(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2006/0072529 A1
`Mujtaba
`(43) Pub. Date:
`Apr. 6, 2006
`
`US 20060072529A1
`
`(54)
`
`(76)
`
`METHOD AND APPARATUS FOR VARYING
`THE NUMBER OF PILOT TONES IN A
`MULTIPLE ANTENNA COMMUNICATION
`SYSTEM
`
`Inventor: Syed Aon Mujtaba, Watchung, NJ
`(US)
`Correspondence Address:
`RYAN, MASON & LEWIS, LLP
`13OO POST ROAD
`SUTE 205
`FAIRFIELD, CT 06824 (US)
`
`(21)
`(22)
`
`Appl. No.:
`Filed:
`
`11/223,775
`Sep. 9, 2005
`
`Related U.S. Application Data
`Provisional application No. 60/608,472, filed on Sep.
`9, 2004.
`
`(60)
`
`Publication Classification
`
`(51) Int. Cl.
`(2006.01)
`H04Q 7/24
`(52) U.S. Cl. ............................................ 370/338; 370/208
`
`ABSTRACT
`(57)
`Methods and apparatus are provided for varying the number
`of pilot tones in a multiple antenna communication system.
`Data is transmitted in a multiple antenna communication
`system by selecting a number of pilot tones to be employed
`to transmit the data; and transmitting an indication of the
`selected number of pilot tones in a preamble of a packet
`containing the data. Data is received in a multiple antenna
`communication system by receiving a preamble having an
`indication of a number of pilot tones embedded in the data;
`and processing the received data based on the indicated
`number of pilot tones. The indication of the selected number
`of pilot tones can be transmitted, for example, in a SIGNAL
`field of an exemplary IEEE 802.11 preamble. The number of
`pilot tones can be selected, for example, based on one or
`more of (i) a delay spread of a channel; (ii) the SNR at the
`receiver, or (iii) a number of antennas at a receiver.
`
`410 N
`
`
`
`
`
`420 N
`
`- DATA TONES
`
`s - - - PILOT TONES
`
`OFDM SYMBOL WITH 4 PILOTS
`
`FREQUENCY
`
`OFDM SYMBOL WITH 2 PILOTS
`
`FREQUENCY
`
`Exhibit 1010
`Panasonic v. UNM
`IPR2024-00364
`Page 1 of 7
`
`

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`Exhibit 1010
`Panasonic v. UNM
`IPR2024-00364
`Page 2 of 7
`
`

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`Patent Application Publication Apr. 6, 2006 Sheet 2 of 3
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`US 2006/0072529 A1
`
`
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`Exhibit 1010
`Panasonic v. UNM
`IPR2024-00364
`Page 3 of 7
`
`

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`Patent Application Publication Apr. 6, 2006 Sheet 3 of 3
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`US 2006/0072529 A1
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`Exhibit 1010
`Panasonic v. UNM
`IPR2024-00364
`Page 4 of 7
`
`

