Reference signal-free transmission for wireless systems

Systems, methods, and instrumentalities are disclosed for separating a channel and data without the use of reference signals. For example, a wireless transmit/receive unit (WTRU) may determine a first orthogonal frequency-division multiplexing (OFDM) symbol based on a data vector. The WTRU may determine a second OFDM symbol by applying a circular time-inverse operation and a conjugate operation to the first OFDM symbol. The WTRU may send the first OFDM symbol and the second OFDM symbol. The first and the second OFDM symbols may be sent to consecutively. Discrete Fourier Transform (DFT)-spread and nonlinear preprocessing (exponential transformation at the transmitter) may be used and/or peak-to-average power ratio (PAPR) may be reduced via randomizer block at the receiver.

BACKGROUND

In Long Term Evolution (LTE), orthogonal frequency-division multiplexing (OFDM) may be used for downlink (DL) transmission. Discrete Fourier Transform spread OFDM (DFT-s-OFDM) may be used for uplink (UL) transmission. In a cyclic prefix (CP) DFT-s-OFDM, the data symbols may be spread with a DFT block, and may be mapped to the corresponding inputs of an inverse DFT (IDFT) block. The CP may be prepended to the beginning of the symbol to avoid inter-symbol interference (ISI) and allow one-tap frequency domain equalization (FDE) at the receiver. A cyclic prefix (CP) DFT-s-OFDM may be referred to as single carrier (SC) frequency-division multiplexing (SC-FDMA) with multiple accessing.

In conventional wireless systems, including LTE, reference symbols may be used to estimate the channel and equalize the received signals. Common reference symbols may be transmitted on subcarriers distributed over the system bandwidth. WTRU-specific reference signals may be distributed over the sub-band that is allocated to a specific user equipment (UE).

SUMMARY

Transmission techniques that allow receivers to decode information via cepstral transformation may be provided. For example, systems, methods, and instrumentalities are disclosed for separating a channel and data without the use of reference signals. For example, a wireless transmit/receive unit (WTRU) may determine a first orthogonal frequency-division multiplexing (OFDM) symbol based on a data vector. The WTRU may determine a second OFDM symbol by applying a circular time-inverse operation and a conjugate operation to the first OFDM symbol. The WTRU may send the first OFDM symbol and the second OFDM symbol. The first and the second OFDM symbols may be sent to consecutively. Discrete Fourier Transform (DFT)-spread and nonlinear preprocessing (exponential transformation at the transmitter) may be used and/or peak-to-average power ratio (PAPR) may be reduced via randomizer block at the receiver.

DETAILED DESCRIPTION

A detailed description of illustrative embodiments will now be described with reference to the various figures. Although this description provides a detailed example of possible implementations, it should be noted that the details are intended to be exemplary and in no way limit the scope of the application.

A wireless communication system called the New Radio (NR) (e.g., 5G NR and/or 5G) may be provided. Applications of NR may include enhanced mobile broadband (eMBB), massive machine-type communications (mMTC), and/or ultra-reliable-and-low-latency communications (URLLC). For mMTC application, NR may support up to 1e6 mMTC devices per km2with extended coverage, low power consumption, and/or low device complexity. To support high connection density, non-orthogonal multiple access techniques may be used. To increase the spectral efficiency, massive MIMO systems may be deployed. In massive MIMO, a large number of antenna elements may be used for transmission and/or reception of wireless signals.

Homomorphic filtering may include a nonlinear transformation to convert a signal obtained from a convolution of multiple (e.g., two) original signals into the sum of multiple (e.g., two) signals. In speech processing, homomorphic filtering may be applied to separate the filter from the excitation in the source-filter model. The cepstrum may be a homomorphic transformation that may allow for performing such separation. The cepstrum may be defined as the spectrum of the log of the spectrum of a time waveform. In this domain, the channel component may create peaks that may be filtered. Filtering may be referred to as ‘liftering’ herein.

Channel estimation may be performed by using reference signals (RS). With non-orthogonal multiple access, and/or for massive MIMO systems, the overhead of reference signals may increase, and/or pilot contamination may occur.

