Channel Estimation for FBMC Modulation

Abstract

Channel estimation with reduced overhead in a filter bank multi-carrier (FBMC) system is enabled by use of frequency-time blocks each comprising a pilot field with two pilot symbols and data symbols outside the pilot field. In embodiments, nearest neighbors of the pilot field are populated with data symbols which fulfill one or more symmetry relations enabling approximate interference cancellation. In a first embodiment, the pilot field consists of two frequency-consecutive and time-coinciding positions; the pilot field may be time-initial in a transmission or may be located in the interior of the transmission. In a second embodiment, a block comprises two frequency-coinciding and time-consecutive pilot symbols; the pilot field may be frequency-initial in a transmission or may be located in the interior of the transmission.

Claims

1. A method implemented in a first node of a communication system employing filter bank multi-carrier (FBMC) modulation with a real symmetric pulse, comprising: transmitting a FBMC-modulated signal comprising data symbols and predetermined pilot symbols, to enable channel estimation at a second node, wherein the signal has a predetermined pilot field populated with two pilot symbols, which are either consecutive in frequency and coinciding in time or coinciding in frequency and consecutive in time, and at least two nearest neighbors of the pilot field are populated with data symbols which fulfill a symmetry relation.

2. The method of claim 1, wherein such nearest neighbors which precede and succeed the pilot field in its longitudinal direction are populated with a same data symbol.

3. The method of claim 1, wherein: the pilot field is neither frequency-initial nor time-initial in a transmission; and such four nearest neighbors, which are adjacent to the pilot field in both time and frequency, are populated with three independent data symbols and one data symbol which is given by a function of the three independent data symbols.

4. The method of claim 1, wherein: the pilot field is frequency-initial in a transmission and the pilot symbols are consecutive in time; and such nearest neighbors, which are adjacent to the pilot field in frequency and coinciding with the pilot field in time, are populated with a same data symbol.

5. The method of claim 1, wherein: the pilot field is frequency-initial in a transmission and the pilot symbols are consecutive in time; and such nearest neighbors, which are adjacent to the pilot field in both frequency and time, are populated with a same data symbol.

6. The method of claim 1, wherein: the pilot field is neither frequency-initial nor time-initial in a transmission, and the pilot symbols are consecutive in time; and such nearest neighbors, which are adjacent to the pilot field in frequency and coinciding with the pilot field in time, are populated with three independent data symbols and one data symbol which is given by a function of the three independent data symbols.

7. The method of claim 1, wherein: the pilot field is time-initial in a transmission and the pilot symbols are consecutive in frequency; and such nearest neighbors, which are coinciding with the pilot field in frequency and adjacent to the pilot field in time, are populated with data symbols of equal magnitudes and opposite phases.

8. The method of claim 7, wherein such nearest neighbors, which are adjacent to the pilot field in both frequency and time, are populated with one independent data symbol and one data symbol which is given by a function of said independent data symbol and of the data symbols which populate said nearest neighbors coinciding with the pilot field in frequency and being adjacent to the pilot field in time.

9. The method of claim 1, wherein: the pilot field is neither frequency-initial nor time initial in a transmission, and the pilot symbols are consecutive in frequency; and such nearest neighbors, which are coinciding with the pilot field in frequency and adjacent to the pilot field in time, are populated with three independent data symbols and one data symbol which is given by a function of the three independent data symbols.

10. The method of claim 3, wherein the function is a linear combination with predetermined coefficients.

11. The method of claim 10, wherein at least one of the independent data symbols is chosen in order for some terms of the linear combination to cancel.

12. The method of claim 3, wherein the transmission is a Long Term Evolution subframe or an equivalent segment.

13. The method of claim 1, further comprising a preceding step of determining, on the basis of a value of a signal quality metric, what proportion of the nearest neighbors is to fulfill a symmetry relation.

14. The method of claim 1, wherein the two pilot symbols are real and imaginary parts of a complex pilot symbol.

15. A method implemented in a second node of a communication system employing filter bank multi-carrier (FBMC) modulation with a real symmetric pulse, comprising: demodulating symbols in a predetermined pilot field in a FBMC-modulated signal received from a first node; and performing channel estimation on the basis of the demodulated symbols and of predetermined pilot symbols, characterized in that the pilot field is populated with two symbols, which are either consecutive in frequency and coinciding in time or coinciding in frequency and consecutive in time.

