Abstract:
The selection of the primary control signal bus to use for a given radio head is based on propagation delay to the relevant radio head. A plurality of radio heads are cascaded in a chain with buffered error correction therebetween. At least first and second control interfaces connect to the chain and supply control information thereto via respective control signal busses, which may be virtual busses. The propagation delay from a first control interface to a first radio head over a first bus is determined. Likewise, the propagation delay from a second control interface to the first radio head over a second bus is determined. Thereafter, the radio head determines which control signal bus to select as its primary control signal bus based on the respective propagation delays between the first radio head and the first and second control interfaces.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to wireless communications, and more particularly to the selection of a primary control signal bus for cascaded radio heads that have buffered communications links between radio heads. 
     In today&#39;s wireless communications environment, it is not uncommon to have localized wireless telephone systems, such as might be installed in a building or on a campus, that are designed to cover a relatively small geographic area. A typical physical configuration of such a localized system includes a central cabinet connected to a plurality of radio heads. The central cabinet (located at the site/premises) includes regional processors and signaling terminals (also known as “control interfaces” or “control interface units”) that oversee communications with the relevant Mobile Switching Center (MSC) and with the corresponding regional processors. The signaling terminals communicate with the regional processors over two or more control signal buses within the control cabinet. The regional processors in turn communicate with the radio heads, typically over a suitable T1 or E1 link. The radio heads are typically arranged in a cascaded fashion, forming a chain of radio heads, with only the end radio heads of the chain directly connected to the central cabinet. 
     Typically, each radio head in the chain is physically located a substantial distance from the next radio head, such as up to 1000 meters apart. Because of this physical separation, it is common to employ error detection and correction schemes to the transmissions between the successive radio heads. Use of such error correction schemes means that data from the central cabinet to a given radio head, say the third radio head, is buffered at the central cabinet for error correction purposes, sent to the first radio head in the chain, buffered again at the first radio head for error correction, sent to the second radio head, and buffered yet again at the second radio head for error correction, before being sent to the third radio head. There can thus be a significant time delay between the central cabinet sending the information and its receipt at the desired radio head. This time delay, sometimes referred to herein as a propagation delay, is generally a function of the error detection and correction schemes employed, the processing speed of the relevant processor(s), the distance between radio heads, and the number of “hops” between the central cabinet and the desired radio head. 
     When installing (or removing) radio heads in a localized system described above, it is necessary to decide which control signal bus each regional processor should consider as its primary bus. Generally, this means that the responsible technician must perform some function that assigns each regional processor to one of the plurality of control signal buses; typically, this involves assigning the regional processor to bus A or bus B based on the physical positioning of the corresponding regional processing board within the central cabinet. Because there is generally a one-to-one relationship between regional processors and radio heads, with a given regional processor controlling a given radio head, this results in the radio heads being effectively assigned to bus A or bus B based on the location of the corresponding regional processing board within the control cabinet. However, such bus assignments sometimes result in unnecessarily long propagation delays for a given radio head. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the present invention, selection of the primary control signal bus to use for a given radio head is based on propagation delay to the relevant radio head. A wireless communications system may include a plurality of radio heads cascaded in a chain and at least first and second control interfaces connected to the chain and supplying control information thereto. The communications between radio heads employ buffered error correction, resulting in store-and-forward propagation delay. A plurality of virtual control signal busses are established connecting the control interfaces to the plurality of radio heads, including at least a first virtual bus connecting the first control interface to the plurality of radio heads and a second virtual bus connecting the second control interface to the plurality of radio heads. The propagation delay from the first control interface to a first radio head over the first virtual bus is determined. Likewise, the propagation delay from the second control interface to the first radio head over the second virtual bus is determined. Thereafter, the first radio head determines which control signal bus, from amongst a plurality of virtual busses, to select as its primary control signal bus based on the respective propagation delays between the first radio head and the first and second control interfaces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a wireless communications system, including a localized wireless telephone system. 
     FIG. 2 shows one possible physical configuration of a localized wireless telephone system according to the present invention. 
     FIG. 3 shows one possible control signal bus assignment architecture. 
     FIG. 4 shows a simplified flowchart of one process according to the present invention. 
     FIG. 5 shows a more detailed view of the system of FIG. 2, including a control signal bus architecture according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention takes a different approach to control signal bus assignment over prior art methods, such that the control signal bus selection is based on propagation delay to the relevant radio head. In order to aid in understanding the context of the present invention, the discussion below will begin with a brief overview of an entire communications system before focusing on a Localized Wireless Telephone System, where radio heads are typically found. 
     Turning now to FIG. 1, a communication system  10  is illustrated. In particular, the communications system  10  includes the Public Switched Telephone Network (PSTN)  20  and the Public Land Mobile Network (PLMN)  30 , which may, in turn, be connected to one or more Localized Wireless Telephone Systems (LWTS)  100 . While not shown, satellites may be used as needed either within the PSTN  20  or the PLMN  30  to provide remote communication links, such as across oceans or the like. 
