Transmit diversity from orthogonal design for FBMC/OQAM

Abstract

How to apply an Alamouti like space-time coding (or transmit diversity) to a Filter Bank Multicarrier (FBMC) transmission using Offset QAM (OQAM). In FBMC, due to the orthogonality in the real domain only, an intrinsic interference results thereof for the imaginary component. Simply adapting the Alamouti scheme to FBMC OQAM is not obvious since the intrinsic interference terms are not equivalent at each antenna since it depends on the surrounding symbols. The application proposes to use a precoding symbol chosen to cancel out (zero) the intrinsic interference individually for each antenna, ie a code rate of 1/2 (sending one data symbol requires two time units). A more elaborated embodiment proposes to choose the contiguous precoding symbols such that a virtual QAM Alamouti scheme is achieved, without rate loss.

Claims

1. A method for transmitting a multicarrier signal, the method comprising: transmitting the multicarrier signal, wherein said signal is of the offset quadrature amplitude modulation, OQAM, type comprising symbols in the time-frequency space, wherein the symbols include a data containing symbol and a precoding symbol, wherein the precoding symbol (x) is selected so that data is carried in both the in-phase and quadrature components of the data containing symbol (y) when received by a receiver, and wherein said data containing symbol in the time-frequency space is formed by modulating a real-valued symbol and an intrinsic interference corresponds to the imaginary part of the demodulated signal at said receiver and is constrained to carry data by a suitably selected value of said precoding symbol, or said data containing symbol in the time-frequency space is formed by modulating an imaginary-valued symbol and an intrinsic interference corresponds to the real-valued part of the demodulated signal at said receiver and is constrained to carry data by a suitably selected value of said precoding symbol.

2. The transmission method of claim 1, wherein said offset quadrature amplitude modulation is applied with the filter bank multicarrier FBMC.

3. The transmission method of claim 1 where the selecting of the precoding symbol is performed in order to realize an orthogonal space-time or space-frequency code in a system using OQAM signalling.

4. The transmission method of claim 1, wherein two complex-valued quadrature amplitude modulation QAM symbols s.sub.1,s.sub.2 are to be transmitted, wherein real parts of the data containing QAM symbols s.sub.1,s.sub.2 are transmitted using resources (m.sub.0,n.sub.0) and (m.sub.0+u,n.sub.0+v) in the time-frequency domain by a first antenna; wherein the real parts of the data containing symbols s.sub.2,s.sub.1 are transmitted using resources (m.sub.0,n.sub.0) and (m.sub.0+u,n.sub.0+v) in the time-frequency domain by a second antenna; and wherein u, v are non-zero.

5. The transmission method of claim 1, wherein two complex-valued quadrature amplitude modulation QAM symbols s.sub.1,s.sub.2 are to be transmitted, wherein real parts of the data containing QAM symbols s.sub.1,s.sub.2 are transmitted using resources (m.sub.0,n.sub.0) and (m.sub.0+u,n.sub.0) in the time-frequency domain by a first antenna; wherein the real parts of the data containing symbols s.sub.2,s.sub.1, are transmitted using resources (m.sub.0,n.sub.0) and (m.sub.0+u,n.sub.0) in the time-frequency domain by a second antenna; and wherein u is non-zero.

6. The transmission method of claim 1, wherein two complex-valued quadrature amplitude modulation QAM symbols s.sub.1,s.sub.2 are to be transmitted, wherein real parts of the data containing QAM symbols s.sub.1,s.sub.2 are transmitted using resources (m.sub.0,n.sub.0) and (m.sub.0,n.sub.0+u) in the time-frequency domain by a first antenna; wherein the real parts of the data containing symbols s.sub.2, s.sub.1, are transmitted using resources (m.sub.0, n.sub.0) and (m.sub.0, n.sub.0+u) in the time-frequency domain by a second antenna; and wherein u is non-zero.

