Abstract:
Conventional technologies using copper tracks to couple integrated circuits (ICs) disposed on printed circuit boards (PCBs) face limitations in scaling beyond a certain transmission rate, restricting their future applications. Described herein is a waveguide network, in which the network comprises ICs on a PCB coupled via a dielectric waveguide, which advantageously overcomes these limitations. The dielectric waveguide is able to transmit radio frequency (RF) signals and has a bandwidth of at least 100 GHz, among other features. Further, the network can be arranged with different topologies such as ring, star or bus based, and is also couplable to other equivalent networks on the PCB using suitable waveguide-based networking devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119 from Singapore Patent Application Number 201106262-7, filed on Aug. 26, 2011. The entire contents of the above application are incorporated herein by reference. 
       FIELD OF INVENTION 
       [0002]    The embodiments of the invention relate generally to waveguide networks. Particularly, but not exclusively, the embodiments of the invention disclose an apparatus and method in which integrated circuits, mounted on printed circuit boards, are interconnected using dielectric waveguides. 
       BACKGROUND 
       [0003]    The adoption of multi-functional digital devices, such as smartphones and tablets, has proliferated in recent years to the point where smartphones and tablets have now grown to be an indispensable part of our daily lives. As a result, there are demands from the consumers for smartphone and tablet devices to be constantly improved with regard to form factor, better data transfer speed, longer battery life and the like. 
         [0004]    Conventionally, smartphone and tablet devices are configured with integrated circuits (ICs) disposed on printed circuit boards (PCBs), and electrically interconnected via copper-based signal traces (i.e. copper tracks) laminated onto the substrate of the PCBs. Each track is configured to be co-shared as a signal channel between designated ICs, similar to the concept of sharing of a physical communication channel in computer networks. For example, in the Ethernet standard, a physical channel may be implemented using twisted copper wires or optical fibres to linkup devices such as PC terminals or standalone modules, which usually have Layer One (i.e. physical layer) and Layer Two (i.e. data link layer) communication capabilities. 
         [0005]    Nevertheless, specific challenges abound with using copper tracks for these purposes. For instance, there is a limit to the maximum data rate (between the ICs) that can be achieved using copper tracks because of the non-liner scaling characteristics in relation to data rate, arising due to frequency dependent losses (e.g. return loss, inter-symbol interference or crosstalk). To compensate for signal impairments due to those losses, equalizers are incorporated to ensure that the link performance is met. Equalizers however consume additional power. Moreover, the losses increase as the date rate increases, which further entails use of stronger equalizers (thereby drawing more power) to ensure the same performance, forming a vicious cycle. 
         [0006]    Therefore, in light of the foregoing problems, an improved apparatus and method for interconnecting ICs on printed circuit boards would thus be useful and advantageous in the art. 
       SUMMARY 
       [0007]    According to a first aspect of embodiments of the present invention, there is provided a waveguide network or waveguide bus comprising a substrate having a plurality of integrated circuits disposed thereon, and a dielectric waveguide on or in the substrate. The plurality of integrated circuits are coupled via the dielectric waveguide. 
         [0008]    The substrate may be a printed circuit board. Each integrated circuit may be coupled to the dielectric waveguide using a waveguide coupler, which is preferably configured as a planar horn antenna. The antenna may be advantageously arranged to be relatively compact, and to exhibit high gain, directivity, and acceptable losses over most of the intended operating frequency range. 
         [0009]    The dielectric waveguide may be configured for transmission of radio frequency signals and may permit the signals to be transmitted concurrently and/or serially. Preferably, transmission may be carried out using Carrier Sense Multiple Access (CSMA) protocol or Frequency Division Multiple Access (FDMA) scheme. In addition, the dielectric waveguide may have a bandwidth of at least 100 GHz. 
         [0010]    Further, the dielectric waveguide may also be arranged to interconnect the plurality of integrated circuits to form a network, which may be configured to have a ring topology, a star topology or a bus topology. Moreover, the network may also be communicably couplable to other equivalently configured networks on the substrate using network bridges. Each network bridge is preferably a passive waveguide component arranged as an inter-coupled waveguide or an end-coupled waveguide. Network bridges are advantageous for interconnecting diverse networks as they provide collision domains isolation via micro-segmentation, and enable bandwidth scaling as the network expands. 
         [0011]    A network hub, preferably comprising a waveguide resonator for signal amplification, may be disposed on the substrate for interconnecting the plurality of integrated circuits, when the network is configured as the tree topology. 
