DFT. The number of subchannels included in each subchannel set may vary from service to service. The distribution of the sub-channels in the same sub-channel group in the whole frequency band can be chosen in many ways, for example, the sub-channel groups of multiple users can each occupy a continuous spectrum, or the sub-channel groups of each user can be spread in the whole frequency band.
The base station is according to its service need and available spectrum resourceThe mobile station U distributes a group of sub-channels, which are marked as a sub-channel group U, and forms sub-channel identification information I of the sub-channel group Uu. For example, the base station may allocate a maximum of sixty-four sub-channel groups to the mobile stations according to the protocol, and each sub-channel group needs six bits to be identified and transmitted to the identification information I of the sub-channel group u of the mobile stationuI.e. the index of the subchannel set u; the number of subchannels in these subchannel groups may be the same or different; identification information I of a group u of subchannels in a situation where the subchannels in the group u of subchannels are completely randomly spread over the entire frequency banduN bits of information are needed for marking, i.e., the base station needs to send N bits of information to the mobile station.
Let DBIs a vector used by the base station to represent the mark information of the subchannel set u, namely:
DB＝{DB(k)，k＝0，1…，N-1}，
DB(k)＝u，1≤u≤Umaxindicating that the kth sub-channel belongs to the u sub-channel group, the u sub-channel group has BuSub-channels, Du(m)＝k，(m＝0，1，…，Bu-1) the global index k of the subchannel denoted local index m of the subchannel set u is denoted.
The mobile station U receives the identification information I of the sub-channel group UuThen, i.e. know Du(m)＝k，(m＝0，1，…，Bu-1)。
Step (2), the base station sends a frame of auxiliary data for channel estimation to the mobile station accessed to the current base station, the mobile stations respectively estimate the channel state information of the used sub-channel group, the mobile station selects the first M with high gain as the available sub-channels in the sub-channel group of the mobile station according to the gain of the sub-channels to form sub-channel mark information, and sends the sub-channel mark information of the sub-channel group of the mobile station to the base station;
there are many methods for channel estimation, such as a channel estimation method based on a training frame, an interpolation pilot symbol estimation method, and the like.
And after the channel state information is acquired, frequency selection is carried out. After obtaining the channel state information of each sub-channel group, the mobile station selects available sub-channels according to the system performance requirement and the current channel state information, and marks the available sub-channels with one bit of information "0" or "1" to form sub-channel mark information, and sends the sub-channel mark information to the base station through a reverse channel, wherein the number of the available sub-channels selected by each mobile station can be different and is determined by the link condition and the service requirement between the base station and different mobile stations.
For example, let the vector representing the subchannel flag information for subchannel set u be:
Au＝{Au(m)，m＝0，1，…，Bu-1}，
Au(m) ═ 1, indicating that the subchannel in subchannel group u, which is locally labeled m, is an available subchannel; a. theuAnd (M) ═ 0, which indicates that the subchannel with local reference number M in the subchannel group u is an unavailable subchannel, and the number of available subchannels in all the subchannel groups u is recorded as M, and M can be changed in the communication process, for example, the number M of the available subchannels selected is generally different each time frequency re-selection is performed. When selecting the available sub-channel, firstly estimating the receiving signal-to-noise ratio and determining the used modulation mode according to the receiving signal-to-noise ratio, wherein the modulation mode can also be agreed by the two communication parties in advance, and the criterion for selecting the available sub-channel is that the number of the selected available sub-channel is as large as possible on the premise of meeting the requirement of the error code performance of the system. The error code performance of the system is determined by the equalized signal-to-noise ratio of the system, the signal-to-noise ratio after the lowest equalization of the error code performance is called the signal-to-noise ratio after the expected equalization, and a certain margin is reserved.