One or more transmission techniques may result in the effective channel to be real-valued. The channel and data may be separated. For example, the channel and data may be separated without the use of reference signals.

An OFDM symbol may be represented by a vector xt. An OFDM symbol may be generated by xt=CFHd. d may be a data vector, F may be a Fourier matrix, the superscript H may denote a Hermitian operation, and/or C may denote a matrix that represents an addition of a cyclic prefix. xmay equal FHd.

The received signal may be represented as y=xh. For example, at the receiver, after removing the CP, the received signal may be represented as y=xh.may denote a circular convolution operator, and/or h may be a channel vector that may be complex valued.

FIG. 2illustrates an example transmitter and an example transmission200. For example, the structures provided inFIG. 2may be implemented in a transmitter. As shown inFIG. 2, data vector210may be processed via an Inverse Fast Fourier Transform (IFFT), for example, via IFFT212. Data vector210may be a data portion of an OFDM symbol. OFDM symbol x213may result from data vector210being processed via the IFFT212. OFDM symbol x213may be an OFDM symbol without a CP. A CP may be added to OFDM symbol x213, for example, via CP214.

A data vector220may be processed via an Inverse Fast Fourier Transform (IFFT), for example, via IFFT222. Data vector220may be a data portion of an OFDM symbol. As an example, data vector220may be the same as data vector210. Data vector220may be processed via IFFT222. OFDM symbol x223may result from data vector220being processed via IFFT222. OFDM symbol x223may be an OFDM symbol without a CP. As an example, OFDM symbol x223may be the same as OFDM symbol x213. A circular time-inverse operation may be applied to OFDM symbol x223, for example, via Circular time-inverse224. The conjugate of OFDM symbol x223may be determined, for example, via Conjugate226. A CP may be added to OFDM symbol x223, for example, via CP228.

Two OFDM symbols may be transmitted. For example, a first OFDM symbol202and a second OFDM symbol204may be transmitted. The first OFDM symbol202and the second OFDM symbol204may be transmitted consecutively. x1may correspond to OFDM symbol213. In an example, x1may equal x=FHd. x2may correspond to the conjugate of OFDM symbol x223, as described herein. In an example, x2may equal x(−n)N*. n may denote a sample index, the subscript N may denote a “modulo N” operation, the superscript * may denote a conjugate, and/or N may represent the DFT size. As an example, if x1=[a b c d]T(N=4), x2may equal [a d c b]H. The superscript T may be the transpose operator. As described herein, OFDM symbol x may not include a CP. The CP may be added. For example, the CP may be added to the OFDM symbol x before transmission.

Signal r may be computed. For example, at the receiver, signal r may be computed after removing the CP. r may be equal to y1(n)+y2(−n)N*. y1may be equal to x1h1, and/or y2may be equal to x2h2. When the channel does not change (e.g., does not change significantly) over the two OFDM symbols, e.g., h=h1=h2, r may be equal to x1h+x2(−n)N*h(−n)N*, which may be equal to xh+xh(−n)N*, which may be equal to x(h+h(−n)N*).

The DFT of the computed signal r may be determined. For example, after the DFT, a vector v may be derived based on v=Fr=F{x(h+h(−n)N*)}=Fx⊙F(h+h(−n)N*)=Fx⊙HR=d⊙HR. HRmay be a real valued channel vector and/or ⊙ may denote point-wise multiplication.

FIG. 3illustrates an example transmitter and an example transmission300. For example, the structures provided inFIG. 3may be implemented in a transmitter. As shown inFIG. 3, data vector310may be processed via an Inverse Fast Fourier Transform (IFFT), for example, via IFFT312. Data vector310may be a data portion of an OFDM symbol. OFDM symbol x313may result from data vector310being processed via the IFFT312. OFDM symbol x313may be an OFDM symbol without a CP. A CP may be added to OFDM symbol x313, for example, via CP314.

Data vector320(e.g., d*320) may be the conjugate of data vector310. Data vector320may be processed via an Inverse Fast Fourier Transform (IFFT), for example, via IFFT322. Data vector320may be a data portion of an OFDM symbol. OFDM symbol x323may result from data vector320being processed via the IFFT322. OFDM symbol x323may be an OFDM symbol without a CP. A CP may be added to OFDM symbol x323, for example, via CP324.