         [0012]    In addition, the dielectric waveguide may comprise a plurality of discrete sections and at least one junction having a plurality of gaps where the discrete sections congregate. The width of each gap is preferably approximately ten percent of the wavelength of a signal frequency transmitted through the dielectric waveguide. This gap feature may improve overall transmission performance by reducing return and signal losses. 
         [0013]    According to a second aspect of the embodiments of the present invention, there is provided a waveguide network or waveguide bus comprising a printed circuit board having a plurality of integrated circuits disposed thereon, and a dielectric waveguide on or in the printed circuit board. The plurality of integrated circuits are coupled via the dielectric waveguide. 
         [0014]    According to a third aspect of the embodiments of the present invention, there is provided a dielectric waveguide configured to be attached to the surface of, or integrated into, a substrate, the dielectric waveguide comprising a first end arranged to be connectable to an integrated circuit disposed on the substrate, and a second end arranged to be connectable to another similar dielectric waveguide. 
         [0015]    These and other aspects of the embodiments of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Embodiments of the invention are disclosed hereinafter with reference to the accompanying drawings, in which: 
           [0017]      FIG. 1  is a illustration showing a prototype waveguide network implemented on a printed circuit board according to a first embodiment of the invention; 
           [0018]      FIG. 2  illustrates a top view of an IC-to-waveguide coupler (i.e. waveguide coupler) used in the network of  FIG. 1 ; 
           [0019]      FIGS. 3A to 3E  show a conventional Y-junction and a slotted Y-junction used in the network of  FIG. 1 , together with their associated performance charts; 
           [0020]      FIG. 4  illustrates a second embodiment of a waveguide network according to the embodiments of the invention, wherein the network is arranged as a ring topology; 
           [0021]      FIG. 5  illustrates a third embodiment of a waveguide network according to the embodiments of invention, wherein the network is arranged as a star topology and includes a network hub; 
           [0022]      FIG. 6  illustrates the network hub in  FIG. 5 , which incorporates a waveguide resonator for signal amplification; 
           [0023]      FIG. 7  illustrates a hybrid network, which comprises the waveguide networks of  FIGS. 1 ,  4  and  5  inter-coupled via network bridges; 
           [0024]      FIG. 8  illustrates a network bridge used in the hybrid network of  FIG. 7 , which is configured as an inter-coupled passive waveguide component; and 
           [0025]      FIG. 9  illustrates another network bridge used in the hybrid network of  FIG. 7 , which is configured as an end-coupled passive waveguide component. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Embodiments of the present invention, as described hereinafter, relate to using dielectric waveguides to provide a radio frequency (RF) based waveguide network or waveguide bus on substrates or printed circuit boards (PCB). Specifically, the dielectric waveguide interconnects several integrated circuits (ICs) that are disposed (i.e. mounted) on the PCB to form a network. The embodiments of the invention may find application in areas where there is a need for ultra high-speed inter-IC communication. Each IC has a waveguide coupler (which may be integrated with the IC, integrated into the PCB or mounted as a separate component) to couple the IC to the dielectric waveguide. The waveguide coupler enables signals to be transmitted to and/or received from the dielectric waveguide. The signals may be transmitted concurrently or serially, based on known transmission techniques and protocols. 
         [0027]    Some advantages of a network formed using a waveguide bus include enabling high data exchange rates between the ICs, reducing power consumed by the devices (due to the excellent low channel loss characteristic of the dielectric waveguide), reducing manufacturing costs through use of low cost dielectric material for the bus channel (as it eliminates the need for costly and messy copper-based signal traces), and allowing realization of a more compact device form factor for a device (as it simplifies the interface coupling between the ICs and waveguide bus). 
         [0028]      FIG. 1  illustrates a waveguide network  100  arranged on a PCB  102  according to a first embodiment of the invention. Particularly, the waveguide network  100  comprises a plurality of integrated circuits (i.e. ICs)  104  connected via a dielectric waveguide (or waveguide path)  106  through being coupled at different ends/ports of the waveguide  106 . The IC  104  may also comprise a plurality of sub IC packages (or IC dies)  107 . In an exemplary embodiment, the ICs  104  and the dielectric waveguide  106  are preferably attached to the surface of the PCB  102  using surface-mount technology known in the art. Optionally, the dielectric waveguide  106  can be formed intermediate to the layers of the PCB  102  to reduce the actual space required, and to allow further miniaturization of the size of the PCB  102 . Therefore, it is advantageous that the dielectric waveguide  106  has a cross section that is rectangular (i.e. substantially planar), semicircular or any geometric shape (all not shown) that would permit easy adhesion or attachment of the dielectric waveguide  106  to the surface of the PCB  102 . 
