Patent Publication Number: US-2007099669-A1

Title: Communication signaling using multiple frequency bands in a wireless network

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      The present application claims priority to U.S. Provisional Patent Application No. 60/730,575, filed Oct. 26, 2005, entitled “FRAME FORMAT STRUCTURES FOR COMMUNICATION WITHIN A WIRELESS NETWORK USING TWO FREQUENCY BANDS.” 
    
    
     TECHNICAL FIELD  
      Embodiments of the present invention relate to the field of data communication, more specifically, to data communication in a wireless network.  
     BACKGROUND  
      In the current state of wireless communication, an increasingly number of communication devices are able to wirelessly communicate with each other. These communication devices include a variety of devices having many different form factors varying from personal computers, mobile or desktop, displays, storage devices, handheld devices, telephones, and so forth. A number of these communication devices are packaged as “purpose” devices, such as set-top boxes, personal digital assistants (PDAs), web tablets, pagers, text messengers, game devices, smart appliances, and wireless mobile phones. Such devices may communicate with each other in various different wireless environments such as wireless wide area networks (WWANs), wireless metropolitan area networks (WMANs), wireless local area networks (WLANs), and wireless personal area networks (WPANs), Global System for Mobile Communications (GSM) networks, code division multiple access (CDMA), and so forth.  
      The growing demand for high throughput applications such as video streaming, real-time collaboration, video content download, and the like, imposes stringent requirements on wireless communications to provide better, faster, and lower cost communications systems. In recent years, unlicensed frequency bands such as 2.4 GHz (Industrial, Scientific, Medical (ISM)) and 5.0 GHz (Universal National Information Infrastructure (UNII)) bands have been utilized for communications up to few hundred Mbps. To achieve these bit rates, relatively complex modulation techniques such as multiple-input/multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) have been proposed to the Institute of Electrical and Electronics Engineers (IEEE). Due to the popularity of the ISM and UNII bands, these bands are becoming crowded resulting in substantial interference for users of these bands.  
      To provide an interference limited Gbps communications, IEEE committees have recently begun looking at communications at higher frequencies such as frequency bands greater than 20 GHz.  FIG. 1  shows the currently available unlicensed frequency bands in selected major industrialized countries/regions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Embodiments of the present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings. To facilitate this description, like reference numerals designate like structural elements. Embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings.  
       FIG. 1  illustrates currently available unlicensed frequency bands in selected major industrialized countries/regions;  
       FIG. 2  illustrates exemplary beamwidths of different frequency bands using antennae with about the same aperture size;  
       FIG. 3  illustrates a wireless network in accordance with various embodiments of the present invention;  
       FIG. 4  illustrates various types of CSMA/CA protocol data that may be transmitted and/or received using a first and a second frequency bands in accordance with various embodiments of the invention;  
       FIG. 5  illustrates a process for communicating by a communication device in a wireless network in accordance with various embodiments of the present invention;  
       FIG. 6  illustrates a communication device in accordance with various embodiments of the present invention;  
       FIG. 7  illustrates a circuitry for transmitting and receiving signals using two frequency bands in accordance with various embodiments of the present invention;  
       FIG. 8  illustrates a frame format in accordance with various embodiments of the present invention;  
       FIG. 9  illustrates another frame format in accordance with various embodiments of the present invention;  
       FIG. 10  illustrates yet another frame format in accordance with various embodiments of the present invention;  
       FIG. 11  illustrates two frame formats using two frequency bands of a soft coupled system adapted to communicate using the two frequency bands in accordance with various embodiments of the present invention; and  
       FIG. 12  illustrates a circuitry of a soft coupled system adapted to communicate using two frequency bands in accordance with various embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION  
      In the following detailed description, reference is made to the accompanying drawings which form a part hereof wherein like numerals designate like parts throughout, and in which is shown by way of illustration embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. Therefore, the following detailed description is not to be taken in a limiting sense, and the scope of embodiments in accordance with the present invention is defined by the appended claims and their equivalents.  
      Various operations may be described as multiple discrete operations in turn, in a manner that may be helpful in understanding embodiments of the present invention; however, the order of description should not be construed to imply that these operations are order dependent.  
      The description may use phrases such as “in one embodiment,” or “in various embodiments,” which may each refer to one or more of the same or different embodiments. Furthermore, the terms “comprising,” “including,” “having,” and the like, as used with respect to embodiments of the present invention, are synonymous.  
      According to various embodiments of the invention, methods and systems are provided in which a communication device communicates with other communication devices in a wireless network using a first and a second frequency band. For the embodiments, the first frequency band may be associated with a first beamwidth while the second frequency band may be associated with a second beamwidth, the first beamwidth being greater than the second beamwidth. Although the following description describes using two frequency bands, in alternative embodiments, more than two frequency bands may be employed.  
      In various embodiments, the first frequency band may be employed to communicate (i.e., transmit and/or receive) first signals to facilitate initial communication between the communication device and the other communication devices of the wireless network, including initial communication of first signals containing signals and/or control information for coarse configuration of the other communication devices to wirelessly communicate with the communication device. The subsequent communication of second signals between the devices may be transmitted using the second frequency band. The second signals further include signals and/or control information for finer configuration of the other communication devices to wirelessly communicate with the communication device.  