7. The transmission method of claim 4, wherein one precoding symbol is used for each QAM symbol transmitted by each antenna, wherein the precoding symbols are selected to force the intrinsic interference at the receiver to deliver desired real or imaginary components of the transmitted symbols to transmit useful data by the intrinsic interference part of the transmitted symbols when received by the receiver.

8. An apparatus for transmitting a multicarrier signal wherein said signal is of the offset quadrature amplitude modulation, OQAM, type comprising symbols in the time-frequency space, the apparatus comprising: a transmitter for transmitting the multicarrier signal, wherein the symbols include a data containing symbol and a precoding symbol, and wherein a selector for selecting the precoding symbol (x) so that data is carried in both the in-phase and quadrature components of the data containing symbol (y) when received by a receiver; and a modulator for forming said data containing symbol in the time-frequency space by modulating a real-valued symbol and an intrinsic interference corresponds to the imaginary part of the demodulated signal at said receiver and is constrained to carry data by a suitably selected value of said precoding symbol, or said modulator for forming said data containing symbol in the time-frequency space by modulating an imaginary-valued symbol and an intrinsic interference corresponds to the real-valued part of the demodulated signal at said receiver and is forced to carry data by a suitably selected value of said precoding symbol.

9. The apparatus of claim 8, further comprising the features as defined in claim 8, wherein said offset quadrature amplitude modulation is applied with the filter bank multicarrier FBMC.

10. A method for receiving a multicarrier signal which is transmitted according to the method for transmitting according to claim 1, the method for receiving comprising data being obtained from demodulating the in-phase and quadrature components of the data containing symbol, when received by a receiver.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the architecture of an FBMC/OQAM transmission scheme with transmit diversity using two transmit antennas.

(2) FIG. 2 shows an Alamouti scheme for transmit diversity as for example used in LTE.

(3) FIG. 3 shows a single Input, single output channel model where a PAM signal is transmitted through a single transmit antenna based on FBMC/OQAM.

(4) FIG. 4 shows a transmission scheme with two transmit antennas based on FBMC/OQAM where orthogonally is lost through intrinsic interference.

(5) FIG. 5 shows an orthogonal transmission scheme with transmit diversity through two transmit antennas based on FBMC/OQAM where intrinsic interference is cancelled using one precoding symbol per PAM signal.

(6) FIG. 6 shows the principle of transmitting one PAM symbol using two resources.

(7) FIG. 7 shows the principle of transmitting one QAM symbol using two resources.

(8) FIG. 8 shows an orthogonal transmission scheme based on FBMC/OQAM with transmit diversity through two transmit antennas where precoding symbols are chosen such that, when being demodulated at a receiver, to carry data in both the in-phase and quadrature components of the data containing symbol.

DETAILED DESCRIPTION

(9) At first, some terms used in the description will be defined in the following list of abbreviations. AWGN Additive White Gaussian Noise FBMC Filter Bank Multicarrier LTE Long Term Evolution (mobile phone standard) OFDM Orthogonal Frequency Division Multiplexing OQAM Offset Quadrature Amplitude Modulation PAM Pulse Amplitude Modulation QAM Quadrature Amplitude Modulation SISO Single-In-Single-Out

(10) The present invention is concerned with filter bank multicarrier (FBMC) offset quadrature amplitude modulation (OQAM) transmission with the so called transmit diversity technique using two transmit antennas as illustrated in FIG. 1.

(11) One objective is to design a transmit diversity scheme from orthogonal design for FBMC/OQAM similar to the above described Alamouti scheme which is, for example, applied to LTE OFDM system.

(12) A first approach to achieve orthogonality in a FBMC/OCAM by cancelling out intrinsic interference is described in the following. In this first approach orthogonality is achieved by introducing precoding symbols, specifically as shown in FIG. 5, four precoding symbols are introduced, i.e., two precoding symbols for each transmit antenna. As it will become apparent, such use of precoding to cancel intrinsic interference achieves orthogonality but has the problem of achieving only a low code rate since some transmission resources are not used to transmit useful data. The present invention will improve this aspect.

