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

Patent metadata

Field	Value
Publication number	US-9780915-B2
Application number	US-201514872217-A
Country	US
Kind code	B2
Filing date	Oct 1, 2015
Priority date	Oct 1, 2014
Publication date	Oct 3, 2017
Grant date	Oct 3, 2017

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.

First claim

The invention claimed is:

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 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 a TTI [n]) of a first OFDM symbol (S 0 ), where r and a are reference letters that denote part of the received signal TTI (r TTI [n]), n is a time index taking values n=0, . . . , (N sym −1)·N OFDM −1, and N OFDM is a natural number equal to the number of subcarriers, the first OFDM symbol (S 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 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 0 ), containing a replica of the last samples of the first OFDM symbol (S 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 sym −1)·N OFDM time-domain complex samples (r b TTI [n]), where b is a reference letter that denote another part of the received signal TTI (r TTI [n]), which samples result from the mapping to time-frequency resources of a set of (N sym −1)·N 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 sym −1)·N OFDM complex subcarriers comprising a concatenation of subcarriers corresponding to the remaining OFDM symbols (S 1 , . . . S Nsym−1 ) of the TTI, the concatenated subcarriers comprising both data subcarriers and pilot subcarriers for channel estimation, and the remaining OFDM symbols (S 1 , . . . S Nsym−1 ) being obtained in the time domain by means of an enlarged inverse Fourier Transform with length equal to the number (N sym −1)·N OFDM of the concatenated subcarriers; and
concatenating the time-domain complex samples (r a TTI [n]) of the first part of the TTI and the time-domain complex samples (r b 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 a TTI [n], r b 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 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 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 0 ) by separating time-domain complex samples (r a 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 b TTI [n]) of the second part of the current TTI;
estimating a carrier frequency offset, CFO, using the detected first OFDM symbol (S 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 0 );
recovering information in the detected first OFDM symbol (S 0 ) through channel equalization and symbol decoding;
reconstructing a time-domain transmitted signal s a TTI [n] corresponding to the detected first OFDM symbol (S 0 ) by applying an inverse Fourier transform of the recovered information with length N 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
⁡

[
n
]

=

∑

j
=
0

N
taps

-
1

⁢

a
j

⁢

δ
⁡

[

n
-

τ
j

]

where h[n] denotes the CIR in time-domain, N taps denotes the number of identified CIR taps, a j is a complex amplitude of the j-th tap, τ 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 b TTI [n]) of the second part of the current TTI by means of the following equation:

r

b
,
ISI

TTI

⁡

[
n
]

=

r
b
TTI

⁡

[
n
]

Assignees

• Telefonica Sa

Inventors

• Lorca Hernando Javier

Classifications

• H04W72/0453

  Resources in frequency domain, e.g. a carrier in FDMA · CPC title

• H04L27/2607

  Cyclic extensions · CPC title

• H04L1/0048 Primary

  in conjunction with detection of multiuser or interfering signals, e.g. iteration between CDMA or MIMO detector and FEC decoder (for spatial equalizer H04L25/03286) · CPC title

• H04L5/0007 Primary

  the frequencies being orthogonal, e.g. OFDM(A) or DMT · CPC title

• H04L5/0048

  Allocation of pilot signals, i.e. of signals known to the receiver (allocation of control signalling H04L5/0053; use of control signalling H04L5/0091) · CPC title