Exhibit 1

Case 2:23-cv-00202-JRG-RSP Document 81-2 Filed 07/15/24 Page 1 of 20 PageID #: 1639
(12) United States Patent 

Crilly, Jr. 

US007 177369B2 

US 7,177,369 B2 

Feb. 13, 2007 

(10) Patent No.: 

(45) Date of Patent: 

(54) 

(75) 

(73) 

(*) 

(21) 

(22) 

(65) 

(60) 

(51) 

(52) 

(58) 

(56) 

MULTIPATH COMMUNICATION METHODS 

AND APPARATUSES 

Inventor: William J. Crilly, Jr., Liberty Lake, 

WA (US) 

Assignee: Vivato, Inc., Portland, OR (US) 

Notice: Subject to any disclaimer, the term of this 

patent is extended or adjusted under 35 

U.S.C. 154(b) by 810 days. 

Appl. No.: 10/131,864 

Filed: Apr. 25, 2002 

Prior Publication Data 

US 2002/O159537 A1 Oct. 31, 2002 

Related U.S. Application Data 

Provisional application No. 60/287,163, filed on Apr. 

27, 2001. 

Int. C. 

H04K L/02 (2006.01) 

H04L 25/03 (2006.01) 

H04L 25/249 (2006.01) 

U.S. Cl. ....................................... 375/296,375/285 

Field of Classification Search ................ 375/267, 

375/279, 147, 150, 220, 228, 229, 231,346, 

375/347, 296,285; 370/203, 226, 229, 252 

See application file for complete search history. 

References Cited 

U.S. PATENT DOCUMENTS 

6,031,866 A * 2/2000 Oler et al. .................. 375,219 

6,052,412 A * 4/2000 Ruether et al. ... ... 375,231 

6,252,914 B1* 6/2001 Yamamoto ........ ... 375,296 

6,314, 135 B1 * 1 1/2001 Schneider et al. .......... 375/232 

6,314,147 B1 * 1 1/2001 Liang et al. ...... ... 375,346 

6,324.220 B1 * 1 1/2001 Sellars ....................... 375,296 

6,377,819 B1 * 4/2002 Gesbert et al. .......... 455/562.1 

6,445,750 B1* 9/2002 Chen et al. ................. 375,300 

6,567,040 B1* 5/2003. Sevaston ... ... 342,354 

6,611,511 B1* 8/2003 Schulz ...... ... 370,342 

6,665,308 B1* 12/2003 Rakib et al. ... ... 370,441 

6,674,808 B1* 1/2004 Griph et al. ... ... 375,285 

6,741,643 B1* 5/2004 McGibney .................. 375,229 

6,763,062 B1* 7/2004 Kohno et al. ............... 375,220 

6,792,049 B1* 9/2004 ... 375,285 

6,804,312 B1 * 10/2004 ... 375,347 

6,868,377 B1 * 3/2005 ... TO4/205 

6,870,515 B2 * 3/2005 Kitchener et al. .......... 343,853 

6,912.247 B2 * 6/2005 Miyashita et al. .......... 375,228 

(Continued) 

Primary Examiner Jean B. Corrielus 

(74) Attorney, Agent, or Firm Brooks & Cameron, PLLC 

(57) ABSTRACT 

Methods, apparatuses and systems are provided for identi 

fying at least one multipath transmission delay within a 

reverse path data signal, determining at least one forward 

path pre-equalization parameter based on the transmission 

delay, and modifying a forward path data signal based on the 

forward path pre-equalization parameter. A reverse link 

transmission is used to help characterize the multipath delays that may exist between communicating devices. The 

reverse path data signal can include a training sequence or 

other like known/substantially-known data that can be ana 

lyzed to detect transmission delays, especially delays that 

extend beyond established guard intervals. Such pre-equal ization techniques can be advantageously combined, for 

E. : 3. E. SE OSCOOK et C. tal. a. 2.i. example, with antenna - arrangements - - that Support transmis 

5,881,108 A * 3/1999 Herzberg et al. ........... 375,296 sity spatial division transmission, and/or other like 

5,881,363 A * 3/1999 Ghosh et al. ............... 725/125 echniques. 

5,930,267 A * 7/1999 Daneshrad et al. ......... 37Of 509 

6,023,242 A 2/2000 Dixon ........................ 342,359 91 Claims, 5 Drawing Sheets 

: to recor " 

------'20s 204 METWORK 

NRFACE BASE 

-202 STATION 

W 02 

200 TRANSCEIVER 

CP 

MECHANISM(s) co 210 

104 

08 / 

OFM 

PROCESSING CODED PRE DATA ECJALIZATION PROCESSING 

ANTENNA YY Y-302 | 204 

Y-304 

REWERSE 

Y-306 

310 LINK PROCESSING 

N-308 

 

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US 7,177,369 B2 

Page 2 

U.S. PATENT DOCUMENTS 

2002fOOO9058 A1* 

2002/0065047 A1* 

2002/00851 18 A1* 

2002.0089927 A1* 

2002fO165626 A1* 

2002fO191540 A1* 

1, 2002 

5, 2002 

T/2002 

T/2002 

11, 2002 

12, 2002 

Kelly et al. ................. 370,316 

Moose ....... ... 455.63 

Harris et al. ... 348/473 

Fischer et al. .... ... 370,229 

Hammons et al. ... ... TOOf 53 

Fujii et al. .................. 370,229 

2003/0058929 A1* 

2004/OO95907 A1* 

2004/O184521 A1* 

2005.0143132 A1* 

2005/02O7334 A1* 

* cited by examiner 

3, 2003 

5, 2004 

9, 2004 

6, 2005 

9, 2005 

Cox et al. ................... 375/15O 

Agee et al. .. 370,334 

Chen et al. .. 372,231 

Proctor et al. 455,561 

Hadad ........................ 370,203 

 

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U.S. Patent Feb. 13, 2007 Sheet 1 of 5 US 7,177,369 B2 

100 

BASE 

STATION 

- 109 

t s 

106 22, t 

BASE 

STATION 

TRANSCEIVER 

208 

MECHANISM(S) 

22, 2 104. 

 

 

 

 

 

 

 

 

 

 

 

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U.S. Patent Feb. 13, 2007 Sheet 2 of 5 US 7,177,369 B2 

PRE 

EQUALIZATION 

REVERSE 

LINK 

PROCESSING 

CROSS 

CORRELATION 

e 

TIME SAMPLES OFDM 

GUARD 

INTERVAL 

 

 

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U.S. Patent Feb. 13, 2007 

RECEIVED 

SIGNAL 

PHASE 

600 N 

FOURIER 

TRANSFORM 

MAGNITUDE/ 

DELAY 

Sheet 3 of 5 US 7,177,369 B2 

500 -1 

-4. 

ANTENNANUMBER 

- - - - - - - - - - - - - - - t---------- 

INTERVAL 

FFT BIN 

 

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U.S. Patent Feb. 13, 2007 Sheet 4 of 5 US 7,177,369 B2 

700 TN 

702 704 708 

N 7O6 

802 N 

POWER 

2 ///b/ 

a- - -74 - - -/ 

O ) 4-24-21 FREQUENCY -- 7- - 7 

1. ^ HEADER/ 

TT PREAMBLE €- - - -> OFDMTONES 

 

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U.S. Patent Feb. 13, 2007 Sheet S of 5 US 7,177,369 B2 

90 

BASE STATION DEVICE TRANSCEIVER CPE DEVICE TRANSCEIVER 

FFT 

 

 

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US 7,177,369 B2 

1. 

MULTIPATH COMMUNICATION METHODS 

AND APPARATUSES 

RELATED APPLICATIONS 

This application is related to and hereby claims priority to 

provisional Patent Application Ser. No. 60/287,163, filed 

Apr. 27, 2001, and titled “Improved Multipath Communi 

cation Methods And Arrangements', and which is included 

herein by reference. 

TECHNICAL FIELD 

This invention relates to data communications, and more 

particularly to wireless communication systems, apparatuses 

and related methods that Support wireless communication in 

a multipath signal propagation environment. 

BACKGROUND 

Computers and other like devices can be interconnected in 

a variety of ways to allow data to be communicated between 

them. One of the most common ways to provide such data 

communication is through a wired network. Wired networks, 

Such as, e.g., wide area networks (WANs) and local area 

networks (LANs) tend to have a high bandwidth and there 

fore can be configured to carry digital data at high data rates. 

One obvious drawback to wired networks is that a user's 

movement is constrained since the computer needs to be 

physically connected to the network. Thus, for example, a 

user of a portable computer will need to remain near to a 

wired network junction to stay connected to the wired 

network. 

An alternative to wired networks is a wireless network 

that is configured to Support similar data communications 

but in a more accommodating manner. Here, the user of a 

portable device will be free to move around a region that is 

supported by the wireless network. A well known example 

of a wireless network is a cellular telephone network. 

Indeed, in the past, cellular telephone modems have proven popular for use with portable laptop computers and other 

like devices, despite their relatively low bandwidth. 

In the future it is expected that higher bandwidth wireless 

networks will become more popular, especially in creating 

metropolitan area networks (MANs) in which users, i.e., 

subscribers, have the ability to freely move their portable communicating devices around within a coverage area. 