      In some embodiments, the first signals may be adapted for signal detection, initial beam forming, and/or initial carrier frequency offset (CFO) estimation, to facilitate subsequent communication using the second frequency band. The second signals communicated through the second frequency band may be adapted for more precise beam forming that supplements the initial beam forming and/or signals that are adapted for fine CFO estimation that may supplement the initial CFO estimation. The second signals may further facilitate timing synchronization of the other communication devices to the communication device. The second signals communicated using the second frequency band, as previously alluded to, may facilitate further communication using the second frequency band in order to facilitate the communication of third signals using the second frequency band. The third signals to be communicated using the second frequency band may include various types of data including, for example, data relating to video streaming, realtime and/or non-realtime collaboration, video content download, audio and text content download and/or upload, and so forth.  
      Various approaches may be used in various alternative embodiments in order to communicate via the first frequency band associated with the first beamwidth (herein “first frequency band”) and the second frequency band associated with the second beamwidth (herein “second frequency band”). For example, in some embodiments, communication using the first frequency band may be as a result of using a relatively low frequency band such as those bands less than about 20 GHz while communication using the second frequency band may be as a result of using a higher frequency band such as those bands centered above about 20 GHz. Various antenna systems that may include various combinations of antennas and/or multi-element antennas may be employed in various alternative embodiments in order to communicate using the first and the second frequency bands.  
      The first frequency band may be a lower frequency band than the second frequency band. For these embodiments, the first frequency band may be the 2.4 GHz ISM band or the 5.0 GHz UNII band, or some other band less than about 20 GHz while the second frequency band may be a higher frequency band such as a band greater than about 20 GHz, including for example, the 24 GHz band or a band centered in the 59 to 62 GHz spectra. Note that for purposes of this description, the process of communicating using the first lower frequency band may be referred to as out-of-band (OOB) communications and the process of communicating using the second higher frequency band may be referred to as in-band communications. Note further that other frequency bands may also be used as the first and second frequency bands in alternative embodiments and that the demarcation between the first lower frequency band and the second higher frequency band may not be at 20 GHz. In still other alternative embodiments, the first and the second frequency bands may be centered at the same frequencies but may be associated with different beamwidths by using, for example, antennas of different aperture sizes.  
      The first frequency band may be used by the communication device to communicate with the other communication devices of the wireless network, OOB control information signals or simply “first control signals” to facilitate data communication using the second frequency band. The first control signals may comprise of “signals” and/or “control information” to facilitate initial or coarse beamforming, CFO estimation, timing synchronization, and so forth, of the device or the other communication devices. In some embodiments, the communication device may use the second frequency band to transmit and/or receive to and/or from the other communication devices of the wireless network, in-band control information signals or simply “second control signals” to further facilitate data communication using the second frequency band. The second control signals may be comprised of signals and control information to facilitate fine beamforming, CFO estimation, timing synchronization, and so forth, of the communication device or the other communication devices. The subsequent data or data signals to be communicated (i.e., transmitted and/or received) using the second frequency band may include signals for tracking of the beamforming, CFO, timing, and so forth, as well as various types of data including, for example, data relating to video streaming, realtime and/or non-realtime collaboration, video content download, audio and text content download and/or upload, and so forth.  
      In order to appreciate various aspects of embodiments of the invention, the characteristics of a frequency band associated with a relative broad beamwidth and the characteristics of a frequency band associated with a relatively narrow beamwidth will now be discussed. This discussion will also describe the characteristics of various types of antennas including, for example, omnidirectional and directional antennas. In addition, a discussion relating to the impact of using a lower as opposed to a higher frequency band will also be provided.  
      This discussion begins with a brief description of beamwidths. A beamwidth is a spatial characteristic typically associated with antennas or dishes. The beamwidth of an antenna may be determined by the ratio of the antenna aperture size to the wavelength of the signals to be transmitted (or received). That is, the greater the aperture size, the narrower the beamwidth if the wavelengths of the signals to be transmitted (or received) are held constant. Alternatively, the beamwidth may also be made narrower by transmitting (or receiving) signals of shorter wavelengths (i.e., higher frequency) while maintaining a constant aperture size. Thus when an antenna or antennas having similar sized apertures transmit signals of different frequency bands, different beamwidths may result. Note that although the above discussion relates to, among other things, the relationship between aperture size and beamwidth, multi-element antennas may be employed to selectively control the beamwidth of the signals to be transmitted, in which case aperture size may not be relevant as to beamwidth of the signals to be transmitted. That is, antenna systems may be employed that have multi-element antennas that may be adaptively configured to selectively transmit (or receive) signals associated with different beamwidths.  
      Thus, in order to obtain a relatively broad beamwidth, one approach is to use an antenna having a small aperture, such as an omnidirectional antenna, instead of or in addition to using a relatively low frequency band (e.g., ISM or UNII bands). In contrast, in order to obtain a narrower beamwidth, one approach is to use an antenna having a large aperture, such as a directional antenna, instead of or in addition to using a relatively high frequency band. Of course, alternatively, a single antenna may provide varying beamwidths simply by varying the frequency bands (i.e., either higher or lower frequency bands) of the signals to be transmitted and/or received. In still other alternative approaches, and as previously alluded to, multi-element antennas may be employed to provide frequency bands with varying beamwidths. That is, a single set of multi-element antennas may be adaptively controlled using, for example, special procedures or protocols to provide specific beam directions and specific beam shapes. Thus, a single set of multi-element antennas may be employed to provide multiple frequency bands of varying beamwidths. Note that in the following description, the phrase “antenna” may refer to a single antenna or multi-element antennas.  
