METHOD AND APPARATUS OF SPACE-TIME-FREQUENCY CODING

Abstract

The invention relates to a space-time-frequency encoding for use in wireless communication systems. According to the encoding scheme provided by the invention, it first transforms a plurality of input element pairs into a plurality orthogonal element pairs respectively, each of the plurality of input element pairs and corresponding orthogonal element pair forming an orthogonal matrix; and then maps the first element, second element and the redundancy of the second element in each of the plurality of input element pairs and corresponding orthogonal element pair as channel elements to three predetermined time-frequency cells in a first and second two-dimension time-frequency matrix so as to make the channel elements in the first and second matrixes suitable for being transmitted via different antennas. As the redundant input element pairs and corresponding orthogonal element pairs transmitted via different antenna are orthogonal in both space-time domain and space-frequency domain at the same time, and thus two-dimension space-time transmit diversity gain and space-frequency transmit diversity gain can be achieved at the same time.

Description

FIELD OF THE INVENTION

The invention relates to wireless communication systems, and more particularly, relates to a method and apparatus of space-time-frequency diversity coding for use in a multi-carrier wireless communication system.

BACKGROUND OF THE INVENTION

In a wireless communication system, it is important to overcome channel fading and interference and therefore provide high quality data service for subscribers. Recently, Space-Time Block Coding (STBC) attracted extensive attention from industry and was selected as one of transmission schemes by 3GPP UMTS because of the simple and efficient encoding and decoding.

STBC can be applied in Orthogonal Frequency Division Multiplexing (OFDM) system as an attractive solution in a multi-path fading environment. The system is considered as space-time block coded OFDM. When the block codes are formed over space and frequency instead of space and time domains, it is considered as space-frequency block coded OFDM.

Patent Application entitled “Space-Time-Frequency Diversity for Multi-carrier systems”, published in Aug. 26, 2004 with publish no WO2004/073275A1, disclosed a technique to use multiple antennas to realize transmission diversity. According o the technique provided by the patent application, it first transforms transmission symbols into a plurality of transmission streams using a predetermined transformation rule, assigns transmission stream elements in frequency and time to multiple sub-carriers available at each antenna, and then transmits the elements. As the scheme adopts orthogonal design to realize space-time-frequency orthogonal and the space-time encoding and space-frequency encoding are independent in the scheme, the transmit diversity gain obtained from the method is one-dimension space-time diversity gain or space-frequency diversity gain.

There is therefore a need in the art for a new technique to further improve transmit diversity gain.

SUMMARY OF THE INVENTION

Amongst others it is an object of the invention to provide a method of encoding to improve transmit diversity gain.

To this end the invention provides a space-time-frequency encoding method comprising steps: transforming a plurality of input element pairs into a plurality orthogonal element pairs respectively, each of the plurality of input element pairs and corresponding orthogonal element pair forming an orthogonal matrix; and mapping the first element, second element and the redundancy of the second element in each of the plurality of input element pairs and corresponding orthogonal element pair as channel elements to three predetermined time-frequency cells in a first and second two-dimension time-frequency matrix so as to make the channel elements in the first and second matrixes suitable for being transmitted via different antennas.

Amongst others it is another object of the invention to provide an apparatus for space-time-frequency encoding, the apparatus comprising: a transforming unit for transforming a plurality of input element pairs into a plurality orthogonal element pairs respectively, each of the plurality of input element pairs and corresponding orthogonal element pair forming an orthogonal matrix; and a mapping unit for mapping the first element, second element and the redundancy of the second element in each of the plurality of input element pairs and corresponding orthogonal element pair as channel elements to three predetermined time-frequency cells in a first and second two-dimension time-frequency matrix so as to make the channel elements in the first and second matrixes suitable for being transmitted via different antennas.

According to the method and apparatus provided by the invention, as the redundant input element pairs and corresponding orthogonal element pairs transmitted via different antenna are orthogonal in both space-time domain and space-frequency domain at the same time, and thus two-dimension space-time transmit diversity gain and space-frequency transmit diversity gain can be achieved at the same time.