However, as the subscribers move about freely there will not 

always be a clear or otherwise unobstructed communication 

path between a transmitting network resource and the 

receiving device. For example, one or more buildings, 

roadways, bridges, or other man-made objects may interfere 

with the transmission of signals in the wireless network. So 

too may non-manmade objects, such as, e.g., mountains, 

hills, valleys, trees, etc., reduce the effectiveness of the 

wireless network. 

For these and other reasons, there is need for improved 

methods and apparatuses that can Support wireless commu 

nications within Such varying environments. 

SUMMARY 

In accordance with certain aspects of the present inven 

tion, improved packet Switched wireless data communica 

tion systems, networks, apparatuses, and related methods are 

provided. 

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65 

2 

The above stated needs and/or others are satisfied, for 

example, by a method in accordance with certain exemplary implementations of the present invention that includes iden 

tifying at least one multipath transmission delay within a 

reverse path data signal, determining at least one forward 

path pre-equalization parameter based on the transmission 

delay, and modifying a forward path data signal based on the 

forward path pre-equalization parameter. 

In certain further implementations, the method also 

includes receiving the reverse path data signal over at least 

one reverse transmission path and transmitting the modified 

forward path data signal over at least one forward transmis 

sion path. Here, it is preferred that the reverse transmission 

path be substantially reciprocal to the forward transmission 

path, for example, at a given moment in time between a base 

station device and a consumer premise equipment device. 

The reverse link transmission is essentially used to help characterize the multipath delays that may exist between the 

communicating devices. As such, in certain implementations 

the reverse path data signal includes a training sequence or 

other like known/substantially-known data that can be ana 

lyzed to detect transmission delays, especially delays that 

extend beyond established guard intervals. 

In accordance with certain other implementations, such 

pre-equalization techniques can be advantageously com 

bined, for example, with antenna arrangements that Support 

transmission diversity, spatial division transmission, and/or 

other like techniques. 

BRIEF DESCRIPTION OF THE DRAWINGS 

FIG. 1 is a block diagram depicting a communication 

system operating in a multipath environment, in accordance 

with certain exemplary implementations of the present 

invention. 

FIG. 2 is a block diagram depicting certain functional 

blocks within a base station device and a CPE device, for 

example, as shown in FIG. 1, in accordance with certain 

exemplary implementations of the present invention. 

FIG. 3 is a block diagram depicting certain functions 

within a transceiver functional block, for example, as shown 

in FIG. 2, in accordance with certain exemplary implemen 

tations of the present invention. 

FIG. 4 is a graph depicting the magnitude of selected 

cross-correlation time samples illustrating, for example, that 

the multipath environment has caused certain delays with a 

transmitted signal, in accordance with certain exemplary implementations of the present invention. 

FIG. 5 is a graph depicting the phase of a transmitted 

signal as received by various antenna elements within an 

antenna array, in accordance with certain exemplary imple 

mentations of the present invention. 

FIG. 6 is a graph depicting the results of certain calcula 

tions showing the Fourier transform magnitude/delay for 

various FFT bins, in accordance with certain exemplary implementations of the present invention. 

FIG. 7 is an illustrative diagram showing data that can be 

transmitted from the CPE device to the base station device 

to help characterize the multipath communication environ 

ment, in accordance with certain exemplary implementa 

tions of the present invention. 

FIG. 8 is an illustrative diagram further showing data that can be transmitted from the CPE device to the base station 

device to help characterize the multipath communication 

environment, in accordance with certain exemplary imple 

mentations of the present invention. 

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US 7,177,369 B2 

3 

FIG. 9 is a block diagram depicting a frequency domain 

based pre-equalizer process Suitable for use in a pre-equal 

ization function, for example, as in FIG. 3, in accordance 

with certain exemplary implementations of the present 

invention. 

DETAILED DESCRIPTION 

Multipath Propagation Delays: 

Wireless communication systems often operate under 

conditions that lead to multipath propagation of transmitted 

signals. Multipath propagation is primarily the result of 

reflections and diffraction from objects in the coverage area 

of the transmitting and receiving antennas. 

High data rate communication systems may be subject to 

detrimental intersymbol interference caused by such multi 

path propagation or fading. As the symbol rate increases, for 

example, multipath fading from one symbol affects the next 

symbol, making it difficult, if not impossible, for the receiver 

to identify the desired (next) symbol and therefore determine 

its information content. 

Multipath fading may be observed by examining the 

frequency domain characteristics of the communication 

channel. Here, for example, multipath fading will often 

cause the channel transfer function to have frequency dependent responses that alter the amplitude and phase 

modulation content of a received signal. Symbols having 

high information content may therefore be difficult to sepa rately identify. This can be true even for small amplitude 

and/or phase shifts that exist across the modulation band 

width of the signal. 

Traditional Modulation Techniques for Dealing with Multi 

path Propagation Delays: 

Various modulation techniques have been used indepen 

dently and/or in combination to ameliorate or avoid 

intersymbol interference. 

By way of example, certain modulation techniques include equalization techniques at the receiving node. Such 

equalization techniques are usually designed to calculate a 

channel correction in the time and/or frequency domain, and 

in response significantly correct for any associated multipath 

fading. Correct symbol identification decisions may also be 

used to estimate the channel at Subsequent times. One of the 

drawbacks to this solution is that it places an additional 

burden on the receiving node to provide Such equalization capabilities. 

Additionally such equalization techniques are usually 

limited by their capability to estimate and correct the chan 

nel. However, some difficulty may be caused by the chang ing nature of the multipath channel. For example, the 

computation of multipath propagation correction parameters 

that is made at a receiver may need to be repeated if/when 

the channel changes in someway. Examples of dynamic 

multipath propagation changes include the movement of the 

receiver?antenna, trees moving in the wind, nearby vehicular 

traffic, etc. Such changes may have time scales ranging from 

a few milliseconds to a few seconds, depending on envi 

ronmental conditions. Unfortunately, implementing Such 

complex equalization techniques in a receiver may not 

always be feasible. 

Other modulation techniques that have been developed include the use of Orthogonal Frequency Division Multi 

plexing (OFDM) to significantly increase the symbol time. 

This prevents a delayed symbol from deleteriously effecting 

a subsequent symbol. Basically, OFDM operates by coding 

a series of bits into a set of modulated orthogonal Sub 

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4 

carriers, thus producing a symbol to be transmitted. The 

transmitted OFDM symbol typically includes a guard inter 

val (GI) that contains a cyclic extension of the basic symbol. When a GI is employed, it is typically designed to be longer than the expected multipath propagation delay spread of the 

channel. 

OFDM is usually generated using an inverse fast Fourier 

transform (IFFT) process. Consequently, OFDM is then 

received using a fast Fourier transform (FFT) process. Techniques that establish the correct timing of FFT blocks 

and channel correction techniques for the GI based on the 

multipath fading are well known. One common technique, 

for example, is to use a complex correction value for each 

OFDM sub-carrier. The magnitude of the complex correc 

tion value can be obtained from the inverse of the sub-carrier 

magnitude, and the phase can be determined from a linear 

regression of the phase of pilot Sub-carriers and other signals in training symbols. 

One of the drawbacks to the OFDMGI is that the system 

may still be susceptible to multipath delays that are beyond 

the GI. 

In still other systems, spread spectrum techniques may be 

used to encode a bit into a long coded sequence having a 

narrow autocorrelation function. A spread spectrum receiver 

would then include, for example, a RAKE receiver with 

cross correlators, which are configured to identify and 

handle individual multipath signals arriving at the receiver. 

Such spread spectrum techniques, however, may not prove 

to be economically viable solutions in every situation. 

Other modulation techniques that can reduce the affects of 

multipath delays on a communication system include ideas 

Such as rapidly changing channels to produce an adequate 

signal at a different time. For example, see “Multicarrier 

Modulation For Data Transmission: An Idea Whose Time 

Has Come' by John A. C. Bingham, published in IEEE 

Communications Magazine, May 1990. 

In general, the various techniques described above utilize 

time and/or frequency diversity to overcome potential mul 

tipath transmission problems. The corresponding modula 

tion systems, therefore, tend to spread a given amount of 

information over a band of frequencies or in time to provide improved performance within the multipath environment. 

Traditional Antenna Techniques for Dealing with Multipath Propagation Delays: 

In addition to the above techniques, various antenna 

designs may be used to further ameliorate the effects of 

multipath propagation and fading. Certain systems, for 

example, employ what is commonly referred to as spatial 

division techniques. Here, transmitting antennas having nar 

row beam widths are employed to reduce multipath propa 

gation caused by reflections from various objects by not 

illuminating those objects in the first place. Thus, usually 

only local diffraction and retro-reflection from objects more 

directly behind the receiving antenna, for example, remain 

to produce multipath fading. 

Similarly, a receiving antenna with a narrow beam width 

may also help to reduce some remaining multipath fading. 

Here, however, one disadvantage is that the intended receiv 

ing node must determine or otherwise be specifically con 

figured to receive signals from a direct path while avoiding certain multipath propagation signals. 

Spatial division techniques may be performed with single 

or multielement transmitting and/or receiving antennas. 

Some multielement antennas, for example, are also referred 

to as phased array antennas. 

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US 7,177,369 B2 

5 

Multielement transmitting antennas are advantageous for 

several reasons. For example, multielement transmitting 

antennas distribute required average signal power over a 

number of separate low power amplifiers, which tends to 

reduce the complexity of each of the associated power amplifiers. 