      Referring now to  FIG. 2  comparing the beamwidths of various frequency bands using antennas with about the same aperture size. As previously alluded to, one of the properties of using a lower frequency band such as the 2.4 GHz (ISM) band or the 5.0 GHz (UNII) band instead of a higher frequency band such as an in-band frequency band (e.g., bands greater than 20 GHz) for communicating in a, for example, wireless network is that the lower frequency bands may be associated with a greater beamwidth. Because of the greater beamwidth, signals transmitted via the lower frequency bands will likely reach more devices in the wireless network. However, because of the greater beamwidth, the drawback in using a lower frequency band is that because of the broader wedge, there is a greater risk of interference and interception.  
      In contrast to the lower frequency bands, when higher frequency bands are used for communicating in a wireless network a narrower beamwidth may result as previously described. As a result, there may be less likelihood of interference. In addition to the narrower beamwidth, another property of a higher frequency band is that if a higher frequency band (such as the 24 or the 60 GHz band) is used then there may be an additional attenuation with distance due to, for example, oxygen absorption. That is, and as depicted in  FIG. 2 , a higher frequency band (e.g., 60 GHz band) may have a smaller beamwidth and a shorter “range” or “reach” than a lower frequency band (e.g., 2.4 or 5.0 GHz bands). Thus, devices operating in the 60 GHz band instead of a lower band such as the 2.4 or 5.0 GHz bands may typically have less interference risk from other remote devices.  
      Another characteristic of using a higher frequency band for communicating in a wireless network is that the higher frequency band may allow higher signal bandwidth to be used (as more spectra is typically available at higher frequencies) which may consequently allow greater data throughput. At the same time, using the larger bandwidth may decrease the power spectral density of the transmit signal and potentially decrease the reliable communication range due to less signal-to-noise ratio at the receiver side.  
      The use of higher frequency bands for communicating in a wireless network may mean that a directional antenna rather than an omnidirectional antenna may be used for such communication. The use of such an antenna by itself may offer certain advantages and disadvantages when used to communicate in a wireless network. For example, one advantage of using a directional antenna and the higher frequency band for transmitting signals is that less power may be needed in comparison to using an omnidirectional antenna to achieve the same level of received power. Thus, less efficient (and less expensive) radio frequency (RF) components may be used with the directional antenna, which may be a significant factor in some situations as costs of RF parts may be significantly higher for higher frequency communication.  
      Of course, there may be certain drawbacks when communicating in a wireless network using a higher frequency band with a directional antenna. For example, adapted or multiple fixed antenna setting that spans 360 degrees may be needed in order to register all of the communication devices in the network. This may be very time-consuming and synchronizing the communication device in the network using, for example, protocols such as carrier sense multiple access and collision avoidance (CSMA/CA) or carrier sense multiple access and collision detection (CSMA/CD) may be very difficult and may not be feasible when a higher frequency band using a directional antenna is employed.  
      In accordance with various embodiments, the characteristics of frequency bands associated with different beamwidths as described above may be combined and used in a wireless communication network in accordance with various embodiments of the invention as described below.  
       FIG. 3  illustrates a wireless network that includes multiple communication devices (CDs) that are in communication with each other via multiple communication links in accordance with various embodiments. For the embodiments, the network  300  may be WWAN, WMAN, WLAN, WPAN, or other types of wireless networks. The communication devices (CDs)  302 - 308  may be desktop computers, laptop computers, set-top boxes, personal digital assistants (PDAs), web tablets, pagers, text messengers, game devices, smart appliances, wireless mobile phones or any other types of computing or communication devices. In some embodiments, at least one of the CDs  302 - 308  may be a master or an access point, while the other CDs may be the client or slave devices. Note that in alternative embodiments, the network  300  may include more or fewer CDs. Each of the CDs  302 - 308  may communicate with the other CDs of the network  300  via links  310  that may be bidirectional. Communication between the CDs may be in accordance with standards such as 802.11a, 802.11b, and other derivatives of these standards.  
      For ease of understanding, embodiments of the present invention will be further described assuming that the network  300  is a WPAN and that CD  302  is the access point and that the other CDs  304 - 308  are the client devices. Note that in alternative embodiments, the network  300  may not include an access point. For example, the network  300  may be an ad-hoc mesh network in alternative embodiments, in which case, the access point is not needed. Returning to  FIG. 3 , in some embodiments, at least some of the client CDs  304 - 308  may arbitrarily and randomly join and/or leave the network  300 . Each time a client CD  304 - 308  enters the network  300 , it may authenticate or associate (herein “associate”) with the network  300  so that the various client CDs of the network  300  may “know” that the client CD is present in the network  300 . In some embodiments, a client CD  304 - 308  may associate with the network  300  by associating with the access point CD  302 . Note that in this illustration, client CD  304  has just entered the network  300  as indicated by reference  312 .  
      The CD  304  upon entering the network  300  may associate itself with the network (e.g., via access point CD  302 ). In accordance with various embodiments, association with the network  300  may be accomplished using, for example, a first frequency band associated with a relatively broad beamwidth. By transmitting the association signals using a frequency band associated with a relatively broad beamwidth (herein “first beamwidth”, the other CDs  302 ,  306 , and  308  in the network  300  may be more likely to receive the authentication signals (e.g., beacons) from CD  304 . In some embodiments, the first frequency band may be a 2.4 GHz (ISM), a 5.0 GHz (UNII), or other bands that may be less than, for example, 20 GHz. Note that the access point CD  302  may listen for (i.e., authentication or association) an entering CD  304  through signals transmitted in the first frequency band. After successfully registering or associating with the network  300  (which may be effectuated via any one of a number of association and/or authentication protocols), the components of CD  304  may then “sleep” until it receives data transmission from one of the other CDs in the network or is ready to transmit data to the network  300  (i.e., to one or more of the other CDs in the network  300 ).  
