Method and system of cyclic prefix overhead reduction for enabling cancellation of inter-symbol and inter-carrier interferences in OFDM wireless communication networks

Abstract

A system and method are provided for reducing the overhead caused by the presence of the cyclic prefix while enabling inter-carrier interference (ICI) and inter-symbol interference (ISI) cancellation in an Orthogonal Frequency Division Multiplexing (OFDM) network that includes an OFDM transmitter and an OFDM receiver.

Claims

1. A method for reducing cyclic prefix overhead while enabling cancellation of inter-symbol and inter-carrier interferences in Orthogonal Frequency-Division Multiplexing (OFDM) wireless networks, wherein information in a set of time-domain complex samples is to be sent by an OFDM transmitter to an OFDM receiver through a wireless channel within a Time Transmission Interval (TTI) the TTI comprising a number N.sub.sym of OFDM symbols, the method comprising: generating, by the OFDM transmitter, a time-domain signal which comprises: a first part of the TTI for carrying time-domain complex samples (r.sub.a.sup.TTI[n]) of a first OFDM symbol (S.sub.0), where r and a are reference letters that denote part of the received signal TTI (r.sup.TTI[n]), n is a time index taking values n=0, . . . , (N.sub.sym−1).Math.N.sub.OFDM−1, and N.sub.OFDM is a natural number equal to the number of subcarriers, the first OFDM symbol (S.sub.0) being obtained in the time domain through an inverse Fourier Transform of the complex samples to be carried by a number of subcarriers in the frequency domain, the inverse Fourier Transform having a length N.sub.OFDM, the subcarriers comprising both data subcarriers and pilot subcarriers for channel estimation; a cyclic prefix (CP) carried in the first part of the TTI and appended to the beginning of the first OFDM symbol (S.sub.0), containing a replica of the last samples of the first OFDM symbol (S.sub.0), which has a length given by the largest expected delay spread of the wireless channel; Forward Error Correction encoded information bits to be transmitted, which are pseudo-random interleaved prior to a mapping of the encoded information bits to time-frequency resources; a second part of the TTI for carrying (N.sub.sym−1).Math.N.sub.OFDM time-domain complex samples (r.sub.b.sup.TTI[n]), where b is a reference letter that denote another part of the received signal TTI (r.sup.TTI[n]), which samples result from the mapping to time-frequency resources of a set of (N.sub.sym−1).Math.N.sub.OFDM complex subcarriers in the frequency domain corresponding to the information to be sent in the second part of the TTI, without appending any cyclic prefix, the set of (N.sub.sym−1).Math.N.sub.OFDM complex subcarriers comprising a concatenation of subcarriers corresponding to the remaining OFDM symbols (S.sub.1, . . . S.sub.Nsym−1) of the TTI, the concatenated subcarriers comprising both data subcarriers and pilot subcarriers for channel estimation, and the remaining OFDM symbols (S.sub.1, . . . S.sub.Nsym−1) being obtained in the time domain by means of an enlarged inverse Fourier Transform with length equal to the number (N.sub.sym−1).Math.N.sub.OFDM of the concatenated subcarriers; and concatenating the time-domain complex samples (r.sub.a.sup.TTI[n]) of the first part of the TTI and the time-domain complex samples (r.sub.b.sup.TTI[n]) of the second part of the TTI, to be sent by the OFDM transmitter to the OFDM receiver which performs cancellation of inter-symbol and inter-carrier interferences using the time-domain complex samples (r.sub.a.sup.TTI[n], r.sub.b.sup.TTI[n]) of the first part and the second part of the TTI.

2. The method according to claim 1, wherein pilot subcarriers in the second part of the TTI are inserted and arranged in the frequency domain to cover a whole bandwidth (BW) with a frequency separation between pilot subcarriers equal to (N.sub.sym−1) times the frequency separation between pilot subcarriers of the subcarriers in the first part of the TTI.

3. The method according to claim 2, wherein the encoded information bits to be transmitted are pseudo-random interleaved by writing input information bits to elements of a rectangular matrix by rows, and reading the output information bits by columns after reordering the columns of the matrix according to a pseudo-random pattern.

4. The method according to claim 3, further comprising reservation of guard bands in the frequency domain before and after the subcarriers corresponding to the information to be transmitted in the second part of the TTI, the guard bands comprising a number of null subcarriers equal to (N.sub.sym−1) times a number of guard subcarriers reserved in the first part of the TTI.

5. The method according to claim 4, further comprising recovering information contained in a current TTI by the OFDM receiver performing the following steps: detecting the first OFDM symbol (S.sub.0) by separating time-domain complex samples (r.sub.a.sup.TTI[n]) of the first part of the current TTI, from the appended cyclic prefix (CP) and from the remaining time-domain complex samples (r.sub.b.sup.TTI[n]) of the second part of the current TTI; estimating a carrier frequency offset, CFO, using the detected first OFDM symbol (S.sub.0) and the appended cyclic prefix (CP); compensating the CFO in both the first part and the second part of the TTI; estimating the channel frequency response (H[f]) in the detected first OFDM symbol (S.sub.0); recovering information in the detected first OFDM symbol (S.sub.0) through channel equalization and symbol decoding; reconstructing a time-domain transmitted signal s.sub.a.sup.TTI[n] corresponding to the detected first OFDM symbol (S.sub.0) by applying an inverse Fourier transform of the recovered information with length N.sub.OFDM; obtaining the channel impulse response, CIR, by performing an inverse Fourier Transform of the channel frequency response (H[f]) estimated in the first OFDM symbol, and identifying a number of CIR taps, the CIR being written as: $h\left[n\right]=\underset{j=0}{\overset{N_{\mathrm{taps}}-1}{.Math.}}{a}_j\delta \left[n-{\tau }_j\right]$ where h[n] denotes the CIR in time-domain, N.sub.taps denotes the number of identified CIR taps, a.sub.j is a complex amplitude of the j-th tap, τ.sub.j is a discrete delay associated to the j-th tap, and δ(•) represents the discrete delta function; removing inter-symbol interference, ISI, from the remaining time-domain complex samples (r.sub.b.sup.TTI[n]) of the second part of the current TTI by means of the following equation: $r_{b,\mathrm{ISI}}^{\mathrm{TTI}}\left[n\right]=r_b^{\mathrm{TTI}}\left[n\right]-\underset{j=0}{\overset{N_{\mathrm{taps}}-1}{.Math.}}{a}_j{\tilde{s}}_a^{\mathrm{TTI}}\left[n-{\tau }_j+N_{\mathrm{OFDM}}\right],$ where r.sub.b,ISI.sup.TTI[n] denotes a signal of the second part of the current TTI after ISI removal, n is a time index taking values n=0, . . . , (N.sub.sym−1).Math.N.sub.OFDM−1, and {tilde over (s)}.sub.a[n] is equal to s.sub.a[n] for 0≦n<N.sub.OFDM and zero outside: ${\tilde{s}}_a^{\mathrm{TTI}}\left[n\right]\equiv \{\begin{array}{cc}{s}_a^{\mathrm{TTI}}\left[n\right],& 0\le n\le N_{\mathrm{OFDM}}\\ 0,& \mathrm{outside}\end{array};$ removing inter-carrier interference from the remaining time-domain complex samples (r.sub.a.sup.TTI[n]) of the second part of the current TTI by performing the following steps: performing a Fourier transform of a received signal r.sub.a.sup.TTI+1[n] in a first part of a subsequent TTI, which is next to the current TTI and comprises a first OFDM symbol corresponding to the received signal r.sub.a.sup.TTI+1[n], and estimating a channel frequency response (H[f]) after discarding a CP appended in the first part of the subsequent TTI; recovering transmitted information from the first OFDM symbol of the received signal r.sub.a.sup.TTI+1[n] in the subsequent TTI by channel equalization and symbol decoding; reconstructing a subsequent time-domain transmitted signal s.sub.a.sup.TTI+1[n] corresponding to the first OFDM symbol of the received signal r.sub.a.sup.TTI+1[n] in the subsequent TTI by applying an inverse Fourier Transform of the recovered information with length N.sub.OFDM; removing inter-carrier interference, ICI from the second part of the current TTI by means of the following equation: $r_{b,\mathrm{ISI},\mathrm{ICI}}^{\mathrm{TTI}}\left[n\right]=r_{b,\mathrm{ISI}}^{\mathrm{TTI}}\left[n\right]+r_a^{\mathrm{TTI}+1}\left[n\right]-\underset{j=0}{\overset{N_{\mathrm{taps}}^{\prime }-1}{.Math.}}{a}_j^{\prime }{\tilde{s}}_a^{\mathrm{TTI}+1}\left[N_{\mathrm{OFDM}}-N_{\mathrm{CP}}+n-{\tau }_j^{\prime }\right],$ where r.sub.b,ISI,ICI.sup.TTI[n] denotes a received signal in the second part of the current TTI after removing the inter-carrier and inter-symbol interferences, n=0, . . . , (N.sub.sym−1).Math.N.sub.OFDM−1, N′.sub.taps denotes the number of CIR taps in the first OFDM symbol of the subsequent TTI, a′.sub.j is a complex amplitude of the j-th tap, τ′.sub.j is a discrete delay associated to the j-th tap, and ${\tilde{s}}_a^{\mathrm{TTI}+1}\left[n\right]\equiv \{\begin{array}{cc}{s}_a^{\mathrm{TTI}+1}\left[n\right],& N_{\mathrm{OFDM}}-N_{\mathrm{CP}}\le n<N_{\mathrm{OFDM}}\\ 0,& \mathrm{outside}\end{array};$ performing channel estimation, channel equalization and symbol decoding in the second part of the current TTI after removing the inter-carrier and inter-symbol interferences; and FEC decoding and de-interleaving of the complex symbols to obtain the information contained in the current TTI.

