MXPA00009092A - Modular base station with variable communication capacity - Google Patents

Abstract

The present invention provides a base station architecture that is modular in configuration, lowering the initial cost of implementing a new CDMA telecommunication system for a defined geographical region while allowing for future capacity. The scalable architecture is assembled from a digital base station unit that is configured to support a plurality of simultaneous wireless calls connecting to a conventional public switched telephone network. For initial startup, two base station units are deployed for redundancy in case of a single failure. Additional base station units may be added when the need arises for extra traffic capacity. If sectorization is required, the base station units may be directionally oriented. Coupled to and remote from each base station unit are two amplified antenna modules that contain an omni-directional or an external directional antenna, a high power RF amplifier for transmitted frequencies and a low noise amplifier for received frequencies. A separate power supply module capable of supporting two base station units provides continued service in the event of a mains power outage.

Description

BASIC MODULAR STATION WITH VARIABLE COMMUNICATION CAPACITY BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to communication systems. More specifically, the invention relates to a communication system using a multiple access air interface with code division between a plurality of individual subscri distributed within a cellular community and a plurality of small capacity base stations, some placed per cell to increase the operating economy in proportion to the numof subscri.

DESCRIPTION OF THE PREVIOUS TECHNIQUE Advanced cellular communication makes use of a state of the art known as multiplexing by code division, or more commonly, as multiple access with code division or CDMA. An example communication system of the prior art is shown in Figure 1. The CDMA is a communication technique in which data is transmitted with a widened band (extended spectrum) by modulating the data to be transmitted with a pseudo-noise signal. The data signal that is to be transmitted can have a bandwidth of only a few thousand Hertz distributed over a frequency band that can be several million Hertz wide. The communication channel that is being used simultaneously by m independent subchannels. For each sub-channel, all other sub-channels appear as noise. As shown, an individual sub-channel of a given bandwidth is mixed with a unique propagation code that repeats a given pattern generated by a broad-band pseudo-noise (pn) sequence generator. These unique user propagation codes are typically orthogonal to each other so that it is approximately the cross-correlation between the propagation codes. The data signal is modulated with the pn sequence that produces an extended-spectrum, digital signal. Then a carrier signal is modulated with the extended-spectrum, digital signal that establishes a direct link and is transmitted. A receiver demodulates the transmission by extracting the extended spectrum, digital signal. The transmitted data is reproduced after correlation with the coupling p sequence. When the propagation codes are orthogonal to each other, the received signal can be correlated with a particular user signal related to the particular propagation code such that only the user's desired signal related to the particular propagation code is enhanced while the other signals Do not intensify or improve for all other users. The same process is repeated to establish an inverted link. If a coherent modulation technique such as phase shift modulation or PSK is used for a plurality of subscri, whether stationary or mobile, a global pilot is continuously transmitted by the base station to synchronize with the subscri. The subscriunits are synchronized with the base station at all times and use the information from the pilot signal to estimate the channel magnitude and phase parameters. For the reverse link, a common pilot signal is not feasible. Typically, only non-coherent detection techniques are suitable for establishing reverse link communications. For initial acquisition by the base station to establish an inverted link, a subscritransmits a random access packet over a predetermined random access channel (RACH). Most of the prior art CDMA communication schemes used to date, whether they are communicating with mobile or fixed subscri that include personal communication services (PCS), have been designed for large scale traffic considerations, immediate A communication system specification proposed by a service provider establishes a required numof base stations that determines the communication coverage region. The specification locates each cell geographically and establishes a traffic capacity that determines the numof prospective subscri per cell including fixed and mobile. The maximum capacity of communication traffic in each cell is typically set by this design.