A second OFDM symbol304may be used to transmit the conjugate of the data vector310transmitted in the first OFDM symbol302. x1may correspond to OFDM symbol x313. x2may correspond to OFDM symbol x323. In an example, if x1=x=FHd, x2may be equal to FHd*.

x2may be equal to x(−n)N*.

FIG. 4illustrates an example transmitter and an example transmission400. For example, the structures provided inFIG. 4may be implemented in a transmitter. Data symbol410and/or the conjugate420of the data symbol410may be transmitted on one or more (e.g., adjacent) subcarriers, such as subcarriers412and422. The data symbol and/or the conjugate may be processed via an Inverse Fast Fourier Transform (IFFT). For example, after being transmitted on the one or more subcarriers412,422, the data symbol410and/or the conjugate420may be processed via an Inverse Fast Fourier Transform (IFFT). The data symbol410and/or the conjugate420may be processed via an Inverse Fast Fourier Transform (IFFT), for example, via IFFT414. A CP may be added to data symbol410and/or the conjugate420, for example, via CP416. A OFDM symbol418may be used to transmit the data symbol410and/or the conjugate420. At the receiver, after DFT, the received symbols on the two adjacent subcarriers may be given as Rk=hkdk, and Rk+1=hk+1dk*. If hk=hk+1, Rk+Rk* may be equal to (hk+hk*)dk. The effective channel (hk+hk*) may be a real valued channel.

Channel impact may be removed from the received signal. For example, the transmitter may generate signals with fast varying components in a domain, such as a spectrum or cepstrum. The generated signals may be random data, independent data symbols, a pre-coded data symbol (e.g., a pre-coded data symbol that may be spread), and the like. The propagation channel may vary slower than the data. The receiver may separate the signal and the channel. For example, the receiver may separate the signal and the channel based on the difference between the variations of the signals and/or the channels. The receiver may remove the impact of the channel. For example, the impact of the channel may be removed from the received signal by using a homomorphic deconvolution technique, such as a complex cepstrum technique.

FIG. 5illustrates an example removal of channel impact. As shown inFIG. 5, two OFDM symbols (e.g., consecutive OFDM symbols), y1502and y2504, may be preprocessed. For example, two consecutive OFDM symbols, y1502and y2504, may be preprocessed via Preprocessing506. The OFDM symbols may be preprocessed at the receiver. The preprocessing of the OFDM symbols may generate a computed signal r508, as r=y1(n)+y2(−n)N*. The DFT of r508may be computed via DFT510to determine v512, wherein v=Fr. The complex logarithm of v512may be computed via514to determine log[v]516. An inverse DFT may be computed, for example, via IDFT518. The inverse DFT may be implemented via an Inverse Fast Fourier Transform (IFFT). The inverse DFT may be used to determine the cepstrum of the input signal r. The channel spectral components may be filtered (e.g., filtered out) via Filtering520. By following inverse steps, the signal without (or with little) impact of the channel may be restored. The filtered channel spectral components may be processed via a DFT, such as DFT522. A complex exponential may be determined, via complex exponential block524, of the values resulting from DFT522. The complex exponential value may be processed via an inverse DFT, for example, via IDFT526.

Preprocessing may be performed after computing the DFT of r. For example, preprocessing may operate on an OFDM symbol. An OFDM symbol may be operated on. For example, a single OFDM symbol may be operated on, one at a time. The real valued effective channel may be generated by addition (e.g., proper addition) of the symbols on two subcarriers (e.g., two adjacent subcariers).

The complex log block514ofFIG. 5may perform the following operation:

rn=adn⁢ahn⁢ej⁡(ϕdn+ϕhn)⁢⟹ln⁡(·)⁢ln⁡(rn)=ln⁡(adn⁢ahn)+j⁡(ϕdn+ϕhn).
The operation may be calculated (e.g., equivalently calculated) as the amplitude of the received signals for ln(adnahn) and the angle of the received signal for the imaginary component of (ϕdn+dhn).