     The operation of the PSTN  20  is well established and subject to extensive documentation beyond the scope of the present invention and therefore a more detailed discussion is omitted. 
     PLMN  30  may include a plurality of proprietary mobile networks  40 , and each mobile network  40  may include a plurality of Mobile Switching Centers (MSC)  42 . Typically, at least one MSC  42  in the PLMN  30 , and more advantageously one MSC  42  in each mobile network  40 , is connected via a gateway to the PSTN  20 . Some MSCs  42  may also serve as gateways connecting the various mobile networks  40  within the PLMN  30 . Gateway functions may be all consolidated at a single MSC  42  within a mobile network  40  or dispersed amongst a plurality of MSCs  42  within a mobile network  40  as needed or desired. Typically, at least one MSC  42  within a particular mobile network  40  connects to, or includes, a Home Location Register (HLR)  44  and a Visitor Location Register (VLR)  46 , whose functions are well known in the art. Additionally, each mobile network  40  may be equipped with a message center  48  communicatively connected to an MSC  42  for handling short message service and the like. Each MSC  42  may further be communicatively connected to a plurality of base stations  50 . Each base station  50  may communicate with one or more mobile terminals  60  over appropriate RF communications channels, as is well known in the art. 
     The LWTS  100  is a wireless telecommunications system designed to cover a relatively small geographic area, such as a building or a campus, to provide public or private wireless communications services. Just by way of example, the LWTS  100  may be used to provide wireless service inside buildings where the RF signals from the PLMN  30  may be inadequate and/or installed to allow employees working in the building or on the campus to use a mobile terminal  60  as an office telephone. LWTS  100  typically connects with an MSC  42  in the PLMN  30  to allow subscribers of the LWTS  100  to move seamlessly between the LWTS  100  and the PLMN  30 . The MSC  42  responsible for a LWTS  100  may treat the LWTS  100  merely as another base station  50  or a plurality of base stations  50  depending on the internal structure of the LWTS  100  in question. One of many physical configurations of a LWTS  100  is shown in more detail in FIG.  2 . 
     The LWTS  100  of FIG. 2 includes a plurality of radio heads  200  connected in a chain  150 , with the end radio heads RH 1 ,RH 8  connected to an MSC  42 . The physical connections between the various radio heads  200  may take the form of a T1/E1 line  260 , typically with pulse code modulation, as is known in the art. The physical connections between the end radio heads RH 1 ,RH 8  and the MSC  42  may also take the form of a T1/E1 line. In some arrangements of the present invention, each radio head  200  may have three conceptual parts—a control interface  210 , a regional processor  220 , a transceiver circuit  240 , and a device processor  230  that controls the transceiver circuit  240  and its communications with the regional processor  220 . See FIG.  3 . In radio heads RH 1  and RH 8 , the control interface  210  is enabled and handles the communications with the MSC  42  in a known fashion. In radio heads RH 2 -RH 7 , the unused control interfaces  210  may be disabled to conserve power, as they are unnecessary. The regional processors  220  of each radio head  200 , and the respective device processors  230  and transceivers  240 , are enabled. The functioning of the hierarchical processor structure (regional processors  220  and device processors  230 ) and the transceivers  240  is well known in the art, and detailed understanding thereof is not necessary for understanding the present invention. 
     The radio heads  200  of the chain  150  may be physically separated by a substantial amount, such as up to 1000 meters. Due to this distance between the radio heads  200 , or for other reasons, it is common to employ error detection and correction schemes (sometimes referred to herein as “error correction” for simplicity). For example, the communications between radio heads  200  may be Link Access Procedure-D Channel error coded. The use of most error correction schemes in cascaded communications leads to so called store-and-forward delay, basically as a result of the processing required to detect and correct any errors at each “hop” in the communications chain  150 . In addition, the physical distances between the radio heads  200  may cause some transmission delays. The combination of the store-and-forward delays and the transmission delays may be referred to herein as propagation delay. Assuming identical processors at each radio head  200  and identical coding schemes for each link, the propagation delay for signals from RH 8  to RH 2  will be longer than the propagation delay for signals from RH 1  to RH 2 . 
     While FIG. 2 shows a single physical transmission line  260  (e.g., a T1/E1 coaxial cable or “category five” UTP) between each radio head  200 , this line may contain several channels through the use of multiplexing (e.g., time division multiplexing or the like). Two of these channels may be used as control signal buses  250 , as is known in the art. This is graphically shown in FIG. 3, where the solid line represents bus A, while the dashed line represents bus B. Because these buses  250  are logically distinct, but share some physical components, they are sometimes referred to as “virtual buses.” Virtual bus A operates under the control of control interface A in radio head RH 8 , while virtual bus B operates under the control of control interface B in radio head RH 1 . FIG. 3 shows the radio heads  200  connected such that each radio head  200  is connected to both virtual bus A and virtual bus B. However, each radio head  200  has only one primary control signal bus  250 , with the alternate bus being used only as a backup in case the primary bus fails. 