The CDMA communication systems of the prior art have been designed and sized to immediately handle many simultaneous communications and therefore are expensive startup facilities for the service provider. These systems have not addressed the need for a flexible base station architecture that allows initial, small-scale, cost-effective installation that can accommodate future subscriber growth. Accordingly, the object of the present invention is to decrease the initial cost of installing a CDMA communication system insofar as it allows future expansion when the need arises.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides a base station architecture that is modular in configuration, lowering the initial cost of implementing a new CDMA telecommunication system for a defined geographic region insofar as it allows future capacity. The scalable architecture is assembled from a digital base station unit that is configured to support a plurality of simultaneous, wireless calls that connect to a public, switched, public, telephone network. For the initial start-up, two base station units are deployed for redundancy in the case of an individual failure. Additional base station units may be added when the need for extra traffic capacity arises. If sectorization is required, the base station units can be oriented in a directional manner. Coupled to and away from each base station unit are two amplified antenna modules that contain an external or omni directional directional antenna, a high power RF amplifier for the transmitted frequencies and a low noise amplifier for the received frequencies. A separate power supply module capable of supporting the two base station units provides continuous service in the event of an energy shutdown to the main components. The present invention supports both small and large size sectors or omni-cells with an architecture that allows easy growth to support the expanding capacity of the traffic without incurring a large, fixed initial cost. Accordingly, it is an object of the present invention to allow for easy expansion when subscriber communication traffic is increased. Other advantages may become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified block diagram of a typical CDMA communication system of the prior art. Figure 2 is a communication network mode of the present invention. Figure 3 is a physical installation of a modular, scalable base station. Figure 4 is a block diagram of a modular, scalable base station power supply. Figure 5 is a block diagram of a base station unit. Figure 6 is a block diagram of two base station units. Figure 7A is a block diagram of two amplified antenna modules and radiofrequency control modules for the first base station as shown in Figure 6. Figure 7B is a block diagram of a six baseband transceiver module. air interface modules for the first base station unit as shown in Figure 6. Figure 7C is a block diagram of two amplified antenna modules and radiofrequency control modules for the second base station unit as shown in FIG. Figure 6. Figure 7D is a block diagram of a baseband transceiver module and six air interface modules for the second base station unit as shown in Figure 6. Figure 8 is a block diagram of a scalable baseband station that uses two base station units.

DESCRIPTION OF THE PREFERRED MODALITY The present invention is described with reference to the figures of the drawings in which like reference numbers similar elements are presented throughout. A system diagram illustrating a CDMA communication system 15 employing modular, scalable base stations is shown in Figure 1. Four cells 17, 19, 21, 23 of a multimodal telecommunication system are shown with respect to their transceivers 17 ', 19', 21 ', 23' base station. A subscriber unit 25 is shown inside a cell. A plurality of individual direct and inverted signals are transmitted in respective regions of the common CDMA frequency bandwidth between the base station 17 'and the subscriber unit 25. The BSU base station units employed in the modular, scalable base station allow a scalable configuration providing the number of subscribers 25. As an example, 150 subscribers whose average utilization during the busy period is less than 10 percent, will require a base station unit with 16 modems that support up to 15 simultaneous calls. For redundancy in the case of an individual failure, the modular, scalable base station requires two placed BSUs (which have twice the minimum capacity) to service the same communication population to provide limited service in the event that it pays for a BSU . The placed modular approach supports additional growth, expanding beyond the two BSUs as the need arises. Each BSU is either nidirectional or can be configured with a directional antenna for sectorization. Likewise, as growth arises in a particular area of the cell, the BSUs that favor a specific address will be deployed to serve a higher density sector. Each BSU connects to the connected, public or PSTN telephone network via any of the private or normal terrestrial interfaces. In