(13) In the first approach, precoding symbols are chosen such as to cancel the intrinsic interferences as follows:

(14) $\begin{array}{cc}\begin{array}{c}{y}_{m_0,n_0}=H^{\left(1\right)}\left(a_1+j\stackrel{\stackrel{.Math.0}{}}{I_{m_0,n_0}^{\left(1\right)}}\right)-H^{\left(2\right)}\left(a_2-j\stackrel{\stackrel{.Math.0}{}}{I_{m_0,n_0}^{\left(2\right)}}\right)+{}_{m_0,n_0}\\ y_{m_0+1,n_0}=H^{\left(1\right)}\left(a_2+j\stackrel{\stackrel{.Math.0}{}}{I_{m_0+1,n_0}^{\left(1\right)}}\right)+H^{\left(2\right)}\left(a_1+j\stackrel{\stackrel{.Math.0}{}}{I_{m_0+1,n_0}^{\left(2\right)}}\right)+{}_{m_0+1,n_0}\end{array}& \left(13\right)\end{array}$

(15) Here, y.sub.m.sub.0.sub.,n.sub.0 is the received signal at the resource at (m.sub.0,n.sub.0) in the time-frequency domain, .sub.m.sub.0.sub.,n.sub.0 is AWGN and I.sub.m.sub.0.sub.,n.sub.0.sup.(1) and I.sub.m.sub.0.sub.,n.sub.0.sup.(2) being the Intrinsic interference from the first and the second antenna at resource (m.sub.0,n.sub.0) respectively. The precoding symbols x.sub.1,x.sub.2,x.sub.3,x.sub.4 are chosen to cancel (zero) the intrinsic interference individually for each antenna. Specifically, the symbols x.sub.1, x.sub.2 are chosen to cancel the intrinsic interferences I.sub.m.sub.0.sub.,n.sub.0.sup.(1) I.sub.m.sub.0.sub.+1,n.sub.0.sup.(1) of the first antenna and x.sub.3, x.sub.4 are chosen to cancel the intrinsic interferences I.sub.m.sub.0.sub.,n.sub.0.sup.(2) I.sub.m.sub.0.sub.+1,n.sub.0.sup.(2) of the second antenna.

(16) The transmission scheme according to the first approach leads to an orthogonal design and achieves a code rate of 1/2, i.e. the transmission of one data symbol requires two time units since one precoding symbol is transmitted per data symbol.

(17) FIG. 6 shows the principles of FIG. 5 yet simplified where only two transmission resources and only a single antenna is considered for simplicity. One of the resources is used for sending the precoding signal that is intended to protect useful data sent using another resource to somewhat combat with pure imaginary intrinsic interference observed at the receiver. This means that one real-valued PAM symbol is transmitted using two resources. Hence FIG. 6 illustrates this principle of using a precoding symbol x to protect a real-valued data symbol to be transmitted, wherein the precoding symbol serves to cancel out intrinsic interference and the value received at the resource used for transmission of the precoding symbol is not otherwise used (useless) at the receiver side.

(18) It can be seen from FIG. 5 and FIG. 6 that real-valued PAM symbols are considered for transmission, i.e., real values are transmitted so that the quadrature component of the data symbol obtained at the receiver is not used for the transfer of useful data; instead, the quadrature component is undesired and is forced to zero using precoding. Alternatively, real values are transmitted so that the quadrature component of the data symbol obtained at the receiver is not used for the transfer of useful data; instead, the in-phase component is undesired and is forced to zero using precoding.

(19) The present invention uses complex-valued QAM symbols for transmission as illustrated in FIG. 7. FIG. 7 highlights the difference to the previously described first approach summarized in FIG. 6, and the present invention. An important difference and aspect of the present invention is to exploit precoding symbols to force the intrinsic interference to a specific value so that the quadrature component of the data signal received at the receiver carries useful data, i.e. the imaginary part of the transmitted QAM symbol.

