# Patent

## Title
Techniques to time vary pilot locations in wireless networks

## Abstract

An embodiment of the present invention provides an apparatus, comprising a receiver capable of selecting optimal pilot locations and providing feedback of the pilot locations to a transmitter in communication with the receiver. The optimal pilot locations may be selected by locations that avoid strong interference or platform noise at the receiver, by locations that avoid deep fading, by locations that maximize the spacing between pilot tones at the two ends of a wireless frequency band or by locations that equalize the interspacing between any two adjacent pilots.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation application of Ser. No. 11/317,311 filed Dec. 23, 2005 U.S. Pat. No. 7,532,675, entitled “TECHNIQUES TO TIME VARY PILOT LOCATIONS IN WIRELESS NETWORKS” which became allowed on Jan. 27, 2009. The Issue Fee was submitted on Apr. 6, 2009.


BACKGROUND
Wireless communications, including wireless networks, have become pervasive throughout society. Improvements in wireless communications are vital to increase their reliability and speed. Present wireless communication standards, such as but not limited to, current 802.11a/g and TGnSync proposals, may use pilot locations that are constant over time. Namely, a fixed set of subcarriers may be assigned for pilots for the whole system all the time. This may cause a problem in the presence of platform noise and co-channel narrow band interference. Because the receiver can't gain a synchronization signal from the pilots for a certain period, the receiver's PLL loses synchronization with the transmitter, and thus the packets may get lost.
Extensive measurements demonstrate that mobile platform noise may be a significant factor of WLAN performance degradation. Further, the performance of 802.11a loses more than 20 dB if one of the pilot tones is corrupted by platform noise or co-channel interference of −83 dBm. In contrast, the corruption of data tones with the same interference power only causes a 10 dB lose. Simulation results for 802.11n demonstrate that loss of pilot tones due to fading significantly degrades performance by 0.5-6 dB.
Thus, a strong need exists for techniques to time vary pilot locations in wireless networks.



BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:
FIG. 1 provides a table with SNR loss under narrow band interference of an embodiment of the present invention;
FIG. 2 illustrates the loss of two pilots in two spatial streams due to fading;
FIG. 3 illustrates the loss of one pilot due to platform noise or co-channel interference;
FIG. 4 is an illustration of one technique to time vary pilot locations with feedback in a wireless network of one embodiment of the present invention;
FIG. 5 is an illustration of one technique to time vary pilot locations in a wireless network without feed back and with pilot patterns of one embodiment of the present invention; and
FIG. 6 depicts how a transmitter may vary the pilot location across OFDM symbols of one embodiment of the present invention.
FIG. 7 depicts how a transmitter may vary the pilot location across OFDM symbols and antennas in accordance with one embodiment of the present invention.