         [0029]    The dielectric waveguide  106  is fabricated by way of one of the following processes: printing, injection stamping, etching, or attaching prefabricated waveguide components to the PCB  102 . 
         [0030]    The dielectric waveguide  106  essentially serves as a bus (i.e. providing a shared medium channel) to facilitate data transfer between various ICs  104  and is preferably configured to permit concurrent and/or serial data (i.e. signals) communication. Hence, all ICs  104  are designed or programmed for dual transmission modes, serial and concurrent. The ICs  104  may optionally be programmed for a specific transmission mode, depending on the prior configuration of the dielectric waveguide  106 . Furthermore, the waveguide  106  is configured with a bandwidth of at least (or exceeding) 100 GHz. 
         [0031]    An exemplary method for performing serial transmission may be similar to that of the Media Access Control (MAC) protocol, Carrier Sense Multiple Access (CSMA) as known in the art. Optionally, other suitable protocols such as CSMA with Collision Detection (CD) or Token Ring technology can also be adopted. Applying the corresponding concept to the current context, all ICs  104  will be pre-assigned with a common frequency for transmission in the same network bandwidth. A carrier sensing mechanism is implemented in which, before every transmission, each IC  104  checks if there are any existing data transmissions on the dielectric waveguide  106 . If no activity is detected (i.e. implies that the dielectric waveguide  106  is free), an IC  104  commences signal transmission. However, any IC  104  that detects another signal while transmitting a data frame (i.e. a RF signal) is required to immediately stop transmission and instead transmit a jam signal. Subsequently, the IC  104  waits for a random time interval before retransmitting the previous data frame. Each IC  104  adheres to the above steps of the protocol to serially transmit signals. 
         [0032]    For concurrent transmissions, the Frequency Division Multiple Access (FDMA) scheme based on the Frequency-Division Multiplex (FDM) technique may preferably be adopted. Under this scheme, each pair of associated ICs  104  is allocated a unique frequency band as the designated transmission frequency. Alternatively, the plurality of ICs  104  may be subdivided into several subgroups (not shown) and each subgroup is assigned a distinct frequency band. Communication within members of each subgroup may (optionally) adopt the serial transmission method as afore described. It is to be appreciated that allocation of different frequency bands under this scheme for different pairs of ICs  104  or subgroups may easily be realizable due to the large bandwidth available (i.e. equal to or greater than 100 GHz). Further, the allocated frequency bands are distinctively separated from neighboring bands to prevent signal interference due to crosstalk. Therefore, independent of any ongoing transmissions over the dielectric waveguide  106 , each pair of ICs  104  or subgroup is able to promptly and reliably exchange data without the constraints of serial transmission. 
         [0033]    The network  100 , as shown in  FIG. 1 , is organized as a bus-topology (although not limited to this arrangement, as seen in the embodiments described below). Each IC  104  preferably communicates to other ICs  104  using radio frequency (RF) signals transmitted or received over the dielectric waveguide  106 . Alternatively, signal communication between the ICs  104  may also be carried out in any desired range of frequencies of the electromagnetic spectrum. Consequently, depending on the adopted communicating frequency for the network  100 , a suitable material (i.e. with matching characteristics for specific signal propagation) that enables transmission via the selected frequency is used to form the dielectric waveguide  106 . 
         [0034]    Each IC  104  additionally interfaces with the dielectric waveguide  106  at the respective ports using an IC-to-waveguide coupler (i.e. waveguide coupler)  200 , which is illustrated in  FIG. 2 . The waveguide coupler  200  may be integrally formed as part of each IC  104  (e.g. integrated into the interposer of the IC  104 ), integrated into the PCB or alternatively made available as an external add-on component. The determination of the particular form factor of the waveguide coupler  200  to adopt depends on the demands of a specific application. In one preferred embodiment, the waveguide coupler  200  comprises an ultra wideband transverse-electromagnetic-mode (TEM) planar horn antenna  202  as depicted in  FIG. 2 . Particularly, each IC  104  is attached to the planar horn antenna  202  by bonding to the Ground-Signal-Ground (GSG) pads (not shown) of the waveguide coupler  200  by using bonding wires. Signals can then be transmitted through the planar horn antenna  202  to an associated dielectric waveguide  106  connected thereto. The tolerance for aligning an end portion of the dielectric waveguide  106  attached to the planar horn antenna  202  is substantially large (according to one embodiment), such that the dielectric waveguide  106  is simply disposed in the central portion of the planar horn antenna  202  in order to effect a coupling. Furthermore, the planar horn antenna  202  is characterized by high pass frequency response (i.e. high pass filtering) and is preferably configured to be relatively compact for its directivity, and to exhibit device properties such as high gain, directivity, and acceptable losses over most of the intended range of operating frequencies. The compactness feature is useful for convenient attachment to the IC  104 , when the planar horn antenna  202  exists as a separate component. Additionally, the waveguide coupler  200  is configured to match the frequency response of the dielectric waveguide  106  to ensure optimal device interoperability. 
