Abstract:
A wireless duplex communication system, particularly for high density service areas serviced by a plurality of radio port units each including receiver-transmitter circuitry and an antenna; characterized in that each radio port unit further includes a first port for connecting the receiver-transmitter circuitry of one radion port unit to the antenna of another radio port unit in the same service area and a second port for connecting the antenna of said one radio port unit to the receiver-transmitter circuitry of said another radio port unit, whereby each of the two radio port units shares the antenna of the other radio port unit to provide space diversity.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to a wireless duplex communication system, and also to a method of operating such a system. The invention is particularly applicable to Wireless Local Loop (WLL) Time Division Duplex (TDD) digital systems, and is therefore described below with respect to such systems. 
     Wireless radio communication is subject to the adverse effects of signal fading and multi-path reflections in which the received signal exhibits distortion and fluctuations in strength. This in turn detrimentally affects the reliability of the communication link. Space diversity is a well known technique for coping with such phenomena. Generally, the term “space diversity” refers to the technique wherein the system uses two or more receiver antennas and selects the better antenna. The two antennas head in the same direction but are physically spaced from each other so as to have little correlation with respect to interferential fading or multi-path reflections, and do not undergo deterioration in quality at the same time. 
     Space diversity may be applied to both the uplink and downlink transmissions using the two antennas spaced apart in a way that minimizes the correlation between the signals at the antennas. Hence, a base station system is normally comprised of three units: the base station hardware circuitry box, and the two antennas. S/N evaluation for each antenna is accomplished either by switching the antennas to a single receiver (a time-consuming in-line process), or by applying a receiver to each antenna for simultaneous reception (hardware consuming). The signal from the better receiving antenna is further processed and is used for controlling the downlink transmission. 
     WLL systems operating in high density populated urban areas have to deal with high traffic capacity situations, which leads to solutions of multiple, co-located base stations. Because of the large number of units, and the required spacing between the antennas, multiple co-located base station systems, each utilizing space diversity, demand a substantial amount of installation space, i.e., tower height and/or volume, popularly known as “Christmas Tree” sites. 
     Environmental regulations, customer requirements for a less conspicuous distribution site and for a more aesthetic appearance, simplified installation processes, and significant reduction of the currently-high installation costs and resources, all demand a more efficient system configuration than the existing ones. 
     OBJECTS AND BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a wireless duplex communication system, and also a method of operating a communication system, having advantages in the above respects. 
     Another object of the present invention is to provide an improved diversity transmission and reception architecture and method particularly useful for WLL TDD co-located multiple based stations, which architecture and method reduce significantly the number of antennas in each site and minimize the installation process and resource requirements. 
     According to one aspect of the present invention, there is provided a wireless duplex communication system, particularly for high density service areas serviced by a plurality of radio port units each including receiver-transmitter circuitry and an antenna; characterized in that each radio port unit further includes: a first port for connecting the receiver-transmitter circuitry of the radio port unit to the antenna of another radio port unit in the same service area; and a second port for connecting the antenna of the radio port unit to the receiver-transmitter circuitry of the other radio port unit; whereby each of the two radio port units shares the antenna of the other radio port unit to provide space diversity. 
     According to further features in the preferred embodiments of the invention described below, the receiver-transmitter circuitry of each radio port unit includes: a first receiver; a second receiver; a transmitter; a multi-port coupler for splitting the signal energy received by the antenna of the radio port unit and for directing a portion thereof as a first signal to the first receiver of the respective radio port unit, and another portion thereof as a second signal to the second receiver of the another radio port unit, such that the first receiver of each radio port unit receive signals from the antenna of the respective radio port unit, and the second receiver of each radio port unit receives signals from the antenna of the another radio port unit; and a selector for selecting the antenna providing the better received signal for connection to the transmitter of the respective radio port unit for transmission. 
     According to further features in the described preferred embodiments, each radio port unit includes: compensation circuitry in the receiver path to the multi-port coupler to compensate for losses in the multi-port coupler; compensation circuitry in the transmitter path to compensate for losses in the multi-port coupler; and isolation circuitry between the transmitter of the respective radio port unit, and the transmitter in the another radio port unit to be connected thereto, to reduce intermodulation products in the transmitted signal. 
     According to still further features in the described preferred embodiments, the radio port units are enclosed within boxes, each box including within it at least one radio port unit, its antenna, and its receiver-transmitter circuitry, each box further including first and second ports exposed externally of the box for connection to corresponding ports in a radio port unit of another box. 
     As will be described more particularly below, the invention may be implemented in a communication system wherein some of the boxes in the services area are: of the single radio port configuration wherein the box includes a single antenna, a single radio port unit, and a two-port coupler for coupling its antenna to another box; and/or of a dual radio port configuration wherein each box includes a single antenna, a single radio port unit, and a two-port coupler for coupling the respective internal antenna to another box; and/or of a dual radio port coupler configuration wherein each box includes a single antenna, two radio port units, and a four-port coupler for coupling the respective internal antenna to each of the two radio port units of its box as well as to each of the two radio port units of another box. 
