Abstract:
In one aspect, the method of synchronizing a communications device in a wireless communications network comprises receiving a beacon signal at a first frequency; and receiving a data signal at a second frequency, the beacon signal being used to synchronize reception of the data signal. In another aspect, the method comprises transmitting a beacon signal at a first frequency; and transmitting a data signal at a second frequency, the beacon signal being useable to synchronize reception of the data signal. The first frequency is substantially less than the second frequency such that the beacon signal experiences substantially different frequency-dependent propagation effects to the data signal.

Description:
TECHNICAL FIELD 
     The present invention relates to wireless data communications, and in particular to methods of synchronising a communications device in a wireless communications network, a receiving device, a transmitting device, a wireless communications system comprising the receiving device and the transmitting device, and computer program to implement the method. 
     BACKGROUND OF THE INVENTION 
     When a radio frequency (RF) signal propagates from a transmitter (TX) through a transmission media to the receiver (RX), the signal strength is reduced due to path loss. The path loss is proportional to f L , where f is the frequency of the signal and L is an environment-dependent parameter. In a practical non-line-of-sight (NLOS) environment, the parameter L is normally between 2 and 4. This physical law suggests that the higher the operating frequency is, the higher its path loss is within the same operating range. This means that more energy per information bit has to be radiated, compared to a lower frequency band. 
     One characteristic of high frequency bands, such as an extremely high frequency (EHF) band ranging from 30 GHz to 300 GHz, is that atmospheric loss is substantial. To overcome extra path loss on high frequency bands, traditionally the prior art EHF communication systems use directional antennas, such as horn antennas, with substantial antenna gains in both transmission and reception. Another approach was to use multiple antenna arrays and beam-forming techniques to form directional beams between the transmitter and receiver. 
     While high-gain directional antennas work well for fixed point-to-point communications, they create problems when the stations or nodes are portable or mobile. The hidden node/network is a typical problem, where a network is composed of two or more connected nodes. In the prior art, a node normally scans/tests the channel on the operating frequency band immediately after it is powered on. For example, a new node intending to use to a channel in the EHF band will tune its receiver to the EHF channels and determine the channel status (free or occupied) by analyzing the signals received. 
     If a node/network is detected, the new node may decide to join the existing node/network or initiate a new network on another free channel. However, the fact that both the transmitter and receiver antennas are highly directional means that the new node can only detect an existing node/network if their antenna directions happen to be aligned by chance. As a result, the probability of missing the detection of an existing node/network is very high. If a new node misses the detection of the existing nodes on the channel, the new node will either miss the opportunity to join the existing nodes or the new node will use the channel and cause interference for the others. Interference between existing nodes can also happen. For example, a receiver receiving data from a transmitter can be disturbed by an another transmitter of a 3 rd  node, as the 3 rd  node cannot hear the communication between the first two nodes and assumes the first two nodes are idle. 
     To establish and maintain a network on the EHF channel under these traditional scenarios, many management messages have to be exchanged. Because each node in the network uses highly directional antennas, effective broadcast and multicast schemes cannot be applied. One solution is to let the transmitter and receiver repeatedly transmit and receive at all directions for a long time, to increase the chance of antenna direction alignment. This solution is however not power efficient. Another problem is that it is very hard for a new node to become synchronized to the existing nodes in time and frequency. 
     Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. 
     Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, there is provided method of synchronising a communications device in a wireless communications network, the method comprising:
         receiving a beacon signal at a first frequency; and   receiving a data signal at a second frequency, the beacon signal being used to synchronise reception of the data signal;   wherein the first frequency is substantially less than the second frequency such that the beacon signal experiences substantially different frequency-dependent propagation effects to the data signal.       