(20) Like FIG. 6, FIG. 7 focuses on only two resource grids and only a single antenna is considered for transmission. In the present invention, the principles of which are demonstrated in FIG. 7, two resources are used, like in FIG. 6, one of which is a precoding signal. In contrast to the scheme of FIG. 6, the precoding signal is designed to deliver useful data in the pure real or imaginary domain observed at the receiver. This means that one complex-valued QAM symbol can be transmitted using two resources, one resource for the transmission of the real-valued data to be transmitted as first useful data, and one resource for transmitting the real-valued preceding symbol, which forces the imaginary or real part respectively to a certain value, which corresponds to the second useful data, which is to be transmitted.

(21) An embodiment of the present invention is illustrated in FIG. 8 where two QAM symbols are to be transmitted and two antennas are used for transmit diversity.

(22) To further elaborate, formula (5) can be written as follows:
y.sub.m.sub.0.sub.,n.sub.0=H.sub.m.sub.0.sub.,n.sub.0(a.sub.m.sub.0.sub.,n.sub.0+jg.sub.m.sub.0.sub.,n.sub.0.sub.1x+I.sub.m.sub.0.sub.,n.sub.0))+.sub.m.sub.0.sub.,n.sub.0(14)
where, corresponding to formula (6), it can be defined
I.sub.m.sub.0.sub.,n.sub.0=I.sub.m.sub.0.sub.,n.sub.0g.sub.m.sub.0.sub.,n.sub.0.sub.1x.(15)

(23) Here, the choice of x highlights the difference between the above-described approach to achieve orthogonality and the present invention. With the above-described approach, x may be chosen, for example, as

(24) $\begin{array}{cc}x=\frac{-I_{m_0,n_0}^{}}{{.Math.g.Math.}_{m_0,n_0-1}}& \left(16\right)\end{array}$
so that formula (14) at the receiver becomes
y.sub.m.sub.0.sub.,n.sub.0=H.sub.m.sub.0.sub.,n.sub.0a.sub.m.sub.0.sub.,n.sub.0+.sub.m.sub.0.sub.,n.sub.0.(17)

(25) It can be seen that the intrinsic interference is cancelled by the precoding symbol and one real-valued interference-free PAM symbol a.sub.m.sub.0.sub.,n.sub.0 is transmitted and received.

(26) In the case of the present invention, x is chosen as follows (compare to formula (16)):

(27) $\begin{array}{cc}x=\frac{-I_{m_0,n_0}^{}+b_{m_0,n_0}}{{.Math.g.Math.}_{m_0,n_0-1}}& \left(18\right)\end{array}$
so that formula (14) at the receiver becomes (compare to formula (17)):
y.sub.m.sub.0.sub.,n.sub.0=H.sub.m.sub.0.sub.,n.sub.0(a.sub.m.sub.0.sub.,n.sub.0+jb.sub.m.sub.0.sub.,n.sub.0)+.sub.m.sub.0.sub.,n.sub.0(19)

(28) It can be observed that one complex-valued interference-free QAM symbol s.sub.m.sub.0.sub.,n.sub.0=a.sub.m.sub.0.sub.,n.sub.0+jb.sub.m.sub.0.sub.,n.sub.0 is received. This means twice the data rate as compared to the first approach with a received signal according to formula (17), where one real-valued PAM symbol a.sub.m.sub.0.sub.,n.sub.0 is transmitted and received.

(29) To clarify, the precoding symbol as used in this embodiment can be interpreted to contain Information which at the receiver side correspond to data symbols, i.e., the precoding symbol is to carry certain bits of transmission data streams.

(30) Next, it will be explained how the new transmission principle according to FIG. 7 can be utilized for the transmit diversity from orthogonal design for FBMC/OQAM. It should be noted however, that the transmission principle according to FIG. 7 is much more general, so that it is not limited to this specific application of transmit diversity but that there might be many other application areas.