BRIEF DESCRIPTION OF THE FIGURES

The above and other objects and features of the present invention will become more apparent from the following detailed description considered in connection with the accompanying drawings, in which:

FIG. 1 is a flowchart illustrating an embodiment of encoding method in accordance with the invention;

FIG. 2 shows a first embodiment of mapping channel elements to time-frequency matrixes in accordance with the invention;

FIG. 3 shows a second embodiment of mapping channel elements to time-frequency matrixes in accordance with the invention;

FIG. 4 shows a third embodiment of mapping channel elements to time-frequency matrixes in accordance with the invention;

FIG. 5 shows a fourth embodiment of mapping channel elements to time-frequency matrixes in accordance with the invention;

FIG. 6 shows a fifth embodiment of mapping channel elements to time-frequency matrixes in accordance with the invention; and

FIG. 7 is a block diagram illustrating an embodiment of encoding apparatus in accordance with the invention.

In the figures, the same reference number represents the same, similar or corresponding feature or function.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The basic idea of the encoding scheme provided by the invention is to realize space-time orthogonality and space-frequency orthogonality between two time-frequency matrixes transmitted by two antennas through smartly allocating channel elements and thus achieve two-dimension space-time transmit diversity gain and space-frequency transmit diversity gain at the same time. And meanwhile, the orthogonal space-time coding and orthogonal space-frequency coding have similar architecture with conventional space-time block coding so that at corresponding receiver the received channel elements can be linearly combined to recover the transmitted symbols, and results in very simple decoding process.

FIG. 1 is a flowchart illustrating an embodiment of encoding method in accordance with the invention. FIG. 2 shows a first embodiment of mapping channel elements to time-frequency matrixes in accordance with the invention. FIG. 3 shows a second embodiment of mapping channel elements to time-frequency matrixes in accordance with the invention. FIG. 4 shows a third embodiment of mapping channel elements to time-frequency matrixes in accordance with the invention. The method provided by the invention becomes more apparent from the following detailed description considered in connection with the accompanying drawings FIG. 1 to FIG. 4.

In the process of the method as shown in FIG. 1, a plurality of input element pairs are first transformed into a plurality orthogonal element pairs respectively in step S10. Each of the plurality of input element pairs and corresponding orthogonal element pair form an orthogonal matrix. Then, the first element, second element and the redundancy of the second element in each of the plurality of input element pairs and corresponding orthogonal element pairs as channel elements are mapped to three predetermined time-frequency cells in a first and second two-dimension time-frequency matrixes in step S20. The channel elements in the first and second matrixes are to be transmitted via different antennas. The elements transforming in step S10 and matrixes mapping in step S20 are explained with embodiments shown in FIGS. 2 to 4.

The elements transforming in step S10 is realized by carrying out negation and conjugation operations on the input element pairs that are similar to the operations of space-time block coding. Assuming the input element pairs are {b₁,b₂} and {b₄,b₃}, performing space-time block coding on them, the corresponding orothognal element pairs are {−b*₂,b*₂} and {b*₃,−b*₄}, wherein [ ]* denotes conjugation operation. The first and second input element pairs and their corresponding orothognal element pairs form two matrixes respectively as

$A_1=\left[\begin{array}{cc}{b}_1& -b_2^{*}\\ b_2& b_1^{*}\end{array}\right]\mathrm{and}$ $A_2=\left[\begin{array}{cc}{b}_3& -b_4^{*}\\ b_4& b_3^{*}\end{array}\right],$

which are orothognal matrixes meeting A₁·A₁^T=I and A₂·A₂^T=I, I is identity matrix, [ ]^T denotes transpose operation.

Mapping the input elements and orthogonal elements to time-frequency matrix is performed in step S20. The transmit diversity obtained from coding provided in the invention comprises two parts: space-time transmit diversity and space-frequency diverity that are achieved at same time by smart allocation of channel elements.

The first and second time-frequency matrixes in FIG. 2 are transmitted respectively by a first and second antennas. The row and column in the two matrixes denote time unit and frequency unit respectively. The input element pairs {b₁,b₂}, {b₄,b₃} are mapped to the first matrix and the orthogonal elements pairs {−b*₂,b*₁},{b*₃,−b*₄} are mapped to the second matrix as illustrated in FIG. 2.