     The present method is useful in communication systems where a receiving device has to synchronise with a transmitting device, such as its frequency and/or timing. By receiving the beacon signal at a first frequency that is substantially less than the second frequency, the beacon signal can be more reliably received by the receiving device due to substantially lower path loss experienced by the beacon signal at the first frequency when compared to that of the data signal at the higher second frequency. Advantageously, the present method lowers total power consumption at the receiving device due to lower current drain in transmitting and receiving a beacon signal. For portable devices, lower power consumption translates into higher battery life and stand-by time, and throughput. 
     The present method is suitable for, but not limited to, use in power-efficient mobile communications systems primarily operating on the extremely high frequency (EHF) band from 30 GHz to 300 GHz, or higher frequency bands such as on the super high frequency (SHF) band from 3 GHz to 30 GHz. In some embodiments, the second frequency is in the range 57 to 64 GHz. In these or other embodiments, the first frequency may be substantially 900 MHz, or 2.4 GHz, or 5 GHz. 
     In these embodiments, the present method is to be contrasted with existing methods that perform synchronization on the EHF channel, which are inefficient in both power and time due to the high path loss of the EHF channel. Other losses affecting the EHF such as a 60 GHz radio may include the extra attenuation due to oxygen and moisture or water, larger noise in the analogue transmitter and receiver circuits, larger phase noise of the analogue transmitter and receiver circuit, and larger in-phase and Quadrature channel (I-Q) imbalance of the analogue transmitter and receiver circuits. 
     The beacon signal may be received using an omni-directional antenna. The data signal may be received using a directional antenna. 
     The method may further comprise determining timing or frequency, or both, of a transmitting device from the beacon signal. In this case, the method may further comprising adjusting timing or frequency, or both, of the receiving device based on the determined timing or frequency, or both of the transmitting device. 
     The beacon signal may be a carrier-less digital signal. In this case, the method may further comprise sampling the beacon signal without down-mixing. 
     The beacon signal and data signal may be time division multiplexed. The method may further comprise converting the beacon signal and data signal using a shared digital-to-analogue converter (DAC). 
     The beacon signal may be received at the first frequency as part of a sync signal. The sync signal may further include at least one of the following: an acknowledgement (ACK) signal, a feedback signal, a command signal, or a control signal. 
     According to a second aspect, there is provided a method synchronising a communications device in a wireless communications network, the method comprising:
         transmitting a beacon signal at a first frequency; and   transmitting a data signal at a second frequency, the beacon signal being useable to synchronise reception of the data signal;   wherein the first frequency is substantially less than the second frequency such that the beacon signal experiences substantially different frequency-dependent propagation effects to the data signal.       

     Similar to the first aspect, the second frequency may be in the range 57 to 64 GHz. In these or other embodiments, the first frequency may be substantially 900 MHz, or 2.4 GHz, or 5 GHz. 
     Similar to the first aspect, the beacon signal may be received using an omni-directional antenna. The data signal may be received using a directional antenna. 
     In a third aspect, there is provided a receiving device configured to carry out the method of the first aspect. In a fourth aspect, there is provided a transmitting device configured to carry out the method of the second aspect. 
     In a fifth aspect, there is provided a system comprising a transmitting device in accordance with the fourth aspect and a receiving device in accordance with the third aspect. In a sixth aspect, there is provided computer program to implement the method in accordance with the first aspect or the second aspect, or both. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting example(s) of the invention will now be described with reference to the following drawings, in which: 
         FIG. 1  is a schematic diagram of a system comprising a transmitting device and a receiving device. 
         FIG. 2  illustrates the time and power requirements to convey a reference amount of energy to a receiving device when using the 2.4 GHz band and the 60 GHz band respectively. 
         FIG. 3  is a flowchart of steps performable by a transmitting device. 
         FIG. 4  is a flowchart of steps performable by a receiving device. 
         FIG. 5  is a schematic diagram of a communications device. 
         FIG. 6  is a schematic diagram of a data packet suitable for use with the system in  FIG. 1 . 
         FIG. 7  is a schematic diagram of another data packet suitable for use with the system in  FIG. 1 . 
         FIG. 8  is a schematic diagram of a further data packet suitable for use with the system in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , the system  100  comprises a first communications device  110  and a second communications device  130  operable to communicate with each other in a wireless communications network. 
     The devices  110  and  130  may be a mobile phone, a personal digital assistant (PDA) device, a desktop computer, a laptop computer or a tablet computer. The network may be a Institute of Electrical and Electronics Engineers (IEEE) 802.11 wireless network using wireless standards such as 802.11a, 11b, 11g, or 11ad. The specifications of IEEE 802.11 can be found, for example, on the IEEE Standards website at http://standards.ieee.org/getieee802/802.11.html, and are incorporated herein by reference. 
     In one example, the first communications device  110  is a transmitting device that wishes to transmit data signals to the second communications device  130 , which is the receiving device. While not shown, the second communications device  130  may also be a transmitting device and vice versa. Before any data signals sent by the transmitting device  110  can be reliably received by the receiving device  130 , the receiving device  130  has to synchronise its carrier frequency and time or clock with those of the transmitting device  110 . 
     To facilitate synchronisation between the devices  110   130 , a beacon signal is sent by the transmitting device  110  periodically so that the receiving device  130 , after power-up, is able to search for the beacon signal. 
     In particular, the transmitting device  110  has two transmit antennas:
         Omni-directional transmit antenna (TX_L)  112  is operable to transmit a beacon signal  120  to the receiving device  130  at a lower frequency band, f L , which may be 900 MHz, or 2.4 GHz, or 5 GHz.       