      When the client CD  304  is ready to transmit signals to one or more of the other CDs  302 ,  306 , and  308  in the network  300  (including the access point CD  302 ), it may initially transmit first control signals that include control information using again the first frequency band associated with the first beamwidth. In using the first frequency band associated with the first beamwidth, the other CDs  302 ,  306 , and  308  in the network  300  are more likely to “hear” or receive the signals transmitted by the client CD  304 . This may provide the opportunity to reduce the interference in the second frequency band because the devices are now aware of intentions of the CD  304  and may therefore defer their transmission for the appropriate time period. In various embodiments, the other CDs  302 ,  306 , and  308  may determine the signal parameters of the first control signals transmitted by the client CD  304 . By measuring the signal parameters, the other CDs  302 ,  306 , and  308  may determine the signal strength and the angle of arrival of the first control signals. As a result, the other CDs  302 ,  306 , and  308  may be facilitated in determining the distance between the other CDs  302 ,  306 , and  308 , and the client CD  304 .  
      Further, the location, at least in part of CD  304  relative to the other CDs (e.g., in terms of azimuth and elevation) may be determined by the other CDs  302 ,  306 , and  308  based at least in part on the angle of arrival of the initial signals using the first frequency band. These determinations, in effect, may facilitate further communication using a second frequency band associated with a relatively narrow beamwidth. That is, the antenna systems employed by the other CDs  302 ,  306 , and  308  may be properly configured and/or aligned based on the determinations to facilitate further communication using the second frequency band between the CDs  302 ,  306 , and  308 , and the client CD  304 .  
      The first control signals transmitted through the first frequency band may facilitate initial communication between the CD  304  and the other CDs  302 ,  306 , and  308  of the network  300 ; including signals and/or control information for coarse configuration by the other CDs  302 ,  306 , and  308  to communicate with CD  304 . The devices subsequently communicate using a second frequency band that is associated with a second beamwidth that may be a narrower beamwidth than the first beamwidth of the first frequency band. In some embodiments, the first control signals may include signals for medium access control (MAC) mechanism data such as data associated with CSMA/CA or CSMA/CD. Again, by using the first frequency band associated with the relatively broad beamwidth for communicating data, such as MAC mechanism data, each of the other CDs  302 ,  306 , and  308  are more likely to receive the MAC mechanism data. The first control signals may further include signals as well as control information for initial beam forming parameters such as beam forming coefficients, synchronization parameters, initial CFO estimation, detection, and so forth. In particular, in some embodiments, the first control signals may be adapted to facilitate beam forming, CFO estimation, and/or synchronization of the other CDs  302 ,  306 , and  308 .  
      In some embodiments, where one or more of the CDs  302 - 304  employ antenna systems that include multi-element antennas, the first control signals transmitted using the first frequency band may include signals that facilitate different diversity techniques (e.g., antenna selection and maximum ratio combining), space-time codes (e.g., Alamouti code), and MIMO techniques.  
      The second frequency band may be a higher frequency band than the first frequency band. For example, the second frequency band may be an in-band band (i.e., greater than 20 GHz) such as the 24 GHz band or a frequency band in the  59-62  GHz spectra. The higher frequency bands, such as those greater than 20 GHz, may provide greater bandwidth than lower frequency bands (e.g., 2.4 GHz and 5.0 GHz). In various embodiments, communication using the second frequency band may be in accordance with a particular technique such as OFDM or other modulation techniques. Note that in some alternative embodiments, the first and the second frequency bands may be substantially the same frequency bands but may be associated with different beamwidth by using, for example, antennas of different aperture sizes or using an antenna system that employs multi-element antennas. Further note that if CD  304  is unable to communicate using the second frequency band, then CD  304  may operate in a fall-back operation mode in which communication is entirely via first frequency band at least until the second frequency band is made available. Such a fall-back mode may be needed, for instance, if the transmitting and receiving devices cannot “see” each other using the second frequency band.  
      After the first control signal has been transmitted using the first frequency band to facilitate communication, second control signals may be transmitted using the second frequency band to further establish communication. The second control signals may include signals and/or control information to facilitate fine beam forming, fine CFO estimation, synchronization, and so forth, by the other CDs  302 ,  306 , and  308 . Once further communication using the second frequency band has been established, signals for tracking of beam forming, CFO, timing, and so forth, as well as signals that include data such as video streaming, real-time collaboration, video content download, and the like may be communicated using the second frequency band.  
      When client CD  304  is to leave the network  300  as indicated by reference  314 , the client CD  304  may exchange various exit information or parameters with the network  300  (e.g., access point CD  302 ) prior to exiting the network  300 . Upon exiting the network  300 , CD  304  may transmit exit information through the first frequency band. The exit information may include the reason code such as bad signal quality, or just does not want to communicate any more (the application has closed), or was not authorized to enter the network, and so forth.  