Furthermore, multielement transmitting and receiving 

antennas often have the advantage of reducing or eliminat 

ing antenna pointing requirements. Phased arrays, for 

example, are capable of providing flexible antenna patterns 

and simultaneously transmitting and receiving in several 

directions using the same antenna aperture. Due to this 

flexibility and the other advantages, phased arrays are very 

helpful in the reception and amelioration of signals that are 

affected by multipath fading. 

Antenna spatial diversity is yet another Subset of antenna 

techniques used to reduce the effects of multipath fading. 

Here, for example, multiple transmitting antennas that are 

physically spaced apart, e.g., by more than a wavelength, 

have the ability to produce different amplitude and phase 

shifts over the frequency band that is used. Computations 

based on the received signals from the antennas may be used 

to establish one or more paths between the transmitting and 

receiving sites. In addition, the signal-to-noise ratio (SNR) 

may be improved by combining signal paths coherently, 

and/or errors reduced in multiple post-detection decoders. 

Coherent combining techniques basically overlap spatial 

division techniques, because coherent combining enhances 

antenna patterns in some directions and minimizes the 

pattern in other directions. 

Indeed, antenna processing techniques may also be 

employed to actually take advantage of multipath propaga 

tion. By way of example, in an extension of the spatial diversity techniques, multiple transmitting antennas having 

physical spacing of more than a few wavelengths may be 

used to illuminate scattering areas with slightly different 

wave fronts. As such, the separate transmit signals diffract or 

reflect off the illuminated areas and are picked up by a 

receiver antenna array. 

In effect, the scattering regions become somewhat poor, 

yet at times very large, pseudo-lenses that can be intention 

ally illuminated by the distant transmitting antennas. A large 

pseudo-lens has the ability to resolve closely spaced distant 

points, as determined by the Aires Disk of the effective 

pseudo-lens aperture. Receiver signal processing may then 

act to correct the phase and amplitude errors of Such large pseudo-lenses. 

Thus, in one example, a number of separate possible paths 

between the transmitter and receiver may be determined by the rank of a matrix establishing the paths between the 

transmitter and receiver antennas. This ranking may be 

determined by the richness of the scattering paths and the 

number of receiving and transmitting antennas. When the 

ranking is high, it is often possible to provide high spectral 

efficiency as measured in bits per second per HZ. 

The antenna signal processing techniques described 

above are limited, however, by a fundamental capability of 

the average-power-limited transmitter to adequately trans 

mit the signals to the receiver. Here, Shannon's Capacity 

Theorem determines the bit rate capacity of each of the 

effective multipath channels between the transmitter and 

receiver. For example, in a multipath channel with equal 

amplitude direct and reflecting paths, an identical minimum 

SNR is required for each received path. Hence, adequate 

received signal power needs to be available to maintain the 

SNR above this minimum value. 

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6 

The SNR may be improved by increasing the average 

transmitter power, by directing greater power at the receiver 

and at the scattering object, and/or by gathering more power 

at the receiver, e.g., by using a larger receiver antenna 

aperture. In the case of a larger receiver antenna aperture, a 

multielement large aperture receive antenna may be used in 

combination with signal processing to separately extract 

data from each multipath propagation. At the transmitting 

site, single element antennas or multielement transmit anten 

nas with array signal processing may be used to direct power 

in specific directions. 

Conclusion Regarding Such Traditional Techniques: 

Although the various techniques described above greatly improve the capacity of a wireless system/network to carry 

information, there are remaining difficulties. 

By way of example, channel equalization of high symbol 

rate systems typically requires many taps in a transversal 

filter, and/or requires long IFFT/FFTs to be calculated. 

Another example relates to the use of a GI or the like. Low 

symbol rate systems, such as, e.g., OFDM symbols, can be 

very robust for a pre-determined multipath delay spread by 

providing an appropriate guard interval (e.g., a guard inter 

Val that is longer than the longest expected multipath delay 

spread). However, Such potentially long GIs essentially set 

the time duration of the FFT processing step in OFDM. 

Thus, a long GI tends to reduce capacity. A long FFT 

processing tends to increase latency. Long FFTs also usually require data to be placed in large blocks, and/or Zero-padded, 

which adds overhead. 

Spread spectrum systems, such as those described above, 

tend to lose the orthogonality of sequences when multipath fading is present, thereby reducing the capacity of the 

system. 

While transmit spatial division antenna processing is 

Sometimes capable of eliminating highly-delayed multipath 

fading, it may not be capable of eliminating retro-reflected 

multipath fading. 

Spatial receive diversity techniques usually increase the 

reliability, and potentially the capacity of the channel. How 

ever, spatial diversity techniques have the disadvantage that 

significant antenna signal processing is required at the 

receiver. Indeed, such receive antenna signal processing usually requires separate receive antennas and receivers for 

each path. 

Consequently, none of the various techniques provide a 

simple and cost effective solution to multipath fading. For 

this reason, it may be advantageous to combine a plurality 

of these techniques and/or others within one system. For 

example, the combination of OFDM and other techniques 

has been shown, see, e.g., U.S. Pat. Nos. 5.282,222, and/or 

5,029,184. 

The Wi-LAN transmitter described by U.S. Pat. No. 

5,282,222 uses a measurement of the differential channel 

amplitude and estimates the differential channel phase based 

on the assumption of a minimum phase multipath charac 

teristic. A training signal and GI are not required. However, multiple point channel equalization is not possible within a 

single OFDM sub-channel. This limits the maximum delay spread to approximately the symbol duration. 

The OFDM equalizer in the receiver taught by U.S. Pat. 

No. 5,029,184 requires two symbols to be transmitted per desired symbol. 

Note that the above-described techniques/systems do not 

provide equalization for a large delay spread when the radio 

chipsets and associated physical layer standards do not 

include amelioration of these long delay spreads. 

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US 7,177,369 B2 

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In accordance with certain aspects of the present inven 

tion, improved methods, apparatuses and systems are pro 

vided to overcome these and/or other shortcomings of 

conventional wireless systems/networks. 

Introducing Novel Pre-Equalization Techniques: 

As signal processing costs are reduced, techniques involv 

ing greater computation are usually preferred over those 

requiring more transmitter power. Furthermore, as part of a 

design process it is usually preferred that any required 

additional processing complexity be implemented at the 

node/location which is least Subject to complexity restric 

tions, such as a base station or the like in a wireless 

communication system/network. With these and/or other 

design considerations in mind, in accordance with certain 

aspects of the present invention equalization techniques are 

provided for use at a transmitting node and configured to 

perform pre-equalization that Substantially reduces 

unwanted effects associated with multipath fading, including retro-reflected propagation delays. Such pre-equalization 

techniques are discussed in greater detail in later sections. 

As is well known, many materials are electromagnetically isotropic, which is a property resulting from Symmetry in 

their associated permittivity and permeability tensors. The 

Lorentz Reciprocity Theorem applies to Such materials. 

Refraction and dielectric reflection from materials therefore 

often show reciprocity, or equivalence of forward and reverse channel characteristics. Diffraction and reflection are 

inherently reciprocal due to the minimal media affecting the 

electromagnetic wave. Thus, reciprocity can be used to 

determine channel characteristics that are used while pre equalizing a transmitted path. The use of a reciprocal 

channel is very useful, for example, when Time Division 

Duplex (TDD) channels are implemented. 

In accordance with certain implementations of the present invention, a wireless system/network is provided which is 

capable of operating in a multipath environment. For 

example, improved methods and apparatuses are provided 

for use in the wireless system/network to significantly minimize the complexity of signal processing required at the 

receiving node. Here, the receiving node may include a 

single receive antenna or multiple receive antennas. 

As described above, Some conventional techniques use 

OFDM signal sequences to correct for multipath effects due 

to multipath delay spread shorter than the guard interval. A 

new wireless system/network is described herein, which 

uses a novel combination of new and known techniques to 

further improve multipath propagation delay handling and to 

also reduce the complexity of signal processing required at 

the receiver. 

Although several OFDM-based system examples are pro 

vided herein, it should be recognized that the various meth 

ods and apparatuses are fully adaptable/applicable to other 

communication protocols, standards, Schemes, and/or sys 

tems. Moreover, Such pre-equalization and like methods and 

apparatuses may also be advantageously combined with 

other existing techniques to improve communication within 

a multipath environment. 

With this in mind, in certain exemplary implementations 

a signal processing technique combines or otherwise 

includes transmitter antenna spatial division processing, 

antenna diversity processing, and/or receiver OFDM sub 

carrier equalization, with OFDM pre-equalization. The 

exemplary OFDM pre-equalization technique basically acts 

within the frequency band of OFDM sub-carriers to pre equalize the multipath channel to handle multipath delay 

spread greater than the delay protection provided by con 

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8 

ventional OFDM signal processing techniques, such as, e.g., 

the OFDMGI. Thus, the receiver can still correct multipath 

effects that are less than the GI. 

The pre-equalization techniques use training sequences, 

which are contained within a reverse link received OFDM 

burst, to specifically adjust associated transmission signals/ 

parameters applied to subsequent OFDM transmissions back 

towards that particular node. In accordance with certain 

implementations, for example, the training sequences may 

or may not be part of the header and equalization sequences 

provided for in a conventional chipset or standard. Further 

more, in certain implementations substantially fewer OFDM 

symbols may be transmitted, than may be in more traditional 

systems. 