         [0035]    As illustrated in  FIG. 1 , the dielectric waveguide  106  is formed from a plurality of discrete sections, and coupled together at the signal junctions  108  (i.e. arranged as Y-junctions  108 ), where signals may be split or combined. Alternatively, the dielectric waveguide  106  may also be formed of a single contiguous portion, depending on the topology type prescribed for the network  100 . It is to be appreciated that any discrete section on the network  100  that is not coupled to an IC  104  needs to be terminated using a signal terminator (not shown) to prevent signal reflection, which would otherwise cause interference. With reference to a conventional Y-junction  302  illustrated in  FIG. 3A , there will typically be detectable signal losses when signals are bifurcated at a junction  108  due to the sudden change in the geometric dimension, consequently triggering an impedance change in the dielectric waveguide  106  at that section, which would result in undesired electromagnetic wave reflection and radiation. The associated signal loss performances of the conventional Y-junction  302  due to this observed phenomenon are depicted in the chart of  FIG. 3B , which shows that the return loss of each discrete section and the propagation loss between any two sections, are considerably large thereby substantially affecting performance. 
         [0036]    Therefore, to minimize the signal loss incurred due to signal splitting, a slotted Y-junction  304 , as shown in  FIG. 3C , is proposed and adapted for use in the network  100  of  FIG. 1 . Specifically, all discrete sections (i.e. sub-branches) of the slotted Y-junction  304  are each configured to have a substantially similar symmetrically-shaped structure at the end (i.e. arrowhead shaped) arranged to meet ends of other sections of the associated junction  108 . By avoiding abrupt change to the shape of the waveguide path  106 , unwanted signal loss effects seen in the conventional design are beneficially mitigated. This configuration also further simplifies the design and fabrication of the junction  108  (e.g. allows easy assembly of the waveguide path  106  for complex networks). Accordingly in this manner, the signal transmitted at a particular section of a junction  108  can be symmetrically split (i.e. to achieve an even split ratio) and propagated to other sections and vice-versa, signals from other sections of the junction  108  can be combined in a converse manner for transmission to a destination section, with a reduced loss rate. 
         [0037]    To further improve performance, the slotted Y-junction  304  is configured such that there are narrow gaps (as shown in an enlarged view in  FIG. 3E ) arranged between the adjacent discrete sections at the junction point where they congregate. These discontinuities may reduce the mutual coupling effect between the discrete sections, thereby further eliminating signal reflection and radiation. Preferably, the width of each gap is approximately ten percent of the wavelength of a signal frequency transmitted through the dielectric waveguide  106 .  FIG. 3D  shows the associated signal loss performance of the slotted Y-junction  304 . In comparison with the conventional Y-junction  302  (as illustrated in  FIG. 3B ), it may be observed that both the return loss and signal loss for the slotted Y-junction  304  are considerably improved. 
         [0038]    Another embodiment shown in  FIG. 4  illustrates a waveguide network  400  arranged as a ring topology. This network  400  comprises a dielectric waveguide configured as a loop  402  on the PCB  404  and a plurality of ICs  406  coupled (via respective waveguide couplers  200 ) at different points along the loop  402 . Since the loop  402  is formed as a single contiguous waveguide path, there is no necessity that the loop  402  include signal terminators or be configured with signal junctions  108 , as may be the case for the bus-topology network  100  of  FIG. 1 . 
         [0039]    A further embodiment of a waveguide network  500 , organized as a star-topology arrangement, is depicted in  FIG. 5 . Under this arrangement, the network  500  comprises a plurality of discrete dielectric waveguide sections  502  on the PCB  504 , all centrally connected through a network hub  506 . One end of each section  502  is coupled to an IC  508  and the opposing end is connected to the network hub  506 . Therefore, all the ICs  508  are indirectly linked together by the network hub  506 , being the common connection point. The network hub  506  preferably provides functionalities such as acting as a signal repeater (which may also include signal boosting), detecting signal collisions (which may include forwarding a jam signal to all ICs  506  if a collision is detected) and the like. Advantages to the star-topology network  500  include (but are not limited to) preventing non-centralized failure from affecting the network  500  (due to inherent isolation of each IC  508  by the discrete section connecting it to the network hub  506 ), enabling easy detection of faulty components, offering better performance by preventing unnecessary transmission of signals through excessive number of nodes (i.e. ICs  506 ), and allowing relatively easy upgrading of network capabilities (e.g. increasing hub capacity or connecting additional ICs  506 ) due to its highly extensible characteristic. 