       FIG. 4  illustrates some types of CSMA/CA data that may be communicated via a first and a second frequency band in a wireless network in accordance with various embodiments. In particular,  FIG. 4  shows three nodes A, B, and C communicating with each other in accordance with the CSMA/CA protocol. The first frequency band is associated with a first beamwidth and the second frequency band is associated with a second beamwidth, and the first beamwidth is wider or larger than the second beamwidth. For the embodiments, the Distributed Coordination Function (DCF) Inter Frame Space (DIFS), the Short Inter Frame Space (SIFS), and the Contention Window (CW) may be facilitated using the first and the second frequency band, while the MAC Protocol Data Unit (MPDU) and the Acknowledge (Ack) may be communicated using the first and/or the second frequency bands.  
       FIG. 5  illustrates a process for communication between devices of a wireless network using a first and a second frequency band, where the first frequency band has a first beamwidth that is broader than a second beamwidth associated with the second frequency band. The process  500  may be practiced by various communication devices and may begin with a communication device entering the network at  504 . After entering the network, the communication device may use a first frequency band (e.g., 2.4 GHz ISM band or 5.0 GHz UNII band) associated with a first beamwidth to register with the network at  506 . If the communication device has finished communicating (e.g., transmitting and/or receiving) at  508  then that device may exchange exit information with the network and proceed to exit the network at  510 .  
      On the other hand, if the communication device is not yet finished communicating with the network (i.e., one or more communication devices of the network) at  508 , then the communication device may exchange control signals with other devices using the first frequency band, and then communicate with the other devices using a second frequency band associated with a second beamwidth at  512 . Note that the term “exchange” as used herein may be a bidirectional or a unidirectional exchange of signals. The second frequency band may then be used to communicate second control signals having signals and/or control information that facilitate further communication using the second frequency band at  514 . The second control signals may include, for example, signals and/or control information for fine beam forming, fine CFO estimation, and/or synchronization, that may supplement the first control signals that were exchanged using the first frequency band in order to further establish communication using the second frequency band. Once communication has been further established using the second frequency band, signals carrying various data may be exchanged at  516 . After the communication device has finished communicating with the devices of the network using the second frequency band, the process  500  may repeat itself by returning to  508 .  
       FIG. 6  depicts portions of a communication device (CD)  600  that includes a protocol stack  604  having a number of layers including an application layer  606 , a network layer  608 , a medium access control (MAC) layer  610 , and a physical (PHY) layer  612 . The CD  600  may further include a controller  602  such as a processor or microcontroller to coordinate the activities of various components associated with the various layers of the CD  600 . The components of PHY layer  612  may be coupled to two antennae  614  and  616 . In some embodiments, one antenna  614  may be an omnidirectional antenna while the other antenna  616  may be a directional antenna. For these embodiments, the omnidirectional antenna may be adapted to transmit and/or receive signals of a first frequency band associated with a first beamwidth while the directional antenna may be adapted to transmit and/or receive signals of a second frequency band associated with a second beamwidth. Again, the first beamwidth may be greater than the second beamwidth. In some embodiments, the first frequency band may be a lower frequency band than the second frequency band. In alternative embodiments, only a single antenna may be coupled to the PHY layer  612 . In still other alternative embodiments, the PHY layer  612  may include or may be coupled to an antenna system that may employ, for example, one or more multi-element antennas to transmit and/or receive signals using the first and the second frequency bands associated with the first and the second beamwidths, respectively.  
      Various embodiments of the invention may be practiced by the components of the MAC and PHY layers  610  and  612  of the CD  600  (hereinafter, simply MAC and PHY layers). PHY layer  612  may be adapted to transmit and/or receive first signals (i.e., first control signals) using a first frequency band to facilitate establishment of initial communication using a second frequency band. The PHY layer  612  may be further adapted to transmit and/or receive second signals (i.e., second control signals) using the second frequency band to facilitate further communication using the second frequency band to communicate third signals carrying data. The MAC layer  610 , in contrast, may be adapted to select the first or the second frequency bands to be used by the PHY layer  612  to transmit and/or receive the first, the second and/or the third signals.  
      The omnidirectional antenna  614  may be used to transmit and/or receive the first signals via the first frequency band to facilitate initial communication between the CD  600  and other CDs of a wireless network using the second frequency band. In contrast, the directional antenna  616  may be used to transmit and/or receive the second and third signals using the second frequency band, the communication using the directional antenna  616  at least in part being initially established via the first signal transmitted and/or received using the omnidirectional antenna  614 . In order to practice the various functions described above for CD  600  as well as the functions described previously, the CD  600  may include a physical storage medium adapted to store instructions that enables the CD  600  to perform the previously described functions.  
       FIG. 7  illustrates a circuitry for transmitting and/or receiving signals using a first and a second frequency band in accordance with various embodiments. The circuitry  700  may operate in a wireless network environment and may include, among other things, transmitter circuitry  702 , receiver circuitry  704 , frequency synthesizer  706 , and antennae  708 - 714 . Note that in alternative embodiments, the circuitry  700  may employ any number of antennas. Note further that the term “antennae” and “antennas” as used herein are synonymous.  
      In various embodiments, the circuitry  700  may operate in an Orthogonal Frequency Multiple Access (OFMA) environment. The circuitry  700  may include zero intermediate frequency (ZIF) circuitry, super heterodyne circuitry, direct conversion circuitry, or other types of circuitry. In some embodiments, the circuitry  700  may be one of the circuitries as disclosed in co-pending application &lt;to be inserted when available&gt; (attorney docket number  111027-145374 ), entitled “Systems For Communicating Using Multiple Frequency Bands In A Wireless Network.” 