16. The method of claim 15, wherein the channel estimation comprises neglecting interference.

17. The method of claim 15, further comprising demodulating data symbols which populate nearest neighbors of the pilot field, wherein the demodulating of data symbols includes utilizing a symmetry relation between the data symbols.

18-25. (canceled)

26. The method of claim 15, wherein the demodulating of symbols in the pilot field includes reconstructing a real and an imaginary part of a symbol.

27. A first node adapted to operate in a communication system employing filter bank multi-carrier (FBMC) modulation with a real symmetric pulse, comprising: a modulator for FBMC-modulating a signal comprising data symbols and predetermined pilot symbols; and a transmitter for transmitting the FBMC-modulated signal to enable channel estimation at a second node, a processor configured to generate the signal with a predetermined pilot field populated with two pilot symbols, which are either consecutive in frequency and coinciding in time or coinciding in frequency and consecutive in time, and in that at least two nearest neighbors of the pilot field are populated with data symbols which fulfill a symmetry relation.

28. (canceled)

29. A second node adapted to operate in a communication system employing filter bank multi-carrier (FBMC) modulation with a real symmetric pulse, comprising: a receiver for receiving a FBMC-modulated signal from a first node; a demodulator for demodulating symbols in a predetermined pilot field in the received FBMC-modulated signal; and a processor configured to perform channel estimation on the basis of the demodulated symbols and of predetermined pilot symbols, wherein the pilot field is populated with two symbols, which are either consecutive in frequency and coinciding in time or coinciding in frequency and consecutive in time.

30-32. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Example embodiments of the invention will now be described in greater detail, with reference to the accompanying drawings, on which:

[0037] FIG. 1 illustrates two nodes of a wireless or wired communication system;

[0038] FIGS. 2 and 3 are flowcharts of methods related to channel estimation in a communication system.

[0039] Unless otherwise indicated, the drawings show only such elements that are vital to the comprehension of the invention, whereas other elements may be implied or merely suggested.

DETAILED DESCRIPTION

[0040] FIG. 1 illustrates an example communication network comprising a first node 10 and a second node 20 operative to communicate over a wired or wireless connection indicated by a dashed line on the drawing.

[0041] The first node 10 comprises a modulator 11 for FBMC-modulating a signal, a transmitter 12 for transmitting the FBMC-modulated signal to a second node, and a processor 13 configured to generate the signal to be modulated. For instance, the modulator may prepare a FBMC-modulated signal according to equation (1) on the basis of a set of symbols {d.sub.n,k: 0≦n≦N−1, 0≦k≦K−1}. In order to enable channel estimation at a second node, the processor 13 may supply the modulator 11 with symbols in such manner that it generates a signal with a predetermined pilot field populated with two pilot symbols. In embodiments, the pilot symbols may be either consecutive in frequency and coinciding in time or coinciding in frequency and consecutive in time, and in that at least two nearest neighbors of the pilot field are populated with data symbols which fulfill a symmetry relation.

[0042] The second node 20 comprises a receiver 22 for receiving a FBMC-modulated symbol transmitted from the first node 10, a demodulator 21 for demodulating symbols in a predetermined pilot field of the received FBMC-modulated signal, and a processor 23 configured to perform channel estimation on the basis of the demodulated symbols and of predetermined pilot symbols. In embodiments, the pilot field is populated with two symbols which are either consecutive in frequency and coinciding in time or coinciding in frequency and consecutive in time.

[0043] FIG. 2 illustrates a method 200 suitable for implementation at the first node 10. The method includes an initial step 201 of generating a FBMC-modulated signal comprising data symbols and predetermined pilot symbols. In embodiments, the generated signal has a predetermined pilot field populated with two pilot symbols, which are either consecutive in frequency and coinciding in time or coinciding in frequency and consecutive in time, and at least two nearest neighbors of the pilot field are populated with data symbols which fulfill a symmetry relation. The initial step 201 may be implemented at the modulator 11 and the processor 13. In a second step 202, the FBMC-modulated signal is transmitted so as to enable channel estimation at a second node 20. The second step 202 may be implemented at the transmitter 12.