6. The method according to claim 5, wherein identifying the number of CIR taps N.sub.taps in the current TTI and the number of CIR taps N′.sub.taps of the subsequent TTI by the OFDM receiver comprises a comparison of the absolute CIR amplitude with a threshold to discard radio channel components being below the threshold.

7. The method according to claim 6, wherein the OFDM wireless network is a Long Term Evolution wireless network.

8. The method according to claim 2, further comprising reservation of guard bands in the frequency domain before and after the subcarriers corresponding to the information to be transmitted in the second part of the TTI, the guard bands comprising a number of null subcarriers equal to (N.sub.sym−1) times a number of guard subcarriers reserved in the first part of the TTI.

9. The method according to claim 1, wherein the encoded information bits to be transmitted are pseudo-random interleaved by writing input information bits to elements of a rectangular matrix by rows, and reading the output information bits by columns after reordering the columns of the matrix according to a pseudo-random pattern.

10. The method according to claim 1, further comprising reservation of guard bands in the frequency domain before and after the subcarriers corresponding to the information to be transmitted in the second part of the TTI, the guard bands comprising a number of null subcarriers equal to (N.sub.sym−1) times a number of guard subcarriers reserved in the first part of the TTI.

11. The method according to claim 1, further comprising recovering information contained in a current TTI by the OFDM receiver performing the following steps: detecting the first OFDM symbol (S.sub.0) by separating time-domain complex samples (r.sub.a.sup.TTI[n]) of the first part of the current TTI, from the appended cyclic prefix (CP) and from the remaining time-domain complex samples (r.sub.b.sup.TTI[n]) of the second part of the current TTI; estimating a carrier frequency offset, CFO, using the detected first OFDM symbol (S.sub.0) and the appended cyclic prefix (CP); compensating the CFO in both the first part and the second part of the TTI; estimating the channel frequency response (H[f]) in the detected first OFDM symbol (S.sub.0); recovering information in the detected first OFDM symbol (S.sub.0) through channel equalization and symbol decoding; reconstructing a time-domain transmitted signal s.sub.a.sup.TTI[n] corresponding to the detected first OFDM symbol (S.sub.0) by applying an inverse Fourier transform of the recovered information with length N.sub.OFDM; obtaining the channel impulse response, CIR, by performing an inverse Fourier Transform of the channel frequency response (H[f]) estimated in the first OFDM symbol, and identifying a number of CIR taps, the CIR being written as: $h\left[n\right]=\underset{j=0}{\overset{N_{\mathrm{taps}}-1}{.Math.}}{a}_j\delta \left[n-{\tau }_j\right]$ where h[n] denotes the CIR in time-domain, N.sub.taps denotes the number of identified CIR taps, a.sub.j is a complex amplitude of the j-th tap, τ.sub.j is a discrete delay associated to the j-th tap, and δ(•) represents the discrete delta function; removing inter-symbol interference, ISI, from the remaining time-domain complex samples (r.sub.b.sup.TTI[n]) of the second part of the current TTI by means of the following equation: $r_{b,\mathrm{ISI}}^{\mathrm{TTI}}\left[n\right]=r_b^{\mathrm{TTI}}\left[n\right]-\underset{j=0}{\overset{N_{\mathrm{taps}}-1}{.Math.}}{a}_j{\tilde{s}}_a^{\mathrm{TTI}}\left[n-{\tau }_j+N_{\mathrm{OFDM}}\right],$ where r.sub.b,ISI.sup.TTI[n] denotes a signal of the second part of the current TTI after ISI removal, n is the time index taking values n=(N.sub.sym−1).Math.N.sub.OFDM−1, and {tilde over (s)}.sub.a[n] is equal to s.sub.a[n] for 0≦n<N.sub.OFDM and zero outside: ${\tilde{s}}_a^{\mathrm{TTI}}\left[n\right]\equiv \{\begin{array}{cc}{s}_a^{\mathrm{TTI}}\left[n\right],& 0\le n\le N_{\mathrm{OFDM}}\\ 0,& \mathrm{outside}\end{array};$ removing inter-carrier interference from the remaining time-domain complex samples (r.sub.b.sup.TTI[n]) of the second part of the current TTI by performing the following steps: performing a Fourier transform of a received signal r.sub.a.sup.TTI+1[n] in a first part of a subsequent TTI, which is next to the current TTI and comprises a first OFDM symbol corresponding to the received signal r.sub.a.sup.TTI+1[n], and estimating a channel frequency response (H[f]) after discarding a CP appended in the first part of the subsequent TTI; recovering transmitted information from the first OFDM symbol of the received signal r.sub.a.sup.TTI+1[n] in the subsequent TTI by channel equalization and symbol decoding; reconstructing a subsequent time-domain transmitted signal s.sub.a.sup.TTI+1[n] corresponding to the first OFDM symbol of the received signal r.sub.a.sup.TTI+1[n] in the subsequent TTI by applying an inverse Fourier Transform of the recovered information with length N.sub.OFDM; removing inter-carrier interference, ICI from the second part of the current TTI by means of the following equation: $r_{b,\mathrm{ISI},\mathrm{ICI}}^{\mathrm{TTI}}\left[n\right]=r_{b,\mathrm{ISI}}^{\mathrm{TTI}}\left[n\right]+r_a^{\mathrm{TTI}+1}\left[n\right]-\underset{j=0}{\overset{N_{\mathrm{taps}}^{\prime }-1}{.Math.}}{a}_j^{\prime }{\tilde{s}}_a^{\mathrm{TTI}+1}\left[N_{\mathrm{OFDM}}-N_{\mathrm{CP}}+n-{\tau }_j^{\prime }\right],$ where r.sub.b,ISI,ICI.sup.TTI[n] denotes a received signal in the second part of the current TTI after removing the inter-carrier and inter-symbol interferences, n=0, . . . , (N.sub.sym−1).Math.N.sub.OFDM−1, N′.sub.taps denotes the number of CIR taps in the first OFDM symbol of the subsequent TTI, a′.sub.j is a complex amplitude of the j-th tap, N.sub.CP denotes the length of the cyclic prefix, τ′.sub.j is a discrete delay associated to the j-th tap, and ${\tilde{s}}_a^{\mathrm{TTI}+1}\left[n\right]\equiv \{\begin{array}{cc}{s}_a^{\mathrm{TTI}+1}\left[n\right],& N_{\mathrm{OFDM}}-N_{\mathrm{CP}}\le n<N_{\mathrm{OFDM}}\\ 0,& \mathrm{outside}\end{array};$ performing channel estimation, channel equalization and symbol decoding in the second part of the current TTI after removing the inter-carrier and inter-symbol interferences; and FEC decoding and de-interleaving of the complex symbols to obtain the information contained in the current TTI.