order to withstand fault tolerance, it is necessary that each subscriber unit 25 be able to communicate with a minimum of two BSUs. If 1 to n BSU share the coverage of a given area of the cell or sector, each subscriber unit 25 can communicate with any of the n BSUs. In a currently preferred mode, n = 6. Each subscriber unit 25 with the cell selects the BSU that has the smallest route loss. The modular, scalable base station for a CDMA air interface requires a set of global channels to support the operation. The global pilot signal supports initial acquisition by the subscriber and provides channel estimation for consistent processing. One or more global broadcast channels provide signaling information. Each BSU requires its own set of global channels. However, global channels use air capacity so they are expensive to assign to a set of global channels of full intensity for each BSU. The modular, scalable base station supports the operation of the subscriber in standby mode of the battery during power shutdowns. To do so, it requires a rest mode where the subscriber unit 25 is briefly reactivated, for example, once per second, to verify the radiolocation messages indicating an incoming call. However, when the period of reactivation of the subscriber is short, the overall pilot signal of the base station must be strong. The intensity of the pilot signal should be higher than the level needed to simply provide a reference signal for coherent demodulation and channel estimation. Each subscriber unit 25 is assigned to a set of BSUs placed and alternately acquires each one in sequence, once per reactivation period. The subscriber unit 25 acquires a first BSU in seconds pairs and a second BSU in seconds nons. If they are deployed more than twice BSU, the subscriber acquires each BSU in sequence returning to the first for the next interval. In direct correspondenceEach BSU transmits its pilot signal to alternating levels of high and low power depending on how many BSU are deployed in the particular cell. Once a BSU transmits a high power global pilot signal in a given time. The BSUs are preprogrammed to specify that BSU is selected to send its pilot signal at high power and which is selected to send its pilot signal at low power. All BSUs placed in the same group are pre-programmed to store two indicia, I group, which designates the group entity and I group, which designates the identity of the BSU within the group. Each subscriber unit 25 is assigned to a group, designated by the group I. For a fixed wireless access, this can be designed and entered during registration. For noble subscribers, this can be derived by the subscriber unit 25 which tests the intensities relative to the BSU pilots and selects the strongest as used ~ for the roaming and the call transfer process. Once a subscribing unit 25 is associated with an I group, when it is synchronized with it, it has access to each BSU member of the group; I group, I unity. Each time a subscriber unit 25 is reactivated, it is synchronized again with the pilot signal of the BSU (I unit) that transmits the pilot signal to full power. The subscriber unit 25 derives the identity of the BSU based on the time of the day. Other subscriber units 25 associated with I anity use the same method to specify which BSU the strong pilot and broadcast channels are transmitting. The effect is that all the subscriber units 25 are reactivated and listen to the pilot signal and the broadcast channels of the respective BSU transmitting at full power. Each subscriber unit 25 receives the day time of the PSTN. Network operations and maintenance functions provide messages that contain the exact day time in the space of 2 milliseconds. Messages are sent over the terrestrial link from the O and M function to each base station location and in each BSU. Each BSU sends the time of day a v z "that is on a slow broadcast channel. The subscriber unit 25 uses the message to synchronize its internal clock. The day time (tod) is converted to the identity of a BSU when using modular arithmetic I uni dad = tod mod (n) Equation 1 where n is the stored value of the BSU number within an I group. Both the BSÜ and all the subscribers of the 1st group know which BSU will be broadcasting at a specific time. When it is reactivated, the subscriber unit 25 synchronizes the time, reads the messages in its assigned time segment and measures the intensity of the pilot signal received from the transmission BSU. The subscriber unit 25 also measures the activity of the transmission BSU. The BSUs indicate the amount of capacity over the slow or fast broadcast channels. Slow broadcast channels indicate the amount of activity. The fast broadcast channel indicates the activity through the use of traffic lights. Each traffic channel has an indicator called a traffic light resident in the fast broadcast channel that tells the subscribing unit 25 the availability. By using the traffic lights as an indication of capacity, the subscriber unit 25 can derive which of the BSUs is least busy. All BSUs send radiolocation