`

`US 2006/0072529 A1
`
`Apr. 6, 2006
`
`METHOD AND APPARATUS FOR VARYING THE
`NUMBER OF PILOT TONES IN A MULTIPLE
`ANTENNA COMMUNICATION SYSTEM
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`0001. The present application claims priority to U.S.
`Provisional Patent Application Ser. No. 60/608,472, filed
`Sep. 9, 2004, incorporated by reference herein.
`
`FIELD OF THE INVENTION
`0002 The present invention relates generally to multiple
`antenna wireless communication systems, and more particu
`larly, to phase and frequency offset estimation techniques for
`a multiple antenna communication system.
`
`BACKGROUND OF THE INVENTION
`0003 Multiple transmit and receive antennas have been
`proposed to provide both increased robustness and capacity
`in next generation Wireless Local Area Network (WLAN)
`systems. The increased robustness can be achieved through
`techniques that exploit the spatial diversity and additional
`gain introduced in a system with multiple antennas. The
`increased capacity can be achieved in multipath fading
`environments with bandwidth efficient Multiple Input Mul
`tiple Output (MIMO) techniques. A multiple antenna com
`munication system increases the data rate in a given channel
`bandwidth by transmitting separate data streams on multiple
`transmit antennas. Each receiver receives a combination of
`these data streams on multiple receive antennas.
`0004. In order to properly receive the different data
`streams, receivers in a multiple antenna communication
`system must acquire the channel matrix through training.
`This is generally achieved by using a specific training
`symbol, or preamble, to perform synchronization and chan
`nel estimation. The preamble helps the receiver (i) estimate
`the power of the received signal to set an automatic gain
`control (AGC) function; (ii) acquire the timing offset to
`perform optimal placement of a Fast Fourier Transform
`(FFT) window; (iii) estimate the frequency offset between
`the transmitter and receiver, and correct for the frequency
`offset prior to FFT demodulation; and (iv) estimate the
`channel transfer function to help demap the Quadrature
`Amplitude Modulation (QAM) symbols after the FFT has
`been performed.
`0005. In addition, a number of pilot tones are embedded
`in the OFDM data symbols to estimate the phase noise and
`residual frequency offset. Phase noise at the local oscillators
`of the transmitter and receiver creates a common phase error
`(CPE) at the FFT output that generally needs to be corrected
`for every OFDM symbol. Residual frequency offset at the
`input of the FFT also creates CPE.
`0006.
`In general, the accuracy of the CPE estimation
`increases with the number of pilots, thereby reducing the
`packet error rate, and increasing the reliability of the trans
`mission. A greater number of pilots, however, reduces the
`effective data rate, since actual data is now replaced by pilots
`(which are known at both the transmitter and the receiver).
`The number of pilots needed to meet a certain packet error
`rate (PER) at the receiver is a function of several parameters,
`Such as the delay spread of the channel, the signal to noise
`
`ratio (SNR) at the receiver, and the number of antennas at
`the receiver. If the channel has a low delay spread, then the
`frequency selectivity of the channel is low as well, and thus
`a fewer number of pilots are required. On the other hand, a
`larger number of pilots would be required for a channel
`exhibiting a larger delay spread. If the SNR at the receiver
`is low, larger number of pilots are needed to get an accurate
`estimate of the CPE. Likewise, if there are diversity anten
`nas present at the receiver, and the RF chains are fed from
`a single LO source, then Maximal Ratio combining (MRC)
`can be used at the receiver to improve the accuracy of the
`estimate of the CPE. Thus, the accuracy of the CPE estimate
`can be improved with diversity antennas and fewer pilots are
`needed to achieve the same level of performance.
`0007. A need therefore exists for methods and apparatus
`for varying the number of pilot tones in a multiple antenna
`communication system
`
`SUMMARY OF THE INVENTION
`0008 Generally, methods and apparatus are provided for
`varying the number of pilot tones in a multiple antenna
`communication system. According to one aspect of the
`invention, data is transmitted in a multiple antenna commu
`nication system by selecting a number of pilot tones to be
`employed to transmit the data; and transmitting an indication
`of the selected number of pilot tones in a preamble of a
`packet containing the data. The indication of the selected
`number of pilot tones can be transmitted, for example, in a
`SIGNAL field of an exemplary IEEE 802.11 preamble. The
`number of pilot tones can be selected, for example, based on
`one or more of (i) a delay spread of a channel; (ii) the SNR
`at the receiver, or (iii) a number of antennas at a receiver.
`0009. According to another aspect of the invention, data
`is received in a multiple antenna communication system by
`receiving a preamble having an indication of a number of
`pilot tones embedded in the data; and processing the
`received data based on the indicated number of pilot tones.
`0010. A more complete understanding of the present
`invention, as well as further features and advantages of the
`present invention, will be obtained by reference to the
`following detailed description and drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`0011 FIG. 1 is a schematic block diagram of a conven
`tional 802.11a/g transceiver;
`0012 FIG. 2 illustrates a typical packet format for an
`exemplary IEEE 802.11 or another IP-based OFDM system:
`0013 FIG. 3 illustrates an exemplary format of the
`preamble of FIG. 2 for IEEE 802.11n; and
`0014 FIG. 4 illustrates an exemplary OFDM symbol
`with four pilot tones and an exemplary OFDM symbol with
`two pilot tones.
`
`DETAILED DESCRIPTION
`0015 The present invention recognizes that the optimum
`number of pilots varies with operating parameters. Thus, the
`present invention provides methods and apparatus for vary
`ing the number of pilot tones to help maximize data transfer
`without compromising link robustness and reliability. In one
`exemplary implementation discussed below in conjunction
`
`Exhibit 1010
`Panasonic v. UNM
`IPR2024-00364
`Page 5 of 7
`
`