12. The method according to claim 11, wherein identifying the number of CIR taps N.sub.taps in the current TTI and the number of CIR taps N′.sub.taps of the subsequent TTI by the OFDM receiver comprises a comparison of the absolute CIR amplitude with a threshold to discard radio channel components being below the threshold.

13. The method according to claim 1, wherein the OFDM wireless network is a Long Term Evolution wireless network.

14. An Orthogonal Frequency-Division Multiplexing (OFDM) transmitter for reducing cyclic prefix overhead while enabling inter-symbol and inter-carrier interferences cancellation at an OFDM receiver to which the OFDM transmitter sends information within a Time Transmission Interval (TTI) in a set of time-domain complex samples through a wireless channel of an OFDM wireless network, the TTI comprising a number N.sub.sym of OFDM symbols, the OFDM transmitter comprising: a generator of a time-domain signal for generating the time-domain signal comprising a first part of the TTI for carrying time-domain complex samples (r.sub.a.sup.TTI[n]) of a first OFDM symbol (S.sub.0) and a second part of the TTI for carrying (N.sub.sym−1).Math.N.sub.OFDM time-domain complex samples (r.sub.b.sup.TTI[n]), where r, a and b are reference letters that denote parts of the received signal TTI (r.sup.TTI [n]), n is a time index taking values n=0, . . . , (N.sub.sym−1).Math.N.sub.OFDM−1, and N.sub.OFDM is a natural number equal to the number of subcarriers; an inverse Fourier Transform performer applied to the complex samples to be carried by a number of subcarriers in the frequency domain for obtaining the first OFDM symbol (S.sub.0) in the time domain, the inverse Fourier Transform having a length N.sub.OFDM, the subcarriers comprising both data subcarriers and pilot subcarriers for channel estimation; a cyclic prefix (CP) adder for appending the cyclic prefix (CP) with a length given by the largest expected delay spread of the wireless channel to the beginning of the first OFDM symbol (S.sub.0), to be carried in the first part of the TTI containing a replica of the last samples of said first OFDM symbol (S.sub.0); a Forward Error Correction encoder for encoding information bits to be transmitted; a pseudo-random interleaver for interleaving the encoded information prior to a mapping of the encoded information bits to time-frequency resources; means for mapping the second part of the TTI to time-frequency resources to obtain a set of (N.sub.sym−1).Math.N.sub.OFDM complex subcarriers in the frequency domain, without appending any cyclic prefix, the set of (N.sub.sym−1).Math.N.sub.OFDM complex subcarriers comprising a concatenation of subcarriers corresponding to the remaining OFDM symbols (S.sub.1, . . . S.sub.Nsym−1) of the TTI, the concatenated subcarriers comprising both data subcarriers and pilot subcarriers for channel estimation; an enlarged inverse Fourier Transform with length equal to the number (N.sub.sym−1).Math.N.sub.OFDM of the concatenated subcarriers for obtaining the remaining OFDM symbols (S.sub.1, . . . S.sub.Nsym−1) in the time domain; and a concatenator for concatenating the time-domain complex samples (r.sub.a.sup.TTI[n]) of the first part of the TTI and the time-domain complex samples (r.sub.b.sup.TTI[n]) of the second part of the TTI, to be sent by the OFDM transmitter to the OFDM receiver which performs cancellation of inter-symbol and inter-carrier interferences using said time-domain complex samples (r.sub.a.sup.TTI[n], r.sub.b.sup.TTI[n]).

15. The OFDM transmitter according to claim 14, further comprising a pilot subcarriers insertion means for inserting pilot subcarriers in the second part of the TTI which are arranged in the frequency domain to cover a whole bandwidth (BW) with a frequency separation between pilot subcarriers equal to (N.sub.sym−1) times the frequency separation between pilot subcarriers of the subcarriers in the first part of the TTI.