messages. By identifying a radiolocation, the subscriber unit 25 will select the optimal BSU to connect. The choice is determined based on the information such as the level of use and signal strength. The subscriber unit 25 will select the BSU that is associated with the strongest level of the received pilot signal unless the BSU is close to the maximum capacity determined by the traffic lights and / or the activity level. Since a BSU pilot signal is always programmed to be stronger when a subscriber unit 25 is reactivated, the reactivation time can be minimized. The strong pilot signal is required to simplify the re-acquisition by a subscriber unit 25 after the reactivation. Subsequently, the subscriber unit 25 returns to a cycle of little work and a low energy consumption. The lower level pilot signal with a signal energy level of about 1/2 of a normal traffic channel is transmitted at all times. Since each time BSU is transmitting a global pilot at a lower power level when it does not support the reactivation process, each BSU supports the coherent demodulation of the established traffic channels at all times with a negligible effect on the total air capacity. For each reactivation cycle, the subscriber units 25 derive the BSU from the choice of the unit, based on the time of day, and load the propagation codes of PN that correspond to the global pilot signal and the broadcast channels of the BSU chosen. The subscriber unit 25 then measures the relative intensity of the received pilot signal, once per reactivation cycle and stores the relative level and averages the most recent set of measurements for each of the candidate BSUs. The subscriber unit 25 reads the amount of traffic currently supported by the given BSU, if that information is transmitted in the slow broadcast channel or observes and stores the number of red traffic lights in each BSU that maintain a short-term average. The subscriber unit 25 performs a selection process to identify a favorable BSU. When a subscriber unit 25 requests an access channel, the BSU is selected by loading the appropriate codes and initiating a normal hooking process. BSUs maintain a time-of-day clock, reading time either once per millisecond or once per sub-time. Daytime is used to identify your period of transmission of the global channel. Subsequently, their respective global channels are assigned and the transmission power is adjusted to the desired level. The traffic messages and the signals normally sent by the BSUs over their broadcast channels continue. When the synchronization between the subscriber unit 25 and a BSU is complete, the subscriber unit 25 transmits the symbol length short code while gradually increasing the transmit power level. The subscriber unit 25 monitors the BSUL for a recognition signal, which acts as a traffic light to determine whether the BSU receives and recognizes the short code. The process of subscriber unit 25 for the selection of BSU includes maintaining a database in memory with the following information: • Rel Power (I unit); where I uni dad = 1 a n where RelPotencia is the relative power of BSU (I uni dad) and there are n total units. • Activity (I unit); where I uni dad = 1 a n For each reactivation cycle Rel Power (I unit) is maintained as a low-pass filtered estimate of the measured pilot signal power, received: • Rel Power (I unit) = Rel Power (I unit) + § (measured pilot signal power - Rel Power) Equation Activity (I unit) = level of traffic as sent in broadcast channel, or Activity (I unit) = number of red traffic lights counted in reactivation current cycle for BSU.

When a subscriber unit 25 attempts an access request, the allocation of the BSU is determined as a function of the power level of the received pilot signal relative to the relative activity. For example, the subscriber unit 25 may select the BSU with the received pilot signal, stronger on the condition that its activity is below a threshold. A person skilled in the art will recognize that other performance criteria may be used. The architecture and physical implementation for a modular, scalable, exemplary base station 62 is shown in Figures 3, 4 and 5. The physical configuration for a base station 61 includes four separate enclosures or boxes: 1) a base station cabinet Digital (DBC) 63; 2) a base station power supply module (BSPM) 65; and 3 and 4) two amplified antenna modules (AAM) 67? , 672. The base station module 63 is a conventional enclosure that supports indoor or outdoor installations. The DBC 63 houses the BSU 69. The AAM 671, 672 is mounted away from the BSU 69, at a high elevation 71. Each BSU 69 requires two AAM 67. The BSPM 65 is shown in Figure 4 and includes storage batteries 73 , a AC / DC 75 rectifier / inverter and a regulation. 