     FIG. 3 shows one possible primary control signal bus assignment architecture, where the odd numbered radio heads (RH 1 , RH 3 , RH 5 , RH 7 ) use bus B as their primary bus, while the even numbered radio heads (RH 2 , RH 4 , RH 6 , RH 8 ) use bus A as their primary bus. The conrol bus assignments are indicated by the letter A or B within parentheses. Under such an arrangement, the control information for radio head RH 2  must travel through RH 8 , RH 7 , RH 6 , RH 5 , RH 4 , and RH 3  before reaching RH 2 . Thus, each message for RH 2  from RH 8  would have to travel through six links between radio heads before reaching RH 2 . In the art, this path is said to have six “hops.” In contrast, the path from RH 1  to RH 2  is only one “hop.” 
     If the control information being sent to RH 2  is time-critical control information, the propagation delays associated with use of control bus A (routing through RH 8 ) may, in some situations, result in the information not arriving in sufficient time to be properly processed. However, because the propagation delay associated with use of control bus B (routing through RH 1 ) may be much less, the time-critical information has a better chance of arriving in time. In addition, the routing on bus B may be less prone to errors, as there are less “hops” to RH 2  on bus B. 
     Accordingly, the present invention contemplates that primary assignment of one or more radio heads  200  to the respective control signal buses  250  be based on the propagation delay between the respective control interfaces  210  and the respective radio heads  200 . The overall process may be seen in the flowchart of FIG.  4 . The selection of primary control bus may be triggered by the “definition” of a radio head associated with the addition or deletion of a radio head from the radio head chain  150  (box  310 ). Of course, other events may also trigger such selection, such as initial installation, a change in error coding employed at a given radio head, and the like, or as a simple maintenance operation. After being triggered, the propagation delay between each control interface  210  and each radio head  200  is automatically determined (box  320 ). This may be accomplished by each control interface  210  pinging a radio head  200  on its associated bus  250  and noting the time elapsed until a response is received. Alternatively, the control interface  210  may use some other measure of propagation delay, such as counting the number of “hops” between itself and the radio head  200  of interest. A table is then created, listing the various propagation delays to each radio head  200  for the various virtual buses  250  (box  330 ). The listing of propagation delays may be in units of time, or in other units, such as “hops”, or some other related metric, such as a normalized scale value. There may be one table for each control interface  210 , or there may be a combined table. The table(s) is then distributed to the regional processors  220  of each radio head  200  (box  340 ). Each radio head  200  then determines which bus  250  provides the shortest propagation delay to/from a control interface  210  (box  350 ). The primary bus  250  for that radio head  200  is then assigned based on that determination (box  360 ). 
     Applying the primary control signal bus selection logic of FIG. 4 results in the bus assignments shown in FIG. 5, where the control bus assignments are indicated by the letter A or B within parentheses. Assuming that the radio heads  200  employ the same error correction schemes, have the same processing speed, and that the distance between radio heads  200  is roughly similar, then the propagation delay between the respective control interfaces  210  and each radio head  200  in FIG. 5 should be proportional to the number of “hops” between that control interface  210  and the radio head  200 . While these assumptions may overlook real world variances, it is nonetheless useful for demonstrating the present invention. Under these conditions, application of the logic of FIG. 4 results in radio heads RH 1 , RH 2 , RH 3 , and RH 4  deciding, based on the associated propagation delays, to select bus B as their primary control signal bus, while radio heads RH 5 -RH 8  chose bus A as their primary control signal bus. This arrangement is shown in FIG.  5 . Thus, according to the present invention, control information may be supplied to radio head RH 2  about six times faster than under the approach of FIG.  3 . Of course, if the primary bus fails, then a secondary bus, such as the un-selected bus, may be used to send information to a radio head  200 , but additional propagation delays would be encountered in this backup mode. 
     The discussion above has focused on a chain  150  of eight radio heads  200 , with two possible control signal buses  250 ; however, such is not required and the present invention contemplates any plurality of radio heads  200  and any plurality of possible control buses  250 . Further, as the present invention solves a problem associated with propagation delay of any sort, not just geographic separation induced propagation delay, some or all of the radio heads  200  in the radio head chain  150  may be essentially co-located if desired without departing from the present invention. 
     The disclosures of U.S. patent application Ser. Nos. 09/690,258 and 09/705,093, commonly assigned, are incorporated herein by reference. 
     The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.