16. An Orthogonal Frequency-Division Multiplexing (OFDM) receiver for cancelling inter-symbol and inter-carrier interferences in OFDM networks, which receives time-domain complex samples (r.sub.a.sup.TTI[n]) in a first part of a current Time Transmission Interval (TTI) and remaining time-domain complex samples (r.sub.b.sup.TTI[n]), where r, a and b are reference letters that denote parts of a received signal TTI (r.sup.TTI[n]), n is a time index taking values n=0, . . . , (N.sub.sym−1).Math.N.sub.OFDM−1, and N.sub.OFDM is a natural number equal to the number of subcarriers, the remaining time-domain complex samples (r.sub.b.sup.TTI[n]) in a second part of the current TTI, the OFDM receiver comprising: a symbol detector for detecting a first OFDM symbol (S.sub.0) in the first part of the current TTI by separating time-domain complex samples (r.sub.a.sup.TTI[n]) of the first part of the current TTI, from a cyclic prefix (CP) appended to the beginning of the time-domain complex samples (r.sub.a.sup.TTI[n]) and from the remaining time-domain complex samples (r.sub.b.sup.TTI[n]) of the second part of the current TTI; a carrier frequency offset estimator for estimating the carrier frequency offset, CFO, by using the detected first OFDM symbol (S.sub.0) and the appended cyclic prefix (CP); a carrier frequency offset corrector for compensating the CFO in both the first part and the second part of the TTI; a channel frequency response estimator for estimating the channel frequency response (H[f]) in the detected first OFDM symbol (S.sub.0); a channel equalizer and symbol decoder for recovering information in the detected first OFDM symbol (S.sub.0); an inverse Fourier transform with length N.sub.OFDM for reconstructing a time-domain transmitted signal s.sub.a.sup.TTI[n] corresponding to the detected first OFDM symbol (S.sub.0); a channel impulse response estimator for estimating the channel impulse response, CIR, by performing an inverse Fourier Transform of the channel frequency response (H[f]) estimated in the first OFDM symbol, and identifying a number of CIR taps, the CIR being written as $h\left[n\right]=\underset{j=0}{\overset{N_{\mathrm{taps}}-1}{.Math.}}{a}_j\delta \left[n-{\tau }_j\right]$ where h[n] denotes the CIR in time-domain, N.sub.taps denotes the number of identified CIR taps, a.sub.j is a complex amplitude of the j-th tap, τ.sub.j is a discrete delay associated to the j-th tap, and δ(•) represents the discrete delta function; inter-symbol interference cancellation means for removing inter-symbol interference, ISI from the remaining time-domain complex samples (r.sub.b.sup.TTI[n]) of the second part of the current TTI by means of the following equation: $r_{b,\mathrm{ISI}}^{\mathrm{TTI}}\left[n\right]=r_b^{\mathrm{TTI}}\left[n\right]-\underset{j=0}{\overset{N_{\mathrm{taps}}-1}{.Math.}}{a}_j{\tilde{s}}_a^{\mathrm{TTI}}\left[n-{\tau }_j+N_{\mathrm{OFDM}}\right],$ where r.sub.b,ISI.sup.TTI[n] denotes a signal of the second part of the current TTI after ISI removal, n is a time index taking values n=0, . . . , (N.sub.sym−1).Math.N.sub.OFDM−1, and {tilde over (s)}.sub.a[n] is equal to s.sub.a[n] for 0≦n<N.sub.OFDM and zero outside: ${\tilde{s}}_a^{\mathrm{TTI}}\left[n\right]\equiv \{\begin{array}{cc}{s}_a^{\mathrm{TTI}}\left[n\right],& 0\le n\le N_{\mathrm{OFDM}}\\ 0,& \mathrm{outside}\end{array};$ inter-carrier interference cancellation means for removing inter-carrier interference from the remaining time-domain complex samples (r.sub.b.sup.TTI[n]) of the second part of the current TTI by performing the following steps: performing a Fourier transform of a received signal r.sub.a.sup.TTI+1[n] in a first part of a subsequent TTI, which is next to the current TTI and comprises a first OFDM symbol corresponding to the received signal r.sub.a.sup.TTI+1[n], and estimating a channel frequency response (H[f]) after discarding a CP appended in the first part of the subsequent TTI; recovering transmitted information from the first OFDM symbol of the received signal r.sub.a.sup.TTI+1[n] in the subsequent TTI by channel equalization and symbol decoding; reconstructing a subsequent time-domain transmitted signal s.sub.a.sup.TTI+1[n] corresponding to the first OFDM symbol of the received signal r.sub.a.sup.TTI+1[n] in the subsequent TTI by applying an inverse Fourier Transform of the recovered information with length N.sub.OFDM; removing inter-carrier interference, ICI from the second part of the current TTI by means of the following equation: $r_{b,\mathrm{ISI},\mathrm{ICI}}^{\mathrm{TTI}}\left[n\right]=r_{b,\mathrm{ISI}}^{\mathrm{TTI}}\left[n\right]+r_a^{\mathrm{TTI}+1}\left[n\right]-\underset{j=0}{\overset{N_{\mathrm{taps}}^{\prime }-1}{.Math.}}{a}_j^{\prime }{\tilde{s}}_a^{\mathrm{TTI}+1}\left[N_{\mathrm{OFDM}}-N_{\mathrm{CP}}+n-{\tau }_j^{\prime }\right],$ where r.sub.b,ISI,ICI.sup.TTI[n] denotes a received signal in the second part of the current TTI after removing the inter-carrier and inter-symbol interferences, n=0, . . . , (N.sub.sym−1).Math.N.sub.OFDM−1, N′.sub.taps denotes the number of CIR taps in the first OFDM symbol of the subsequent TTI, a′.sub.j is a complex amplitude of the j-th tap, τ′.sub.j is a discrete delay associated to the j-th tap, and ${\tilde{s}}_a^{\mathrm{TTI}+1}\left[n\right]\equiv \{\begin{array}{cc}{s}_a^{\mathrm{TTI}+1}\left[n\right],& N_{\mathrm{OFDM}}-N_{\mathrm{CP}}\le n<N_{\mathrm{OFDM}}\\ 0,& \mathrm{outside}\end{array};$ a channel estimator, channel equalizer and symbol decoder for estimating and equalizing a radio channel and decoding the symbols in the second part of the current TTI after removing the inter-carrier and inter-symbol interferences; and a FEC decoder and de-interleaver of the complex symbols to obtain an information block contained in the current TTI.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For the purpose of aiding the understanding of the characteristics of the invention, according to a preferred practical embodiment thereof and in order to complement this description, the following figures are attached as an integral part thereof, having an illustrative and non-limiting character:

(2) FIG. 1 shows an OFDM frame structure in a time transmission interval, as known in prior art;

(3) FIG. 2 shows an OFDM frame structure in a time transmission interval, according to a preferred embodiment of the invention;

(4) FIG. 3 shows a process for generating the two parts of a time transmission interval, in accordance with a possible embodiment of the invention;

(5) FIG. 4 shows a process for mapping the complex symbols to time-frequency resources for the two parts of the time transmission interval, in accordance with a possible embodiment of the invention;

(6) FIG. 5 shows a process for allocating OFDM pilot subcarriers into the frame structure, as known in prior art;

(7) FIG. 6 shows a process for allocating OFDM pilot subcarriers into the frame structure, according to a possible embodiment of the invention;

(8) FIG. 7 shows a schematic diagram of a network scenario for a possible application case of the invention in performing cancellation of inter-symbol and inter-carrier interference;

(9) FIG. 8 shows a diagram of the channel impulse response and its associated channel frequency response;

(10) FIG. 9 shows a block diagram of the architecture of an OFDM transmitter, according to a possible embodiment of the invention;

(11) FIG. 10 shows a block diagram of the architecture of an OFDM receiver, according to a possible embodiment of the invention; and

(12) FIG. 11 shows a block diagram of the architecture of a system comprising the OFDM transmitter and the OFDM receiver described in FIGS. 9-10, according to a preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(13) The matters defined in this detailed description are provided to assist in a comprehensive understanding of the invention. Accordingly, those of ordinary skill in the art will recognize that variation changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, description of well-known functions and elements are omitted for clarity and conciseness.