     An advantage of this invention is that the Radio Port Coupler (RPC) maintains the flexibility to perform as a conventional base station system in cases where special antennas are required (i.e., omnidirectional, hi-gain, 120° sector. etc.), for applications of single base station sites, or different sector angle configurations. Reconfiguration of the RPC is simple and can be performed in the field as it merely involves unplugging the internal antenna and relocating an internal coaxial cable. Also, built into the RPC is the flexibility to perform as a conventional base station system using the internal antenna and a single external antenna for space diversity. 
     According to another aspect of the present invention, there is provided a method of operating a wireless duplex communication system in a high density service area, comprising: providing the service area with a plurality of co-located radio port units located spatially apart from each other; providing at least some of the radio port units with a single antenna; and externally connecting each of the radio port units having a single antenna to the antenna of another radio port unit heading in the same direction to enable each radio port unit provided with a single antenna to share the antenna of another radio port unit in order to produce space diversity for improving single reception under fading and multi-path channel conditions. 
     As will be described more particularly below, the present invention improves the facilities of a co-located cluster of fixed base stations which communicate with fixed subscriber units, and provides a space diversity system which reduces error probability caused by fading and multi-path channel conditions. Installations may thus be erected with a large number of co-located base stations in a substantially reduced installation space, while still complying with environmental regulatory requirements and also producing a more pleasing aesthetic appearance. 
     Further features and advantage of the invention will be apparent from the description below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 illustrates the configuration of a typical prior art base station system; 
     FIG. 2 illustrates a single RPC (Radio Port Coupler) configuration in accordance with the present invention comprising a single radio port unit; 
     FIG. 3 illustrates a dual RPC configuration each comprising a single radio port unit; 
     FIG. 4 illustrates a dual RPC configuration each comprising two radio port units: 
     FIG. 5 is a detailed block diagram of an RPC with a single radio port unit such as shown in FIG. 2; 
     FIG. 6 shows the interconnection configuration of a dual RPC, each comprising a single radio port unit as shown in FIG. 3; and 
     FIG. 7 shows the interconnection configuration of a dual RPC, each comprising two radio port units as shown in FIG.  4 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a typical prior art base station system including three units: the radio port unit  12 , and two antennas  13 ,  14  for space diversity located spatially apart from each other in a way that produces minimum correlation with respect to interfering signals. Each antenna  13 ,  14  is connected to the radio port unit  12  via a relatively short coaxial cable  15 ,  16  respectively. In case of multiple co-located base station sites, the same system is duplicated and mounted appropriately apart from each other, side by side, and/or one on top of the other. 
     FIGS. 2,  3  and  4  illustrate the present invention implemented in RPC (Radio Port Coupler) systems of various configurations including: a single radio port unit (FIG.  2 ), two radio port units (FIG.  3 ), and four radio port units (FIG.  4 ), respectively. 
     The system in FIG. 2 performs the same as the conventional base station of FIG. 1, but with only two boxes  21 ,  22  connected to each other by a single cable  23 , rather than with the three boxes in FIG.  1 . Box  21  of FIG. 2 is a Radio Port Coupler (RPC) constructed in accordance with the present invention. It contains a single internal antenna and is connectable to an external antenna  22  by a cable  23  to provide space diversity. One construction that may be used for RPC unit  21  of FIG. 2 is described below with respect to FIG.  5 . 
     FIG.  3 . illustrates a dual RPC configuration containing two boxes  31 ,  32 , each including a single radio port unit (RPU) having a single internal antenna. The two RPC boxes  31 .  32 , are interconnected by two cables  33 ,  34 . A specific construction that may be included in the dual RPC configuration of FIG. 3 is shown in FIG. 6, described below. Such a configuration, containing but two boxes, is equivalent to two sets of the conventional base station of FIG. 1, namely to six boxes. 
     FIG. 4 illustrates a dual RPC configuration also containing two boxes  41 ,  42 . Here, however, each box includes two RPU&#39;s; and the two boxes are interconnected by four cables  43 ,  44 ,  45  and  46 . Such a dual RPC configuration, sometimes called a quad RPU system, is equivalent to four sets of the conventional station of FIG. 1, containing  12  boxes, rather than two boxes. FIG. 7, described below, illustrates a specific construction that may be used for the dual RPC configuration of FIG.  4 . 
     It will thus be seen that the RPC configurations illustrated in FIGS. 2-4 drastically reduce the number of boxes required in a service area. For example, in a densely populated urban area, having a traffic density of 0.1 Erlang per subscriber in a 60° sectored system, and capable of handling eight simultaneous calls per radio port, the number of conventional base stations (FIG. 1) needed to serve 2000 subscribers with a GOS (grade of service) of 99% is 36. As each conventional base station (FIG. 1) comprises three boxes, a total of 108 boxes would be needed, averaging 18 boxes per sector (6 base station sets). By using the present invention architecture for this example, only four boxes per sector would be required instead of 18, these being two RPC boxes configured as in FIG. 3, and two RPC boxes configured as in FIG.  4 . The first dual RPC configuration would comprise two radio ports, and the second one four radio ports. The total number of boxes for the whole site would be 24 instead of 108; thus in this case, the number of boxes would be reduced by a factor of 4.5, by using dual RPC configurations of the type illustrated in FIG. 4 only (in case the original number of base stations per sector is a multiple of 4), the maximal reduction factor would be 6. The RPC boxes of the present invention may maintain the same features and parameters as the standard radio port boxes. The RPC box could have the same front area, which is determined by the antenna; however the depth would be slightly larger to include the radio port circuitry. 