     Directional transmit antenna  114  (TX_H) is operable to transmit at least one data signal  122  to the receiving device  130  in a higher frequency band, f H , which may be in the range of 57 to 64 GHz. 
     Correspondingly, the receiving device  130  has two receive antennas to receive signals transmitted by the transmitting device  110 :
         Omni-directional receive antenna (RX_L)  132  is operable to receive a beacon signal  120  transmitted by transmit antenna (TX_L)  112  at a lower frequency band, f L , which may be 900 MHz, or 2.4 GHz, or 5 GHz.       

     Directional receive antenna (RX_H)  134  is operable to receive at least one data signal  122  transmitted by transmit antenna (TX_H)  114  at a higher frequency band, f H , which may be in the range of 57 to 64 GHz. 
     The beacon signal  120  can then be used to synchronise reception of the data signal  122 . Upon receiving the beacon signal  120 , the receiving device  130  analyses the beacon signal  120  and adjusts its frequency and timing to match those of the transmitting device  110 . 
     By transmitting the beacon signal  120  at the lower frequency band, the beacon signal can therefore be received by the receiving device  130  more reliably. This also substantially reduces power consumption of the receiving device  130  and therefore increases stand-by time. Further, transmitting the data signal  122  in the large band of 57 to 64 GHz permits increased data throughput. 
     Compared to the higher frequency band, f H , the path loss of a channel on the lower frequency band, f L  is lower due to lower levels of oxygen absorption and rain attenuation. For example, we compare the path loss of a 60 GHz channel and that of a 2.4 GHz channel, both bands being for unlicensed use. The extra path loss of the 60 GHz channel, above the path loss of the 2.4 GHz channel, is within the range of 42 dB to 56 dB using environment-dependent parameter L=2 and 3 respectively:
 
[10 log 10 (60/2.4) 3  dB, 10 log 10 (60/2.4) 4  dB]=[42 dB, 56 dB].
 