When b₁,b₂,b₃ and b₄ are data symbols, the time unit and frequency unit are time-slot and sub-carrier respectively. The data symbols b₁,b₂,b₃ and b₄ occupy six time-frequency cells corresponding to three sub-carriers and two time-slots. The input element pair {b₁,b₂} transmitted at time-slot t₁ and sub-carrier f₁ and f₂ via the first antenna and the orthogonal element pair {−b*₂,b*₁} transmitted at time-slot t₁ and sub-carrier f₁ and f₂ via the second antenna constitute space-frequency transmit diversity. The input element pair {b₁,b₂} transmitted at time-slot t₁ and t₂ and sub-carrier f₁ via the first antenna and the orthogonal element pair {−b*₂,b*₁} transmitted at time-slot t₁ and t₂ and sub-carrier f₁ via the second antenna constitute space-time transmit diversity. Here the symbol b₁ and −b*₂ transmitted on time-slot t₁ and sub-carrier f₁ are reused for space-time transmit diversity and space-frequency transmit diversity.

The transmit diversity gain generated by the encoding method provided by the invention is explained as below with conjunct consideration of mathematics expression. Without lossing generality, for conventional wireless communication systems such as 3GPP or WLAN, it is reasonable to assume that the channel response of adjacent time-slots or sub-carriers has time-invariant characteristics. When the wireless channels suffer very deep and slow fading, the channel response remains constant over the time and frequency corresponding to an input elements pair or an orthogonal elements pair, i.e.:

h _m,i,j =h _m,i+1,j+1 =h _m   (1)

wherein, m is number of antennas, i is sequence number of time-slot and j is sequence number of sub-carriers. In this embodiment, m=2. At receiving side, the received channel elements are linearly combined and maximum likelihood decision is taken to recover the input elements.

The received channel element corresponding to time-slot t₁ and sub-carrier f₁ can be expressed as:

R ₁ =h ₁ b ₁ −h ₂b*₂+n₁   (2)

The received channel element corresponding to time-slot t₂ and sub-carrier f₁ can be expressed as:

R ₂ =h ₁ b ₂ +h ₂ b* ₁ +n ₂   (3)

The received channel element corresponding to time-slot t₁ and sub-carrier f₂ can be expressed as:

R ₃ =h ₁ b ₂ +h ₂b*₁+n₃   (4)

b₁ and b₂ are decoded according to equation as below:

$\begin{array}{cc}\begin{array}{c}{\tilde{b}}_1=h_1^{*}R_1+h_2R_2^{*}+h_1^{*}R_1+h_2R_3^{*}\\ =2\left({h_1}^2+{h_2}^2\right)·b_1+2h_1^{*}n_1+h_2n_2^{*}+h_2n_3^{*}\end{array}& \left(5\right)\\ \begin{array}{c}{\tilde{b}}_2=h_2R_1^{*}+h_1^{*}R_2+h_2R_1^{*}+h_1^{*}R_3\\ =2\left({h_1}^2+{h_2}^2\right)·b_2+2h_2n_1^{*}+h_1^{*}n_2+h_1^{*}n_3\end{array}& \left(6\right)\end{array}$

The maximum likelihood (ML) decision is just to make the decision as the follows:

{circumflex over (b)} ₁=mixmum({tilde over (b)}₁−2(|h₁|²+|h₂|²)·b₁)   (7)

{circumflex over (b)} ₂=mixmum({tilde over (b)}₂−2(|h₁|²+|h₂|²)·b₂)   (8)

where {circumflex over (b)}₁,{circumflex over (b)}₂ is the decision results, b₁, b₂ is the symbols used for ML decision.

In equ.5, the signal energy of b₁ is (2(|h₁|²+|h₂|²))²·E_b, and the noise power spectrum density is (4|h₁|²+|h₂|²+|h₂|²)·N₀, so the signal to noise ratio is:

$\begin{array}{cc}\mathrm{SNR}=\frac{{\left(2\left({h_1}^2+{h_2}^2\right)\right)}^2·E_b}{\left(4{h_1}^2+2{h_2}^2\right)·N_0}>\left({h_1}^2+{h_2}^2\right)·\frac{E_b}{N_0}& \left(9\right)\end{array}$

The diversity gain

$\frac{{\left(2\left({h_1}^2+{h_2}^2\right)\right)}^2}{\left(4{h_1}^2+2{h_2}^2\right)}$

is total diversity gain obtained from space-time transmit diveristy and space-frequency diversity provided by the invention. It is clear that the total diversity gain is more than (|h₁|²+|h₂|²), which is the diversity gain of conventional space time block coding or space frequency block coding. Therefore, the space-time-frequency coding has has better performance than the conventional schemes.