      The frequency synthesizer  706 , in some embodiments, may be a frequency synthesizer that provides both a first lower modulation frequency signal  716  and a second higher modulation frequency signal  718 , such as a 2.4/60 GHz frequency synthesizer, to the transmitter and receiver circuitries  702  and  704 . The first and the second modulation frequency signals  716  and  718  may be used to modulate and/or demodulate signals to be transmitted or received using the first and the second frequency bands, respectively. The transmitter circuitry  702  may be coupled to a first antenna  708  that may be an omnidirectional antenna, and a second antenna  710  that may be a directional antenna. The receiver circuitry  704  may be coupled to a third antenna  712  that may be a directional antenna, and a fourth antenna  714  that may be an omnidirectional antenna.  
      In various embodiments, the relative CFO for circuitry  700  may be defined by the reference oscillator stability. Thus the same oscillator may be employed for both the OOB (e.g., first frequency band) and the in-band band (e.g., second frequency band) operations. Accordingly, the absolute value of the CFO may be much higher for the in-band (second frequency band) operations.  
      The initial CFO estimation and compensation problem for such a system is solved using the OOB operations. For example, the frequency synthesizer  706  is designed in such a way that both the in-band frequency synthesis circuitry and OOB frequency synthesis circuitry use the same reference clock oscillator. In this case, the signals transmitted at both OOB frequency and in-band frequency may have the same relative (in ppm) CFOs. An initial estimation of the CFO at the receiving end may be done for the OOB signal, and after that, an estimate may be recalculated and used for the coarse frequency offset compensation at the in-band frequency. The entire system may also use OOB signaling for tracking of, for example, timing, carrier frequency offset and so forth.  
       FIG. 8  illustrates a frame format for communicating in a wireless network using a first and a second frequency band in accordance with various embodiments. Frame format  800  may represent the format of the signals to be transmitted and/or received by a communication device to and/or from another communication device of a wireless network. The first frequency band (i.e., out-of-band (OOB) frequency band) may be a lower frequency band such as a frequency band less than about 20 GHz while the second frequency band (i.e., in-band frequency band) may be a frequency band above about 20 GHz. Further note that because of the greater spectra available in the higher frequency bands, the second higher frequency band may have a bandwidth of about 1-2 GHz or more while the first lower frequency band may only have a bandwidth of several MHz.  
      The frame format  800  includes an OOB preamble  802  to be communicated via the first frequency band that may be embodied in signals adapted for signal detection, initial carrier frequency offset (CFO) estimation, and/or initial beam forming. Note that the term “preamble” as used herein is to be broadly interpreted and may mean any type of data packet or portion of a data packet. In some embodiments, the OOB preamble may include medium access control data such as data relating to CSMA/CA or CSMA/CD data.  
      The frame format  800  may further include an in-band preamble  804  and in-band data  806  to be communicated using the second frequency band. The in-band preamble  804  may be embodied in signals that are adapted for finer timing synchronization, finer CFO estimation, and/or finer beam forming. The signals for the in-band preamble  804  may supplement the control signals (e.g., initial CFO estimation, initial beam forming, and so forth) exchanged using the first frequency band. As a result, the in-band preamble  804  may further facilitate communication using the second frequency band in order to facilitate communication of the in-band data  806 . Special field symbols may be placed after the OOB preamble  802  to provide encoded service information that may be needed for consequent data symbols and in-band packet decoding (e.g., modulation and coding scheme used, and so forth).  
      In order to appreciate certain aspects of the signals that embody the frame format  800 , a more detailed explanation of CFO will now be provided. CFO is the difference between the carrier frequencies that the transmitter and the receiver are tuned at. Although CFO estimation may be more accurately determined when it is determined using the preamble (i.e., preamble signals) of a higher frequency band such as the in-band preamble  804 , an initial CFO estimation may be initially determined using the OOB preamble  802  (i.e., OOB preamble signals) to partially determine the CFO prior to fine estimation of the CFO using the in-band preamble  804 . As a result, by including signals for initial CFO estimation in the signals embodying the OOB preamble  802 , the task of fine CFO estimation may be simplified.  
      The in-band preamble  804  (i.e., in-band preamble signals) may be adapted for fine CFO estimation, which may supplement the initial CFO estimation performed using the OOB preamble  802 . The CFO is the frequency difference between the reference clock oscillator in the transmitting device and the reference clock oscillator in the receiving device. Since the reference oscillators determine the “time scales” of the transmitting device and the receiving device, the CFO may be determined by the product of the difference of the reference oscillator frequencies expressed in percent with respect to the absolute value of those frequencies, and the value of carrier frequency expressed in Hertz. CFO estimating schemes are typically more sensitive to the absolute value of the difference between the carrier frequencies of the receiver and the transmitter, noting that the greater the carrier frequency, the higher the achievable CFO values. Thus, improved accuracy may be obtained for CFO estimates when they are determined using preamble signals that are communicated using a higher frequency band such as an in-band frequency band.  
      The signals embodying the OOB preamble  802  may be adapted for initial beam forming. As used herein, initial beam forming refers to an initial process in beam forming calculations that may include preliminary estimation of angle of arrival of a signal wave front from a remote transmitting device. This operation may facilitate preliminary adjustments of the antenna system of the receiving device in order for the receiving device to receive the subsequent in-band preamble. This operation may also reduce the search interval for angle of arrival of the in-band signals. For example, initial beam forming may point to a sector where the remote transmitting device is operating. If the antenna of the receiving device has multiple substantially narrow sectors, then the initial beam forming may reduce the number of sectors to search for the subsequent in-band signals.  