77 active voltage. The BSPM 65 requires external power 79 from a main Vac power supply of 120/220 (not shown) and provides a filtered, isolated output 81 to a DBC 63. The operation is similar to an uninterruptible power supply commonly known in the technique of electronic products. Batteries 63 provide up to four hours of continuous operation for a DBC 63 (two BSU 69) configured for maximum capacity in a main power supply failure. The power or power is coupled via a main cord to the respective BSU (s) 69. Since a DBC 63 can be located externally, the BPSM 65 is remote and environmentally sealed as well. As shown in Figure 5, the BSU 69 is a card shelf rack 83 having a common communication backplane 85 that uses a common high-speed parallel data bus 87 and a common power distribution bar 89 or power. The removable card add-on for a base station 61 requires: 1) a system control module (SCM) 91; 2) a baseband transceiver module (BTM) 93; 3) a power supply module (PSM) 95; 4) two radio frequency control modules (RFC) 97; and 5) up to six air interface modules (AIM) 99 each having 16 transmit / receive modems 101. The PSM 95 couples the external BSPM 65 with a BSU 69 via the male / female connectors 103 and provides regulation and local power supply filter. The SCM 91 contains a microprocessor of the levels of the system with collateral memory to control the selection of the transmission / receiver modem and control the failure of the components with another placed BSU 69. Each SCM 91 includes a port 105 of communication common bar for allow communication over a data transport 107 such as an EthernetR line between the placed BSUs 69. The communication bus 107 also allows the external interrogation of each SCM 91 for loading or unloading of operating parameters or computer counting programs. operation. The identification of SCM 91 is achieved via DIP switches or the like. External connections to the modular base station are made via F 108 ports on this module and can support copper HDLC lines or fiber optic lines to receive a 111 POTS line that can carry up to 60 EDPCM calls. The BTM93 coordinates the transmission by combining the baseband analog signals from the active transmission AIM 99 and distributes the received communication signals to the active reception AIM 99. If the required capacity of an installation requires two BSU 69, each BTM 93 per BSU 69 is coupled together. RFC 97 accepts the signal from a BTM 93 and converts upwards 113 for the transmission L0, L1. Similarly, RFC 97 downconverts 115 to received signals A, B for BTM 93. Conversion from digital to analog together with transmission 114 and reception 119 of digital or selectable delays takes place in RFC 97. The AAM 67 encloses a printed circuit, 121 unidirectional antenna for transmission L0, Lx and reception A, B of communication signals. A directional antenna 123 can be used and the cell sectioning is a design requirement. Antenna 123 can be configured to support three and six sector operations. The high 125 and low 127 power duplexers separate the transmission frequencies L0, reception A, with separate amplifiers 129, 131 located between each respective frequency direction. Remote manufacture of the transmission amplifiers 129 and reception 131 allows the use of low cost coaxial cable 123 between an RFC 97 and an AAM 67. A cd potential is recorded by the BTM 93 in the coaxial cable to drive both amplifiers 129, 131. Each AIM 99 includes up to 16b individual modems 135 for either transmission L0, Lx or reception A, B depending on the assignment. A BSU 69 can be configured with a minimum of up to a complement of six AIM 99. Each AIM 99 contains 16 modems (15 simultaneous calls plus a broadcast modem). Depending on the traffic need, a maximum of six AIM 99 can support up to 98 PCM or 180 LD-CELP calls. The modular architecture 61 can support sectors of both small and large size in a cell or in an omni-cell. Each BSU 69 is initially configured to support the number of calls and the specific type of service required depending on the modem number 135 _ (AIM 99) installed. A minimum of two BSUs 69 are required for redundant operation at a designated cell location. Since each BSU 69 does not have internal redundancy if an individual failure occurs, redundancy is achieved by allowing any fixed or mobile subscriber unit 25 to communicate with a BSU placed 69 at the site of the base station of the cell. Redundancy is achieved by allowing any subscriber 25 to associate with any BSU 69 in a sector. If a BSU 69 fails, the capacity is lost, but a subscriber 25 can have access to another placed BSU 69. A BSU 69 in a sector can be configured with access capability thus providing a cushioning or damping in the unlikely event of a failure in that sector. Each BSU 69 communicates independently with an assigned subscriber. As previously described, to save this function each BSU 69 must have unique global channels for the overall pilot signal 137, the fast diffusion channel 139 and the slow diffusion channel 141. The single global pilot signal 137 allows each subscriber 25 to synchronize with an individual BSU 69. The fast broadcast channel 139 provides a traffic light function to the subscriber 25 informing it of the availability of the BSU 69 and the latching state of power from the respective BSU 69. The slow-diffusion channel 141 transports the activity and radiolocation information from the BSU 69 to the subscriber 25 for the personal communication services (PCS). As discussed above, if each global pilot signal of BSU 69 is transmitted as in the prior art, the capacity availability of the cell or sector will be severely affected due to the effect of air capacity. Different from the prior art, each BSU 69 continuously transmits a weak overall pilot signal 137 about half the signal strength of a normal 32 bps POTS traffic channel. Each placed BSU 69 recognizes and communicates with or placed BSUs 69 via line 103, the system communication, external, coupling each BTM 93 / SCM 91 of the BSU 69 together to "coordinate the transmission of the overall pilot signal 137 from each base station location." The 103 line interrogates each of the placed BSUs 69 to coordinate the each of its unique, global, pilot signals 137. Each BSU 69 increases its level from the overall pilot signal 137 to a normal level of traffic channel for a finite period of time.Each different BSU 69 continues the transmission of its respective global pilot signals 137 but at a weaker energy level This method ensures that only one BSU is transmitting its respective global pilot signal 137 at a high power level, the fast diffusion 139 and slow 141 channels are transmitted from each BSU 69 at a nominal power level, if many BSU 69 are placed, the total air capacity above what is required to transmit the fast diffusion channels 139-141, the global pilot signals s 137 and a strong overall pilot signal 137 is increased when compared to a base station. However, the maximum capacity of 98 PCM calls per sector or per cell is not affected since the overload occurred only on the direct link. The reverse link is more problematic because the pilots assigned 133 of each subscriber mimic air capacity. The energy modulation of each pilot signal 137 of a BSU 69 benefits the acquisition of the subscribers 25. Since each BSU 69 broadcasts its pilot signal 137 at the normal power level for a finite period of time, a subscriber 25 will most likely acquire the strongest pilot signal 137. If the BSU 69 at maximum power has all of its modems 135 active (either transmitting or receiving), the subscriber unit 25 will pass over and attempt to acquire the next, consecutive full power pilot signal 137. Each BSU 69 requires unique codes to transmit the unique global pilot signals 137. A common seeding is provided to all BSU 69s for each pilot signal 137, but they are manufactured in unique entities by compensating the code for z-thousand chips to effectively produce a code only for BSU 69. From. a pilot, global, common seeding, a plurality of unique codes will be produced for each BSU 69. With reference to Figures 6 and 7A through 7D, a modular, scalable base station installation 61 includes at least one, two (as shown) ) or a plurality of BSU if required. The adjustable reception delay units 19 located in each AAM 67 change the arrival time of Xaa received signals A, B, C, D. An individual BSU 6 9 facility processes two adjustable arrival times 119 where each assumes 145 that produces a signal 147 which will have 2 copies of the received signal with different time delays. A modular base station 61 that is sectorized or configured for a large number of subscribers 25 will have a plurality of BSU 69. All AAM 67 associated with that facility will share their received signals with each BSU 69. The output of the individual antenna 121 it is coupled to the adders 145, 149 located in each respective BTM 93 of a BSU "69. All the adjustable arrival times 119 are summed and entered into each BSU 69 producing a signal that will have and copies of the received signal with different delays of time, where y is an integer, Each AAM receiving delay unit 119 has a different predetermined delay, Preferably, each delay unit 119 imparts a delay of at least two chips which allows for additional processing to achieve a Net increment in signal strength Each CDMA communication is associated with a unique code The modems 135 of the AIM 99 allow simultaneous multiplex processing CDMA communications, each processing of a communication associated with a different CDMA code. The combination of x signals with a known distortion allows the reduction of the required transmission power, increasing the number of subscribers 25 (the number of simultaneous communications) with a given base station. A cellular base station with the maximum number of BSUs in a two-link configuration is shown in Figure 8. A standby relationship is formed between the BSUs 69 in the case of an individual failure. A radio distribution unit (RDU) 153, a line The individual 111 that carries up to 68 PCM calls is coupled to the two BSU 69.

The topology also eliminates individual failures so that it increases the signal production through the modules. While the present invention has been described in terms of the preferred embodiment, other variations that are within the scope of the invention as summarized in the claims will be apparent later to those skilled in the art.