`

`US 2006/0072529 A1
`
`Apr. 6, 2006
`
`with FIG. 3, the number of pilots is signaled to the receiver
`in the SIGNAL field, or any such field that indicates to the
`receiver the parameters that are required to Successfully
`decode the payload. For example, other parameters could
`include data rate, coding rate and modulation level.
`0016 FIG. 1 is a schematic block diagram of a conven
`tional 802.11a/g transceiver 100. At the transmitter side 105,
`the information bits are first encoded at stage 110 and then
`frequency interleaved at stage 120. The encoded and inter
`leaved bits are then mapped onto Subcarriers (tones) at stage
`130 and form a frequency domain OFDM signal. The
`frequency domain OFDM signal is translated to the time
`domain by an inverse Fourier transform (IFFT) during stage
`130. At stage 140, the data is serialized and a guard interval
`is added to each OFDM symbol. Finally, a preamble includ
`ing training and signal fields is added during stage 145 at the
`beginning of each packet.
`0017. At the receiver side 150, the received signal is
`initially processed by the RF front end 155, and then the
`serial data is parallelized and the guard interval is removed
`at stage 160. The time domain signal is translated to the
`frequency domain using an FFT 170 and the subcarriers are
`demapped to encoded and interleaved bits. Meanwhile, the
`preamble is processed at stage 165. The interleaved bits are
`deinterleaved at stage 180 and decoded at stage 190 to
`provide the transmitted information bits.
`0018 FIG. 2 illustrates a typical packet format 200 for an
`exemplary IEEE 802.11 or another IP-based OFDM system.
`As shown in FIG.2, each packet 200 or PLCP Protocol Data
`Unit (PPDU) (for wireless LANs) comprises a preamble 210
`and a payload 220. Each preamble 210 contains all relevant
`information needed to decode the payload 220, such as
`synchronization information and a SIGNAL field. The pay
`load 220 contains data and one or more pilot tones.
`0.019
`FIG. 3 illustrates an exemplary preamble format
`300 for IEEE 802.11n. According to one aspect of the
`invention, the existing HTSignal field can be used to signal
`to the receiver the number of pilot tones that were employed.
`As shown in FIG. 3, the exemplary preamble 300 includes
`a legacy short training symbol 310, a legacy long training
`symbol 320, a legacy SIGNAL field 330, a high throughput
`(HT) SIGNAL field 340, a HT long training symbol 350 and
`a HT data field (payload) 360. Thus, the HT SIGNAL field
`340 can convey to the receiver the number of pilot tones. For
`example the HT SIGNAL field 340 can be encoded with a
`value indicating whether two, four, or six pilot tones were
`employed.
`0020 For example, some manufacturers have proposed
`using four pilot tones in 20 MHz and six pilot tones in 40
`MHz. Other manufacturers have proposed using only two
`pilot tones in 20 MHz and four pilot tones in 40 MHz.
`0021. The present invention recognizes that the optimum
`number of pilots varies with operating parameters. The
`various criteria used to select the number of pilot tones is
`application specific and outside the scope of the present
`invention. In general, the number of pilot tones can be
`selected, for example, based on the delay spread of the
`channel, or the SNR at the receiver, or the number of
`antennas at the receiver (or all of the above). In this manner,
`the present invention allows the increased accuracy provided
`by a greater number of pilots to be balanced against the
`
`resulting reduction in data rate. As previously indicated, a
`greater number of pilots increases the accuracy of the CPE
`estimation, thereby reducing the packet error rate, and
`increasing the reliability of the transmission. A greater
`number of pilots, however, reduces the effective data rate,
`since actual data is now replaced by the pilots (which are
`known at both the transmitter and the receiver).
`0022. From an efficiency perspective, the lowest number
`of pilots to achieve the desired robustness is desirable. In
`other words, from an efficiency perspective, data should be
`transmitted on as many tones as possible. It is noted that the
`position of the pilots are configured in advance. Thus, only
`the number of pilot tones, but not their position, need not be
`signaled to the receiver. The receiver simply selects one of
`several possible pilot patterns, which indicate number of
`pilots and their positions
`0023 The number of pilot tones must be signaled to the
`receiver, for example, in the SIGNAL field, either explicitly
`or implicitly. Implicit signaling for instance would be in the
`case of low modulation order QAM symbols, which typi
`cally operate at low SNR. In such a case, whenever a BPSK
`symbol is received, more pilots would be used, and when
`ever 64-QAM symbols are used, fewer pilots would be used.
`0024 FIG. 4 illustrates an exemplary OFDM symbol 410
`with four pilot tones and an exemplary OFDM symbol 410
`with two pilot tones.
`0025. It is to be understood that the embodiments and
`variations shown and described herein are merely illustrative
`of the principles of this invention and that various modifi
`cations may be implemented by those skilled in the art
`without departing from the scope and spirit of the invention.
`
`I claim:
`1. A method for transmitting data in a multiple antenna
`communication system, said method comprising the step of
`selecting a number of pilot tones to be employed to
`transmit said data; and
`transmitting an indication of said selected number of pilot
`tones in a preamble of a packet containing said data.
`2. The method of claim 1, wherein said indication of said
`selected number of pilot tones is transmitted in a SIGNAL
`field.
`3. The method of claim 1, wherein said preamble is an
`IEEE 802.11 preamble.
`4. The method of claim 1, wherein said number of pilot
`tones is selected based on a delay spread of a channel.
`5. The method of claim 1, wherein said number of pilot
`tones is selected based on the SNR at the receiver.
`6. The method of claim 1, wherein said number of pilot
`tones is selected based on a number of antennas at a receiver.
`7. A transmitter in a multiple antenna communication
`System, comprising:
`a memory; and
`at least one processor, coupled to the memory, operative
`tO:
`select a number of pilot tones to be employed to transmit
`said data; and
`transmit an indication of said selected number of pilot
`tones in a preamble of a packet containing said data.
`
`Exhibit 1010
`Panasonic v. UNM
`IPR2024-00364
`Page 6 of 7
`
`