(31) FIG. 8 illustrates a system model with two transmit antennas where 2 complex-valued QAM symbols s.sub.1=s.sub.1.sup.R+j.Math.s.sub.1.sup.I and s.sub.2=s.sub.2.sup.R+j.Math.s.sub.2.sup.I are transmitted using four real-valued precoding symbols x.sub.1, x.sub.2, x.sub.3, and x.sub.4, two for each transmit antenna.

(32) To simplify the notations, the ambiguity functions are also illustrated in the FIG. 8.

(33) An arrow labelled a.sub.i from a first transmission resource to a neighbouring second transmission resource specifies that the first transmission resource Induces an Interference component with weight w.sub.i on the second transmission resource.

(34) The present invention exploits such interference components from neighbouring resources by choosing precoding symbols, which are transmitted on the neighbouring resources, to deliver the desired quadrature components of the QAM symbols s.sub.1 and s.sub.2 by taking into account Alamouti design as follows:

(35) 0$\begin{array}{cc}\begin{array}{c}{y}_{m_0,n_0}=H^{\left(1\right)}\left(s_1^R+j\stackrel{\stackrel{.Math.s_1^I}{}}{I_{m_0,n_0}^{\left(1\right)}}\right)+H^{\left(2\right)}\left(-s_2^R+j\stackrel{\stackrel{.Math.s_2^I}{}}{I_{m_0,n_0}^{\left(2\right)}}\right)+{}_{m_0,n_0}\\ y_{m_0+1,n_0}=H^{\left(1\right)}\left(s_2^R+j\stackrel{\stackrel{.Math.s_2^I}{}}{I_{m_0+1,n_0}^{\left(1\right)}}\right)+H^{\left(2\right)}\left(s_1^R+j\stackrel{\stackrel{.Math.-s_1^I}{}}{I_{m_0+1,n_0}^{\left(2\right)}}\right)+{}_{m_0+1,n_0}\end{array}& \left(20\right)\end{array}$

(36) In formula (20) there are four constraints and four unknowns (precoding symbols) and thus, these equations are solvable. From the construction of the constraints, transmitting precoding symbols obtained by solving the equations should lead to the equivalent system as Alamouti. Hence, a transmit diversity technique from orthogonal design is realized for FBMC/OQAM without rate loss.

(37) To be more specific, formula (20) may be rewritten as follows:
y.sub.m.sub.0.sub.,n.sub.0=H.sup.(1)(s.sub.1.sup.R+j(w.sub.1x.sub.1+w.sub.4x.sub.2+I.sub.m.sub.0.sub.,n.sub.0.sup.(1)))+H.sup.(2)(s.sub.2.sup.R+j(w.sub.1x.sub.3+w.sub.4x.sub.4+I.sub.m.sub.0.sub.,n.sub.0.sup.(2)))+.sub.m.sub.0.sub.,n.sub.0(21)
y.sub.m.sub.0.sub.+1,n.sub.0=H.sup.(1)(s.sub.2.sup.R+j(w.sub.2x.sub.2+w.sub.3x.sub.1+I.sub.m.sub.0.sub.+1,n.sub.0.sup.(1)))+H.sup.(2)(s.sub.1.sup.R+j(w.sub.2x.sub.4+w.sub.3x.sub.3+I.sub.m.sub.0.sub.+1,n.sub.0.sup.(2)))+.sub.m.sub.0.sub.,n.sub.0(22)