FIGS. 6aand 6billustrate example receivers. For example, the structures provided inFIG. 6aandFIG. 6bmay be implemented in a receiver. A signal may be received by the receiver. For example, the signal y602(FIG. 6a) and/or signal y622(FIG. 6b) may be received by the respective receiver. If a CP is added (e.g., prepended) to an OFDM symbol, the CP may be removed from the signal y602(FIG. 6a) and/or signal y622(FIG. 6b). For example, block B604(FIG. 6a) and/or block B624(FIG. 6b) may remove the CP from the respective signal y602(FIG. 6a),622(FIG. 6b). Block B may be a matrix B. The signal y may be processed via a Fourier matrix. For example, signal y602may be processed via a Fourier matrix606. Signal y622may be processed via Fourier matrix626.

Signal r may result from signal y being processed via a Fourier matrix. For example, signal r607may result from the signal y602being processed via the Fourier matrix606. Signal r627may result from the signal y622being processed via Fourier matrix626. As shown inFIG. 6a, signal r607may be processed via log components608aand/or608b. Log components608a,608bmay include one or more log operations. The one or more log operations may include a point wise operation, which may be applied to one or more elements of r607. The base for the log operation may be a number, such as 2, e, or 10. As shown inFIG. 6b, signal627may be processed via log component628aand/or angle component628b. The angle component628bmay include an angle operation. The angle operation may be an operation that may return the phase of the elements of r607.

A liftering operation (e.g., removing one or more of the components after the DFT operation, seeFIG. 6aandFIG. 6b) may use two-ends of DFT output. The liftering operation may be performed in an unwrapping component. For example, the liftering operation may be performed in the unwrapping component609(FIG. 6a) and/or the unwrapping component629(FIG. 6b) of the respective receiver. A Fourier matrix with a Hermitian operation may be performed upon signal r607and/or signal r627. For example, the Fourier matrix with Hermitian operation denoted as610aand/or the Fourier matrix with Hermitian operation denoted as610bmay be performed upon signal r607. The Fourier matrix with Hermitian operation denoted as630aand/or the Fourier matrix with Hermitian operation denoted as630bmay be performed upon signal r627. The signal r607(FIG. 6a) and/or the signal r627(FIG. 6b) may be processed via a Fourier matrix. For example, the signal r607may be processed via Fourier matrix612aand/or612b. The signal r627may be processed via Fourier matrix632aand/or632b. The order of Fourier matrix612a,612band the Fourier matrix with Hermitian operation610a,610bmay be substituted (e.g., reversed). For example, the order of Fourier matrix612amay be reversed with the Fourier matrix with Hermitian operation610a. The order of Fourier matrix612bmay be reversed with the Fourier matrix with Hermitian operation610b. The order of Fourier matrix632a,632band the Fourier matrix with Hermitian operation630a,630bmay be substituted (e.g., reversed). For example, the order of Fourier matrix632amay be reversed with the Fourier matrix with Hermitian operation630a. The order of Fourier matrix632bmay be reversed with the Fourier matrix with Hermitian operation630b. The order of Fourier matrix612a,612band Fourier matrix with Hermitian operation610a,610bafter the log operation may be substituted based on the duality of the time and frequency. The order of Fourier matrix632a,632band Fourier matrix with Hermitian operation630a,630bafter the log operation may be substituted based on the duality of the time and frequency. The impact of the channel may be circular and/or the low-frequency component may appear on both ends.

The exponent of the signal r607,627may be determined. For example, as shown inFIG. 6a, the exponent of signal r607may be determined via component614aand/or the exponent of signal r607may be determined via component614b. Component614a,614bmay include one or more exponent operations. For example, component614a,614bmay include an exponent operation that corresponds to the operation that inverses the log operation. The base used for the exponent operation may be 2, e, or 10 for log 2, ln, or log 10 operations, respectively. Data vector d616may result from the operations described herein.