`

`US 2006/0072529 A1
`
`Apr. 6, 2006
`
`8. The transmitter of claim 7, wherein said indication of
`said selected number of pilot tones is transmitted in a
`SIGNAL field.
`9. The transmitter of claim 7, wherein said preamble is an
`IEEE 802.11 preamble.
`10. The transmitter of claim 7, wherein said processor is
`further configured to select said number of pilot tones based
`on a delay spread of a channel.
`11. The transmitter of claim 7, wherein said processor is
`further configured to select said number of pilot tones based
`on the SNR at the receiver.
`12. The transmitter of claim 7 wherein said processor is
`further configured to select said number of pilot tones based
`on a number of antennas at a receiver.
`13. A method for receiving data in a multiple antenna
`communication system, said method comprising the steps
`of:
`receiving a preamble having an indication of a number of
`pilot tones embedded in said data; and
`processing said received data based on said indicated
`number of pilot tones.
`14. The method of claim 13, wherein said indication of
`said selected number of pilot tones is transmitted in a
`SIGNAL field.
`
`15. The method of claim 13, wherein said preamble is an
`IEEE 802.11 preamble.
`16. The method of claim 13, wherein said number of pilot
`tones is selected based on one or more of (i) a delay spread
`of a channel; (ii) the SNR at the receiver; or (iii) a number
`of antennas at a receiver.
`17. A receiver in a multiple antenna communication
`System, comprising:
`at least one receive antenna for receiving a preamble
`having an indication of a number of pilot tones embed
`ded in said data; and
`means for processing said received data based on said
`indicated number of pilot tones.
`18. The receiver of claim 17, wherein said indication of
`said number of pilot tones is transmitted in a SIGNAL field.
`19. The receiver of claim 17, wherein said preamble is an
`IEEE 802.11 preamble.
`20. The receiver of claim 17, wherein said number of pilot
`tones is selected based on one or more of (i) a delay spread
`of a channel; (ii) the SNR at the receiver; or (iii) a number
`of antennas at said receiver.
`
`Exhibit 1010
`Panasonic v. UNM
`IPR2024-00364
`Page 7 of 7
`
`