       FIG. 2  illustrates the transmit power (dBm) and time needed by an equivalent transceiver to deliver equal energy via the 60 GHz and 2.4 GHz channels, respectively. The box  200  at bottom left illustrates a reference power and time required by a 2.4 GHz transmitter. The two larger boxes in  FIG. 2  show the transmit power  210  and time  220  required by a 60 GHz transmitter to effect the same energy transfer. This means that omni-directional antennas are more suitable for transmitting the beacon signal  120  on the lower frequency band, while directional antennas are more suitable for transmitting the data signal  122  on the higher frequency band. 
       FIG. 3  and  FIG. 4  illustrate steps performed by the transmitting device  110  and receiving device  130  for synchronisation respectively. 
     Referring first to  FIG. 3 , the transmitting device  110  first generates a beacon signal  120  that includes information on timing and/or frequency of the device  110  itself; step  310 . 
     The beacon signal  120  is then transmitted at a first frequency on a lower frequency band, f L , using an omni-directional transmit antenna  112  at the transmitting device  110 ; see step  320 . 
     At power-up, the receiving device  130  searches for a beacon signal  120  and may decide to join and synchronize with the transmitting device  110 . Referring also to  FIG. 4 , the receiving device  130  receives the beacon signal  120  from the transmitting device  110  at the first frequency on the lower frequency band f L ; see step  410 . As mentioned, the first frequency may be 900 MHz, 2.4 GHz or 5 GHz. The beacon signal  120  is received using an omni-directional receive antenna  132  at the receiving device  130 . 
     From the received beacon signal, the receiving device  130  then determines timing and/or frequency of the transmitting device  110 , and adjusts its own timing and/or frequency according to the beacon signal; see steps  430  and  440 . For example, the frequency on the higher band f H  is synchronized to the beacon frequency on the lower frequency band f L . Consequently, no calibration or accurate external RF source is required for frequency synchronization. In terms of time synchronization, the receiving device  130  determines its frame time boundary from the time by which the beacon signal is received. 
     In one example, the receiving device  130  does not try to use an ‘absolute’ accurate frequency. The receiving device  130  only tries to synchronize to the frequency used for beacon signal, even if the beacon frequency itself is inaccurate. Since the receiving device  130  periodically tracks the beacon frequency, the scheme can cope well with frequency drifting. The ‘absolute’ frequency accuracy in the receiving device  130  is not important as long as the receiving device  130  can track the frequency used by the transmitting device  110 . 
     Once the receiving device  130  has adjusted its own timing and/or frequency based on the beacon signal  120 , it sends a request signal to the transmitting device  110  to request for data; see step  440  in  FIG. 4 . The request signal may be sent at a frequency on the lower frequency band f L  using an omni-directional transmit antenna  112  at the receiving device  130 . There may also be an association phase, where the receiving device  130  notifies the transmitting device  110  of its presence and initiates an association with the transmitting device  110 . Where applicable, quality of service parameters are negotiated between the transmitting  110  and receiving device  130 . Further, there may be an authentication phase, where the receiving device  130  authenticates its identity with the transmitting device  110 . 
     Upon receiving the request signal, the transmitting device  110  transmits data signal to the receiving device  130  at a second frequency on a higher frequency band f H . As mentioned, the second frequency may be in the range of 57 to 64 GHz; see step  340  in  FIG. 3 . However, if no request signal is received, the transmitting device  110  continues to transmit the beacon signal  120  periodically at the lower frequency band. 
     The data signal  122  is transmitted using a directional transmit antenna  114  at the transmitting device  110 . The receiving device  130  then receives the data signal  122  at the second frequency within the range of 57 to 64 GHz; see step  450  in  FIG. 4 . The data signal  122  is received using a directional receive antenna  134  at the receiving device  130 . 
     While not shown in  FIG. 1 , it will be appreciated that the transmitting device  110  may also have an omni-directional receive antenna (RX_L) for receiving a signals on the lower frequency band and a directional receive antenna (RX_H) for receiving signals on the higher frequency band. Similarly, while not shown, the receiving device  130  may have corresponding omni-directional (TX_L) and directional (TX_H) transmit antennas operating at low and high frequency bands respectively. 
     Referring now to  FIG. 5 , the periodic beacon signal may be transmitted as part of a Sync signal on a lower-frequency band f L , which in this example is 2.4 GHz. The Sync signal also includes command and control signals exchanged between the transmitter (TX) and the receiver (RX). 
     The data transmitter (TX_H) operates on a higher-frequency band f H , which in this embodiment is the 57 to 64 GHz band. The data and the Sync signals can thus be thought of as operating in a type of frequency-division multiplexing (FDM). In addition to being transmitted in FDM in this way, the data and Sync signals are transmitted using time-division multiplexing (TDM). This allows a single analog to digital converter (ADC) to be used in the receiver for both the Sync signal and the data signal. Similarly, while not shown in  FIG. 5 , the transmitter may use a single digital to analog converter (DAC) for both the data and Sync signals. 
     In this embodiment, it is further noted that the ADC of the receiver and the DAC of the transmitter are able to operate at a very high sampling frequency in order to effect the high data rates possible in the 57 to 64 GHz band. Accordingly, the Sync signal in this embodiment is not up converted by the transmitter nor down converted by the receiver, and is instead directly sampled. Notably, in this embodiment, the lower-frequency band f L  carries a modulated signal and not an unmodulated carrier. 
     To enable fine receiver tuning, such as carrier frequency and sampling frequency, and timing tuning, it is possible to transmit the pre-amble  1  on the first frequency f L , followed by a guard interval, and the rest of the data packet on the second frequency f H ; as shown in  FIG. 6 . 
     To enable fine receiver tuning, such as carrier frequency and sampling frequency, and/or timing tuning, and/or reliable reception of the packet header, it is possible to transmit the pre-amble  1  and the packet header on the first frequency f L , followed by guard interval, and the rest of the data packet on the second frequency f H  as shown in  FIG. 7 . 
     To enable fine receiver tuning, such as carrier frequency and sampling frequency, and/or timing tuning, and/or reliable reception of Sync/Manage message, it is possible to transmit the pre-amble  1  and the Sync/Manage message on the first frequency f L , followed by guard interval, and the rest of the data packet on the second frequency f H  as shown in  FIG. 8 . 
     Other packet arrangements are also possible. 
     The present embodiment is particularly suited to low-power 60 GHz or millimeter wave radio applications. For example, one suitable use for some embodiments of the present invention may be in an office conference room or the like, with the device and method of the present invention enabling laptops to wirelessly transmit large amounts of data within the room, such as transmitting lightly compressed video files to a projector, or a “Sync and go” file transfer. 
     Another suitable use for some embodiments of the present invention may be in an enterprise cubicle, allowing a laptop to transmit lightly compressed video to a monitor or display. Similarly, the laptop may wirelessly transmit to a printer or hard drive in near proximity. 
     A further use for some embodiments of the invention may be in the home environment, in allowing uncompressed video to be wirelessly transmitted within a room in a residence. 
     The data transport types in the above uses of some embodiments of the invention could include uncompressed video, lightly compressed video, local file transfer TCP/IP, web browsing TCP/IP, and/or hard disk transfer. 
     Further, it is estimated that the present embodiment provides at least a 10 times reduction in battery power during Sync periods, compared to a traditional approach. Further, this embodiment is estimated to provide improved mobility by providing a reduction of around 10 5  in the time required for Sync, compared to a traditional approach applied at the same frequency. 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. 
     For example, if no beacon signal is received in step  410  in  FIG. 4 , the device itself shall send the a beacon signal periodically. The system design is thus symmetrical, although this device can be applied to an asymmetrical scenario. 
     The first frequency may in some embodiments be used partially for beam-forming and/or tracking.