The exponent of signal r627may be determined via component634aand/or the exponent of signal r627may be determined via component634b. For example, as shown inFIG. 6b, the exponent of signal r627may be determined via component634aand/or the exponent of signal r627may be determined via component634b. Components634a,634bmay include exponent operations that may be applied to an (e.g., each) element of r627. Data vector d636may result from the operations described herein.

FIG. 7illustrates an example transmitter and receiver. For example, the structures provided inFIG. 7may be implemented in a transmitter and/or in a receiver. Reduced-complexity Cepstrum-based receivers may be provided. DFT-spread and/or exponential pre-processing may be performed. In an example, the transmitter may use a preprocessing with DFT-spread and/or an exponential operation (e.g., a subsequent exponential operation), for example, to enable less complex receiver structures.

FIG. 8illustrates an example transmitter and receiver. For example, the structures provided inFIG. 8may be implemented in a transmitter and/or in a receiver. The parts after the liftering operation inFIG. 7may be moved to the transmitter side, as show inFIG. 8. Data vector d801may be received. The transmitter may include a permutation operation P802. DFT may be performed upon data vector801, for example, via DFT-spread operation D804. Exponential precoder operations may be performed upon data vector801, for example, via exponential component e(.)806. Two-ends of a DFT-spread block (e.g., before preprocessing with DFT-spread and/or exponential operations) may be nulled. Nulling two-ends of the DFT-spread block may avoid damage (e.g., potential damage) to the data symbol after the liftering operation. The liftering operation may remove the upper-end and the lower-end of the data at the receiver.

A randomizer block (e.g., R808inFIG. 8) may be provided. For example, R808may be provided after the exponential operation806. The randomizer block808may be used to reduce the peak to average power ratio (PAPR). The PAPR may be reduced by performing a point-to-point multiplication with one or more coefficients. The coefficients may be random coefficients and/or may be developed based on a predefined criterion. Guard subcarriers may be added. For example, guard subcarriers may be added to the coefficients of R via block B810(e.g., matrix B). A Fourier matrix with a Hermitian operation may be performed upon the coefficients of R, such as the Fourier matrix with a Hermitian operation denoted as812. The transmitter may include component814, which may add a CP.

As shown inFIG. 8, a receiver may be provided. The receiver may consist of operations that are inverse of those provided in the transmitter. The inverse operations at the receiver may be provided in reverse order of operations (e.g., corresponding operations) provided at the transmitter. The receiver may remove the CP, for example, at820. A Fourier operation may be performed, for example, at822. The transpose of the matrix B may be performed, at824. Guard subcarriers may be removed at824. The coefficients of R may be received and/or may be multiplied with the inverse of R and/or the Hermitian of R, at826. The PAPR Reducer may be removed, at826. Log component828may include one or more log operations. Log components830,832may include one or more log operations. A liftering operation may be performed in the unwrapping component834. DFT with a Hermitian operation may be performed at DH836. The transpose of the permutation operation P may be performed at838. The data vector d may be provided, at840.

Phase modulation may be performed. The channel (e.g., effective channel) may become real-valued. The channel may scale the transmitted data symbols. For example, the channel may scale the transmitted data symbols without impacting the phase of the transmitted data symbols. If phase modulation is used, channel estimation may be bypassed. Data detection may be achieved. For example, data detection may be used by using the phase of the received data symbols.

RS-free non-orthogonal multiple access transmission may be performed. For example, two WTRUs may be transmitting to the eNB on the same subcarriers. A WTRU may transmit a (e.g., one) data symbol and the conjugate of the data vector on two subcarriers (e.g., two adjacent subcarriers). The received signal per WTRU, after combining the signals on two subcarriers, as described herein, may be written as (the subscripts1and2denote WTRUs1and2): Rk+Rk*={(hk1+h*k1)dk1}+{(hk2+hk2*)dk2}=2R(hk1)dk1+2R(hk2)dk2

The real-valued channels R(hk1) and R(hk2) may be separated from the received signal. For example, the real-valued channels R(hk1) and R(hk2) may be separated from the received signal via the cepstrum technique, as described herein. The individual user data (dk1and dk2) may be decoded (e.g., decoded separately, based on the respective orthogonal codes of the individual user data.