The present invention relates generally to a wireless communication system, and more particularly, to a method and apparatus for space-time-frequency diversity encoding and decoding in a multi-carrier wireless communication system.
It's of great importance to overcome wireless channel fading and channel interference in order to provide high quality data service for users in a wireless communication system. In recent years, STBC (Space-Time Block Coding), with which a space-time transmission diversity gain can be obtained, has been accepted widely in the industry due to its simple encoding and decoding characteristics, and has been chosen as one of the transmission diversity schemes in 3GPP UMTS. The STBC scheme can also be used for an OFDM (Orthogonal Frequency Division Multiplexing) system, that is, an OFDM system employing STBC. In the case that block coding is performed in the space-frequency domain instead of the space-time domain, an OFDM system employing SFBC (Space-Frequency Block Coding) is implemented.
A method for implementing transmission diversity with a plurality of antennas in a multi-carrier communication system is disclosed in a patent application with publication No. WO2004/073275A1 published on Aug. 26, 2004, entitled “Space-Time-Frequency Diversity for Multi-carrier Systems”. According to the method disclosed in this patent application, firstly, a set of transmission symbols is converted according to a predetermined rule, to generate a plurality of transmission streams; then, transmission elements in each of the transmission streams are allocated to time-frequency units associated with the antennas and corresponding to a plurality of carriers and symbol intervals, and transmitted via corresponding antennas. This method achieves a space-time-frequency orthogonality through an orthogonal design. Since the space-time coding and space-frequency coding in this scheme are independent of each other, the transmission diversity gain obtained through coding is one dimensional space-time diversity gain or space-frequency diversity gain.
Therefore, there is a need for a more effective encoding method, in order to further enhance the data transmission quality.
The technical problem to be solved in the present invention is to provide an effective encoding method, in order to enhance the data transmission quality.
For this purpose, the present invention provides a space-time-frequency encoding method. The method according to the invention comprises steps of: coding a set of transmission symbols according to a predetermined orthogonal STBC rule, so as to obtain a plurality of code words; and mapping a plurality of elements in each of the plurality of code words and redundancy for at least part of the plurality of elements, as channel elements, to a plurality of predetermined time-frequency units in one of a plurality of two dimensional time-frequency matrixes corresponding to the code word, so that the channel elements in each of the matrixes can be transmitted via an antenna corresponding to the matrix.
In an embodiment, the predetermined orthogonal STBC rule is an Alamouti STBC for two transmitting antennas.
In another embodiment, the predetermined orthogonal STBC rule is an extended Alamouti STBC for three or four transmitting antennas.
The present invention also provides a decoding method corresponding to above encoding method. The decoding method according to the invention comprises steps of: extracting a plurality of sets of faded channel elements corresponding to a set of transmission symbols among a plurality of signal streams received from different transmitting antennas, wherein each of the sets of channel elements includes code word elements and at least partial redundancies thereof; combining the redundant channel elements and the code word elements corresponding to the redundant channel element in each of the sets of channel elements, so as to obtain a transmission code word composed of the channel elements that are obtained through the combination and the remaining code word elements in the set of channel elements; and performing a linear combination on a plurality of transmission code words according to a predetermined orthogonal STBD (Space-Time Block Decoding) rule, so as to recover a set of transmission symbols.
In an embodiment, the predetermined orthogonal STBD rule is an Alamouti STBD rule for two transmitting antennas.
In another embodiment, the predetermined orthogonal STBD rule is an extended Alamouti STBD rule for three or four transmitting antennas.
Another technical problem to be solved in the invention is to provide an effective encoding apparatus, which could enhance the gain of data transmission diversity.
For this purpose, the present invention provides a space-time-frequency encoding apparatus, comprising: a coding unit configured to code a set of transmission symbols according to a predetermined orthogonal STBC rule, so as to obtain a plurality of corresponding code words; and a mapping unit configured to map a plurality of elements in each of the plurality of code words and at least partial redundancies thereof, as channel elements, to a plurality of predetermined time-frequency units in one of a plurality of two dimensional time-frequency matrixes corresponding to the code word, so that the channel elements in each of the matrixes can be transmitted via an antenna corresponding to the matrix; wherein the predetermined orthogonal STBC rule is one of an Alamouti STBC rule for two transmitting antennas, an extended Alamouti STBC rule for three transmitting antennas, and an extended Alamouti STBC rule for four transmitting antennas.
Furthermore, the present invention also provides a decoding apparatus, comprising: an extracting unit configured to extract a plurality of sets of faded channel elements corresponding to a set of transmission symbols among a plurality of signal streams received from different transmitting antennas, wherein each the sets of channel elements includes code word elements and redundant elements for at least part of the code word elements; a combination unit configured to combine the redundant channel elements and the code word elements corresponding to the redundant channel elements in each of the sets of channel elements, so as to obtain a transmission code word composed of the channel elements that are obtained through the combination and the remaining code word elements in this set of channel elements; and a decoding unit configured to perform a linear combination on a plurality of transmission code words according to a predetermined orthogonal STBD rule, so as to recover a set of transmission symbols; wherein the predetermined orthogonal STBD rule is one of an Alamouti STBD rule for two transmitting antennas, an extended Alamouti STBD rule for three transmitting antennas, and an extended Alamouti STBD rule for four transmitting antennas.