The method for calculating the received signal-to-noise ratio refers to relevant documents. Let the global index of M available sub-channels currently selected by the mobile station be kmAnd (M-0, 1, …, M-1), which are all subchannels in subchannel set u. The following is a brief description of the calculation of the post-equalization snr by taking zero-forcing equalization as an example, and the same considerations as above are not taken into considerationInfluence of step error:
due to the cyclic prefix, a linear convolution of the signal and the channel impulse response can be converted into a product in the discrete frequency domain in the discrete time domain. Let S' (k), Hu(k) W (k), R' (k), (k ═ 0, 1, …, N-1) are frequency domain transmit signal, channel complex gain, noise and frequency domain receive signal with CP removed, respectively, where w (k), (k ═ 0, 1, …, N-1) is gaussian noise, then:
R′(k)＝S′(k)Hu(k)+W(k)，(k＝0，1，…，N-1)
after zero-forcing equalization is performed on M available subchannels in the subchannel group u, the following results are obtained:
the post-equalization signal-to-noise ratio is:
wherein, <math> <mrow> <msubsup> <mi>σ</mi> <mi>n</mi> <mn>2</mn> </msubsup> <mo>=</mo> <mi>E</mi> <mrow> <mo>(</mo> <msup> <mrow> <mo>|</mo> <mi>W</mi> <mrow> <mo>(</mo> <mi>k</mi> <mo>)</mo> </mrow> <mo>|</mo> </mrow> <mn>2</mn> </msup> <mo>)</mo> </mrow> <mo>,</mo> </mrow></math>
(k-0, 1, …, N-1) is the power of the noise on each subchannel.
Step (3), the base station carries out orthogonal transformation on the modulated signals of each mobile station according to the received subchannel marking information of each subchannel group, expands the orthogonal transformation into N-dimensional vectors, and transforms the N-dimensional vectors back to time domain for transmission, and establishes a downlink communication link from the base station to the mobile station;
the base station symbol-maps the service data of each mobile station according to the modulation scheme adopted by each mobile station, and then transforms the service data after symbol mapping according to the subchannel flag information of the subchannel group of the mobile station, which is not illustrated by the example of transformation of the service data of the mobile station by the base station:
the base station performs symbol mapping on the service data according to the modulation mode adopted by the mobile station U to form a frame of M symbols to be transmitted, performs orthogonal transformation on the M symbols to obtain M transform domain symbols, and expands the M transform domain symbols into N-dimensional vectors according to the subchannel marking information to obtain the frequency domain form of the signal to be transmitted.
The transmission signals of each mobile station are not overlapped in frequency domain, and the non-overlapped N-dimensional vectors are combined into an N-dimensional vector, namely the frequency domain form of the signal to be transmitted of the base station is changed into time domain and CP for transmission.
The specific method for expanding the M transform domain symbols into N-dimensional vectors according to the subchannel flag information is as follows:
after receiving the subchannel marking information sent back by the mobile station U, the base station transmits signals by using only M available subchannels, so that M-point orthogonal transformation is performed on a frame of M block transmission system symbols s (n), (n ═ 0, 1, …, M-1) of the mobile station U to a transformation domain:
S＝Fs
where F is an M-point orthogonal transform matrix, S ═ { S (n) ═ 0, 1 … M-1} is M time domain symbols of the block transmission system, S ═ { S (i) ═ 0, 1 …, M-1} is M transform domain symbols.
Expanding M-dimensional transform domain symbols S ═ { S (i), i ═ 0, 1 … M-1} into N-dimensional vectors S '═ { S' (k), k ═ 0, 1 … N-1}, where M transform domain symbols correspond one-to-one to M available subchannels of subchannel set u, e.g., S corresponds to M available subchannels in order of their overall index:
let S '{ S' (k), k ═ 0, 1, …, N-1} th kmOne component is equal to S (m), and zero or some non-information data is placed on the components corresponding to other sub-channels of the sub-channel group u; and all the sub-channels which do not belong to the sub-channel group u are set to be zero.
Where k ismAnd (M-0, 1, …, M-1) is an overall index of the M available subchannels in the subchannel set U. The above process is performed on the data sets of all the mobile stations, so as to obtain the frequency domain signals S ' ═ { S ' (k), k ═ 0, 1, …, N-1} of a frame of block transmission system, and then, for S ' (k), (k ═ 0, 1, …, N-1), an Inverse Discrete Fourier Transform (hereinafter referred to as IDFT: Inverse Discrete Fourier Transform) of N points is performed, which can be implemented by an Inverse Fast Fourier Transform (hereinafter referred to as IFFT: Inverse Fast Fourier Transform) algorithm:
the time domain signal is changed into a time domain signal, the IFFT points are more than N when the time domain signal is over-sampled, the high frequency part is set to be zero, and the time domain signal is sent out after being subjected to D/A conversion and then subjected to carrier modulation.