An Exemplary Multipath Environment: 

Reference is now made to FIG. 1, which is a block 

diagram depicting an exemplary communication system 100 

operating in a multipath environment. Here, a base station 

device 102 is configured to exchange data with at least one 

customer premise equipment (CPE) device 104 through an 

antenna 108 and over a wireless communication link. Base 

station device 102 is further operatively configured to send 

and receive data via one or more networks 106. In this 

manner, data can be received through network 106 by base 

station 102, and then forwarded over a wireless communi 

cation link to CPE device 104 using antenna 108. Likewise, 

in reverse order, CPE device 104 can transmit data over a 

wireless communication link to base station device 102 

through antenna 108, and base station 102 may then forward 

the received data to other device(s) via network 106. 

As illustrated in this example, a transmission 109, which 

is generated by base station device 102, is emitted from 

antenna 108. Transmission 109 takes a direct path to CPE 

device 104. However, given the physical Surroundings, 

transmission 109 radiates and is reflected or diffused, for 

example, by one or more objects 110 that are closer to 

antenna 109. This causes a multipath propagation signal 111 

to be directed towards CPE device 104. Similarly, some of 

the electromagnetic energy in transmission 109 can be 

retro-reflected back towards CPE device 104, as multipath propagation signal 115, by one or more objects 114 located 

further away from antenna 109. Other types of multipath propagation signals may also be created by various other 

objects within the coverage field of transmission 109. 

CPE device 104 is representative of any type of device 

that may be capable of sending and receiving data over a 

wireless communication link. By way of example, CPE 

device 104 may include a computer, a portable computer, a 

personal digital assistant (PDA), a mobile communication 

device. Such as, e.g., a mobile telephone or other like 

handheld device. 

Exemplary Base Station and CPE Nodes: 

FIG. 2 is a block diagram depicting functional units 

within base station device 102 and CPE device 104, in 

accordance with certain exemplary implementations of the 

present invention. Here, base station device 102 includes a 

network interface 202 that is configured to send and receive 

information in the form of data over one or more networks 

106. Network interface 202 is operatively coupled to a 

transceiver 204. Transceiver 204 is also operatively coupled 

to logic 206 and antenna 108. Transceiver 204 is configured 

to send and receive information in the form of data to/from 

network interface 202 and/or logic 206. Transceiver 204 is 

further configured to send and receive information in the 

form of data to/from antenna 108. In this example, antenna 

108 is representative of any number of transmitting and/or 

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receiving elements provided in any suitable type of antenna 

configuration. For example, in certain implementations, 

antenna 108 includes a phased array antenna having a 

plurality of elements. Logic 206 is provided in this example 

to represent any applicable additional logic/circuitry that 

may be used to implement all or part of the methods and 

apparatuses of the present invention. In certain implemen 

tations, logic 206 may be integrated within transceiver 204. 

For this reason, logic 206 is depicted within a dashed line 

box. 

CPE device 104 includes a corresponding transceiver 208, 

which is operatively coupled to an antenna 200, one or more 

CPE mechanisms 210 and logic 212. Here, as with antenna 

108, antenna 200 is representative of any applicable type of 

antenna configuration. In accordance with certain imple 

mentations of the present invention, for example, antenna 

200 includes an omnidirectional antenna. Although depicted 

outside of the representative CPE device box, it should be 

understood that antenna 200 may be internal and/or external 

of an actual CPE device. 

Transceiver 208 is configured to receive data from and 

provide data to CPE mechanism(s) 210. CPE mechanism(s) 

210 is representative of one or more mechanisms associated 

with the functionality of CPE device 104. Thus, for example, 

in certain implementations CPE device 104 includes a 

computer or like computing device. As such, CPE mecha 

nism(s) 210 may take the form of a processor that is coupled 

to memory and operatively configured to perform functions 

in accord with one or more computer instructional programs, 

e.g., application programs. In still other implementations, 

CPE device 104 may include a mobile telephone, in which 

case, CPE mechanism(s) 210 may include a signal processor and other circuitry for converting digital audio data into an 

applicable analog audio signal and converting an analog 

audio signal into corresponding digital data. 

Logic 212 is provided in this example to represent any 

applicable additional logic/circuitry that may be used to 

implement all or part of the methods and apparatuses of the 

present invention. In certain implementations, logic 212 may 

be incorporated in transceiver 208. For this reason, logic 212 

is depicted within a dashed line box. 

Providing a Pre-Equalization Capability at an Exemplary 

Base Station Node: 

Attention is now drawn to the block diagram in FIG. 3, which depicts exemplary functions provided within in trans 

ceiver 204, in accordance with certain implementations of 

the present invention. 

FIG. 3 is a functional block diagram, wherein a set of 

coded data to be transmitted from base station device 102 to 

CPE device 104 is further prepared, pre-equalized and 

eventually transmitted. Here, for example, a set of coded 

data may comprise ninety-six bits of binary data, which is 

applied in pairs to (or otherwise processed by) OFDM 

processing block 302. 

In this example, OFDM processing block 302 is config 

ured to generate corresponding Quadrature Phase Shift 

Keying (QPSK) modulation values for each of forty-eight 

OFDM Sub-carriers. These OFDM modulated Sub-carriers 

may, for example, be output by an OFDM IFFT (not shown). 

The OFDM modulated sub-carriers are then provided to a 

pre-equalization block 304. 

Pre-equalization block 304 is configured to modify one or more of the OFDM modulated sub-carriers based on infor 

mation from a reverse link processing block 308. Reverse 

link processing block 308 is basically configured to identify multipath propagation delays and/or problems for packets 

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transmitted from CPE device 104 to base station device 102. 

One exemplary way to recognize multipath delays is to have 

CPE device 104 transmit a known sequence of data/tones. 

The received signal can then be compared or contrasted with 

the known sequence and differences detected, the differ 

ences being at least partially associated with the multipath 

communication environment. The differences, or informa 

tion derived from such differences can then be used within 

pre-equalization block 304 to modify, in Some manner, as 

applicable, the OFDM modulated sub-carriers. The modified 

ODFM modulated sub-carriers are then provided to antenna 

processing block 306, wherein they are further processed 

and eventually transmitted using one or more antennas 108. 

Those skilled in the art will recognize that reverse link 

processing block 308 and pre-equalization block 304 may be 

implemented in a variety of ways. In certain exemplary 

implementations, for example, reverse link processing block 

308 may include a tapped transversal filter, or FFT/multi 

pliers/IFFT filter of length on the order of the longest 

expected multipath delay. The complex coefficients of the 

tapped filter, or the complex multiplier values, may be 

derived from the complex cross-correlation of the known 

OFDM signal sequences and the sequences received from 

the reverse path signal 310. 

The result of a sample cross-correlation output is shown 

in FIG. 4, which shows a graph 400 illustrating the magni 

tude of cross-correlation time samples plotted as line 402. 

Here, the period covered by an exemplary OFDMGI is also 

depicted. Line 402 illustrates that a multipath delay spread 

that is greater than the OFDM guard interval should be used. 

Thus, pre-equalization block 304 may be configured, in 

this example, to invert the measured channel response so 

that the pre-equalization and the actual Subsequently 

induced channel response errors Substantially cancel out. As 

a result, transceiver 208 in the receiving CPE device will 

receive a signal that is essentially significantly free of 

troublesome multipath signals that are longer than the GI. 

Transceiver 208, therefore, will be able to reproduce and 

output the set of coded data, perhaps only with correctable 

errors. Such errors can be overcome/corrected is by further 

employing error control coding techniques, such as, e.g., 

CRC and other like techniques. 

In certain systems, it may be likely that the reverse link 

bursts transmitted to base station device 102 fall below the 

noise level in the full bandwidth of the system. However, 

header signals, e.g., SIGNAL and pre-SIGNAL headers in 

IEEE 802.11a can be detected at lower levels. For example, 

the cross-correlation of the known header with signals received on two phased arrays would usually allow for 

improved sensitivity detection. This detection may be nec 

essary for pre-equalization of the channel and for diversity summing to take place. An OFDM signal, for example IEEE 

802.11a, may be detected at lower levels by using cross 

correlation with a known PILOT tone sequence. If fewer 

pilots are transmitted, then power may be concentrated in the 

pilots (provided, regulatory limitations are not exceeded). 

The offset of the cross correlation of known headers is to 

reduce the effective bandwidth, essentially establishing an 

effect similar to a RAKE receiver receiving a direct 

sequence waveform. 

Continuing with this example, the Subsequent multipath 

estimate can then be used to estimate antenna pointing and/or determine the near-optimal Summation of signals 

received by the two phased array antennas. The base station 

device 102 produces a directed signal to CPE device 104 

using Substantially the same antenna and pre-equalized 

settings/parameters. If TDD is used, then the channel can be 

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assumed to be reciprocal for durations (coherence time) of 

approximately 10 ms. The 10 ms duration can be determined 

in a fixed wireless system by passing vehicles, moving 

people, blowing trees, and/or other interruptions nearby or 

within the communication path/region. 

An important aspect in certain implementations is that no 

attempt need be made to pre-equalize multipath delays less 

than the provided GI of the OFDM signal. Only those 

multipath signals that have an impact on the signal through 

OFDM inter symbol interference need be pre-equalized for 

at base station 102. Moreover, base station 102 can be 

further configured to post equalize the signals received from 

CPE device 104 based on the results of the reverse link 

processing. 