In the encoding method provided in the invention, through properly allocating transmission code words and their redundant elements to space-time-frequency unit, the transmission code words and their partial redundancies can be transmitted via different antennas. Accordingly at the receiving side, the redundant elements and the code word elements corresponding to the redundant elements can be combined, so as to enhance the SNR (signal-to-noise ratio) and diversity gains for a part of the elements in the code words. Meanwhile, the space-time orthogonal coding design and the space-frequency orthogonal coding design have a similar orthogonal structure with a conventional STBC. At the receiving side, a conventional linear combination can be performed on code words after the combinations of redundant elements and corresponding code word elements, so as to recover the corresponding transmission symbols or symbol blocks. This makes the decoding process quite simple.
Other aspects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following descriptions and claims taken in conjunction with the accompanying drawings.
Throughout all the above drawings, like reference numerals will be understand to refer to similar or corresponding features or functions.
A detailed description will be made below to the encoding and decoding methods and apparatuses provided in the invention in conjunction with the appended drawings.
The orthogonal STBC rule employed in step S10 will vary with the number of transmitting antennas, and the basic principle is to ensure the orthogonality between respective code words through coding. Assuming that NI is the number of input symbols, NT is the number of the transmitting antennas corresponding to the code words obtained through coding, P is the number of elements in one of the code words obtained through coding. Coding modes are somewhat different for different parameters NI, NT, and P, and can be summarized in the following table 1:
Where A-STBC represents an Alamouti STBC mode for two transmitting antennas, B3′-STBC represents an extended Alamouti STBC mode for three transmitting antennas, and B4′-STBC represents an extended Alamouti STBC mode for four transmitting antennas.
In the Alamouti STBC mode shown in
In the extended Alamouti STBC mode B3′-STBC shown in
In step S20, the elements in each of the obtained code words obtained in step S10 and the partial redundancies in that code word are mapped to time-frequency units in one of a plurality of two dimensional time-frequency matrixes as channel elements, in order that the channel elements for each of the code words can be modulated on predetermined sub-carriers and transmitted at predetermined symbol intervals via an antenna corresponding to that matrix in the following process, wherein channel elements in different matrixes correspond to different transmitting antennas.
In the description herein and hereafter, in a time-frequency unit {fi, ti} in the matrix, fi denotes a sub-carrier on which channel elements are modulated, and ti denotes a time unit in which channel elements are transmitted and which corresponds to the duration of a symbol.
In the embodiment illustrated in
In the description above and hereafter, the channel elements shown in the matrixes but not described represent code words obtained through coding other input symbols and the partial redundancies of the code words. Moreover, the time units and frequency units in the matrix can be extended in accordance with the number of sub-carriers and the number of sets of input symbols.
In the embodiment illustrated in
In the embodiment illustrated in
In the above embodiments, there can be one or more redundant elements and one or more redundant times, which can be adjusted according to the requirement of a practical system.
The encoding method provided in the invention can be used in an OFDM system. In this case, before channel elements are output to a plurality of antennas for transmission, there generally comprises a step of modulating the channel elements in a plurality of time-frequency matrix units on OFDM sub-carriers, and transforming the channel elements on each of the sub-carriers from the frequency domain to the time domain with Fourier Inverse Transformation. While at the receiving side, the channel elements are transformed from the time domain to the frequency domain with Fourier Transformation, and then decoded.
It is assumed that the input symbols or the symbol block at the transmitting side are {x1,x2,x3}, which are encoded into output code words {x1,−x2*,x3*,0}, {x2,x1*,0,x3*}, and {x3,0,−x1*,−x2*} with B3′-STBC mode. The elements of each of the code words and the redundancies thereof {x1,_31 x2*,x3*,0,x1,−x2*}, {x2,x1*,0,x3*,x2, x1*}, and {x3,0,−x1*,−x2*,x3,0} are mapped, as channel elements, to three time-frequency matrixes as shown in
Generally, for conventional wireless communication systems such as 3GPP/WLAN, it can be suitably assumed that the channel response for adjacent symbols and adjacent sub-carriers has time invariant characteristic. While wireless channels undergo deep slow fading, channel elements corresponding to a set of code words transmitted via the same transmitting antenna undergo the same channel response on the wireless channels, that is:
h
m
=[h
m,1
,h
m,2
, . . . h
m,N
]T (1)
where hm denotes NR dimensional column vector for the channel response, m=1,2, . . . , NT denotes the sequence number of a transmitting antenna, n=1,2, . . . , NR denotes the sequence number of a receiving antenna, hm,n denotes the channel response that the channel elements transmitted via the mth antenna and received via the nth antenna undergoes in the time-frequency units {t1,f1}, {t1,f2}, {t1,f3}, {t1,f4}, {t2,f1}, and {t2,f2}. A plurality of sets of channel elements corresponding to a set of transmission symbols are extracted among a plurality of signal streams received from different transmitting antennas in step 50, which can be expressed as:
where rt,f denotes NR dimensional column vector extracted from received signal streams, wherein the NR dimensional column vector corresponds to channel elements transmitted in time-frequency unit {t,f} via antennas and received via NR receiving antennas, nt,f denotes additive white noises, wherein it can be assumed that additive white noise in each of the time-frequency units is independent, with the bilateral noise power spectral density or variance being N0.