When the number of available subchannels for each mobile station is not an integer power of 2, the orthogonal transformation may be implemented in blocks, different blocks may use the same or different orthogonal transformations;
step (4), the mobile station transforms the received sampling signal to the frequency domain, performs frequency domain equalization on the received signal according to the sub-channel marking information of the sub-channel group of the mobile station U, selects the useful signal on the available sub-channel, transforms the useful signal back to the time domain and completes the judgment to obtain the information data;
when the number of available sub-channels of each mobile station is not an integer power of 2, if the original orthogonal transformation is realized by blocks, the inverse orthogonal transformation is also realized by blocks, and different blocks adopt the same or different inverse orthogonal transformations according to the respectively adopted orthogonal transformations;
the specific implementation method for selecting the signal on the subchannel available to the mobile station according to the subchannel flag information is, for example, taking the mobile station U as an example:
the time domain discrete signal with the CP removed from the signal received by the receiving end is set as follows:
and performing N-point DFT on the obtained data:
and:
R′(k)＝S′(k)H(k)+W(k)，(k＝0，1，…，N-1)
in this way, the signals r (M) on the M available subchannels can be collectively selected based on the flag information of the subchannel set u and the corresponding subchannel flag information, (M ═ 0, 1, …, M-1), and for example, for the above-mentioned spectrum transformation method, there are:
R(m)＝R′(km) Where k ismAnd (M-0, 1, …, M-1) is an overall index of M available subchannels.
Equalizing the selected signal by using the channel state information of the available sub-channels in the estimated channel state information; one of three equalization modes can be selected:
1. zero forcing equalization;
2. balancing the minimum mean square error;
3. hybrid equalization, i.e., one part of the subchannels is equalized with zero-forcing and the other part of the subchannels is equalized with minimum mean square error.
The equalized signal is transformed back to the time domain by M-point inverse orthogonal transformation:
r＝FHR
wherein FHIs the conjugate transpose of F, which is the inverse transform matrix of F. When M is not an integer of 2When the power is lower, if the original orthogonal transformation is realized by blocks, the inverse orthogonal transformation is also realized by blocks, and different blocks adopt the same or different inverse orthogonal transformations according to the respective adopted orthogonal transformation;
step (5), according to the requirement, the base station can change the sub-channel group u to another sub-channel group in the communication process, or the mobile station keeps the sub-channel group u unchanged and only changes the sub-channel mark information in the sub-channel group u; after the change, the base station sends the identification information of the changed sub-channel group to the mobile station or the mobile station sends the sub-channel marking information of the changed sub-channel group u to the base station, and the mobile station always communicates with the mobile station according to the recently received sub-channel group identification information or the base station always communicates with the mobile station according to the recently received sub-channel marking information of the sub-channel group u.
When the channel state information changes or the system needs to optimize the sub-channel group of multiple users, the base station can change the identification information of the sub-channel group u, that is, the sub-channel group u is changed to another sub-channel group, or the mobile station keeps the identification information of the sub-channel group u unchanged, only the sub-channel marking information in the sub-channel group u is changed, at this time, the previous channel marking information is inaccurate, thereby affecting the system performance, at this time, frequency re-selection is needed, and the second step and the third step are repeated.
Through the above descriptions of the steps, a new system can be constructed, but parameters affecting the error performance and the spectral efficiency of the system need to be explained:
1. determination of the number of available subchannels
The number of available subchannels is an important parameter affecting the performance of the new system. In view of the above, if only available subchannels are used for transmitting useful information, there is a problem how to determine the number of available subchannels, which is not a constant value for different channel types and at different times of a time-varying channel. According to different channel conditions, the system spectrum efficiency and performance are considered, and the selected available sub-channel number M occupies the sub-channel number B in the sub-channel group uuIn generalBetween 5% and 100%.
2. Block orthogonal transformation of signals on available subchannels
Because most orthogonal transformation operations have a fast algorithm when the number of points is the integer power of 2, when the number of the orthogonal transformation points is not the integer power of 2, a blocking method can be adopted to improve the calculation efficiency.