The presence of fewer tones in the header and pilots potentially complicates the process of determining 

prefpost —equalization channel coefficients and/or other 

like parameters. This is due, for example, to the inadequate 

resolution in frequency, and thus inversely in time, which 

leads to aliasing as described in later sections. 

The typical GI used in an OFDM system is usually only 

a small portion of the overall symbol time. For example, an 

OFDM symbol time of ten microseconds results from a 

sixty-four subcarrier OFDM system operating in a 6.4 MHZ 

bandwidth. Thus, in this example, a GI may be chosen to be one microsecond to reduce overhead. 

The delay spread is the time difference between the first 

and last arriving (significant) multipath propagated signal. In 

this example, the first and last arriving multipaths must 

arrive within one microsecond of each other to allow for 

successful conventional OFDM reception. As mentioned, 

however, some retro-reflected multipath propagated signals 

may produce delays that exceed this GI. For example, 

retro-reflected signals propagating from an object 114 (see 

FIG. 1) located at a distance of about one km behind the 

receiving CPE device 104 will result in an approximate 6.6 

microsecond delay spread. Thus, the resulting composite 

signal cannot be handled by a conventional OFDM trans 

ceiver using a one microsecond GI. 

When a phased array antenna 108 is used at the base 

station node to further ameliorate the effect of multipath, 

antenna element coefficients may be determined by evalu 

ating, for example, the discrete Fourier transform (DFT) of 

the signals received on the elements of the phased array. Referring next to graph 500 in FIG. 5, the effect of a direct 

and multipath signal can be seen by line 502, which plots the 

received signal phase for various numbered antenna ele 

mentS. 

After computation, for example, using an FFT to derive 

the angular distribution of the received signals, and cross 

correlated the signals with known sequences, the delay and 

angle of arrival of the direct and multipath signal may be 

determined. Graph 600 in FIG. 6 shows the results of such 

calculations in the form of plotted lines 602 and 604 

corresponding to the Fourier transform magnitude/delay for 

various FFT bins. Also shown is the corresponding delay 

period covered by a traditional GI 606. 

In some situations the direct and multipath signals may be 

combined to actually improve the signal-to-noise ratio 

(SNR). However, in the exemplary situation shown in FIG. 

6, the additional delay of a multipath signal 604 exceeds the 

GI used in the OFDM modulation. Knowing that the channel 

is essentially reciprocal allows a pre-equalization of the 

signals to be applied to the phased array during Subsequent 

forward path transmissions from base station device 102 to 

CPE device 104. Alternatively, the pre-equalized signals 

may be applied at only one angle. In this case, pre-equal 

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ization may not be desired/necessary. In some cases two 

delays, in the same direction, may be determined in the 

cross-correlation/reverse link processing, as a result of retro 

reflected multipath propagation. In which case, pre-equal 

ization can then be employed to substantially correct the 

channel. 

In those systems where only a few transmit antennas are 

used, transmit antenna diversity and like techniques may also be implemented in place of phased array transmission/ 

processing. 

For example, reverse link processing block 308 can be 

configured to determine or otherwise identify the best 

antenna to be used for reception based on measurements of 

the reverse link. Here, SNR, cross-correlation of the known 

sequences, or other measurements may be made. As multi 

path propagated signals longer in duration than the guard 

interval may be present, a determination can be made as to 

which antenna to select for the transmission of the pre equalized signals. Thus, the combination of pre-equalization 

and diversity help to reduce the multipath signals at CPE 

device 104 to a level that allows the detection of user data, 

perhaps with correctable errors. 

In accordance with certain further aspects of the present invention, transmit diversity may be implemented using 

simultaneous transmission of coded user data on diversity 

antennas. In systems where many transmit elements are 

used, transmit antenna diversity may be implemented in 

combination with phased array processing. 

For example, base station device 102 may be configured 

to generate two simultaneous beams illuminating separate 

areas of a multipath environment. The transmit signals applied to the two antennas can be selectively adjusted in 

phase and amplitude to produce a signal at CPE device 104 

that is Substantially free of multipath propagated signals 

greater in delay than the GI. If a strong composite signal is 

received still containing a relatively large delay spread, then 

pre-equalization techniques may be applied by base station 

device 102, thereby driving the two beams to ameliorate the 

long delay spread of one or both of the multipaths. The 

system may further be configured to include antenna pat 

terns other than a dual beam approach. For example, the 

transmitted pattern may be determined by an incident 

reverse path power pattern. 

An example of known headers that may be used to Support training sequences can be found in the IEEE 802.11a stan 

dard. IEEE 802.11a provides short and long sequences 

totaling a duration of about 8 us. The 8 LS may be used to 

ascertain delay spread to approximately 8 us delay. The 

known sequences have Sufficient low frequency energy per 

burst (here, low being between about 100 kHz and 125 kHz. 

/s LS resolution) to determine fine variations in the complex channel response. 

As described, traditional OFDM systems rely on the GI to 

eliminate multipath effects during transmission of the tones 

in the OFDM signal. Longer delays require longer GIs. The 

techniques shown here use the long and short sequences to 

perform channel estimation, and decision feedback equal ization, following the headers to equalize the channel for 

delays that are greater than 1/Af, where Afis the subcarrier 

(in OFDM) spacing. 

Some Exemplary Training Sequences: 

As described, CPE device 104 is configured to transmit a 

signal or signals back to base station device 102 over the 

reverse link transmission path. One or more of these trans 

missions may include training sequences that are designed to 

allow reverse link processing block 308 (see FIG. 3) to 

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determine what if any changes need to be made in pre equalizing the signals transmitted on the forward link trans 

mission path from base station device 102 to CPE device 

104. 

The training sequences preferably include either Substan- 5 

tially known or perfectly known training sequences. The 

training sequences in certain implementations, for example, 

may be included in the data or payload section of the OFDM 

symbols. In other implementations, known portions of a 

header or preamble may be used as the training sequence. 10 

The resulting training sequence signals may take a number 

of modulated forms, such as, e.g., spread spectrum signals, 

OFDMA signals, or other like signals. 

One exemplary implementation includes a training 

sequence within a physical layer PDU in an IEEE 802.11(a) 15 

system as illustrated in FIG. 7. Here, a frame 700 includes 

a preamble portion 702, a signal portion 704, and a payload 

portion 706. Payload portion 706 includes a training 

sequence 708 in the form of OFDM data/symbols. Prefer 

ably, training sequence 708 is adequately long enough to 20 

serve its purpose for the intended system and the OFDM 

data/symbols are not repeated. 

An illustrative waterfall representation is shown in FIG.8. 

Here, power, time and frequency are plotted in a first graph 

802 along with a corresponding constellation map (Q-I) in a 25 

second graph 804 illustrating the trajectory of the training 

sequence for an exemplary 16 QAM system. Here, the 

training sequence includes unique symbols identified by 

blocks 806 lettered A, B, C, D, E, F, G, and H, for example. 

Exemplary Techniques for Dealing with Aliasing: 30 

As described earlier, aliasing may occur, for example, 

when too few is symbols are transmitted. Aliasing may be 

dealt with by estimating on which aliased signal set the delay 

that needs to be prefpost-equalized actually exists, and 

making corrections accordingly. Thus, for example, if sev 

eral potential aliased delays are found and in amongst them 

lies the actual delay to be prefpost-equalized against, then an 

iterative procedure can be followed wherein guesses are 

made, parameters set, and signals pre-equalized and trans 

mitted. When the correct (actual) signal set is found and 

attempted to be equalized, then CPE device 104 will suc 

cessfully receive the transmission and reply accordingly. If 

after transmitting a pre-equalized signal no reply (e.g., an 

acknowledgement) is received then it can be assumed that 

the signal set that was selected is only an aliased signal set. 

Subsequent transmission attempts with other selected signal 

sets (or regions) being pre-equalized can then be made. 

Preferably error correction techniques will also be employed 

to help CPE device 104 to process the transmitted signals. 

35 

40 

45 

50 

Exemplary Low Bandwidth Reverse Link: 

In accordance with certain further aspects of the present invention, it was recognized that in certain implementations 

it may be possible to use an IEEE 802.11a transmission on 

the reverse link to base station device 102 at a bit rate much 55 

less than the minimum 6 Mbps in the current standard. This 

may prove useful, for example, when CPE device 104 

operates a much lower power levels than base station device 

102 (which may operate at 1 watt, for example). Here, a 

slower speed reverse link may actually work better anyway 60 

due to shadow fading or otherwise poor link budget. 

Thus, CPE device 104 can be configured to selectively 

make changes to the transmission process by backing out the 

effects of an interleaver, a FEC convolutional coder, and/or 

scrambler that may be included, for example, within trans- 65 

ceiver 208. Furthermore, the same or other logic within CPE 

device 104 may be configured to force a constant phase shift, 

14 

e.g., QPSK symbol over a long number of OFDM symbols, 

e.g., 16. Within transceiver 204 of base station device 102, 

the GI can then be removed and a long FFT performed on 

the compressed time interval due to GI removal. The long 

FFT length my be, e.g., 16*64 point=1024 point, wherein 

the bandwidth is /16 of the 64 point FFT, resulting in an SNR 

improvement of about 16, i.e., 10 log 16–12.04 dB. 