It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements.
DETAILED DESCRIPTION
In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention.
An algorithm, technique or process is here, and generally, considered to be a self-consistent sequence of acts or operations leading to a desired result. These include physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers or the like. It should be understood, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities.
Embodiments of the present invention may include apparatuses for performing the operations herein. An apparatus may be specially constructed for the desired purposes, or it may comprise a general purpose computing device selectively activated or reconfigured by a program stored in the device. Such a program may be stored on a storage medium, such as, but not limited to, any type of disk including floppy disks, optical disks, compact disc read only memories (CD-ROMs), magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), electrically programmable read-only memories (EPROMs), electrically erasable and programmable read only memories (EEPROMs), magnetic or optical cards, or any other type of media suitable for storing electronic instructions, and capable of being coupled to a system bus for a computing device.
The processes and displays presented herein are not inherently related to any particular computing device or other apparatus. Various general purpose systems may be used with programs in accordance with the teachings herein, or it may prove convenient to construct a more specialized apparatus to perform the desired method. The desired structure for a variety of these systems will appear from the description below. In addition, embodiments of the present invention are not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein. In addition, it should be understood that operations, capabilities, and features described herein may be implemented with any combination of hardware (discrete or integrated circuits) and software.
Use of the terms “coupled” and “connected”, along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” may be used to indicate that two or more elements are in direct physical or electrical contact with each other. “Coupled” my be used to indicated that two or more elements are in either direct or indirect (with other intervening elements between them) physical or electrical contact with each other, and/or that the two or more elements co-operate or interact with each other (e.g. as in a cause and effect relationship).
It should be understood that embodiments of the present invention may be used in a variety of applications. Although the present invention is not limited in this respect, the devices disclosed herein may be used in many apparatuses such as in the transmitters and receivers of a radio system. Radio systems intended to be included within the scope of the present invention include, by way of example only, cellular radiotelephone communication systems, satellite communication systems, two-way radio communication systems, one-way pagers, two-way pagers, personal communication systems (PCS), personal digital assistants (PDA's), wireless local area networks (WLAN), personal area networks (PAN, and the like), wireless wide are networks (WWAN) and Mesh networks.
Some embodiments of the present invention provide pilot techniques that are robust to interference and fading. In an embodiment of the present invention, the transmitter may vary pilot tone locations. The receiver may feed back the pilot tone locations to the sender according to the receiver's interference or fading profile. The pilots may be located at the tones with higher channel gains and less interference. For simplicity, the variation of pilot location may be parameterized by a step size of shift in order to maintain the relative spacing between pilot tones. In another embodiment of the present invention, the sender may change the pilot locations across OFDM symbols. Three advantages are thus provided: First, it prevents the pilot from constantly getting lost. Second, frequency diversity may be obtained. Finally, frequency resolution of channel tracking may be increased. Thus, a major benefit of the techniques provided herein is that the number of pilot tones may be reduced and the throughput may be increased.
In current wireless standards, such as but not limited to, 802.11a/g and TGnSync proposal to IEEE 802.11n, pilot locations are constant. Namely, a fixed set of subcarriers are assigned for pilots for the whole system all the time. This causes a problem in the presence of platform noise and co-channel narrow band interference. Because the receiver can't gain synchronization signal from the pilots for a certain period, the receiver's phase-locked loop (PLL) loses synchronization with the transmitter, and thus the packets may get lost. Measurements demonstrate that mobile platform noise is significant factor of WLAN performance degradation. Further, the performance of 802.11g loses 20 dB if one of the pilot tones is corrupted by platform noise or co-channel interference. In contrast, the corruption of data tones with the same interference power only causes a 10 dB lose.
Turning now to the figures, FIG. 1 at 100 shows a table illustrating the SNR loss 105 with narrow band interference 110. Interference frequency of 2439.1875 MHz 120 corresponds to one of the pilot tones in channel 6. On the other hand 2438 MHz 115 interferer only interferes with data tones.
Although not limited in this respect nor limited to 802.11g, the pilot tones in 802.11g band may occupy 13*4*0.3125=16.25 MHz in the 76 MHz spectrum of the ISM band. Therefore, there is a 16.25/76=21% chance for any narrow band interferer to corrupt pilot tones. Experiments show that there are more than a few narrowband interferers in 802.11g band, making a corrupted pilot a highly probable reality. Simulation results for 802.11n also demonstrate that loss of pilot tones due to fading significantly degrades performance by 0.3-6 dB.
In FIG. 2 at 200, the effect of fading on pilot tones is illustrated. The four pilots may be located on two tones and two spatial channels 205 and 210. Since there is typically some correlation in the received signal of two antennas due to space limitation and platform coupling, it is likely that a pilot tone experiences destructive fading on both spatial channels. Conventional receivers estimate frequency offset using at least two pilots on two different tones per OFDM symbol. If one pilot tone is faded, the frequency offset can't be estimated using the current OFDM symbol (even though the estimation may be still possible using multiple OFDM symbols including the previous).
Turning now to FIG. 3, the effect of platform noise and co-channel interference 305 on pilot tones are illustrated generally at 300 as a function of frequency 315. Platform noise 305 consists of various harmonics of various clocks in the host computer of the receiver while co-channel interference 305 includes other narrow band signals (such as Bluetooth) located in the same wireless local area network (WLAN) band. These two impairments may corrupt the pilot tone 310 and cause synchronization problems.
Depicted in FIG. 4 at 400, is an embodiment of the present invention which provides two robust pilot schemes, one with feedback and the other without. A first technique that provides a receiver first select optimal locations is shown graphically as a function of frequency 430, with mobile pilots selected as shown at 425 and data tones and DC tones at 410, 420 and 415 respectively. The criterion that may be used is as follows—although the present invention is not limited to these criterions.
1. Avoid strong interference or platform noise at the receiver.
2. Avoid deep fading. Criterions 1 and 2 improve the signal to noise plus interference of the received pilots.
3. Subjected to criterions 1 and 2, maximize the spacing between pilot tones at the two ends of the band. This optimizes the estimation of frequency offset because the estimation of frequency offset improves as the spacing between pilots increases.
4. Subjected to criterions 1 and 2, equalize the interspacing between any two adjacent pilots. This optimizes channel tracking because even pilot locations are desired for channel interpolation. However, if this conflicts with criterion 3 when there are more than two pilots, criterion 3 has a higher priority.
After the pilot locations are selected, the receiver may feed back the pilot locations in a piggy back way such as sending the feedback by an ACK packet. In order to reduce the amount of feedback, a set of pilot location patterns may be defined and the receiver may only feed back the index of the selected location pattern. The transmitter may include the feed back index in the transmitting frame to acknowledge received pilot pattern and indicate the current pilot pattern. An example is shown in FIG. 5, where four pilot patterns 505, 510, 515 and 520 are defined. The receiver selects one out of four choices in favor of its channel/interference status, where each pattern has two pilot locations. The feedback only needs two bits. If the feedback is not available, the transmitter may employ a default pattern. It is understood that the present invention is not limited to four choices and this is provided for illustrative purposes only.
In another embodiment of the present invention, in the case that feedback is not available; the transmitter may vary the pilot location across OFDM symbols. Even though the pilots are lost in some symbols, they are recovered in the other symbols. Two examples are illustrated in FIG. 6, at 600 in frequency 605 and 610 vs. OFDM Symbol 615 and 620; although the present invention is not limited to these examples.
In yet another embedment of the present invention spreading the pilot across frequency and time may be extended to frequency, time, and antenna. For systems employing multiple transmit antennas, the pilot may be spread out across antennas to avoid destructive fading at the receiver. As illustrated in FIG. 7, generally as 700, spreading pilots across frequency, time, and antenna may be utilized and have been considered in the Institute for Electronic and Electrical Engineers (IEEE 802.16e) standard. However, the purpose is for channel estimation and not phase tracking of PLL (or phase offset estimation). The requirement in the IEEE 802.16e standard is to uniformly spread out the pilot in frequency, time, and antenna but in an embodiment of the present invention no uniformity is required. In the following description as it relates to FIG. 7, pilots sent by antenna 2 are depicted in the shading shown as 770 and pilots sent by antenna 1 are depicted in the shading shown as 765,
In an embodiment of the present invention the, pilots of the same OFDM symbol may separate apart to help phase offset estimation as shown in (d) 760 (pilot sent by antenna 1 is shown as 755, and pilot sent by antenna 2 is illustrated at 750). In figure (a) 705 (pilot sent by antenna 1 is shown as 722, and pilot sent by antenna 2 is illustrated at 720), (b) 710 (pilot sent by antenna 1 is shown as 730, and pilot sent by antenna 2 is illustrated at 725) and (d) 760 (pilot sent by antenna 1 is shown as 755, and pilot sent by antenna 2 is illustrated at 750), the pilots may occupy the one subcarrier for two (or more) adjacent OFDM symbols, because space-time coding in the data subcarriers may use the same subcarrier across adjacent OFDM symbols. This is not required for phase offset estimation as shown in (c) 715 (pilot sent by antenna 1 is shown as 745, and pilot sent by antenna 2 is illustrated at 740). In addition, two pilots for the same OFDM symbol may be sent by the same or different antennas for phase offset estimation as shown in (c) 715 and (d) 760.
While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