The method divides the orthogonal transformation operation with more points but not the integer power of 2 into a plurality of orthogonal transformation operations with relatively less points; in the orthogonal transformation operation with less points, at most one point is not an integer power of 2, but the point is very small, and the rest points are all integer powers of 2, namely, the block orthogonal transformation is carried out, and various block methods are proposed to follow the following principle:
a. a block of length 16 or more, the length of which is an integer power of 2;
b. at most 1 block of length less than 16;
c. blocks of length less than 4 are not recommended;
the same processing is performed on the orthogonal inverse transformation, and the operation efficiency of the system is improved by the blocking processing.
The invention better solves the access problem of the downlink in the time-varying environment on the premise of ensuring the system performance. From the simulation results given in the embodiment, it can be seen that, for a single-antenna system with a signal sampling rate of 40MHz and a radio frequency bandwidth of no more than 46MHz, 16QAM is applied under the conditions that the sum Doppler frequency of an IMT2000 mobile channel a reaches 100Hz-300Hz and the received signal-to-noise ratio is 14dB, and the method provided by the invention can ensure that the bit error rate of the system is lower than 5 × 10-3Under the condition of (1), for a mobile station occupying 128 sub-channels, the uplink transmission rate of the system is not lower than 10.0Mbps, and the return information rate of the reverse channel does not exceed 500 Kbps.
(IV) description of the drawings
The figure is a block diagram of a system implementing the proposed method of the invention.
In the figure: 1. the device comprises a source module, a 2 symbol mapping module, a 3 FFT module (M point), a 4 signal spectrum transformation module, a 5 merging module, a 6 IFFT module (N point), a 7 Cyclic Prefix (CP) adding module, a 8D/A module, a 9 intermediate frequency and radio frequency modulation module, a 10 channel, a 11 radio frequency and intermediate frequency demodulation module, a 12A/D module, a 13 CP removing module, a 14 FFT module (N point), a 15 signal spectrum inverse transformation module, a 16 equalization module, a 17 IFFT module (M point), a 18 decision module, a 19 channel estimation module, a 20 adaptive frequency selection decision module, a 21 frequency selection module, a 22 reverse channel, a 23 synchronization module, a 24 multi-address access control module.
(V) detailed description of the preferred embodiments
Example (b):
the orthogonal transform employed in the embodiment is an M-point discrete fourier transform, and the corresponding inverse orthogonal transform is an inverse M-point discrete fourier transform. The embodiment does not perform the blocking process for the M-point DFT and the IDFT.
The attached drawings show a system block diagram for realizing the method provided by the invention, and the functions of all modules are as follows:
the information source module 1: and the general module generates data to be transmitted. Based on the results returned by the multiple access module 24 and the reverse channel 22, data groups of a length corresponding to the number of available subchannels selected by each mobile station are generated, respectively.
The symbol mapping module 2: and the universal module is used for mapping the data generated by the information source to the corresponding points of the constellation diagram according to the adopted modulation mode.
M-point FFT transform module 3: and a general module for performing DFT conversion on the mapped signals of each mobile station U.
The signal spectrum transformation module 4: the special module of the system, the base station sends back the sub-channel marking information through the multiple access module 24 and the reverse channel 22, the M point frequency domain signal output by the module 3 is placed on the spectrum points corresponding to the M available sub-channels, the spectrum points corresponding to the forbidden sub-channels are set to zero, or non-information data is filled, and then the new frequency domain signal of the block transmission system of a frame of N points is obtained. This module needs to be programmed according to the method described in detail step (3) in the summary of the invention, and is implemented by a general digital signal processing chip.
The signal combining module 5: and directly superposing the frequency domain signals of all the mobile stations to obtain a data frame containing data of all the mobile stations.
N-point IFFT module 6: and the universal module is used for transforming the newly obtained frequency domain signal to a time domain.
And a CP adding module 7: and the universal module adds the cyclic prefix to each frame of obtained data.
D/A module 8: and the general module is used for converting the digital signal into an analog signal.
Intermediate frequency and radio frequency modulation module 9: and a general module, if the system is used in a wireless environment, the system needs to perform radio frequency modulation on signals to be transmitted to an antenna. Sometimes, the signal needs to be modulated to an intermediate frequency for intermediate frequency amplification, then radio frequency modulation, and finally the modulated signal is sent to an antenna for transmission.