The systems above may be used with any OFDM modu 

lated CPE devices. For example, by allowing for a slow 

phase rotation during the 16 symbol interval, OFDMA may 

be implemented. This would allow for more than one reverse 

link to be present at a time, thereby improving the reverse 

link capacity. If QAM modulation is used, for example, as 

opposed to QPSK, then slower phase rotation may be used. 

This would allow more of the 16 bins per 64bins to be used. 

The data is provided in the tones in the usual way, but with 

a different phase, amplitude (QAM) per tone (of the 64 

tones). This coding can be performed, for example, by 

applicable computer instructions within the logic of CPE 

device 104. This allows for the applicable power control, 

QAM modulation mapping and virtually all other “normal' 

operations to appear in the resulting modulation. This pro 

vides the ability to deploy IEEE 802.11a, for example, client 

devices and Subsequently provide infrastructure that is 

capable of receiving the weak reverse link signals from Such 

client devices. 

Exemplary Frequency Domain Pre-Equalization Tech 

niques: 

In accordance with certain further implementations of the 

present invention, the pre-equalization techniques described 

herein may, for example, take advantage of frequency 

domain equalization techniques such as those described by 

Van Acker, et al., in an article titled “Per Tone Equalization 

For DMT-Based Systems, as published in IEEE Transac 

tions. On Communications, Vol. 49, No. 1, January 2001, and 

which is incorporated herein by reference. Van Acker et al., 

teach that a receiver can benefit from certain frequency domain equalization techniques. 

Here, the Van Acker et al. equalization techniques are 

adapted for use in pre-equalizing signals in the transmitting 

portion of a base station or like device/node. An example of 

a frequency domain based pre-equalizer process 900 suitable 

for use in pre-equalization process 304 (see FIG. 3) is shown 

in the block diagram of FIG. 9. 

In this example, frequency domain based pre-equalizer 

process 900 is shown to include, within the base station 

device transceiver, a pre-equalization processing block 304 

having its output coupled to an antenna processing block 

306, similar to that shown in FIG. 3. Here, however, pre-equalization processing block 304 includes several 

FFTs and IFFTs that are arranged to allow equalization 

parameters to be operatively applied to signals transformed 

from the time domain to a corresponding frequency domain. 

More particularly, several FFTs 902 are provided to receive 

signals that are to be transmitted. FFTs 902 convert these 

time domain signals, each of which is associated with a 

particular OFDM tone, into corresponding frequency 

domain signals. The outputs from FFTs 902 are provided to 

a scaling process 908 (or other like process) along with the 

desired/calculated channel coefficients or like parameters 

from a reverse link processing block 308 (not shown). 

Scaling process 908 modifies each tone as needed in accor 

dance with the measured/estimated channel characteristics. 

The scaled signals output by scaling process 908 are still 

frequency domain signals. To convert the scaled signals 

back in to a corresponding time domain, the scaled signals 

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are then input to IFFTs 904. IFFTs 904 convert the scaled 

signals into corresponding time domain scaled signals. 

These time domain scaled signals are then selectively input 

to a longer (e.g., 64 input/output) FFT 906 that is used to 

complete the Sub-band frequency domain equalization. In 

this illustrative diagram, each input to FFT 906 is switched 

Such that it selects one of the time domain scaled signals 

output by an IFFT 904. The 64 selected scaled tone signals 

output by IFFT 904 are then provided to antenna processing 

block 306 for further processing and eventual transmission. 

During transmission, the multipath environment alters the 

transmitted signals as represented by a Summation process 

912 that includes channel noise, for example, within the 

transmitted signals. The environmentally modified signals 

are received by the transceiver in the CPE device, wherein 

the signals may be stored in a memory device 914 and then 

provided to an FFT 916. Here, FFT 916 would be a longer (e.g., 64 input/output) FFT configured to process/convert the 

received signals. For example, FFT 916 can provide a one 

point FIR correction for each received tone signal. 

In this implementation, the frequency domain processing 

provided above essentially acts a Sub-band equalizer in the 

transmitting node (here, the base station device). The GI and 

other one point FIR correction is then provided in the 

receiver (here, the CPE device). Another aspect of this type of system is that power may be set to be equal in each of the 

Sub-bands, allowing regulatory rules, such as FCC rules, to 

be satisfied. In this manner, the power may be substantially 

flattened over the spectrum. 

Using Reduced Spectral Power Density Transmissions: 

The power in each sub-carrier in the above example may 

be held substantially constant. A modification to the system 

involves the determination of a worst case path loss over the 

OFDM tones, for example, through the use of reciprocal 

measurements. Then the highest allowed power spectral 

density for that particular sub-carrier is used, while the 

power spectral density for other tones is reduced to allow the 

other tones to sustain the same level of QAM, QPSK, BPSK, 

etc. By reducing the power of the other tones, an improve 

ment may be had with respect to the capacity of the system 

as a result of lower interference being generated. Thus, for 

example, doing so could allow the power amplifier to carry 

additional channels for the same PEP level. 

The requisite power spectral density needed to establish a 

level of QAM can be determined, for example, by path loss 

estimation and the knowledge of the reverse link power level 

transmitted on the reverse link. Thus, logic operating on the 

CPE device may assist in the determination. 

One exemplary method involves establishing a reverse 

link, having the CPE device provide information to the base 

station device regarding the power levels being used on the 

reverse link. Next, reciprocal path loss measurements are 

made by the base station device and a link budget for a 

particular QAM level is determined. Then variations in 

channel path loss can be used to set/reset minimum/maxi 

mum power levels for a particular QAM level, e.g., 16 

QAM. Sub-carrier/channel equalization is then provided by the 

base station device and the signals are transmitted to the 

CPE device. The CPE device then performs single point FIR 

equalization to decode the QAM (e.g., 16 QAM) tones. Then 

a decoder process corrects the errors caused, for example, by 

the tones that are below the QAM threshold. 

Further Extensions of the Above Methods and Apparatuses: 

As described and shown, cross-correlation processing, 

FFT processing and filtering may be implemented in a 

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variety of ways. For example, it is possible to combine 

OFDM FFT processing and pre-equalization filtering. 

Cross-correlation may be best performed in the frequency 

domain using the previously calculated DFTs of the known 

OFDM signal sequences. The resulting calculations reduce 

to the calculation of a single FFT and a set of multiplica tions. Many other signal processing techniques may also be 

used. 

Other implementations of this invention may be derived 

from the disclosed contents herein. By way of example, a 

practitioner may implement a high data rate communication 

system using separately calculated multipaths, with all of the 

paths transmitting on the same frequency. Another imple 

mentation may use receiver OFDM channel equalization for 

delays greater than the guard interval, thus improving per 

formance. 

In still other implementations, a reverse channel quality 

indicator, and/or the measured forward channel impulse 

response, may be transmitted on the reverse path when 

channel reciprocity is not available. Such a system may be 

useful when Frequency Division Duplex or Half Frequency 

Division Duplex is used. 

In Frequency Division Duplex systems, short training 

signals may be periodically transmitted using Time Division 

Duplex on each of the frequencies used for Frequency Division Duplex. Multiple RF signals, operating on the same 

and different frequencies, may also be simultaneously trans 

mitted and received. Furthermore, the operation of the 

described wireless system on the forward path may be 

similarly implemented in a wireless system on the reverse 

path. 

An Example of Channel Measurement and Correction: 

Certain exemplary 64-point FFT OFDM systems have a 

first tone measured on the reverse link to have a normalized 

amplitude slope from about 0.4 to about 0.25 and a phase 

shift from about 10 to about 100 degrees. The measurement 

can be performed using a 256-point FFT, for example. As 

Such, eight samples are available to determine channel 

characteristics. Pre-equalization, for example, can then be 

applied on the forward link in the frequency domain using 

a 2.5 to 4 normalized amplitude and -10 to -100 degree slope across the eight frequency domain samples. A second 

OFDM tone can be calculated in a similar way. 

Conclusion 

Although some preferred implementations of the various 

methods and arrangements of the present invention have 

been illustrated in the accompanying Drawings and 

described in the foregoing Detailed Description, it will be 

understood that the invention is not limited to the exemplary implementations disclosed, but is capable of numerous 

rearrangements, modifications and Substitutions without 

departing from the spirit of the invention as set forth and 

defined by the following claims. 

What is claimed is: 

1. A method comprising: 

identifying at least one multipath transmission delay 

within a reverse path data signal received from a 

receiving device; 

determining at least one forward path pre-equalization 

parameter based on said at least one transmission delay; 

and 

modifying a forward path data signal that is to be trans 

mitted to the receiving device based on said at least one 

forward path pre-equalization parameter, where said 

modifying includes selectively setting different trans 

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mission power levels for at least two Orthogonal Fre 

quency Division Multiplexing (OFDM) tones in said 

forward path data signal. 

2. The method as recited in claim 1, further comprising: receiving said reverse path data signal over at least one 

reverse transmission path. 

3. The method as recited in claim 2, further comprising: transmitting said modified forward path data signal over 

at least one forward transmission path. 

4. The method as recited in claim 1, wherein said reverse 

path data signal includes at least one type of data selected 

from a group of different types of data comprising Orthogo 

nal Frequency Division Multiplexing (OFDM) data and 

Quadrature Phase Shift Keying (QPSK) data. 