FIG. 3 shows a second embodiment of mapping channel elements to time-frequency matrixes in accordance with the invention. In this case, the data symbols b₁,b₂,b₃ and b₄ occupy six time-frequency cells corresponding to three sub-carriers and two time-slots.

FIG. 4 shows a third embodiment of mapping channel elements to time-frequency matrixes in accordance with the invention. In this case, the input symbol is extended to be symbol blocks.

FIG. 5 and FIG. 6 show a fourth and fifth embodiments of mapping channel elements to time-frequency matrixes in accordance with the invention. The channel element P is specific input element having different attribute from input elements b₁,b₂,b₃ and b₄ . For example, P is symbol representing pilot signal, b₁,b₂,b₃ and b₄ are symbols representing data. As space-time block coding and space-frequency block coding can process input elements in pairs, a single input element may cooperate with space-time block coding and space-frequency block coding to fill the inpar time-frequency cells.

In embodiments of mapping channel elements to time-frequency matrixes as shown in FIG. 2 to FIG. 6, the first element, second element and the redundancy of the second element in each of the plurality of input element pairs and corresponding orthogonal element pair are mapped as channel elements to three predetermined time-frequency cells in a first and second two-dimension time-frequency matrix. The three predetermined time-frequency cells are three of the four time-frequency cells corresponding to two predetermined time units and two predetermined frequency units. The two predetermined time units and/or two predetermined frequency unit must not be adjacent if the channel response of the three cells meet the requirement of time-invariant characteristics.

The encoding method provided by the invention can be also applied to orthogonal frequency division multiplexing systems. In this case, the channel elements are transformed from frequency-domain to time-domain via Inverse Discrete Fourier Transform before transmission. At receiving side, Discrete Fourier Transform is used to transform the received channel elements from time-domain to frequency-domain before decoding.

The above encoding method as illustrated in FIG. 1-6 can be implemented in software or hardware, or in combination of both.

FIG. 7 is a block diagram illustrating an embodiment of encoding apparatus in accordance with the invention. The encoding apparatus 30 comprises a transforming unit 32 and a mapping unit 34.

The transforming unit 32 is arranged to transform a plurality of input element pairs into a plurality orthogonal element pairs respectively. Each of the plurality of input element pairs and corresponding orthogonal element pair forms an orthogonal matrix. The transforming unit 32 performs complex conjugation and negation operations on the input element pair. Assuming the input element pairs are {b₁,b₂} and {b₄,b₃}, performing space-time block coding on them, the corresponding orothognal element pairs are {−b*₂,b*₁} and {b*₃,−b*₄}, wherein [ ]* denotes conjugation operation. The first and second input element pairs and their corresponding orothognal element pairs form two matrixes respectively as

$A_1=\left[\begin{array}{cc}{b}_1& -b_2^{*}\\ b_2& b_1^{*}\end{array}\right]\mathrm{and}$ $A_2=\left[\begin{array}{cc}{b}_3& -b_4^{*}\\ b_4& b_3^{*}\end{array}\right],$

which are orothognal matrixes meeting A₁·A₁^T=I and A₂·A₂^T=I, I is identity matrix, [ ]^T denotes transpose operation.

The mapping unit 34 is arranged to map the first element, second element and the redundancy of the second element in each of the plurality of input element pairs and corresponding orthogonal element pair as channel elements to three predetermined time-frequency cells in a first and second two-dimension time-frequency matrix so as to make the channel elements in the first and second matrixes suitable for being transmitted via different antennas. More particularly, the mapping unit 34 allocates the input element pairs {b₁,b₂} and {b₄,b₃} and corresponding orthogonal element pairs {−b*₂,b*₁} and {b*₃,−b*₄} and/or specific channel element, for example a pilot symbol, to predetermined time-frequency cells to obtain two matrixes as illustrated in FIG. 2 to 6. The channel elements in the two matrixes are transmitted via two different antennas for achieving space-time transmit diversity and space-frequency transmit diversity at the same time.