Channel 10: and the universal module is used for transmitting the broadband mobile channel through which the signal passes.
Radio frequency and intermediate frequency demodulation module 11: and the general module is used for shifting the frequency spectrum of the signal received by the receiving antenna from radio frequency or intermediate frequency to low frequency in a wireless environment. Frequency offset caused during signal transmission needs to be corrected by frequency synchronization data before demodulation.
The A/D module 12: and the general module is used for converting the demodulated analog signal into a digital signal. The analog signal needs to be sampled by the a/D module, and the crystal oscillator providing the clock signal needs to have the same frequency as the crystal oscillator of the D/a module of the transmitter, otherwise, a sampling rate error may result. The sampling rate synchronization is performed before the a/D.
The CP removing module 13: and the general module is used for removing the cyclic prefix. There is a problem in determining when a frame of data starts, and therefore timing synchronization is required before the CP is removed.
The N-point FFT module 14: and the general module is used for transforming the CP-removed signal to a frequency domain.
The signal spectrum inverse transformation module 15: the special module of the system finds out M point frequency domain signals carried by available sub-channels in the received signals according to the sub-channel marking information sent by the multiple access module 24 and the channel estimation module 19, thereby forming frequency domain signals. This module needs to be programmed according to the method described in detail step (4) in the summary of the invention, and is implemented by a general digital signal processing chip.
The equalization module 16: and a general module for equalizing the signal selected by the inverse signal spectrum transform module 15 by using Channel State Information (CSI), which is an available subchannel parameter sent by the channel estimation module 19. The equalization mode can select one of the following three equalization modes: zero-forcing equalization, minimum mean square error equalization, and mixed mode equalization.
M-point IFFT transforming module 17: and the general module is used for transforming the M frequency domain signals of the equalized signals to a time domain.
The decision module 18: and the universal module is used for finishing the judgment of the time domain signal according to the modulation mode adopted by the system.
The channel estimation module 19: and the general module is used for acquiring the channel state. The channel state information may be obtained in different ways, such as channel prediction, an auxiliary data based channel estimation method, a decision feedback channel tracking method, etc. The embodiment provides a channel state acquisition method which is training frame and decision feedback tracking. This method is briefly described below:
the method for training frame and decision feedback tracking is that firstly, a training frame is sent to estimate a channel, and a symbol after decision is reconstructed by a following data frame according to a decision result:
assuming that the received discrete time domain signal is R ' (k), (k ═ 0, 1, …, N-1), and the discrete time domain signal is transformed into the frequency domain to obtain R ' (k), (k ═ 0, 1, …, N-1), the time domain symbol after the decision of the data frame is R ' (k), (k ═ 0, 1, …, N-1)
(M-0, 1, …, M-1), symbol mapping is performed according to the modulation scheme adopted by the mobile station, and the reconstructed symbol is obtained
(M-0, 1, …, M-1), and
(M-0, 1, …, M-1) to obtain(M-0, 1, …, M-1), which is the frequency domain symbol reconstructed according to the decision result, and the method for tracking the channel by using the reconstructed frequency domain symbol is as follows:
wherein, H (k)m) Threshold is a threshold for the channel state information of the system in the previous frame. Where k ismAnd (M-0, 1, …, M-1) is an overall index of M available subchannels.
Since only some of the subchannels have decision symbols, tracking is only performed for the available subchannels. And after a period of time, the training frame is sent again to eliminate the error accumulation caused by tracking. It should be noted that, the tracking method does not utilize the reconstructed symbols on all available sub-channels, but only utilizes the frequency domain symbols with power greater than a certain threshold, that is, threshold, and for the sub-channels with amplitude less than the threshold, the frequency domain CSI is not updated, that is, the value at the previous time is kept unchanged.
The adaptive frequency selection judging module 20: the special module of the system obtains the amplitude gain | H (k) of the sub-channel according to the CSI updated by each frame transmitted by the channel estimation 19i) And |, (i ═ 0, 1, …, M-1) and available subchannel flag information. Different decision rules may be used. If the judgment result is that frequency re-selection is needed, controlling the frequency selection module 21 to work; the originating always works according to the latest acquired sub-channel marking information when transmitting a new frame of data. Implementation examples are given below:
after acquiring the CSI on the sub-channel, the used method for determining is as follows: calculating the current signal-to-noise ratio after equalization, namely the actual signal-to-noise ratio after equalization, and making a difference value with the signal-to-noise ratio after the expected equalization, if the absolute value of the obtained difference value is greater than a threshold, re-selecting the frequency, otherwise, keeping the current channel marking information unchanged; the threshold value is 1dB in the simulation of the embodiment;
the frequency selection module 21: the system is characterized by comprising a special module, a channel state information of a subchannel group u obtained by a channel estimation module 19 selects an available subchannel, the available subchannel is marked by 1 bit information (0 or 1) according to the availability of the channel to form subchannel marking information, the subchannel marking information is simultaneously sent to a signal spectrum inverse transformation module 15 and a reverse channel 22, and the signal spectrum inverse transformation module 4 of the mobile station is sent back through the reverse channel; this module needs to be programmed according to the method described in the chinese patent application No. 200410036439.6 mentioned in the background, and is implemented by a general digital signal processing chip.
Reverse channel 22: and the general module is used for transmitting the sub-channel mark information back to the base station.
The synchronization module 23: and the universal module is used for obtaining various synchronous data required by the system through parameter estimation. The synchronization module sends the frequency synchronization data to the radio frequency and intermediate frequency demodulation module 11; sending the sampling rate synchronization data to the a/D module 12; the timing synchronization data is sent to the de-CP module 13.
The multiple access control module 24: when establishing communication, the base station obtains the channel state information of each user by the channel estimation module 19, and allocates a sub-channel group u to each user. The scheme for creating the subchannel set u is that all subchannels of the subchannel set u are spread over the entire bandwidth at intervals, the size of the intervals being equal to the number of users, and the number of subchannels of all the subchannel sets u is the same.
Simulation parameters of this embodiment:
simulation environment: matlab7.0.1
Total number of subchannels: n1024
Modulation mode: QPSK, 16QAM
CP Length: 128
Data sampling rate: 40M
Maximum doppler frequency: 100Hz, 200Hz, 300Hz
Time-varying channel model:
ITU IMT2000 Vehicular Test Environment channel model A
reference to RECOMMENDENDATION ITU-R M.1225
GUIDELINES FOR EVALUATION OF RADIO TRANSMISSION
TECHNOLOGIES FOR IMT-2000
The influence of synchronization errors (including carrier synchronization errors, sampling rate synchronization errors and frame timing synchronization errors) on the system is not considered in the simulation, namely, the errors of all synchronization parameters are assumed to be 0; the influence of transmission delay and transmission error code when the reverse channel transmits back the available sub-channel is not considered, that is, the transmission delay and the error code are both assumed to be 0; the effects of other non-ideal factors (e.g., device non-linearity, etc.) are not considered.
And (3) simulation results:
1.128 subchannel users, subchannel groups: 1mod (8)
2.16 subchannel users, subchannel groups: 1mod (64)
To avoid confusion, some of the terms mentioned in this specification are to be interpreted as follows:
symbol 1: refers to data in which information bits are modulation mapped (also referred to as symbol mapped). Typically a complex number where the real and imaginary parts are integers.
2 one frame signal: for OFDM, a frame signal refers to N symbols for IFFT at the transmitting end and N symbols for FFT at the base station after CP removal. For SC-FDE, a frame of signal refers to N information symbols between two adjacent CPs at the transmitting end, and refers to N symbols for FFT transformation after the CPs are removed at the base station. For the SC-FDE system realized by the method provided by the invention, a frame of signal refers to M symbols for FFT transformation at a transmitting end, and refers to M symbols for IFFT transformation after equalization at a base station.
3, sub-channel: for OFDM, SC-FDE baseband signals, a subchannel refers to a frequency point after the FFT of the base station. For a radio frequency channel, a subchannel refers to a segment of the frequency spectrum of the radio frequency channel.
4 subchannel group: a set of subchannels allocated to a user.
5 signal-to-noise ratio: the ratio of signal power to noise power, where the signal-to-noise ratio mentioned in the summary and claims section is log-log, without units; the signal-to-noise ratio mentioned in the examples is a logarithmic signal-to-noise ratio in dB.
6 signal-to-noise ratio after equalization: the ratio of signal power to noise power after equalization.
Desired post-equalization signal-to-noise ratio: minimum post-equalization signal-to-noise ratio for different error performance requirements.
Claims (4)
1. A downlink frequency division multiple access method of a frequency selective block transmission system, the method comprising the steps of:
(1) initial sub-channel group allocation, wherein the base station allocates a group of sub-channels for each mobile station accessed to the current base station and informs each mobile station of the sub-channel group condition to which the mobile station is allocated;
(2) the base station sends a frame of auxiliary data for channel estimation to a mobile station accessed to the current base station, the mobile stations respectively estimate the channel state information of a subchannel group used by the mobile station, the mobile station selects the first M subchannels with high gains as available subchannels in the subchannel group of the mobile station according to the gain of the subchannels to form subchannel marking information, and sends the subchannel marking information of the subchannel group of the mobile station to the base station;
(3) the base station carries out orthogonal transformation on the modulated signals of each mobile station according to the received subchannel marking information of each subchannel group, expands the orthogonal transformation into N-dimensional vectors, returns the N-dimensional vectors to a time domain for transmission, and establishes a downlink communication link from the base station to the mobile station, wherein N is the number of all available subchannels of the base station;
(4) the mobile station transforms the received sampling signal to a frequency domain, performs frequency domain equalization on the received signal according to the subchannel marking information of the subchannel group of the mobile station U, selects a useful signal on an available subchannel, transforms the useful signal back to a time domain and completes judgment to obtain information data;
(5) according to the requirement, the base station changes the sub-channel group u to other sub-channel groups in the communication process, or the mobile station keeps the sub-channel group u unchanged and changes the sub-channel mark information in the sub-channel group u; after the change, the base station sends the identification information of the changed sub-channel group to the mobile station or the mobile station sends the sub-channel marking information of the changed sub-channel group u to the base station, and the mobile station always communicates with the base station according to the recently received sub-channel group identification information or the base station always communicates with the mobile station according to the recently received sub-channel marking information of the sub-channel group u.
2. The downlink frequency division multiple access method of a frequency selective block transmission system according to claim 1, wherein: in the step (3), the base station performs symbol mapping on the service data according to the modulation mode adopted by the mobile station U to form a frame of M symbols to be transmitted, performs orthogonal transformation on the M symbols to obtain M transform domain symbols, and expands the M transform domain symbols into N-dimensional vectors according to the subchannel marking information to obtain the frequency domain form of the signal to be transmitted;
the transmission signals of each mobile station are not overlapped in the frequency domain, and the non-overlapped N-dimensional vectors are combined into an N-dimensional vector, namely the frequency domain form of the signals to be transmitted of the base station is changed into the time domain and the cyclic prefix CP for transmission;
the specific method for expanding the M transform domain symbols into N-dimensional vectors according to the subchannel flag information is as follows:
after receiving the subchannel marking information sent back by the mobile station U, the base station transmits signals by using only M available subchannels, so that M-point orthogonal transformation is performed on a frame of M blocks of the mobile station U, wherein n is 0, 1, … and M-1, to a transform domain:
S＝Fs
wherein F is an M-point orthogonal transform matrix, S ═ { S (n) ═ 0, 1 … M-1} is M time domain symbols of the block transmission system, S ═ { S (i) ═ 0, 1 …, M-1} is M transform domain symbols;
expanding M-dimensional transform domain symbols S ═ { S (i), i ═ 0, 1 … M-1} into N-dimensional vectors S '═ { S' (k), k ═ 0, 1 … N-1}, wherein M transform domain symbols correspond one-to-one to M available subchannels of subchannel set u, and S sequentially corresponds to M available subchannels in order of their overall index:
let S '{ S' (k), k ═ 0, 1, …, N-1} th kmOne component is equal to S (m), and zero or some non-information data is placed on the components corresponding to other sub-channels of the sub-channel group u; all the sub-channels which do not belong to the sub-channel group u are set to be zero;
where k ismM-0, 1, …, M-1, is an overall index of the M available subchannels in the subchannel set u;
wherein: du(m): subchannel label array, D, for subchannel group uuWhere k denotes the overall index k of the subchannel of subchannel set U, with local index m, this array being stored both at the base station and at the mobile station U, the base station being dependent on DB(k) To obtain Du(m), the mobile station U is based on the identification information I of the sub-channel group U transmitted from the base stationuTo obtain Du(m)；
DB(k) The method comprises the following steps Vector of N dimension, N is the number of all available sub-channels of the base station, if DB(k)＝u，1≤u≤UmaxIndicates that the kth sub-channel belongs to the sub-channel group U, U of the Uth mobile stationmaxRepresenting the maximum number of users allowed to be accessed by the base station;
Au(m): sub-channel marking information, A, of a sub-channel group uu(m) ═ 0, indicating that the subchannel in subchannel group u, which is locally labeled m, is an unavailable subchannel; a. theu(m) ═ 1, indicating that the subchannel in subchannel group u, which is locally labeled m, is an available subchannel;
the above process is performed on the data sets of all the mobile stations, so as to obtain the frequency domain signal S ' ═ { S ' (k), k ═ 0, 1, …, N-1} of a frame of block transmission system, and then the inverse discrete fourier transform of N points is performed on S ' (k), k ═ 0, 1, …, N-1, and the implementation is realized through an inverse fast fourier transform algorithm:
the time domain signal is changed into a time domain signal, the IFFT points are more than N when the time domain signal is over-sampled, the high frequency part is set to be zero, and the time domain signal is sent out after being subjected to D/A conversion and then subjected to carrier modulation.
3. The downlink frequency division multiple access method of a frequency selective block transmission system according to claim 1, wherein: the specific implementation method for selecting the signal on the subchannel available to the mobile station according to the subchannel flag information in the step (4) is as follows:
the time domain discrete signal of the signal received by the receiving end and the cyclic prefix CP is set as follows:
and performing N-point DFT on the obtained data:
and:
R′(k)＝S′(k)H(k)+W(k)，k＝0，1，…，N-1
wherein:
h (N), wherein N is 0, 1, … N-1 is the impulse response of the channel;
w (N), N is 0, 1, …, and N-1 is additive white Gaussian noise;
h (k), W (k) are h (N) and w (N) respectively, and frequency domain symbols obtained by performing N-point DFT are obtained;
h (k), k ═ 0, 1, …, N-1 is the frequency domain response of the channel;
where k ismM-0, 1, …, M-1, is an overall index of the M available subchannels in the subchannel set u;
wherein: du(m): subchannel label array, D, for subchannel group uuWhere k denotes the overall index k of the subchannel of subchannel set U, with local index m, this array being stored both at the base station and at the mobile station U, the base station being dependent on DB(k) To obtain Du(m), the mobile station U is based on the identification information I of the sub-channel group U transmitted from the base stationuTo obtain Du(m)；
DB(k) The method comprises the following steps Vector of N dimension, N is the number of all available sub-channels of the base station, if DB(k)＝u，1≤u≤UmaxIndicates that the kth sub-channel belongs to the sub-channel group U, U of the Uth mobile stationmaxRepresenting the maximum number of users allowed to be accessed by the base station;
Au(m): sub-channel marking information, A, of a sub-channel group uu(m) ═ 0, indicating that the subchannel in subchannel group u, which is locally labeled m, is an unavailable subchannel; a. theu(m) ═ 1, indicating that the subchannel in subchannel group u, which is locally labeled m, is an available subchannel;
this enables signals r (M) on M available subchannels to be jointly selected on the basis of the label information of the subchannel set u and its corresponding subchannel label information, M being 0, 1, …, M-1.
4. The downlink frequency division multiple access method of a frequency selective block transmission system according to claim 1, wherein: selecting the number of available sub-channels M occupying the number of sub-channels B in the sub-channel group u in the step (4)uBetween 5% and 100%.
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| CN1694443A (en) * | 2005-06-06 | 2005-11-09 | 山东大学 | Blocking bit loading method in selective frequency blocking transmission system |
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| CN1617530A (en) * | 2004-12-06 | 2005-05-18 | 山东大学 | Frequency selective mode single carrier blocking transmission method |
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