(38) where w.sub.1, w.sub.2, w.sub.3, and w.sub.4 are weights from the ambiguity functions (see FIG. 8) and where the following is introduced:
I.sub.m.sub.0.sub.,n.sub.0.sup.(1)=I.sub.m.sub.0.sub.,n.sub.0.sup.(1)w.sub.1x.sub.1+w.sub.4x.sub.2
I.sub.m.sub.0.sub.,n.sub.0.sup.(2)=I.sub.m.sub.0.sub.,n.sub.0.sup.(2)w.sub.1x.sub.3+w.sub.4x.sub.4
I.sub.m.sub.0.sub.+1,n.sub.0.sup.(1)=I.sub.m.sub.0.sub.+1,n.sub.0.sup.(1)w.sub.2x.sub.2+w.sub.3x.sub.1
I.sub.m.sub.0.sub.+1,n.sub.0.sup.(2)=I.sub.m.sub.0.sub.+1,n.sub.0.sup.(2)w.sub.2x.sub.4+w.sub.3x.sub.3(23)
and I.sub.m.sub.0.sub.,n.sub.0.sup.(1), I.sub.m.sub.0.sub.,n.sub.0.sup.(2), I.sub.m.sub.0.sub.+1,n.sub.0.sup.(1), and I.sub.m.sub.0.sub.+1,n.sub.0.sup.(2) are defined in a manner similar as in formula (6), for the antennas Tx1, Tx2, and for respective resource grids. The constraints in (20) can be explicitly written as
w.sub.1x.sub.1+w.sub.4x.sub.2+I.sub.m.sub.0.sub.,n.sub.0.sup.(1)=s.sub.1.sup.I
w.sub.2x.sub.2+w.sub.3x.sub.1+I.sub.m.sub.0.sub.+1,n.sub.0.sup.(1)=s.sub.2.sup.I
w.sub.1x.sub.3+w.sub.4x.sub.4+I.sub.m.sub.0.sub.,n.sub.0.sup.(2)=s.sub.2.sup.I
w.sub.2x.sub.4+w.sub.3x.sub.3+I.sub.m.sub.0.sub.+1,n.sub.0.sup.(2)=s.sub.2.sup.I(24)

(39) These equations can be rewritten as the following two sets of equations in a matrix-vector form as

(40) $\begin{array}{cc}\left[\begin{array}{cc}{w}_1& w_4\\ w_3& w_2\end{array}\right]\left[\begin{array}{c}{x}_1\\ x_2\end{array}\right]=\left[\begin{array}{c}{s}_1^I-I_{m_0,n_0}^{{\left(1\right)}^{}}\\ s_2^I-I_{m_0+1,n_0}^{{\left(1\right)}^{}}\end{array}\right]\left[\begin{array}{cc}{w}_1& w_4\\ w_3& w_2\end{array}\right]\left[\begin{array}{c}{x}_3\\ x_4\end{array}\right]=\left[\begin{array}{c}{s}_2^I-I_{m_0,n_0}^{{\left(2\right)}^{}}\\ -s_1^I-I_{m_0+1,n_0}^{{\left(2\right)}^{}}\end{array}\right]& \left(25\right)\end{array}$

(41) These equation systems can be easily solved to obtain the desired precoding symbols x.sub.1, x.sub.2, x.sub.3, and x.sub.4 that satisfy those constraints above. Thus, (21) and (22) reduce to (1) and (2). This means that an equivalent system as Alamouti design is obtained, i.e. the system is orthogonal and without rate loss.

(42) The advantage of the technology according to the embodiments presented herein is the ability to realize transmit diversity from the orthogonal design with full diversity, i.e. diversity order of 2 for two transmit antennas.

(43) It will be readily apparent to the skilled person that the methods, the elements, units and apparatuses described in connection with embodiments of the invention may be implemented in hardware, in software, or as a combination of both. In particular it will be appreciated that the embodiments of the invention may be implemented by a computer program or computer programs running on a computer or being executed by a microprocessor. Any apparatus implementing the invention may in particular take the form of a computing device acting as a network entity. An apparatus for transmitting according to the embodiments of the invention may be implemented by a microprocessor or a signal processor, which is programmed to perform the signal processing steps and the modulating steps as described herein before. A signal processor suitably programmed thereby may be an implementation of a selector and a modulator according to embodiments of the invention. For that purpose the signal processor may<be connected to a memory comprising the program for when being executed enabling the microprocessor or signal processor or computer to act as an apparatus according to the embodiments of the invention, in particular as a selector and/or as a modulator or demodulator according to embodiments of the invention.