         [0040]    The network hub  506  may also incorporate a waveguide resonator  600  as depicted in  FIG. 6  for signal amplification purposes (if it provides signal boosting). The waveguide resonator  600  comprises arranging the dielectric waveguide portions  902  to form an enclosure or cavity (e.g. a ring) on the PCB  604  as is illustrated. Energies of the transmitted electromagnetic signals are subsequently stored within this volume to establish a resonance condition, which amplifies the signals. It is also preferred that the network hub  506  incorporates a reasonable range of differently configured resonators (not shown) to handle diverse frequencies if the star-topology network  500  is connected to external networks. In addition, waveguide resonators are typically categorized based on the quality factor, Q, where the sharpness in the frequency response of a resonator increases with an increase in the Q-factor. It is therefore desirable that the waveguide resonator  600  is configured with a high-Q factor. 
         [0041]    Not restricted to the foregoing described embodiments, the dielectric waveguide  106  may alternatively be configured such that networks (comprising the ICs  104 ) of other topology types such as mesh, fully-connected, line, and tree based (all not shown) are also realizable. 
         [0042]      FIG. 7  shows a hybrid network  700  (on a PCB  702 ) formed by combining the bus-topology network  100  of  FIG. 1 , ring-topology network  400  of  FIG. 4 , and star-topology network  500  of  FIG. 5 . More particularly, the various networks  100 ,  400 ,  500  are inter-coupled, preferably, using network bridges  704 . It is to be appreciated that in this configuration, the network hub  506  provides a point of connection for the star-topology network  500  to other miscellaneous networks  100 ,  400 . Network bridges  704  are advantageous for interconnecting diverse networks as they may provide collision domain isolation (via micro-segmentation), and enable bandwidth scaling as the network  700  expands. Alternatively, other types of devices (e.g. network routers) for connecting multiple network segments at the data-link layer (i.e. Layer Two) or network layer (i.e. Layer Three) may also be used in place of the network bridges  704 . 
         [0043]    Matching devices known as “irises” (not shown) or equivalently configured circuits may be included into the hybrid network  700  for impedance matching the respective networks  100 ,  400 ,  500  to the respective loads (i.e. other connected networks). In particular, an iris is used to introduce capacitance (i.e. act as a shunt capacitive reactance), inductance (i.e. act as a shunt inductive reactance) or a combination of both into a waveguide to reduce induced signal reflections due to a mismatch between the waveguide and the load, which may otherwise result in malperformance issues such as power loss, reduction in power-handling capability and an increase in frequency sensitivity. 
         [0044]    Further, the network hub  506  of  FIG. 5  and network bridges  704  of  FIG. 7  are configured as passive, waveguide components. As commonly known in the art, passive and active components are respectively incapable and capable of power gain. According to an exemplary embodiment, the dielectric material used to form the network hub  506  and bridges  704  should preferably show no appreciable additional electromagnetic effect for switching or modulation, and should be relatively insensitive to temperature drifts in order to ensure operational stability of the hybrid network  700 . 
         [0045]      FIG. 8  shows a circuit implementation of a network bridge  800  used in the hybrid network  700 . The network bridge  800 , formed on a PCB  802 , consists of two adjacently arranged dielectric waveguide portions  804 . A device having such an arrangement is known as an inter-coupled waveguide. Preferably, the inter-coupled waveguide is configured as a 4-port coupler, wherein a signal being relayed is coupled to one half of the waveguide portions  804  and also transmitted to the other half. The coupling strength of the network bridge  800  may be altered by adjusting the gap width between the waveguide portions  804 . 
         [0046]      FIG. 9  depicts an alternative circuit implementation of a network bridge  900  for use in the hybrid network  700 . Specifically, the dielectric waveguide portions  902  on the PCB  904  are arranged as an end-coupled waveguide. In one embodiment, the end-coupled waveguide is configured as a 3-port coupler, wherein the energy of the signal to be relayed can be transmitted equally among all sections of the waveguide portions  902 . Moreover, in order to avoid the use of a signal terminator in this configuration, it is to be appreciated that the coupling length of the waveguide portions  902  is approximately a quarter wavelength of the transmitted signal frequency or a multiple thereof. 
         [0047]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary, and not restrictive; the invention is not limited to the disclosed embodiments. 
         [0048]    Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention. In the claims, the term “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in different dependent claims does not mean that a combination of these measures cannot be used to advantage.