      In order to supplement the initial beam forming, signals embodying the in-band preamble  804  may be adapted for fine beam forming. Fine beam forming may refer to the process of fine or precise antenna adjustment to improve the receiving quality of, for example, in-band signals (i.e., signals transmitted through second frequency band). Depending on the beam forming algorithm used, this may include choosing the optimal antenna or optimal sector within the antenna where the signal quality metrics are the best. Fine beam forming may also include calculations of complex coefficients (or only phase shift values) for combining the signals coming from different antennae or from different sectors within the sectored antenna.  
      The signals embodying the OOB preamble  802  may be adapted for signal detection. That is, the signals containing the OOB preamble  802  may be adapted to facilitate signal detection and to indicate to the receiving devices that the signals are “valid” signal. The signals containing the OOB preamble may be adapted to indicate to the receiving device or devices that it is a signal containing a “valid” message from a network communication device rather than just noise or interference. Currently, the Federal Communications Commission (FCC) allows greater power spectral density in the lower bands (e.g., 2.4 GHz and 5.0 GHz bands), and therefore, signal detection may be more easily performed in these lower bands because of the higher probability that “valid” signals will be properly detected when the lower bands are used.  
      The signals embodying the in-band preamble  804  may be adapted for fine timing synchronization. Fine timing synchronization may relate to a process that finds boundaries of informational symbols within a received signal. Since the signals of the in-band preamble  804  have greater spectrum bandwidth (relative to the OOB preamble signals), these signals may be designed to have, for example, better correlation properties than the signals embodying the OOB preamble  802 . Therefore, by including fine timing synchronization signals with the signals embodying the in-band preamble  804 , more precise timing estimation and therefore better synchronization may be obtained.  
      Once communication using the second frequency band has been fully established as a result of communicating the OOB preamble  802  and the in-band preamble  804 , in-band data  806  may be communicated via the second frequency band as shown in  FIG. 8 . The in-band data  806  may include for example, video streaming, real-time collaboration, video content download, and so forth.  
       FIG. 9  depicts frame format  900  that includes OOB preamble  802 , in-band preamble  804 , and in-band data  806 , similar to the frame format  800  of  FIG. 8 , as shown. However, unlike the frame format  800  of  FIG. 8 , the frame format  900  includes a time gap  902 . The time gap  902  separates the OOB preamble  802  and the higher-frequency part of the frame (e.g., in-band preamble  804 ) to allow the receiver circuitry of the receiving device to switch between the first and second frequency bands and to allow the subsequent relaxation processes in the circuitries, such as filters, to finish (see, for example,  FIG. 7 ).  
       FIG. 10  depicts still another frame format for communicating in a wireless network using a first and a second frequency band in accordance with various embodiments. The frame format  950  is similar to the frame format  900  of  FIG. 9  except that the first frequency band may be used, after the time gap  902 , for tracking and/or sending service information as indicated by reference  952 . That is, the first frequency band may be used for tracking of beamforming, CFO, timing, and so forth, and/or for sending service information such as channel access signals. Note that in alternative embodiments, the time gap  902  may not be present. Further note that the OOB part of the frame format  950  may contain signals such as pilot or training signals  
      The previous embodiments refer to “hard” coupled systems that communicate using a first and a second frequency band, wherein communication using the second frequency band is a result of the communication using the first frequency band. In other words, the hard coupled systems use the first frequency band to communicate signals (e.g., first control signals) to facilitate subsequent communication using the second frequency band.  
      In alternative embodiments, however, “soft” coupled systems are contemplated that may use two frequency bands independently so that signal transmission or reception using a first frequency band may overlap the signal transmission or reception by the same system using a second frequency band. For these embodiments, the first frequency band may be a lower frequency band such as those below 20 GHz (e.g., 2.4 GHz or 5.0 GHz bands) and the second frequency band may be a higher frequency band such as those above 20 GHz (e.g., in-band bands).  
      The soft coupled system may use the first lower frequency band for procedures that may not require a high data throughput rate such as network entry, bandwidth requests, bandwidth grants, scheduling the transmissions in a second higher frequency band, transferring feedback information that may comprise beam forming information and power control information, and so forth. In contrast, the second higher frequency band may be used for data transmission at relatively high data throughput rates.  
       FIG. 11  depicts frame formats for both a first and a second frequency band for a soft coupled system. The first frame format  1102  is associated with a first frequency band  1100  while the second frame format  1104  is associated with a second frequency band  1101 . The first frequency band  1100  may be a frequency band below 20 GHz while the second frequency band  1101  may be a frequency band above 20 GHz. The frame formats  1102  and  1104  may include respective preambles  1110  and  1116 , frame PHY headers  1112  and  1118 , and frame payloads  1114  and  1120 . Each of the preambles  1110  and  1116  may be adapted for frame detection, timing and frequency synchronization, and so forth, similar to that of the hard coupled system previously described. However, unlike the hard coupled system, the preambles  1110  and  1116  of these frame formats  1102  and  1104  may be processed independently with respect to each other. The preambles of both frame formats  1102  and  1104  may be embodied in signals adapted for coarse and fine estimations of CFO, timing synchronization, beam forming, and so forth.  
      Both of the frame formats  1102  and  1104  may include PHY headers  1112  and  1118  to indicate at least the amount of data carried in their associated frame payloads  1114  and  1120 . The PHY headers  1112  and  1118  may also indicate the modulation and/or coding type to be applied to the frame payloads  1114  and  1120 , beam forming control information, power control information of the payload, and/or other parameters. The frame PHY headers  1112  and  1118  may be modulated and coded using, for example, a predetermined modulation and coding type, a predetermined beam forming, and a predetermined power control that may be applied to the PHY headers  1112  and  1118 .  
      Both frame formats  1102  and  1104  may include a frame payload  1114  and  1120  to carry payload data. The frame payloads  1114  and  1120  of both frame formats  1102  and  1104  may include additional sub-headers to control the interpretation of the information within the payload, such as MAC layer headers that may indicate, for example, the source and/or destination addresses of the frame.  
      The frame payload  1114  of the first frame format  1102  may contain channel access control information such as bandwidth requests and grants. It may also contain special messages used for network entry, and test signals for measurement of distance between stations in the network, although these functionalities may be carried by the preamble  1110  in alternative embodiments. The first frame format  1102  may further include fields for sending feedback information from the destination of the packet back to its source, the feedback information relating to, for example, power control, rate control, beam forming control, for sending channel state information, receiver and/or transmitter performance indicators such as bit error ratio, current transmit power level, and so forth.  
      The frame payload  1120  of the second frame format  1104  may include information relating to higher network protocol layers.  
      The PHY headers  1112  and  1118  and/or the frame payloads  1114  and  1120  of both the first and the second frame formats  1102  and  1104  may include pilot signals for estimation and/or tracking of channel transfer functions, maintaining timing and/or frequency synchronization, and other service tasks.  
      Accessing of a wireless channel of a wireless network using the first frequency band  1100  may be based on contention between communication devices (e.g., stations) of the wireless network. Different techniques may be applied to resolve the collisions that may be possible due to contention. These techniques may include, for example, CSMA/CA, CSMA/CD, and so forth. Different division techniques may be used to reduce the number of collisions and include, for example, code division and frequency or time division of contention opportunities, and so forth. Accessing of the wireless channel using the first frequency band  1100  may include deterministic mechanisms provided that contention-based access takes place. Frame exchange sequences in the first frequency band  1100  may include special beacon frames transmitted periodically to facilitate the frame exchange in the first frequency band  1100 . The transmission of frames in the first frequency band  1100  other than beacons may occur in substantially random moments of time.  
      In contrast to the above approaches for accessing a wireless channel using the first frequency band  1100 , accessing of a wireless channel using the second frequency band  1101  may be deterministic and may be based on a schedule that may be derived as a result of communications using a lower frequency band (e.g., first frequency band  1100 ). This may allow for more effective use of the high-throughput channel in the higher second frequency band  1101  as a result of reducing the time overhead for channel access by reducing the overhead of the backing-off and retransmissions caused by collisions taking place when using, for example, random channel access methods.  
      The first frequency band  1100  may be a lower frequency band while the second frequency band  1101  may be a higher frequency band. The first frequency band  1100  may be associated with a first bandwidth  1106  while the second frequency band  1101  may be associated with a second bandwidth  1108 , the second bandwidth  1108  being greater than the first bandwidth  1106 . Selected types of payloads may be communicated via the first frequency band  1100  while other types of payloads may be communicated using the second frequency band  1101 . For example, network control messages are typically short and comprised of few tens of bytes of data, while higher layer payload information may contain several thousand bytes or more. Therefore, network control messages may be communicated using the first frequency band  1100  while the second frequency band  1101  may be used in order to communicate the higher layer payload information.  
       FIG. 12  illustrates a transmitter/receiver circuitry of a soft coupled system for independent dual-band communication. The circuitry  1200  may be comprised of a transmitter circuitry  1202  and a receiver circuitry  1204 . The circuitry  1200  may be coupled to a MAC layer that may control various functionalities and may include, among other things, a frequency synthesizer  1206 , a 90 degree phase splitter  1208 , antennae  1210  and  1212 , and switches  1214  and  1216 . The frequency synthesizer  1206  may be a 2.4/5.0/60 GHz frequency synthesizer. As depicted, the transmitter and receiver circuitry  1202  and  1204  are coupled to the two antennae  1210  and  1212  via switches  1214  and  1216 . In alternative embodiments, however, the transmitter and receiver circuitry  1202  and  1204  may be coupled to any number of antennas. In some embodiments, the first antenna  1210  and the second antenna  1212  may be adapted to transmit and/or receive a first and a second frequency band, respectively, wherein the first frequency band being a lower frequency band (e.g., UNII/ISM frequency bands) than the second frequency band (e.g., in-band bands). In various embodiments, switches  1214  and  1216  may be coupled to and controlled by the MAC layer to selectively communicate using, for example, an UNII/ISM frequency band and/or an in-band band.  
      Although certain embodiments have been illustrated and described herein for purposes of description of the preferred embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent embodiments or implementations calculated to achieve the same purposes may be substituted for the embodiments shown and described without departing from the scope of the present invention. Those with skill in the art will readily appreciate that embodiments in accordance with the present invention may be implemented in a very wide variety of ways. This application is intended to cover any adaptations or variations of the embodiments discussed herein. Therefore, it is manifestly intended that embodiments in accordance with the present invention be limited only by the claims and the equivalents thereof.