[0044] FIG. 3 illustrates a method 300 suitable for implementation at the second node 20. It includes an initial step 301 of demodulating symbols in a predetermined pilot field in a FBMC-modulated signal that the second node 20 has received. The initial step 301 may be implemented at the demodulator 21, possibly in cooperation with the receiver 22. The initial step 301 may include neglecting interference. In a second step 302, channel estimation is performed on the basis of the demodulated symbols and on the basis of predetermined pilot symbols. In embodiments, the pilot field is populated with two symbols which are either consecutive in frequency and coinciding in time or coinciding in frequency and consecutive in time. The second step 302 may be implemented at the processor 23.

[0045] In embodiments, the second step 302 may include an optional substep 303 of utilizing a symmetry relation which at least some of the data symbols are assumed to fulfill. Such symmetry relations have been described above and will be further discussed below.

[0046] The set of interference weights according to equation (2) were derived by calculations similar to those of Kofidis et al. (see above), pp. 2051-2052. The interference weights approximately describe observable and measurable phenomena which arise in connection with FBMC/OQAM modulation as a consequence of the laws of nature. Accordingly, by applying the symmetry relations resulting from calculations based on such interference weights, it is possible to generate an observable and measurable transmission with characteristics advantageous for channel estimation.

[0047] Introducing matrices C and I(n, k), respectively denoting transmitted symbols and interference weights around position (n, k), a nearest neighbor approximation of the demodulated signal at this position may be written as

[00013]$y_{n,k}\approx H\left(\underset{\underset{.Math.k^{\prime }-k.Math.\le 1}{.Math.n^{\prime }-n.Math.\le 1}}{.Math.}.Math..Math.{I\left(n,k\right)}_{n^{\prime },k^{\prime }}.Math.C_{n^{\prime },k^{\prime }}\right)+w_0$

where A.sub.i,j denotes the element at position (i,j) of matrix A and w.sub.0 is measurement noise.

[0048] For a pilot field with time-consecutive positions, the sum y.sub.n,k+y.sub.n+1,k is of interest. Hence, interference weights

[00014]$I\left(n,k\right)=\left[\begin{array}{ccccc}& \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}& \\ \mathrm{.Math.}& -\delta & -\beta & \delta & \mathrm{.Math.}\\ \mathrm{.Math.}& -\gamma & G_{n,k}& \gamma & \mathrm{.Math.}\\ \mathrm{.Math.}& -\delta & \beta & \delta & \mathrm{.Math.}\\ & \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}& \end{array}\right],.Math.I\left(n+1,k\right)=\left[\begin{array}{ccccc}& \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}& \\ \mathrm{.Math.}& \delta & -\beta & -\delta & \mathrm{.Math.}\\ \mathrm{.Math.}& \gamma & G_{n+1,k}& -\gamma & \mathrm{.Math.}\\ \mathrm{.Math.}& \delta & \beta & -\delta & \mathrm{.Math.}\\ & \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}& \end{array}\right]$

are used. It easily verified that these weights applied to signal (5) will return the approximate equality (2). In the special case where the pilot field is frequency-initial in a transmission, the combination of all symmetry relations may correspond to setting the symbol matrix to

[00015]$C=\left[\begin{array}{ccc}\mathrm{.Math.}& \mathrm{.Math.}& \\ a& b& \mathrm{.Math.}\\ d_{n,0}& c& \mathrm{.Math.}\\ d_{n+1,0}& c& \mathrm{.Math.}\\ a& b& \mathrm{.Math.}\\ \mathrm{.Math.}& \mathrm{.Math.}& \end{array}\right],$

where symbols d.sub.n,0, d.sub.n+1,0 may be set to the real and the imaginary part of a complex pilot symbol. It is easily verified that approximate equality (2) holds in this case as well. As used herein, a transmission may be an LTE subframe or an equivalent segment.