(14) Of course, the embodiments of the invention can be implemented in a variety of architectural platforms, operating and server systems, devices, systems, or applications. Any particular architectural layout or implementation presented herein is provided for purposes of illustration and comprehension only and is not intended to limit aspects of the invention.

(15) It is within this context, that various embodiments of the invention are now presented with reference to the FIGS. 1-11.

(16) FIG. 1 shows the frame structure 100 of a sequence of standard OFDM symbols, SS.sub.1, SS.sub.2, . . . SS.sub.N, in a time transmission interval, TTI. Every OFDM symbol, SS.sub.1, SS.sub.2, . . . SS.sub.N, comprises a cyclic prefix, CP, at the beginning of each symbol in the TTI, which is followed by a useful part, UP.sub.1, UP.sub.2, . . . UP.sub.N, containing control data or user data.

(17) FIG. 2 presents the frame structure 200 of a sequence of OFDM symbols, S.sub.0, S.sub.1, . . . S.sub.Nsym−1, according to a preferred embodiment of the invention, in a time transmission interval, TTI. In order to reduce the overhead caused by the cyclic prefix, CP, the proposed frame structure 200 is built according to the following features: Leave only the first CP at the beginning of the first symbol S.sub.0 in every TTI. Thus, the frame structure 200 carries overhead associated with only a single one CP located at the beginning of the first part 201 of the TTI. The cyclic prefix CP contains a replica of the last samples of the first OFDM symbol S.sub.0. Introduce an additional interleaver operation after Forward Error Correction (FEC) encoding and prior to mapping to time-frequency resources for the remaining part 202 of the TTI, in order to increase the frequency diversity. Change the distribution of pilot subcarriers along the frequency domain of the remaining part 202 of the TTI, in order to enable channel estimation. Change the way in which the time-domain samples for the remaining part 202 of the TTI are generated from the corresponding subcarriers in the frequency domain, so that ISI and ICI cancellation can be performed at the receiver side for the whole remaining part 202 of the TTI (and not on a per-symbol basis) without compromising the maximum supported carrier frequency offset (CFO).

(18) The cyclic prefix CP introduces a loss in efficiency that can be quantified by the expression:

(19) $\eta =\frac{N_{\mathrm{CP}}}{N_{\mathrm{CP}}+N_{\mathrm{OFDM}}},$
where η is the loss in efficiency, N.sub.CP is the length of the cyclic prefix CP and N.sub.OFDM is the length of the useful part UP.sub.1 of the first symbol S.sub.0, i.e., the OFDM symbol length excluding the CP length, (both N.sub.CP and N.sub.OFDM given as a number of samples). This loss in efficiency directly translates into a loss in throughput compared to the Shannon bound.

(20) The second and subsequent symbols in the remaining part 202 of the TTI have no appended CP, therefore reducing the loss in efficiency to:

(21) ${\eta }^{\prime }=\frac{N_{\mathrm{CP}}}{N_{\mathrm{CP}}+N_{\mathrm{sym}}.Math.N_{\mathrm{OFDM}}}<\eta ,$
where η′ is the efficiency loss after application of the proposed invention and N.sub.sym is the number of OFDM symbols in the TTI.

(22) The way for transforming the OFDM subcarriers 30, 30′ in frequency-domain (f) into time-domain (t) samples, in order to obtain the proposed frame structure 300 is depicted in FIG. 3. Two parts of a TTI can be identified in FIG. 3: the first part 301 which comprises the first OFDM symbol including the CP and the second (remaining) part 302 of the TTI with no CP. The sets of time-domain samples of the first part 301 and the remaining part 302 of the TTI are denoted respectively as r.sub.a.sup.TTI[n] and r.sub.b.sup.TTI[n], wherein r.sub.a.sup.TTI[n] comprises the useful part of the first OFDM symbol S.sub.0 after the CP and r.sub.b.sup.TTI[n] comprises the remaining samples of the TTI (beginning with index zero).

(23) FIG. 3 illustrates the Inverse FFT operations, IFFT, required to generate 310 the sets of time-domain samples r.sub.a.sup.TTI[n] and r.sub.b.sup.TTI[n] of the proposed first part 301 and second part 302 of the TTI in the time domain, and the concatenation 320 of the resulting samples to obtain the received signal r[n] in the given TTI.

(24) The first symbol S.sub.0 contains a regular CP with a length N.sub.CP given by the maximum delay spread supported by the OFDM system, thus avoiding ISI and ICI. The remaining part 302 of the TTI however lacks from any CP in order to increase efficiency, and transmission and reception procedures must be changed for this remaining part 302 in order to overcome the resulting ISI and ICI.

(25) FIG. 4 illustrates the mapping process of complex OFDM symbols to time-frequency resources. The time-domain samples from each symbol are obtained in a different way for both parts of the TTI. For the first OFDM symbol, samples are obtained 400 using standard techniques through application of an inverse FFT of the complex subcarriers that carry the information as well as the pilot symbols for channel estimation. However, the remaining part 402 of the TTI is obtained from a longer inverse FFT of the concatenated set of subcarriers corresponding to the (N.sub.sym−1) remaining original OFDM symbols. As the system bandwidth BW is unchanged, the subcarriers are more densely “packed” in the second part of the TTI by a factor (N.sub.sym−1), as shown in FIG. 4. Thus, all samples are mapped 410 to one enlarged OFDM symbol. The length of the second part 402 in the time domain also grows by the same factor hence keeping the sampling frequency unchanged, and the same amount of information per TTI is thus included. Appropriate guard bands must also be observed by leaving a number of unmodulated subcarriers before and after the set of non-null subcarriers prior to the IFFT operation. The length of the guard band in the frequency domain is increased by a factor (N.sub.sym−1) compared to prior art techniques; however the frequency resolution is also increased by the same factor, therefore yielding the same guard bands overhead as in prior art techniques.

(26) It is to note that this way of translating information from the frequency domain f to the time domain t is completely different than in prior art techniques. Concatenation of the subcarrier amplitudes in the second part 402 of the TTI leads to a time-domain signal which is completely different than the one obtained using standard techniques, where a cascade of inverse FFTs provides the symbols in a serial way (after inclusion of the CP). In fact, the original number of subcarriers per each OFDM symbol in prior art techniques is now replaced by a proportionally higher number of subcarriers for the remaining part of the TTI. The occupied bandwidth BW is not changed, as it comprises the same information over the same time interval, but the samples are obtained from a single IFFT of the concatenation of all the subcarriers after the first OFDM symbol, thus increasing the frequency resolution by a factor (N.sub.sym−1) compared to prior art. The main advantage of this arrangement is a more efficient way to deal with ISI and ICI, while not impacting the maximum supported CFO that is determined by the first OFDM symbol. In addition, the ISI and ICI cancellation algorithm only depends on proper channel estimation in the first OFDM symbol, thus improving reliability thanks to the preserved CP.

(27) In order to perform channel estimation over the second part of the TTI for successful decoding, pilot subcarriers must be arranged so as to cover the system bandwidth with a frequency separation given by the minimum channel coherence bandwidth to be supported (see FIG. 5). Although the figure does not accurately reflect the increased frequency resolution in the second part of the TTI, the separation between pilot subcarriers in the frequency domain should be increased by a factor (N.sub.sym−1) compared to that in the first symbol, there being a larger number of data subcarriers between each pair of pilot subcarriers. Channel estimation accuracy is however not impacted as the channel coherence bandwidth also extends over (N.sub.sym−1) times the subcarriers involved in the first OFDM symbol, therefore leading to the same channel estimation capabilities.

(28) FIG. 5 shows the allocation of pilot subcarriers PS for channel estimation in accordance with the prior art solutions. FIG. 6 shows the allocation of pilot subcarriers PS for channel estimation according to a proposed embodiment of the invention.

(29) A fundamental difference of the proposed frame structure with respect to prior art lies in the CFO compensation capabilities. It could be argued that the proposed structure is essentially similar to having a single, longer OFDM symbol with the duration of a TTI and a cyclic prefix appended to it, thereby allowing detection of the samples with a single FFT/IFFT. However, with only one OFDM symbol in the TTI, the maximum allowed CFO would be decreased in the same way as the width of the subcarriers would do because of the longer OFDM symbol. In the proposed frame structure, the maximum supported CFO is however unchanged, as determined by the first OFDM symbol, while the second part of the TTI can effectively collect the samples from the remaining original OFDM symbols for easier ISI/ICI removal without impacting the maximum CFO. The proposed structure also has advantages in terms of an easier decoding process of the control signals in the first OFDM symbol, which is a key aspect in order to reduce battery consumption at the user device, provide robust control signaling and allow discontinuous reception (DRX). The proposed arrangement of subcarriers in the frequency domain of the second part of the TTI, as shown in FIG. 6, also has implications in the way users are scheduled by a base station. Prior art OFDM systems (such as e.g. LTE) allocate portions of the spectrum to the users over a number of OFDM symbols, whereas in this invention the users are capable of comprising a higher number of subcarriers over a single “enlarged” symbol which includes the remaining part of the TTI. In practice, the scheduler of the base station have to deal with a number of subcarriers that could generally experience different frequency responses, while in prior art techniques the subcarriers corresponding to different OFDM symbols undergo approximately similar channel conditions (at least for moderate UE speeds). The consequence of this feature is a higher granularity in the frequency domain, thus allowing for more precise allocation of resources. However, the higher frequency resolution results in lower frequency diversity for the complex samples, because adjacent subcarriers could experience lower channel variations in the frequency domain than in prior art, thus impairing the FEC decoding process. To compensate this undesired effect, an additional interleaver prior to subcarrier mapping can effectively increase frequency diversity and thus enhance the FEC decoding process.

(30) FIG. 7 shows a very simplified scenario for application of the proposed frame structure, in a possible embodiment of the invention, providing a system for reduction of the overhead caused by the presence of the cyclic prefix, while enabling inter-symbol interference (ISI) and inter-carrier interference (ICI) cancellation. The system comprises an OFDM transmitter 701 and at least one OFDM receiver 702, the OFDM transmitter 701 and each OFDM receiver 702 being connected through a wireless radio channel 700 which introduces a number of impairments 710, mainly noise (in the form of Gaussian noise and other types of interference) and multipath. The latter introduces significant inter-symbol interference (ISI) and inter-carrier interference (ICI) that must be cancelled at the receiver side prior to successful decoding of the symbols, containing information 740 suffering from ISI and ICI, received by the OFDM receiver 702. The OFDM transmitter 701 sends the information blocks 730 to be transmitted in the TTI according to the proposed frame structure.

(31) Let us denote with r.sup.TTI[n] the signal received in a given TTI received by the OFDM receiver 702, and the two parts of the received TTI are denoted as r.sub.a.sup.TTI[n] and r.sub.b.sup.TTI[n] respectively, where r.sub.a.sup.TTI[n] refers to the first OFDM symbol (excluding CP) and r.sub.b.sup.TTI[n] refers to the remaining original (N.sub.sym−1) OFDM symbols. For ease of notation, the time indices run from zero in both parts, thus:
r.sub.a.sup.TTI[n]=r.sup.TTI[n+N.sub.CP], n=0, . . . ,N.sub.OFDM−1,
r.sub.b.sup.TTI[n]=r.sup.TTI[n+N.sub.CP+N.sub.OFDM], n=0, . . . , (N.sub.sym−1).Math.N.sub.OFDM−1.
r.sub.a.sup.TTI[n] comprises the useful part of the first OFDM symbol after the CP, and
r.sub.b.sup.TTI[n] comprises the remaining samples of the TTI (beginning with index zero).

(32) From the first part r.sub.a.sup.TTI[n], it is possible to detect the transmitted information through standard OFDM detection techniques, including estimation of the channel frequency response H[f], shown in FIG. 8, with the aid of suitable pilot sub-carriers PS or training signals. The time-domain transmitted signal corresponding to the first OFDM symbol (excluding the cyclic prefix), denoted here by s.sub.a.sup.TTI[n], can be reconstructed through proper equalization, demodulation and application of inverse FFT. It is to note that equalization and detection are not limited to the subcarriers scheduled for the user, but performed over the whole system bandwidth including other users and control channels. This is required in order to be able to reconstruct the complete time-domain first OFDM symbol that produces inter-symbol interference towards the next symbols. The user could detect all the control information included in the first symbol (performing FEC decoding of all the control information), but this may involve additional signalling from the network in order to aid in the decoding process. Instead, the user can perform simpler hard-decision decoding of the complex modulated symbols, thus estimating the constellation symbols that are nearest to the equalized symbols for each of the subcarriers. This procedure is suitable for low-order modulations (like QPSK), as typically employed for the control information.

(33) The remaining part r.sub.b.sup.TTI[n] of samples in the TTI suffers from ISI and ICI due to the absence of cyclic prefix. At the OFDM receiver 702, an objective is to effectively remove ISI and ICI according to the procedure explained below.

(34) FIG. 8 illustrates an example channel impulse response (CIR) and its associated channel frequency response. The time-domain channel impulse response (CIR) at the first OFDM symbol can be obtained by performing an inverse FFT of the channel frequency response estimated at the first OFDM symbol, i.e., the CIR can be written by the expression h[n]=IFFT{H[f]}. The CIR in general comprises a number of delayed discrete delta functions representing the multipath components of the radio channel (commonly known as taps), each having different amplitudes, phases and associated delays, as shown in FIG. 8). The receiver 702 can then extract the most significant multipath components from the CIR. Identification of the peaks or taps of the CIR and their respective delays τ.sub.0, τ.sub.1, . . . , τ.sub.Ntaps-1 (N.sub.taps denotes the number of significant taps) can be based on a suitable threshold for the CIR amplitudes below which the influence of the tap can be considered negligible, but any other procedure is also valid for the purpose of the present invention.

(35) After identification of the most significant or strongest taps, the CIR in the first OFDM symbol can be written in the form:

(36) $h\left[n\right]=\underset{j=0}{\overset{N_{\mathrm{taps}}-1}{.Math.}}{a}_j\delta \left[n-{\tau }_j\right],$
where N.sub.taps is the number of significant taps, a.sub.j is the complex amplitude of the j-th tap, τ.sub.j is the delay associated to the j-th tap, and δ(•) represents the discrete delta function.

(37) The ISI component towards the second (remaining) part r.sub.b.sup.TTI[n] of the TTI can then be written in the form:

(38) $\mathrm{ISI}=\underset{j=0}{\overset{N_{\mathrm{taps}}-1}{.Math.}}{a}_j{\tilde{s}}_a^{\mathrm{TTI}}\left[n-{\tau }_j+N_{\mathrm{OFDM}}\right],0\le n\le {\tau }_{\max },$
where n is the time index, τ.sub.max is the maximum value of the channel delays, and {tilde over (s)}.sub.a.sup.TTI[n] is equal to s.sub.a.sup.TTI[n] for 0≦n<N.sub.OFDM and zero outside:

(39) 0${\tilde{s}}_a^{\mathrm{TTI}}\left[n\right]\equiv \{\begin{array}{cc}{s}_a^{\mathrm{TTI}}\left[n\right],& 0\le n<N_{\mathrm{OFDM}}\\ 0,& \mathrm{outside}\end{array}.$

(40) The ISI component can be subtracted from r.sub.b.sup.TTI[n], thus yielding the signal r.sub.b,ISI.sup.TTI[n] with ideally no ISI from the first OFDM symbol:

(41) $r_{b,\mathrm{ISI}}^{\mathrm{TTI}}\left[n\right]\equiv r_b^{\mathrm{TTI}}\left[n\right]-\mathrm{ISI}=r_b^{\mathrm{TTI}}\left[n\right]-\underset{j=0}{\overset{N_{\mathrm{taps}}-1}{.Math.}}{a}_j{\tilde{s}}_a^{\mathrm{TTI}}\left[n-{\tau }_j+N_{\mathrm{OFDM}}\right],$
where the index n in this equation can take the values n=0, . . . , (N.sub.sym−1).Math.N.sub.OFDM−1. r.sub.b,ISI.sup.TTI[n] will thus be a “cleaned” version of the second part r.sub.b.sup.TTI[n] of the TTI after ISI removal.

(42) Prior to performing frequency-domain equalization, it is also necessary to restore the cyclicity of the OFDM signal, i.e. to cancel ICI. Cyclicity is lost because, as a consequence of the multipath components, the delayed replicas of the signal do not appear as cyclic shifts (as would happen with a proper CP). The OFDM samples which are “lost” to the right of the TTI are then introduced to the left as in a circular shift register, after being affected by the complex amplitudes and delays of the multipath components.

(43) The samples to be reconstructed are in principle unknown, but they can be found in the form of ISI towards the first OFDM symbol of the next TTI. This ISI component can in turn be obtained by subtracting the delayed replicas of the reconstructed cyclic prefix from the beginning of the first symbol in the next TTI. Denoting with r.sub.a.sup.TTI+1[n] the received signal of the first symbol in the next TTI, and s.sub.a.sup.TTI+1[n] the corresponding reconstructed samples in the time-domain after equalization, demodulation and inverse FFT, we have:

(44) ${\mathrm{ISI}}^{\mathrm{TTI}+1}\left[n\right]=r_a^{\mathrm{TTI}+1}\left[n\right]-\underset{j=0}{\overset{N_{\mathrm{taps}}^{\prime }-1}{.Math.}}{a}_j^{\prime }{\tilde{s}}_a^{\mathrm{TTI}+1}\left[N_{\mathrm{OFDM}}-N_{\mathrm{CP}}+n-{\tau }_j^{\prime }\right],0\le n<N_{\mathrm{CP}},$
where ISI.sup.TTI+1[n] represents the ISI component from the current TTI that extends towards the first symbol of the next TTI; N′.sub.taps, a′.sub.j and τ′.sub.j refer to the channel tap components estimated in the first OFDM symbol of the next TTI; and:

(45) ${\tilde{s}}_a^{\mathrm{TTI}+1}\left[n\right]\equiv \{\begin{array}{cc}{s}_a^{\mathrm{TTI}+1}\left[n\right],& N_{\mathrm{OFDM}}-N_{\mathrm{CP}}\le n<N_{\mathrm{OFDM}}\\ 0,& \mathrm{outside}\end{array}.$

(46) The equation exploits the fact that the CP is present at the end of the reconstructed first OFDM symbol, which can therefore be subtracted from the received signal in order to obtain the ISI term. This component is exactly the same term that is added to the beginning of the second part r.sub.b.sup.TTI[n] of the current TTI in order to remove ICI. Denoting with r.sub.b,ISI,ICI.sup.TTI[n] the received second part of the TTI after removing the ISI and ICI components, we can write:

(47) $r_{b,\mathrm{ISI},\mathrm{ICI}}^{\mathrm{TTI}}\left[n\right]=r_{b,\mathrm{ISI}}^{\mathrm{TTI}}\left[n\right]+r_a^{\mathrm{TTI}+1}\left[n\right]-\underset{j=0}{\overset{N_{\mathrm{taps}}^{\prime }-1}{.Math.}}{a}_j^{\prime }{\tilde{s}}_a^{\mathrm{TTI}+1}\left[N_{\mathrm{OFDM}}-N_{\mathrm{CP}}+n-{\tau }_j^{\prime }\right],$

(48) for n=0, . . . , (N.sub.sym−1).Math.N.sub.OFDM−1.

(49) The resulting signal in the second part r.sub.b.sup.TTI[n] of the TTI ideally has no ISI and ICI from the absence of the CP. The only requirement is that the receiver additionally decodes the first OFDM symbol of the TTI next to the one to be detected. However, this is not a major problem as receivers usually have to decode that first symbol except in DRX mode, in order to obtain important control information such as scheduling and paging.

(50) In contrast to prior art techniques, the algorithm does not operate over each of the OFDM symbols for ICI cancellation, but rather cancels ICI over the whole remaining part of the TTI after the first symbol thus leading to significantly lower complexity. In addition it does not rely on complex iterative methods that can suffer from chained estimation errors. The first OFDM symbol can be estimated with very low error probability given that it is usually QPSK-modulated for robust detection, which improves reliability of the ISI and ICI cancellation algorithm.

(51) Channel estimation in the second part r.sub.b.sup.TTI[n] of the current TTI can then be performed with the aid of the pilot subcarriers PS, arranged as in FIG. 6, after ISI and ICI cancellation.

(52) An added advantage of the proposed invention is that the ISI and ICI cancellation algorithm only relies on accurate channel estimation in the first OFDM symbol of both the current and the next TTIs, which can be performed almost ideally with the aid of the CP.

(53) After ISI and ICI cancellation, the second part r.sub.b.sup.TTI[n] of the TTI can be detected by means of an FFT of length (N.sub.sym−1).Math.N.sub.OFDM, followed by standard channel equalization and demodulation according to prior art techniques. It is assumed that the channel frequency response remains valid over the whole second part r.sub.b.sup.TTI[n] of the TTI (as there is only one set of pilot subcarriers PS for channel estimation), that is, the channel coherence time is greater than the duration of the second part r.sub.b.sup.TTI[n] of the TTI.

(54) The main fundamental difference of this procedure with respect to prior art techniques is that it can be applied over the whole second part r.sub.b.sup.TTI[n] of the TTI after the first OFDM symbol, instead of having to cancel ISI and ICI successively for each of the symbols. The ICI cancellation procedure particularly benefits from this, as it comprises a relatively complex iterative algorithm. Arranging the information after the first symbol with a longer FFT allows for easier ISI and ICI cancellation, while at the same time retaining the desirable properties of CFO robustness and channel estimation thanks to the presence of the first OFDM symbol. At the same time, the proposed ISI and ICI cancellation procedure relies on reconstruction of the first OFDM symbol of both the current and the next TTI, which by definition are ISI- and ICI-free.

(55) The maximum supported CFO is not changed with respect to prior art solutions as the first symbol allows for estimation of the CFO up to half the subcarrier width. This can be exploited for CFO compensation in the whole TTI in spite of the higher FFT length required for the second part of the TTI, which gives rise to narrower subcarriers.

(56) Applicability of this algorithm relies on the invariance of the channel impulse response along the second part r.sub.b.sup.TTI[n] of the TTI. The channel coherence time T.sub.c should be higher than the duration of the second part r.sub.b.sup.TTI[n] of the TTI according to the formula:

(57) $T_c=\frac{0.423}{f_d}=\frac{0.423c}{{\mathrm{vf}}_c},$
where f.sub.d is the Doppler frequency, c is the speed of light, v is the speed of the user and f.sub.c is the carrier frequency. Comparison of T.sub.c with the duration of the second part of the TTI would yield a maximum practical limit for user speed. If the second part r.sub.b.sup.TTI[n] is small enough (in order to keep the end-to-end latency of the system at a low value), then this assumption is also valid for a significant range of user speeds.

(58) FIG. 9 depicts in detail the steps of the transmission process to be followed by the OFDM transmitter 701 of the proposed system for reducing the overhead caused by the cyclic prefix and easing ISI and ICI cancellation (dashed blocks represent processing steps already present in prior-art techniques, while solid blocks represent new procedures as described in this invention): a) In a first path to obtain the first part r.sub.a.sup.TTI[n] of the TTI, including the useful part of the first OFDM symbol S.sub.0 and the cyclic prefix CP: Firstly, generating 711 the necessary control information to be mapped (next step) into the first OFDM symbol S.sub.0 in the TTI. Subcarrier mapping 712 to time-frequency resources in the first part r.sub.a.sup.TTI[n] of the TTI. Performing an inverse FFT 713 to generate the corresponding time-domain samples, CP introduction 714. b) In a second path to obtain the remaining part r.sub.b.sup.TTI[n] of samples in the TTI: FEC encoding 715 of the information block 70. Interleaving 716 of the encoded bits. The interleaver can be based on an interleaver matrix, a permutation polynomial or any other suitable procedure that avoids adjacent bits to be mapped to adjacent subcarriers in the frequency domain, therefore increasing frequency diversity. Mapping 717 to time-frequency resources in the second part r.sub.b.sup.TTI[n] of the TTI. Note that interleaving 716 is performed prior to this mapping. Generation 718 of appropriate pilot symbols to be mapped on time-frequency resources for channel estimation in the second part r.sub.b.sup.TTI[n] of the TTI. Performs an “enlarged” inverse FFT 719 to generate the time-domain samples corresponding to the second part r.sub.b.sup.TTI[n] of the TTI (without CP addition). c) Finally, concatenation 720 of the two parts of samples, r.sub.a.sup.TTI[n] and r.sub.b.sup.TTI[n], which comprise the complete signal r[n] to be transmitted in the TTI at baseband level.

(59) FIG. 10 depicts in detail the steps of the reception process to be followed by the OFDM receiver 702 of the proposed system for enabling ISI and ICI cancellation (dashed blocks represent processing steps already present in prior-art techniques, while solid blocks represent new procedures as described in this invention): The current TTI, TTI i, is first analysed in order to estimate 811 the eventual carrier frequency offset CFO, with the aid of the first OFDM symbol and its associated cyclic prefix. CFO is further compensated 812 for the whole TTI using standard techniques. Also CIR is estimated 813 in the first OFDM symbol of the current TTI, TTI i, from the CFO-compensated signal. CIR estimation 813 involves obtaining the channel frequency response as well as identifying the most significant multipath components from it. The first part of the TTI, i.e, the first OFDM symbol, is detected 814 through proper channel equalization and demodulation. From the OFDM symbols comprising the second part of the TTI, ISI is removed 815 with the aid of the first OFDM symbol detected in the step 814 and its CIR estimated in the step 813. For the next (subsequent) TTI, TTI i+1, CIR estimation 816 is performed for the first OFDM symbol in said subsequent TTI, TTI i+1. Using the CIR estimated 816 in the subsequent TTI, TTI i+1, the receiver performs ICI cancellation 817. An additional CIR estimation 818 is then performed for the second part of the current TTI, TTI i, that comprises the “enlarged” OFDM symbol. The second part of the TTI, with the remainging part of the “enlarged” OFDM symbol is detected 819. The receiver performs equalization and detection of this remaining part before de-interleaving 820. The received information bits are de-interleaved 820 prior to FEC decoding in order to increase frequency diversity. Finally, FEC decoding 821 is performed to allow the receiver to recover the original information block 80.

(60) FIG. 11 shows an exemplary detailed embodiment of the OFDM transmitter 701 and the OFDM receiver 702 of the proposed system according to the processes described before. An OFDM transmitter 701 is willing to transmit an information block in the form of a set of complex modulated symbols 91, which are first FEC-encoded and interleaved 911, and then subcarrier-mapped 912. Simultaneously, control information 91 undergoes a first inverse FFT 913 (of one OFDM symbol length), and a cyclic prefix is appended 915 to obtain the first part r.sub.a.sup.TTI[n] of the TTI. The second part r.sub.b.sup.TTI[n] of the TTI is first completed with a number of pilot subcarriers in the pilot insertion block 914. A second inverse FFT 916 is then performed (with a length equal to the sum of the lengths of the remaining OFDM symbols). An adder 917 sums together the first and second parts of the TTI, r.sub.a.sup.TTI[n] and r.sub.b.sup.TTI[n], thus constructing the signal r[n] to be transmitted 900. The signal r[n] arrives 900′ at an OFDM receiver 702, through the air interface 90, after suffering the impairments from the air interface 90. At the OFDM receiver 702, the first OFDM symbol is detected 921 by exploiting the presence of the cyclic prefix. The OFDM receiver 702 performs a FFT of the first OFDM symbol after correcting the CFO and discarding the CP, in order to estimate the channel frequency response by means of the pilot subcarriers. ISI cancellation is performed 922 from the remaining part of the TTI according to the process proposed before. The OFDM receiver 702 also removes ICI 923 from the remaining part of the TTI, and finally de-interleaver and FEC decoding operations are performed 924 to deliver the recovered information bits 93.

(61) The proposed embodiments can be implemented as a collection of software elements, hardware elements, firmware elements, or any suitable combination of them.

(62) Note that in this text, the term “comprises” and its derivations (such as “comprising”, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

(63) Changes and modifications to the specifically described embodiments may be carried out without departing from the principles of the present invention, which is intended to be limited only by the scope of the appended claims as interpreted according to the principles of patent law including the doctrine of equivalents.