5. The method as recited in claim 1, wherein said modified 

forward path data signal includes at least one type of data 

selected from a group of different types of data comprising 

Orthogonal Frequency Division Multiplexing (OFDM) data 

and Quadrature Phase Shift Keying (QPSK) data. 

6. The method as recited in claim 5, wherein said modified 

forward path data signal includes Sub-carrier pre-equalized 

OFDM data. 

7. The method as recited in claim 6, further comprising: generating corresponding Quadrature Phase Shift Keying 

(QPSK) modulation values based on said sub-carrier 

pre-equalized OFDM data. 

8. The method as recited in claim 4, wherein identifying said at least one multipath transmission delay includes: 

identifying at least one multipath transmission delay 

spread that is greater than a guard interval. 

9. The method as recited in claim 1, wherein said reverse 

path data signal includes identifiable training data. 

10. The method as recited in claim 9, further comprising: comparing said identifiable training data to a local version 

of said training data to identify said at least one 

multipath transmission delay within said reverse path data signal. 

11. The method as recited in claim 3, wherein modifying 

said forward path data signal based on at least one forward 

path pre-equalization parameter further includes: 

Selectively inverting arid applying a measured channel 

response to said forward path data signal to signifi 

cantly reduce Subsequent induced deleterious channel 

response changes to said modified forward path data 

signal during transmission over said at least one for 

ward transmission path. 

12. The method as recited in claim3, wherein said at least 

one reverse transmission path is Substantially reciprocal to 

said at least one forward transmission path. 

13. The method as recited in claim 1, wherein identifying said at least one multipath transmission delay, determining said at least one forward path pre-equalization parameter, 

and modifying said forward path data signal are performed 

by a transmitting device. 

14. The method as recited in claim 13, wherein said 

transmitting device includes a base station device that is 

operatively configured for use in a wireless communication 

system. 

15. The method as recited in claim 13, further comprising: using at least one transmitting device receive antenna 

operatively coupled to said transmitting device to 

receive said reverse path data signal over at least one 

reverse transmission path from the receiving device. 

16. The method as recited in claim 15, wherein said 

receiving device includes a customer premise equipment 

(CPE) device that is operatively configured for use in a 

wireless communication system. 

10 

15 

25 

30 

35 

40 

45 

50 

55 

60 

65 

18 

17. The method as recited in claim 16, wherein said CPE 

device includes at least one device selected from a group of 

devices comprising a computer, a portable computer, a 

personal digital assistant (PDA), and a mobile communica 

tion device. 

18. The method as recited in claim 15, wherein said at 

least one transmitting device receive antenna includes at 

least one type of antenna selected from a group of Suitable 

types of antennas comprising at least one omnidirectional 

type of antenna, at least one beam type of antenna, at least 

one phased array type of antenna, and at least one adaptive 

antenna. 

19. The method as recited in claim 15, wherein said 

transmitting device is operatively coupled to a plurality of 

first device receive antennas. 

20. The method as recited in claim 13, further comprising: receiving said reverse path data signal over at least one 

reverse transmission path; and 

causing said transmitting device to post equalize at least 

a portion of said received reverse path signal. 

21. The method as recited in claim 15, wherein determin 

ing said at least one forward path pre-equalization parameter 

based on said at least one transmission delay further 

includes: 

determining at least one angle of arrival of said reverse 

path data signal with respect to said at least one 

transmitting device receive antenna. 

22. The method as recited in claim 15, further comprising: using at least one receiving device transmit antenna 

operatively coupled to said receiving device and con 

figured to transmit said reverse path data signal over 

said at least one reverse transmission path to said 

transmitting device. 

23. The method as recited in claim 22, wherein a plurality of receiving device transmit antennas are operatively 

coupled to said receiving device and configured to transmit 

said reverse path data signal over said at least one reverse 

transmission path to said transmitting device. 

24. The method as recited in claim 22, further comprising: causing said receiving device to transmit at least two 

reverse path data signals over said at least one reverse 

transmission path to said transmitting device. 

25. The method as recited in claim 24, wherein said at 

least two reverse path data signals include Orthogonal Frequency-Division Multiple Access (OFDMA) data. 

26. The method as recited in claim 22, further comprising: causing said receiving device to transmit said reverse path 

data signal over said at least one reverse transmission 

path to said transmitting device at a bit rate less than 

about 6 Mbps. 

27. The method as recited in claim 22, further comprising: causing said receiving device to transmit corresponding 

receiving device transmission power level information 

to said transmitting device; and 

causing said transmitting device to adjust at least one 

transmission power level associated with said forward 

path data signal based on said receiving device trans 

mission power level information. 

28. The method as recited in claim 13, further comprising: using at least one transmitting device transmit antenna 

operatively coupled to said transmitting device to trans 

mit said modified forward path data signal over at least 

one forward transmission path to the receiving device. 

29. The method as recited in claim 28, wherein said at 

least one transmitting device transmit antenna is operatively configured to provide transmission diversity when transmit 

ting said modified forward path data signal. 

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30. The method as recited in claim 28, wherein said at 

least one transmitting device transmit antenna is operatively configured to provide spatial division transmission utilizing 

a spatial division technique when transmitting said modified 

forward path data signal as a narrow signal to reduce 

multipath propagation. 

31. The method as recited in claim 28, wherein said at 

least one transmitting device transmit antenna includes at 

least one type of antenna selected from a group of Suitable 

types of antennas comprising at least one omnidirectional 

type of antenna, at least one beam type of antenna, at least 

one phased array type of antenna, and at least one adaptive 

antenna. 

32. The method as recited in claim 28, further comprising: setting at least one antenna pointing parameter associated 

with said at least one transmitting device transmit 

antenna based on said at least one forward path pre equalization parameter. 

33. The method as recited in claim 28, further comprising: setting at least one phased array antenna transmission 

directing parameter associated with said at least one 

transmitting device transmit antenna based on said at 

least one forward path pre-equalization parameter. 

34. The method as recited in claim 28, further comprising: Selectively transmitting said modified forward path data 

signal at one angle with respect to said transmitting 

device transmit antenna. 

35. The method as recited in claim 28, further comprising: Selecting said at least one transmitting device transmit 

antenna from a plurality of transmitting device transmit 

antennas that are each operatively coupled to said 

transmitting device. 

36. The method as recited in claim 35, further comprising: Selectively transmitting a plurality of beams using two or 

more transmitting device transmit antennas. 

37. The method as recited in claim 36, wherein each of 

said transmitted plurality of beams is selectively adjusted in 

phase and amplitude to reduce multipath affects when 

received by said receiving device. 

38. The method as recited in claim 28, further comprising: using at least one receiving device receive antenna opera tively coupled to said receiving device and configured 

to receive said modified forward path data signal over 

said at least one forward transmission path from said 

transmitting device. 

39. The method as recited in claim 38, wherein a plurality of receiving device receive antennas are operatively coupled 

to said receiving device and configured to receive said 

modified forward path data signal over said at least one 

forward transmission path from said transmitting device. 

40. The method as recited in claim 1, wherein determining said at least one forward path pre-equalization parameter 

based on said at least one transmission delay further 

includes: 

iteratively selectively identifying different forward path pre-equalization parameters over time to operatively 

identify and Subsequently ignore aliased multipath 

delays within said reverse path data signal. 

41. The method as recited in claim 1, wherein determining said at least one forward path pre-equalization parameter 

based on said at least one transmission delay further 

includes: 

Sub-band equalizing said forward path data signal using corresponding frequency domain reverse path data. 

42. A system comprising: 

a first device operatively configured to connect to at least 

a second device over a wireless communication path, 

10 

15 

25 

30 

35 

40 

45 

50 

55 

60 

65 

20 

said first device having logic that is configured to detect 

a multipath transmission delay in at least one reverse 

path data signal transmitted by said second device, 

determine at least one forward path pre-equalization 

parameter based on said detected multipath transmis 

sion delay, and pre-equalize a forward path data signal 

that is to be transmitted to said second device based on 

said at least one forward path pre-equalization param 

eter, said first device further comprising: 

at least one first device antenna; and 

at least one transceiver operatively coupled to said at least 

one first device antenna and said logic, said at least one 

transceiver being configured to receive said reverse 

path data signal over at least one reverse transmission 

path using said at least one first device antenna and 

provide said reverse path data signal to said logic, and 

transmit said forward path data signal from said logic 

over at least one forward transmission path; 

wherein said logic is further configured to cause said 

transceiver to selectively establish different transmis 

sion power levels for at least two Orthogonal Fre 

quency Division Multiplexing (OFDM) tones in said 

forward path data signal based on said at least one 

forward path pre-equalization parameter. 

43. The system as recited in claim 42, wherein said 

reverse path data signal includes at least one type of data 

selected from a group of different types of data comprising Orthogonal Frequency Division Multiplexing (OFDM) data 

and Quadrature Phase Shift Keying (QPSK) data. 

44. The system as recited in claim 42, wherein said 

forward path data signal includes at least one type of data 

selected from a group of different types of data comprising Orthogonal Frequency Division Multiplexing (OFDM) data 

and Quadrature Phase Shift Keying (QPSK) data. 

45. The system as recited in claim 44, wherein said 

forward path data signal includes Sub-carrier pre-equalized 

OFDM data. 

46. The system as recited in claim 45, wherein said logic is further configured to produce corresponding Quadrature 

Phase Shift Keying (QPSK) modulation values based on 

said sub-carrier pre-equalized OFDM data. 