     FIG. 5 is a detailed block diagram of a single RPC box, e.g., corresponding to box  31  in FIG. 3, which would be the same as box  32  in FIG.  3 . Such a single RPU (Radio Port Unit) has a single internal antenna  51 , and a two-port coupler  52 . It further includes an RF connector port P 1  which couples its receiver-transmitter circuitry, via a coaxial cable (e.g.,  33 , FIG.  3 ), to the internal antenna of the second RPC box, (e.g., box  32  in FIG. 3) which antenna is used as the second antenna to perform the space diversity. In the same way, the internal antenna  51  of RPC box  31  is coupled to the receiver-transmitter circuitry of RPC box  32  via connector port P 2  to perform space diversity for the other box. 
     As shown in FIG. 5, each RPC box includes two receivers  53   a ,  53   b , and a single transmitter  54 . As will be described more fully below, the coupler  52  splits the signal energy received by the antenna of the respective box, and directs a portion thereof as a first signal to one of the receivers  53   b  of the respective box, and another portion as a second signal to the second receiver  53   a  of the other box to which the one box is connected via port P 2 . Thus, the first and second receivers of each box receive signals both from the antenna of its respective box and from the antenna of the other box connected to it, respectively. 
     Each box further includes a logic selector circuit  55  which compares the outputs of the two receivers  53   a ,  53   b , and from this, determines the antenna providing the better received signal (using standard methods for analyzing received signal quality), and connects that antenna to the transmitter  54  for transmission. 
     Each RPC box further includes two switches, SW 1 , SW 2 , for directing the signal received by the internal antenna  51  to the two port coupler  52 ; via a bandpass filter and low noise amplifier  52   a , and during transmission bypassing said amplifier and two further switches SW 3 , SW 4 , which serve as transmitter receiver switches enabling, during transmission periods, the appropriate antenna to be connected to the transmitter circuitry, and during reception to connect both antennas to the receivers circuitry. Each RPC box further includes another switch SW 5 , which serves as a transmitter selector switch controlled by the selector logic circuit  55  for selecting the appropriate antenna to be used for transmission during the transmission periods. 
     The illustrated system has full transmission-receiver synchronization. This means that all base stations (RPC boxes) and subscribers are synchronized so that when all RPCs transmit all subscribers receive, and when all subscribers are transmitting all RPCs are receiving. 
     For example, assuming the TDD system accommodates eight simultaneous subscribers, there would be eight time slots for down-link transmission from the base station to each subscriber, and eight time slots for up-link transmission from each subscriber to the base station. With each data burst for each subscriber, the signal is received during the receive period from its internal antenna  51 , and from the antenna of the other RPC box via port P 2 , as controlled by the switches SW 1 , SW 2 . The multi-port coupler  52  of the respective RPC box splits the received signal energy from its internal antenna and directs a portion thereof as a first signal to its receiver  53   b  via switch SW 3  in the respective box, and another portion as a second signal to receiver  53   a  of the other RPC box connected via port P 2  and its switch SW 4 . Thus, each RPC box receives signals from its respective antenna and from the antenna of the other RPC connected to it. These signals are processed and compared by the selective logic circuit  55  which selects the antenna providing the better received signal, and then actuates the switch SW 5  for connecting that antenna to the transmitter  54  for transmission during the transmission period. 
     The system illustrated in FIG. 5 includes compensating circuitry for compensating for various losses. This compensating circuitry includes a low-noise amplifier  52   a  in the receiver path between the internal antenna  51  and the two port coupler  52 . Low-noise amplifiers  56   a ,  56   b , and band path filters  57   a ,  57   b  are included in the receiver paths of the receivers  53   a ,  53   b  respectively. 
     In the transmitter path, the compensating circuitry includes a power amplifier  58 , for compensating for the losses in the coupler, typically 3.5 db for a two port coupler, and 7 db for a four port coupler. The output of the transmitter  54  includes an isolator  59  which increases the isolation between the transmitters to improve the intermodulation products in the transmitted spectrum. 
     FIG. 6 is a simplified block diagram illustrating the two RPC boxes  31 ,  32  of FIG. 3, each box according to the construction illustrated in FIG. 5, connected together by the two cables  33 ,  34  of FIG.  3 . 
     FIG. 7 is a simplified block diagram illustrating the dual RPC configuration corresponding to the diagram of FIG. 4, including two RPC boxes  41 ,  42 , connected together by four cables  43 ,  44 ,  45 , and  46 . In this configuration, each antenna is shared by both RPU 1  and RPU 2  of box  42 , as well as by RPU 3  and RPU 4  of box  41 . 
     While the invention has been described with the respect to several preferred embodiments, it will be appreciated that these are set forth merely for purposes of example, and many other variations, modifications and applications of the invention may be made.