## Claims

1. An apparatus, comprising:
a receiver capable of selecting optimal pilot locations and providing feedback of said pilot locations to a transmitter in communication with said receiver; and
wherein said optimal pilot locations are selected by locations that avoid strong interference or platform noise at said receiver;
wherein said optimal pilot locations are selected by locations that avoid deep fading; and
wherein said optimal pilot locations are selected by locations that maximize the spacing between pilot tones at the two ends of a wireless frequency band.

2. The apparatus of claim 1, wherein said receiver operates conforming to the Institute for Electronic and Electrical Engineers 802.11 standard.

3. The apparatus of claim 1, wherein said optimal pilot locations are selected by locations that equalize the interspacing between any two adjacent pilots.

4. The apparatus of claim 1, wherein the providing feedback of said pilot locations to a transmitter in communication with said receive is accomplished in a piggy back method.

5. The apparatus of claim 4, wherein said piggy back method is sending the feedback by an ACK packet.

6. The apparatus of claim 4, wherein said feedback is accomplished by defining a set of pilot location patterns and wherein said receiver only feeds back the index of the selected location pattern.

7. The apparatus of claim 6, wherein if said feedback is not available, said transmitter may employ a default pattern.

8. The apparatus of claim 6, wherein if said feedback is not available, said transmitter varies said pilot location across OFDM symbols.

9. A method, comprising:
selecting optimal pilot locations that avoid strong interference or platform noise at receiver; and
providing feedback of said pilot locations to a transmitter in communication with said receiver
selecting pilot locations that avoid deep fading; and
selecting pilot locations that equalize the interspacing between any two adjacent pilots.

10. The method of claim 9, wherein said receiver operates conforming to the Institute for Electronic and Electrical Engineers 802.11 standard.

11. The method of claim 9, further comprising selecting pilot locations that maximize the spacing between pilot tones at the two ends of a wireless frequency band.

12. The method of claim 9, wherein the providing feedback of said pilot locations to a transmitter in communication with said receive is accomplished in a piggy back method.

13. The method of claim 12, further comprising sending the feedback by an ACK packet as said piggy back method.

14. The method of claim 12, wherein said feedback is accomplished by defining a set of pilot location patterns and wherein said receiver only feeds back the index of the selected location pattern.

15. The method of claim 14, further comprising/employing a default pattern if said feedback is not available.

16. The apparatus of claim 14, further comprising varying said pilot location across OFDM symbols if said feedback is not available.