The embodiments of the present invention described herein are intended to be taken in an illustrative and not a limiting sense. Various modifications may be made to these embodiments by those skilled in the art without departing from the scope of the present invention as defined in the appended claims.

Claims

1. An encoding method comprising: (a) transforming a plurality of input element pairs into a plurality orthogonal element pairs respectively, each of the plurality of input element pairs and corresponding orthogonal element pair forming an orthogonal matrix; and(b) mapping the first element, second element and the redundancy of the second element in each of the plurality of input element pairs and corresponding orthogonal element pair as channel elements to three predetermined time-frequency cells in a first and second two-dimension time-frequency matrix so as to make the channel elements in the first and second matrixes suitable for being transmitted via different antennas.

2. A method as claimed in claim 1, wherein in step (b) the three predetermined time-frequency cells are three of the four time-frequency cells corresponding to two predetermined time units and two predetermined frequency units.

3. A method as claimed in claim 2, when the input element pairs are {b1,b2},{b4,b3}, the corresponding orthogonal element pairs are {−b*2,b*1} and {b*3,−b*4},b*i is complex conjugation of bi, the result obtained by mapping the two input element pairs and orthogonal element pairs as channel elements to a first and second two-dimensions matrixes is:

4. A method as claimed in claim 2, when the input element pairs are {b1,b2},{b4,b3}, the corresponding orthogonal element pairs are {−b*2,b*1} and {b*3,−b*4},b*i is complex conjugation of bi, the result obtained by mapping the two input element pairs and orthogonal element pairs as channel elements to a first and second two-dimensions matrixes is:

5. A method as claimed in claim 2, when the input element pairs is {b1,b2}, the corresponding orthogonal element pairs are {−b*2,b*1},b*i is complex conjugation of bi, the result obtained by mapping the input element pair, the orthogonal element pair and a specific input element P as channel elements to a first and second two-dimension matrixes is:

6. A method as claimed in claim 3, when the input element is symbol block corresponding to a plurality of sub-carriers and symbol intervals, the number of symbols contained in a block equals to the product of the number of sub-carriers and the number of symbol intervals contained in a time-frequency cell.

7. A method as claimed in claim 3, when the input element is a symbol, the time-frequency cell then corresponds to a sub-carrier and a time-slot.

8. An encoding apparatus comprising: a transforming unit for transforming a plurality of input element pairs into a plurality orthogonal element pairs respectively, each of the plurality of input element pairs and corresponding orthogonal element pair forming an orthogonal matrix; anda mapping unit for mapping the first element, second element and the redundancy of the second element in each of the plurality of input element pairs and corresponding orthogonal element pair as channel elements to three predetermined time-frequency cells in a first and second two-dimension time-frequency matrix so as to make the channel elements in the first and second matrixes suitable for being transmitted via different antennas.

9. An apparatus as claimed in claim 8, wherein the three predetermined time-frequency cells are three of the four time-frequency cells corresponding to two predetermined time units and two predetermined frequency units.

10. An apparatus as claimed in claim 9, when the input element pairs are {b1,b2},{b4,b3}, the corresponding orthogonal element pairs are {−b*2,b*1} and {b*3,−b*4},b*i is complex conjugation of bi, the result obtained by mapping the two input element pairs and orthogonal element pairs as channel elements to a first and second two-dimensions matrixes is:

11. An apparatus as claimed in claim 9, when the input element pairs are {b1,b2}, {b4,b3}, the corresponding orthogonal element pairs are {−b*2,b*1} and {b*3,−b*4},b*i complex conjugation of bi, the result obtained by mapping the two input element pairs and orthogonal element pairs as channel elements to a first and second two-dimensions matrixes is:

12. An apparatus as claimed in claim 9, when the input element pairs is {b1,b2}, the corresponding orthogonal element pairs are {−b*2,b*1},b*i is complex conjugation of bi, the result obtained by mapping the input element pair, the orthogonal element pair and a specific input element P as channel elements to a first and second two-dimension matrixes is: