System and method for single-carrier space-frequency block coded transmission over frequency selective and fast fading channels

Abstract

A single carrier transmission scheme which utilizes space-frequency block coding and frequency domain equalization (SF-SCFDE) is proposed for frequency selective and fast fading channel. It is shown that employing this technique in slow fading environment depicts the same performance as that obtained with space-time coding scheme. However, in the more difficult fast fading channels, the proposed scheme exhibits much better performance.

Description

FIELD

Embodiments of the present application relate to the field of single carrier transmission schemes in communication systems. Exemplary embodiments relate to a method and system for single carrier space-frequency block coding transmission.

BACKGROUND

The rapid increase of the demand for wireless applications has stimulated tremendous research efforts in developing systems that support reliable high rate transmissions over wireless channels. However, these developments must cope with challenges such as multipath fading, time varying nature of the wireless channel, bandwidth restrictions and handheld devices power limitations. Space-time transmission techniques have been proven to combat the detrimental efforts of the multipath fading. Unfortunately, the large delay spreads of frequency selective fading channels destroy the orthogonality of the received signals which is critical for space-time coding. Consequently, the techniques are only effective over frequency flat block fading channels. Orthogonal frequency division multiplexing (OFDM) has also shown to combat the multipath fading. A space-time OFDM (ST-OFDM) and space-frequency OFDM (SF-OFDM) have been proposed as an effective way to combat the frequency selectivity of the channel. Moreover, SF-OFDM can be applied to fast fading channel wherein the channel doesn't need to be constant for at least two block transmission as it is usually required for ST-OFDM scheme. OFDM is a multicarrier communication technique, with which a single data stream is transmitted over a number of lower rate subcarriers. A multicarrier signal consists of a number of independent modulated subcarriers that can cause a large peak-to-average PAPR when the subcarriers are added up coherently. Also OFDM suffer from phase noice and the frequency offset problems. Therefore, to combat the frequency selectivity of the channel, an alternative solution for OFDM was proposed that utilizes single carrier transmission with frequency domain equalization. In parallel to ST-OFDM scheme, a space-time single carrier (ST-SC) transmission scheme was proposed in that requires the channel to be same for at least two block periods.

Although many high rate wireless communication method and systems have been proposed, none provide a space-frequency single carrier (SF-SC) technique which doesn't require the channel to be the same for two block periods, and hence beneficial for fast fading channel. Prior methods and systems do not use a single carrier transmission technique that implements space-frequency block coding with additional frequency diversity as shown in the next section.

SUMMARY

Aspects of the exemplary embodiments are directed to a single carrier transmission scheme which utilizes space-frequency block coding and frequency domain equalization (SF-SCFDE). Such a technique can be used with frequency selective and fast fading channel.

In one exemplary embodiment, a method for single carrier space-frequency (SF-SCFDE) transmission over frequency selective and fast fading channel includes receiving communication block streams, encoding the received communication block streams to produce communication blocks, adding a cyclic prefix to each communication block to form transmission blocks, and communicating each transmission block through a frequency selective fading channel.

In another exemplary embodiment, an apparatus for single carrier space-frequency (SF-SCFDE) transmission over frequency selective and fast fading channel includes a space-frequency encoder that receives and encodes communication block streams, a cyclic prefix adder that adds a cyclic prefix to each communication block, and antennas that communicate each communication block.

In yet another exemplary embodiment, a system for single carrier space-frequency (SF-SCFDE) transmission over frequency selective and fast fading channel includes a transmitter and a receiver. The transmitter includes a space-frequency encoder that receives and encodes communication block streams, a cyclic prefix adder that adds a cyclic prefix to each communication block, and antennas that communicate each communication block. The receiver includes antennas that receive communication blocks from the transmitter, a structure to remove the cyclic prefix, and a decoder.

These and other features, aspects and advantages of the present invention will become apparent from the following description, appended claims, and the accompanying exemplary embodiments shown in the drawings, which are briefly described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general diagram of a transmitter in accordance with an exemplary embodiment.

FIG. 2 is a flow diagram depicting operations performed in the single carrier space-frequency (SF-SCFDE) transmission scheme of the transmitter of FIG. 1.

FIG. 3 is a graph comparing the single carrier space-frequency (SF-SCFDE) transmission scheme of the transmitter of FIG. 1 with other transmission schemes.

DETAILED DESCRIPTION

Exemplary embodiments are described below with reference to the accompanying drawings. It should be understood that the following description is intended to describe exemplary embodiments of the invention, and not to limit the invention.

FIG. 1 illustrates a transmitter 10 having N_t=2 transmit antennas and N_t=1 receive antenna. The information bearing data symbols d(n) belonging to an alphabet A are first demultiplexed into two block streams of N symbols each for transmission for either transmit antennas using a demultiplexer 12. The communication block is represented by s_k=[s_k(0), s_k(1), . . . s_k(N−1]^T and is transmitted by the k^th transmit antenna, k=1.2 with |s_k(n)|²=1, whose N-point Discrete Fourier Transform (DFT) is given by |S_k=(0), S_k(1), . . . , S_k(N−1)|^T. The two symbols blocks s₁ and s₂ are then fed to a space-frequency encoder 14 suitable for SCFDE to produce the following two blocks:

u ₁ =[s _1,0 ,−s* _2,0 ,s _1,1 ,−s* _2,N−1 , . . . , s _1,N−1 ,−s* _2,1]^T

u ₂ =[s _2,0 ,s* _1,0 ,s _2,1 ,s* _1,N−1 , . . . , s _2,N−1 ,s* _1,1]^T  (1)

where s_k,n=s_k(n). These two blocks u₁ and u₂ are then compressed from symbol duration T_s to T_s/2 and the compressed vector is repeated twice to form the following blocks

v _k =[u _k ^T ,u _k ^T]^T; k=1,2  (2)

The vector v_k is denoted by v_k,e (v_k,o) with the odd (even) elements made zeros i.e. v_k,e=[s_k,e^T,s_k,e^T]^T, k=1, 2 and v_1,o[−s_2,o^T,−s_2,o^T]^T, v_2,o=[s_1,o^T,s_1,o^T]^T where

s _k,e =[s _k(0),0,s_k(1),0, . . . , s₁(N−1),0]^T

s _k,o=[0,s*_k(0),0,s*_k(N−1), . . . , 0,s*_k(2),0,s*_k(1)]^T  (3)

The zeros insertion and repetition operation is performed at blocks 18 in FIG. 1. Before transmission, the vectors v_k,o are multiplied by a phase shift matrix Φ_4N using multipliers 20:

$\begin{array}{cc}{\Phi }_{4N}=\mathrm{diag}\left(1,{}^{\frac{\mathrm{j2\pi }}{4N}},{}^{\frac{\mathrm{j2\pi }2}{4N}},\dots ,{}^{\frac{\mathrm{j2\pi }\left(4N-1\right)}{4N}}\right),& \left(4\right)\end{array}$

to create one element forward shift in their Fourier transform. The transmitted signals vector from the two antennas are then given by

x _k=√{square root over (P_o/8N)}(v_k,e+Φ_4,Nv_k,o), k=1,2  (5)

where P_o is the total transmitted power. As shown in FIG. 1, adders 22 are used for the (v_k,e+Φ_4,Nv_k,o) operation in (5). At blocks 24, a cyclic prefix (CP) is added to each block before transmitting through a frequency selective fading channel of order L. h_k=[h_k(0),h_k(1), . . . , h_k(L−1)] where h_k(l) is the l^th response of the channel impulse response (CIR) between the k^th transmit antenna and the receive antenna. The received signal vector y is given by

$\begin{array}{cc}y=\sum _{k=1}^2x_k^{\left(\mathrm{CP}\right)}\otimes h_k+w& \left(6\right)\end{array}$

where x_k^(CP) is x_k with the CP added and w is an added noise vector assumed AWGN with variance N_o and {circle around (X)} denotes the linear convolution. Since the CP turns the linear convolution into a circular, the received signal vector after the removal of the CP and taking the 4N-point DFT is given by

$\begin{array}{cc}Y=\sum _{k=1}^2{\Lambda }_kX_k+W& \left(7\right)\end{array}$

where Λ_k, k=1, 2 represent diagonal matrices whose elements are the 4N-point DFT of the corresponding CIR h_k·X_k and W represents the 4N-point DFT of x_k and w respectively.

From (5), it follows:

X _k=√{square root over (P_o/8N)}(V_k,e+{tilde over (V)}_k,o), k=1.2  (8)

where V_k,e, V_k,o, and {tilde over (V)}_k,o represent the 4N-point DFT of v_k,e, v_k,o and (Φ_4Nv_k,o) respectively. Now it follows that

$\begin{array}{cc}{V}_{k,e}={\left[S_{k,e}^T·S_{k,e}^T\right]}^T{\tilde{V}}_{2.o}={{\Phi }_{4N}\left[S_{l,o}^T·S_{1,o}^T\right]}^T{\tilde{V}}_{1.0}={{\Phi }_{4N}\left[S_{2.o}^T·-S_{2.o}^T\right]}^T\mathrm{where}& \left(9\right)\\ S_{k,e}={\left[S_k\left(0\right),0,S_k\left(1\right)0,\dots ,S_{k^{\prime }}\left(N-1\right),0\right]}^TS_{k,o}={\left[0,S_k^{*}\left(0\right),0,S_k^{*}\left(N-1\right),\dots ,0,S_k^{*}\left(1\right)\right]}^T{\tilde{\Phi }}_{4N}=\mathrm{diag}\left(0,1,e\frac{\mathrm{jo}*}{4N},e\frac{\mathrm{jo}*o}{4N},\dots ,e\frac{\mathrm{jo}*\left(4N-2\right)}{4N}\right)& \left(10\right)\end{array}$

It is proposed that the 4N -point DFT of a 4N elements vector of the form [p_e^T,p_e^T]^T where p_e=[p(0),0, . . . , p(N−1),0]^T is [p_e^T,p_e^T]^T where P_e=[P(0),0, . . . , P(N−1),0]^T and the vector [P(0),P(1), . . . , P(N−1)]^T is the N-point DFT of [p(0),p(1), . . . , p(N−1)]^T. To prove (9), it can be seen from the foregoing proposition that V_k,e is the 4N-point DFT of v_k,e=[s_k,e^T,s_k,e^T]^T where s_k,e is defined in (3) hence V_k,e=[s_k,e^T,S_k,e^T]^T. Next the vector {right arrow over (s)}_k=[s*_k(0),0,s*_k(N−1), . . . , 0,s*_k(2),0,s*_k(1),0]^T is defined which is one element circular shift to the left of s_k,o in (3), that is s_k,o(n)={right arrow over (s)}_k(n−1) and {right arrow over (v)}_2,o=[{right arrow over (s)}₁^T,{right arrow over (s)}₁^T]^T i.e. v_2,o(n)={right arrow over (v)}_2,o(n−1).

Using the shift property of DFT, the 4N-point DFT of v_2,o is given by

V_2,o=Φ_4NV_2,o  (11)

where V_2,o is the 4N-point DFT of V_2,o. Using the statement above and applying the N-point DFT of [s*₁(0),s*₁(N−1), . . . , s*₁(2),s*₁(1)]^T as [S*₁(0),S*₁(1), . . . , S*₁(N−2),S*₁)N−1)]^T on v_2,o it follows that V_2,o=[ S₁^T, S₁^T]^T where S₁=S*_1,e. With {tilde over (v)}_2,o=Φ_4Nv_2,o and using the inverse of the shift property of DFT, it follows that {tilde over (V)}_2,o(m)=V_2,l(m−1) i.e. {tilde over (V)}_2,o is one element circular shift to the right of V_2,o. Using (11), it follows that {tilde over (V)}_2,o=Φ_4B[S_1,o^T,S_1,o^T]^T where Φ_4N is defined in (10). One can follow the same procedure to show that {tilde over (V)}_1,o=Φ_4N[−S_2,o^T,−S_2,o^T]^T.

Now for a vector A, A^e and A^o are defined to be the even and odd parts of A respectively. From (9), it follows

V_1,e^e=[S_k^T,S_k^T]^T, V_k,e^o=[0_N^T,0_N^T]^T

{tilde over (V)} _1,o ^o=Φ_2N[−S₂^H,−S₁₂^H]^T, {tilde over (V)}_2,o^o=φ_2N[S_x^H,S_l^H]^T.

V_k,o^o=[0_N^T,0_N^T]^T, k=1.2  (12)

where

$\begin{array}{cc}{\Phi }_{2N}=\mathrm{diag}\left(1,{}^{\frac{\mathrm{j2\pi }}{4N}},{}^{\frac{\mathrm{j2\pi }2}{4N}},\dots ,{}^{\frac{\mathrm{j2\pi }\left(2N-1\right)}{2N}}\right),& \end{array}$

is a 2N×2N diagonal matrix whose diagonal elements are the odd diagonal elements of Φ_4N of ((10)) and 0_N is a zero vector of length N. It then follows that

X _k ^e=√{square root over (P_o/8N)}[S_k^T,S_k^T]^T

X ₁ ^o=√{square root over (P_o/8N)}Φ_2N[−S₂^H,−S₂^H]^T

X ₂ ^o=√{square root over (P_o/8N)}Φ_2N[S₁^H,S₁^S]^T  (13)

From ((13)) it can be seen that:

X₂^o=Φ_2NX₁^e*

X ₁ ^o=−Φ_2NX^e*  (14)

Eq ((7)) can then be rewritten as

$\begin{array}{cc}{Y}^e={\Lambda }_1^eX_1^e+{\Lambda }_2^eX_2^e+W^e\begin{array}{c}{Y}^o={\Lambda }_1^oX_1^o+{\Lambda }_2^oX_2^o+W^o\\ =-{\Lambda }_1^o{\Phi }_{2N}X_2^{e*}+{\Lambda }_2^o{\Phi }_{2N}X_1^{e*}+W^o,\end{array}& \left(15\right)\end{array}$

where Λ_k^e and Λ_k^o are diagonal matrices whose diagonal elements are the even and odd diagonal elements of Λ_k respectively. Assuming that the channel gains for adjacent subcarriers are approximately equal, i.e. Λ_k^e≈Λ_k^o, k=1,2; hence combining ((15)) gives

$\begin{array}{cc}\left(\begin{array}{c}{Y}^e\\ Y^{o*}\end{array}\right)=\left(\begin{array}{cc}{\Lambda }_1^e& {\Lambda }_2^e\\ {\Phi }_{2N}^{*}{\Lambda }_2^{e*}& -{\Phi }_{2N}^{*}{\Lambda }_1^{e*}\end{array}\right)\left(\begin{array}{c}{X}_1^e\\ X_2^e\end{array}\right)+\left(\begin{array}{c}{W}^e\\ W^{o*}\end{array}\right)& \left(16\right)\end{array}$

The first and second N terms of Y^e and Y^o are defined respectively for k=1, 2 by

Y _k ^e =[Y ^e((k−1)N), . . . , Y^e(kN−1)]^T

Y _k ^o =[Y ⁰((k−1)N), . . . , Y^o(kN−1)]^T  (17)

Plugging ((13)) and ((17)) in ((16)) arrives at:

$\begin{array}{cc}Z=\mathrm{AS}+\tilde{W}\mathrm{where}& \left(18\right)\\ Z={\left[Y_1^{e^T},Y_2^{e^T},Y_1^{o{*}^T},Y_2^{o{*}^T}\right]}^T,S={\left[S_1^T,S_2^T\right]}^T\tilde{W}={\left[W_1^{e^T},W_2^{e^T},W_1^{o^T},W_2^o\right]}^T\Lambda =\sqrt{P_o/8N}\left(\begin{array}{c}\begin{array}{cc}{\Lambda }_{1,1}^e& {\Lambda }_{2,1}^e\\ {\Lambda }_{1,2}^e& {\Lambda }_{2,2}^e\end{array}\\ {\Phi }_{2N,1}^{*}{\Lambda }_{2,1}^{e*}-{\Phi }_{2N,1}^{*}{\Lambda }_{1,1}^{e*}\\ {\Phi }_{2N,2}^{*}{\Lambda }_{2,2}^{e*}-{\Phi }_{2N,2}^{*}{\Lambda }_{1.2}^{e*}\end{array}\right)& \left(19\right)\end{array}$

where Λ_k,1^e and Λ_k,2^e are N×N diagonal matrices whose diagonal elements are the first and last N diagonal elements of Λ_k^e and Λ_2,1^e, Λ_2,2^e, Φ_2N,1, Φ_2N,2 are similarly defined. Note that |Φ_2N|²=I_2N,|Φ_2N,1|²=|Φ_2N,2|²=I_N where for a diagonal matrix D we defined |D|²=DD*. The proposed space-frequency decoder gives the estimation Ŝ according to the following

{circumflex over (S)}=(Λ^HΛ)⁻¹Λ^HZ  (20)

It can be shown that the matrix Λ^HΛ is diagonal and given by

$\begin{array}{cc}{\Lambda }^H\Lambda =\left(\begin{array}{cc}{\Delta }^2& 0\\ 0& {\Delta }^2\end{array}\right)& \left(21\right)\end{array}$

where

${\Delta }^2=\frac{P_o}{8N}\left({{\Lambda }_{1,1}^e}^2+{{\Lambda }_{1,2}^e}^2+{{\Lambda }_{2,1}^e}^2+{{\Lambda }_{2,2}^e}^2\right)$

hence S₁ and S₂ are completely decoupled. The estimates in (20) are transformed back in time domain for detection.

FIG. 2 illustrates operations performed in an exemplary single carrier space-frequency (SF-SCFDE) transmission technique utilized by transmitter 10 described with reference to FIG. 1. Additional, fewer or different operations may be performed depending on the embodiment. In an operation 30, data symbols d(n) belonging to an alphabet A are demultiplexed into two block streams of N symbols. In an operation 32, the block streams are encoded into communication blocks.

The communication blocks are compressed into vectors in an operation 34 and the compressed vectors are repeated twice to form vector blocks in an operation 36. Once the vector blocks are formed, they are phase shifted using a phase shift matrix (operation 38). This phase shift creates a one element forward shift in the Fourier transform. In an operation 40, a cyclic prefix (CP) is added to each phase shifted vector block. Once the CP prefix is added, the blocks are transmitted through a frequency selective fading channel of order L. Upon receipt of the communicated blocks, the CP prefix is removed and a 4-N point DFT is determined.

The exemplary single carrier space-frequency (SF-SCFDE) transmission scheme over frequency selective and fast fading channel described herein has been shown to be an efficient and effective transmission technique especially for application where channel is fast fading. The bit error rate (BER) performance of the exemplary space-frequency single carrier system was calculated in a simulation. The simulation used a single carrier transmission with N=64 data symbols per block in a frequency selective channel assumed to be a COST207 six-ray (L=6) typical urban channel. The BER performance was shown to outperform the ST-OFDM described by K. F. Lee and D. B. Williams in “A space-time coded transmitter diversity technique for frequency selective fading,” in IEEE Sensor Array and Multichannel Signal Processing Workshop, pp. 149-152, March 2000 and “A space-frequency diversity technique for OFDM system,” IEEE GLOBECOM, pp. 1473-1477, November 2000 (referred to below as “Lee and Williams”). It also outperformed the conventional OFDM system in the same channel. The performance of SF-SCFDE described herein was also compared with that of ST-SCFDE described in W. M. Younis, N. Al-Dhahir, and A. H. Sayed, “Adaptive frequency-domain equalization of space-time block-coded transmissions,” in IEEE Int. Conf. Accoust., Speech, Signal Process., vol. 3, Orlando, Fla. May 2002, pp. 2353-2356 (referred to below as “Younis”), in slow fading channel (where the normalized Doppler frequency is 0.001) and fast fading channel (where the normalized Doppler frequency is 0.05). Simulation results show that the SF-SCFDE scheme described herein depicts much better BER. One reason for the better performance is frequency domain spreading which causes additional frequency domain diversity. Furthermore, the techniques of the exemplary embodiments do not suffer the PAPR (peak to average power ratio) problem.

FIG. 3 is a graph showing a performance comparison of the exemplary SF-SCFDE described herein with ST-SCFDE of Lee and Williams (left) and SF-SCFDE with SF-OFDM of Younis (right) in slow (fdTs=0.001) and fast (fdTs=0.05) fading COST207 six-ray typical urban (TU) channel.

The foregoing description of exemplary embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described in order to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments and with various modifications as are suited to the particular use contemplated.

Claims

1. A method for single carrier space-frequency (SF-SCFDE) transmission over frequency selective and fast fading channel, the method comprising: receiving communication block streams;encoding the received communication block streams to produce communication blocks;adding a cyclic prefix to each communication block to form transmission blocks; andcommunicating each transmission block through a frequency selective fading channel.

2. The method of claim 1, wherein receiving communication block streams further comprises demultiplexing data symbols into multiple communication block streams.

3. The method of claim 1, wherein encoding the received communication block streams to produce communication blocks further comprises: compressing the communication blocks into compressed vectors;repeating the compressed vectors twice to form vector blocks; andphase shifting the vector blocks using a phase shifting matrix.

4. The method of claim 3, wherein the phase shifting matrix is Φ4N and is defined by:

5. The method of claim 1, wherein the frequency selective fading channel is of an order L such that a channel impulse response between a transmit antenna and a receive antenna is represented as hk=[hk(0),hk(1), . . . , hk(L−1)] where hk(l) is the lth response.

6. The method of claim 1, wherein communicating each transmission block through a frequency selective fading channel comprises transmitting the transmission block by a kth transmit antenna using an N-point Discrete Fourier Transform (DFT) where a 4N-point DFT of a 4N elements vector of the form [peT, peT]T where pe=[p(0),0, . . . , p(N−1),0]T is [peT,peT]T where Pe=[P(0),0, . . . , P(N−1),0]T and the vector [P(0), P(1), . . . , P(N−1)]T is the N-point DFT of [p(0), p(1), . . . , p(N−1)]T.

7. The system of claim 1, further comprising, at a receive antenna, removing the cyclic prefix and taking a 4N-point Discrete Fourier Transform (DFT).

8. An apparatus for single carrier space-frequency (SF-SCFDE) transmission over frequency selective and fast fading channel, the apparatus comprising: a space-frequency encoder that receives and encodes communication block streams;a cyclic prefix adder that adds a cyclic prefix to each communication block; andantennas that communicate each communication block.

9. The apparatus of claim 8, wherein the space-frequency encoder further comprises a compressor to compress the encoded communication block streams, circuitry to repeat the communication block streams and form vector blocks, and a phase shifter to perform a phase shift.

10. The apparatus of claim 9, wherein the phase shift is done using a phase shifting matrix, Φ4N, defined by:

11. The apparatus of claim 8, further comprising a demultiplexer that demultiplexes data symbols into multiple communication block streams.

12. The apparatus of claim 8, wherein the antennas that communicate each communication block communicate using a fast fading channel.

13. A system for single carrier space-frequency (SF-SCFDE) transmission over frequency selective and fast fading channel, the system comprising: a transmitter having a space-frequency encoder that receives and encodes communication block streams, a cyclic prefix adder that adds a cyclic prefix to each communication block, and antennas that communicate each communication block; anda receiver having antennas that receive communication blocks from the transmitter, a structure to remove the cyclic prefix, and a decoder.

14. The system of claim 13, wherein the space-frequency encoder further comprises a compressor to compress the encoded communication block streams, circuitry to repeat the communication block streams and form vector blocks, and a phase shifter to perform a phase shift.

15. The system of claim 14, wherein the phase shift uses a phase shifting matrix, Φ4N, defined by:

16. The system of claim 13, wherein the transmitter further includes a demultiplexer that demultiplexes data symbols into multiple communication block streams.

17. The system of claim 13, wherein antennas communicate the communication blocks through a frequency selective fading channel.

18. The system of claim 17, wherein the frequency selective fading channel is of an order L such that a channel impulse response between the transmitter and the receiver is represented as hk=[hk(0), hk(1), . . . , hk(L−1)] where hk(l) is the lth response.

19. The system of claim 17, wherein communication through the frequency selective fading channel uses a kth transmit antenna with an N-point Discrete Fourier Transform (DFT) where a 4N-point DFT of a 4N elements vector of the form [peT, peT]T where pe=[p(0),0, . . . , p(N−1),0]T is [PeT,PeT]T where Pe=[P(0),0, . . . , P(N−1),0]T and the vector [P(0),P(1), . . . , P(N−1)]T is the N-point DFT of [p(0), p(1), . . . , p(N−1)]T.

20. The system of claim 16, wherein the receiver takes a 4N-point Discrete Fourier Transform (DFT) of the communication blocks after the cyclic prefix is removed.