47. The system as recited in claim 43, wherein said logic detects said multipath transmission delay by identifying at 

least one multipath transmission delay spread that is greater 

than a guard interval. 

48. The system as recited in claim 42, wherein said 

reverse path data signal includes identifiable training data. 

49. The system as recited in claim 48, wherein said logic is further configured to compare said identifiable training 

data to a non-multipath delayed version of said training data 

to detect said multipath transmission delay. 

50. The system as recited in claim 43, wherein said logic is configured to selectively invert and apply a measured 

channel response to said forward path data signal to signifi 

cantly reduce Subsequent induced deleterious channel 

response changes to said forward path data signal during 

transmission over said at least one forward transmission 

path. 

51. The system as recited in claim 43, wherein said at least 

one reverse transmission path is Substantially reciprocal to 

said at least one forward transmission path. 

52. The system as recited in claim 42, wherein said first 

device includes a base station device that is operatively configured for use in a wireless communication system. 

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53. The system as recited in claim 43, wherein said second 

device is a customer premise equipment (CPE) device that 

is operatively configured for use in a wireless communica 

tion system. 

54. The system as recited in claim 53, wherein said CPE 

device includes at least one device selected from a group of 

devices comprising a computer, a portable computer, a 

personal digital assistant (PDA), and a mobile communica 

tion device. 

55. The system as recited in claim 43, wherein said first 

device is operatively coupled to a plurality of first device 

receive antennas. 

56. The system as recited in claim 43, wherein said logic is configured to post-equalize at least a portion of said 

received reverse path signal. 

57. The system as recited in claim 43, wherein said logic is further configured to determine at least one angle of 

arrival of said reverse path data signal with respect to said 

at least one first device antenna. 

58. The system as recited in claim 53, wherein said second 

device includes at least one second device antenna that is 

operatively configured to transmit said reverse path data 

signal over said at least one reverse transmission path to said 

first device and receive said transmitted forward path data 

signal from said first device over said at least one forward 

transmission path. 

59. The system as recited in claim 58, wherein said second 

device includes a plurality of second device antennas that 

are operatively configured to transmit said reverse path data 

signal over said at least one reverse transmission path to said 

first device. 

60. The system as recited in claim 58, wherein said second 

device is configured to transmitat least two reverse path data 

signals over said at least one reverse transmission path to 

said first device. 

61. The system as recited in claim 60, wherein said at least 

two reverse path data signals include Orthogonal Frequency 

Division Multiple Access (OFDMA) data. 

62. The system as recited in claim 58, wherein said second 

device is configured to transmit said reverse path data signal 

over said at least one reverse transmission path to said first 

device at a bit rate less than about 6 Mbps. 

63. The system as recited in claim 58, wherein said second 

device is configured to transmit corresponding second 

device transmission power level information to said first 

device, and said first device is further configured to adjust at 

least one transmission power level associated with said 

forward path data signal based on said second device 

transmission power level information. 

64. The system as recited in claim 43, wherein said at least 

one first device antenna is operatively configured to provide transmission diversity when transmitting said forward path 

data signal. 

65. The system as recited in claim 43, wherein said at least 

one first device antenna is operatively configured to provide 

spatial division transmission utilizing a spatial division 

technique when transmitting said forward path data signal as 

a narrow signal to reduce multipath propagation. 

66. The system as recited in claim 43, wherein said at least one first device antenna includes at least one first device 

transmit antenna and at least one first device receive 

antenna. 

67. The system as recited in claim 66, wherein said at least 

one first device transmit antenna includes at least one type of antenna selected from a group of Suitable types of 

antennas comprising at least one omnidirectional type of 

5 

10 

15 

25 

30 

35 

40 

45 

50 

55 

60 

65 

22 

antenna, at least one beam type of antenna, at least one 

phased array type of antenna, and at least one adaptive 

antenna. 

68. The system as recited in claim 66, wherein said at least 

one first device receive antenna includes at least one type of 

antenna selected from a group of Suitable types of antennas 

comprising at least one omnidirectional type of antenna, at 

least one beam type of antenna, at least one phased array 

type of antenna, and at least one adaptive antenna. 

69. The system as recited in claim 66, wherein said logic is further configured to establish at least one antenna point ing parameter associated with said at least one first device 

transmit antenna based on said at least one forward path pre-equalization parameter. 

70. The system as recited in claim 66, wherein said logic is further configured to establish at least one phased array 

antenna transmission directing parameter associated with 

said at least one first device transmit antenna based on said 

at least one forward path pre-equalization parameter. 

71. The system as recited in claim 66, wherein said logic is further configured to cause said at least one transceiver to 

selectively transmit said forward path data signal at one 

angle with respect to said at least one first device transmit 

antenna. 

72. The system as recited in claim 66, wherein said logic is further configured to cause said at least one transceiver to 

select said at least one first device transmit antenna from a 

plurality of first device transmit antennas that are each 

operatively coupled to said first device. 

73. The system as recited in claim 72, wherein said logic 

is further configured to cause said at least one transceiver to 

selectively transmit a plurality of beams using two or more 

first device transmit antennas. 

74. The system as recited in claim 73, wherein each of 

said transmitted plurality of beams is selectively adjusted in 

phase and amplitude to reduce multipath affects when 

received by said second device. 

75. The system as recited in claim 58, wherein said at least one second device antenna includes at least one second 

device transmit antenna and at least one second device 

receive antenna. 

76. The system as recited in claim 75, wherein said second 

device includes a plurality of second device receive antennas 

that are configured to receive said transmitted forward path 

data signal over said at least one forward transmission path 

from said first device. 

77. The system as recited in claim 42, wherein said logic is further configured to iteratively selectively test different 

forward path pre-equalization parameters over time Such 

that aliased multipath delays within said reverse path data 

signal are identifiable. 

78. The system as recited in claim 42, wherein said logic is configured to provide Sub-band equalizing of said forward 

path data signal using corresponding frequency domain 

reverse path data. 

79. An apparatus comprising: 

an Orthogonal Frequency Division Multiplexing (OFDM) 

processor configured to receive coded data and output corresponding OFDM data; 

a pre-equalizer operatively coupled to said OFDM pro 

cessor and configured to receive said OFDM data and 

output pre-equalized OFDM data based on gathered 

reverse link data; 

at least one antenna; 

an antenna signal processing component operatively 

coupled to said pre-equalizer and said at least one 

antenna, said antenna signal processing component 

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being configured to receive said pre-equalized OFDM 

data from said pre-equalizer and apply a corresponding 

transmittable forward link data signal to said at least 

one antenna, the antenna signal processing component 

being further configured to receive a transmitted 

reverse link data signal using said at least one antenna 

and output corresponding reverse link data; and 

reverse link logic operatively coupled to said at least one 

antenna, said reverse link logic being configured to 

detect at least one multipath induced delay in said 

reverse link data and provide said gathered reverse link 

data to said pre-equalizer based on said multipath 

induced delay, wherein said reverse link logic is further 

configured to cause selectively establishing different 

transmission power levels for at least two Orthogonal 

Frequency Division Multiplexing (OFDM) tones in 

said transmittable forward link data signal. 

80. The apparatus as recited in claim 79, wherein said 

pre-equalized OFDM data includes sub-carrier pre-equal 

ized OFDM data. 

81. The apparatus as recited in claim 79, wherein said 

reverse link logic detects said at least one multipath induced 

delay by identifying at least one transmission delay spread 

that is greater than a guard interval. 

82. The apparatus as recited in claim 79, wherein said 

reverse link data signal includes identifiable training data. 

83. The apparatus as recited in claim 82, wherein said 

reverse link logic is further configured to compare said 

identifiable training data to a non-delayed version of said 

training data to detect said at least one transmission delay. 

84. The apparatus as recited in claim 79, wherein said 

pre-equalizer is configured to selectively invert and apply a 

measured channel response to said OFDM data. 

10 

15 

25 

30 

24 

85. The apparatus as recited in claim 79, wherein a reverse 

transmission path carrying said transmitted reverse link data 

signal towards said antenna is at least Substantially recipro 

cal to a forward transmission path carrying said transmit 

table forward link data signal away from said at least one 

antenna. 

86. The apparatus as recited in claim 79, wherein said 

apparatus includes a base station device Suitable for use in 

a wireless communication system. 

87. The apparatus as recited in claim 79, wherein said at 

least one antenna is operatively configured to provide trans 

mission diversity. 

88. The apparatus as recited in claim 79, wherein said at 

least one antenna is operatively configured to provide a 

spatial division technique to transmit the data signal as a 

narrow signal to reduce multipath propagation. 

89. The apparatus as recited in claim 79, wherein said at 

least one antenna includes at least one transmit antenna and 

at least one receive antenna. 

90. The apparatus as recited in claim 89, wherein said at 

least one transmit antenna includes at least one type of 

antenna selected from a group of Suitable types of antennas 

comprising at least one omnidirectional type of antenna, at 

least one beam type of antenna, at least one phased array 

type of antenna, and at least one adaptive antenna. 

91. The apparatus as recited in claim 89, wherein said at 

least one receive antenna includes at least one type of 

antenna selected from a group of Suitable types of antennas 

comprising at least one omnidirectional type of antenna, at 

least one beam type of antenna, at least one phased array 

type of antenna, and at least one adaptive antenna. 

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