[0049] For a pilot field with frequency-consecutive positions, the sum y.sub.n,k+y.sub.n,k+1 is of interest. It may be calculated on the basis of the following interference weights:

[00016]$I\left(n,k\right)=\left[\begin{array}{ccccc}& \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}& \\ \mathrm{.Math.}& -\delta & -\beta & \delta & \mathrm{.Math.}\\ \mathrm{.Math.}& -\gamma & G_{n,k}& \gamma & \mathrm{.Math.}\\ \mathrm{.Math.}& -\delta & \beta & \delta & \mathrm{.Math.}\\ & \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}& \end{array}\right],.Math.I\left(n,k+1\right)=\left[\begin{array}{ccccc}& \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}& \\ \mathrm{.Math.}& -\delta & -\beta & \delta & \mathrm{.Math.}\\ \mathrm{.Math.}& -\gamma & G_{n,k+1}& \gamma & \mathrm{.Math.}\\ \mathrm{.Math.}& -\delta & \beta & \delta & \mathrm{.Math.}\\ & \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}& \end{array}\right].$

It easily verified that these weights applied to signal (6) will return the approximate equality (3). In the special case where the pilot field is time-initial in a transmission, the combination of all symmetry relations may correspond to setting the symbol matrix to

[00017]$C=\left[\begin{array}{cccccc}\mathrm{.Math.}& a& d_{0,k}& d_{0,k+1}& a& \mathrm{.Math.}\\ \mathrm{.Math.}& b& c& -c& \eta \left(b,c\right)& \mathrm{.Math.}\\ & \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}& \end{array}\right],$

whereby approximate equality (3) can be demonstrated to hold.

[0050] As seen above, the symmetry relations may involve linear combinations with predetermined coefficients of the independent data symbols, such as function η in the previous equation. In some embodiments, at least one of the independent data symbols is chosen in order for some terms of the linear combination to cancel.

[0051] As explained in previous sections, some embodiments include applying only a subset of all known symmetry relations. It may be determined based on a value of a signal quality metric what proportion of the nearest neighbors is to fulfill a symmetry relation.

[0052] It is noted that although terminology from 3GPP LTE has been used in this disclosure to exemplify embodiments herein, this should not be seen as limiting the scope of the embodiments to only the aforementioned system. Other wireless systems, including WCDMA, WiMAX, UMB and GSM, may also benefit from exploiting embodiments herein.

[0053] It is noted that the first node 10 and the second node 20 herein may correspond to any pair of nodes configured to transmit radio or other signals and otherwise interact in the way described. In one embodiment, though, the first node 10 comprises a base station (e.g., an eNodeB in LTE) or a relay node, whereas the second node 20 comprises a wireless communication device (e.g., a UE in LTE). Terminology such as eNodeB and UE should be considering non-limiting and does in particular not imply a certain hierarchical relation between the two. Furthermore, while the present detailed description is focused on wireless transmissions in the downlink, embodiments herein are equally applicable in the uplink.

[0054] In some embodiments, a non-limiting term UE is used. The UE herein can be any type of wireless device capable of communicating with a network node or another UE over radio signals. The UE may also be a radio communication device, target device, device to device (D2D) UE, machine-type UE or UE capable of machine-to-machine communication (M2M), a sensor or actuator equipped with an UE, tablet, mobile terminal, smart phone, laptop embedded equipped (LEE), laptop mounted equipment (LME), USB dongles, Customer Premises Equipment (CPE) etc.

[0055] Also in some embodiments generic terminology “base station” is used. This may refer to any kind of network node which may comprise of base station, radio base station, base transceiver station, base station controller, network controller, evolved Node B (eNB), Node B, Multi-cell/multicast Coordination Entity (MCE), relay node, access point, radio access point, Remote Radio Unit (RRU) Remote Radio Head (RRH), or even core network node etc.

[0056] Embodiments herein also include a computer program comprising instructions which, when executed by at least one processor of a first 10 or second 20 node, cause the node to carry out any of the methods herein. In one or more embodiments, a carrier containing the computer program is one of communication media (or transitory media, such as an electronic signal, optical signal, radio signal) or computer readable storage media (or non-transitory media). The term computer storage media includes both volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information; computer storage media includes but is not limited to RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which stores the desired information and is accessible by a computer.

[0057] The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.