Since the vectors rt
where rt
The channel element vectors obtained through the combination and other channel elements extracted among the receiving signals form transmission code word vectors corresponding to the input symbols {x1,x2,x3}, and equation (2) is updated as:
where nt
Compared with the decoding method corresponding to the B3′-STBC mode, the variance of the additive white noise included in the updated channel element vectors in the signal pattern shown in equation (5) is half of that before the combination. By the separation of a linear filter, the SNR for the resulting input symbols can be expressed as:
while the conventional decoding method corresponding to the B3′-STBC mode will obtain an SNR given by:
By comparing equations (6-1), (6-2), (6-3) with equation (7), it can be found that the decoding method provided in the invention is able to achieve a higher SNR, and in turn to enhance the reception quality for the overall transmission data.
The space-time-frequency encoding method described above in connection with
The coding unit 32 is configured to code a set of transmission symbols according to a predetermined orthogonal STBC rule, as so to obtain a plurality of corresponding code words. A set of transmission symbols is coded according to a predetermined orthogonal STBC rule, as so to obtain a plurality of corresponding code words. Wherein the predetermined orthogonal STBC rule is known, and varies with the number of transmitting antennas. The basic principle for coding is to make the code words obtained through coding orthogonal to each other.
The coding unit is configured to generate the code words as shown in
The mapping unit 34 is configured to map a plurality of elements in each of the code words obtained by the coding unit 32 and the redundancies of at least part of the elements, as channel elements, to a plurality of predetermined time-frequency units in each of a plurality of two dimensional time-frequency matrixes corresponding to that code word, so that the channel elements in each of the matrixes can be transmitted via an antenna corresponding to that matrix.
Wherein the channels elements as redundancies can be single-time redundancies for multiple elements, multiple-time redundancies for a single element, or multiple-time redundancies for multiple elements. At the receiving side, elements in a code word can be combined with the redundancies for the elements, so as to enhance the SNR for this part of code word elements, and in turn to enhance the reception quality for the overall code word.
The extracting unit 62 is configured to extract a plurality of sets of faded channel elements as shown in equation (2) corresponding to a set of transmission symbols among a plurality of signal streams received from different transmitting antennas, wherein each of the sets of channel elements includes code word elements and redundant elements for at lest part of the code word elements.
The combination unit 64 is configured to combine the redundant elements in each of the sets of channel elements with the code word elements corresponding to the redundant elements, so that the channel elements obtained by the combination and the remaining code word elements in the set of channel elements make up of a transmission code word. The combination for redundant channel elements can be performed as shown in equations (3) and (4), or can be performed with weighting factors. The channel elements obtained after the combination and the remaining channel elements in the plurality of channel elements obtained by the extracting unit make up of a transmission code word corresponding to a set of transmission symbols. The signal pattern for the transmission code word can be expressed as that shown in equation (5). The signal pattern is similar to the conventional transmission code word corresponding to the B3′-STBC coding mode, with the only difference in that the combined elements in the transmission code word have a noise power spectral density half that before combination.
The decoding unit 66 is configured to perform linear combination on a plurality of transmission code words according to a predetermined orthogonal STBD rule, so as to recover a set of corresponding transmission symbols. The specific decoding rule for two transmitting antennas can be referred to the document: A simple transmit diversity technique for wireless communications, by S. Alamouti, IEEE J. on Select. Areas Commun., vol.16, (1998)10, 1451-1458. The specific decoding rule for three or four transmitting antennas can be referred to the reference document: Space-time block codes from orthogonal designs, by V. Tarokh, H. Jafarkhani, and A. R. Calderbank, IEEE Trans. on Info. Theory, vol.45, (1999)5, 1456-1467.
The encoding method and apparatus provided in the invention can be used in an OFDM system. In this case, before channel elements are output to a plurality of antennas for transmission, there generally comprises a step of transforming a plurality of channel elements corresponding to respective symbol intervals in the time-frequency matrixes from the frequency domain to the time domain with Fourier Inverse Transformation. While at the receiving side, the channel elements are transformed from the time domain to the frequency domain with Fourier Transformation, and then decoded.
It is to be understood by those skilled in the art that the embodiments described herein are intended to illustrate, but not to limit the invention. Various improvements and modifications can be made to the space-time-frequency encoding method and apparatus for wireless communication system as disclosed in the present invention without departing from the spirit and scope of the present invention. The scope of the present invention is to be defined by the attached claims herein.
Number | Date | Country | Kind |
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200610105987.9 | Jul 2006 | CN | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB07/52759 | 7/11/2007 | WO | 00 | 1/20/2009 |