Claims (14)

1. CLAIMS "_ 1. A bidirectional communication system-using a CDMA air interface between a plurality of subscriber units communicating with a base station, the system comprising: a scalable base station configured of a selected maximum number of units modular, placed base stations to support communication growth capacity based on the number of modular base station units, each base station unit to communicate with a predefined number of subscriber units, each base station unit transmitting a channel signal pilot, global single CDMA at a full power level for a limited, discrete time interval, time interval that is distinct from the time intervals of all the other base station units in the scalable base station configuration, and a 'plurality of subscriber units for CDMA communication with the escalab base station Each subscribing unit having a means for selectively receiving global pilot channel signals of up to n modular base station units, such that the reception of each global pilot channel signal is in a discrete time interval synchronized with the interval of transmission time of the complete power level of the respective global pilot channel signal, time interval which is different from reception of all other transmission time intervals of the global pilot channel signals. The communication system according to claim 1, wherein n is calculated based on the maximum desired communication capacity divided by the capacity of a single base station unit. The communication system "according to claim 1, wherein the modular, scalable base station is configured from a selected number n of base station modular units where m < n 4. The communication system according to claim 1 , where n = 6. 5. The communication system according to claim 1, wherein the time interval is determined by the time of day 6. The communication system according to claim 1, wherein the means for receiving Selectively, global pilot channels include reactivating the discrete time interval 7. The communication system according to claim 1, wherein each unit or base station additionally transmits a fast broadcast channel and a slow broadcast channel. according to claim 1, wherein the means for receiving global pilot channels further includes storing the received intensity of the relative, global, pilot channel signal. The communication system according to claim 7, wherein the means for receiving global pilot channels further includes receiving the slow broadcast channel and the fast broadcast channel and deriving and storing which communication capability of the base station unit of the broadcast channels. slow and fast diffusion. The communication system according to claim 9, wherein a subscriber unit initiates communication with one of the modular base station units by choosing from storage the modular base station unit having the strongest overall pilot signal strength. The communication system according to claim 10, wherein the choice of storage further includes the communication capability of that base station unit. 12. A base station for use in a bidirectional communication system using a CDMA air interface within a plurality of subscriber units communicating with the base station, comprising: a scalable base station configured up to a maximum number selected n from modular units, placed from base station to support the communication growth capacity based on the number of modular units of base station, each unit of base station to communicate with a predefined maximum number of subscriber units at any given time; and each base station unit that transmits a single global CDMA pilot channel to a high level during a limited, discrete, time interval that is different from the time slots of the other base station units in the configuration scalable from the base station. 13. A subscriber unit for use in a bidirectional communication system using a CDMA air interface between the plurality of subscriber units communicating with a base station transmitting multiple pilot, global channels, comprising: a means for selectively receiving a predetermined number n of global pilot channels of the base station such that the reception of the global pilot channel is in one of the n discrete time intervals, each interval to receive a different global pilot channel. 14. A method for providing bidirectional communication using a CDMA air interface between a plurality of subscriber units communicating with a base station, the steps comprising: configuring up to a selected maximum number of modular units, placed from a base station, a scalable base station for supporting the increasing communication capacity based on the number of modular base station units, each base station unit for communicating with a predefined number of süscript units; transmitting a single CDMA pilot, global channel signal from each base station unit to a full power level for a limited, discrete time interval, time slot that is distinct from the time intervals of the other base stations in the scalable base station configuration; and selectively receiving the global pilot channel signals of up to n modular base station units in a plurality of subscriber units for CDMA communication with the scalable base station, such that the reception of each global pilot channel signal is in a range of discrete time synchronized with the full power level transmission time slot of the respective global channel which is different from the reception of the other global pilot channel signals. SUMMARY OF THE INVENTION The present invention provides a base station architecture that is modular in configuration, lowering the initial cost of implementing a new CDMA communication system for a defined geographic region that allows future capacity. The scalable architecture is assembled from a digital base station unit that is configured to support a plurality of simultaneous wireless calls that are connected to a public, switched, public, telephone network. For the initial start-up, two base station units are deployed for redundancy in the case of an individual failure. Additional base station units can be added when the need for extra traffic capacity arises. If sectorisation is required, the base station units can be oriented in a directional manner. Attached and away from each base station unit are the amplified antenna modules that contain an external or omnidirectional directional antenna, a high-power RF amplifier. power for the transmitted frequencies and a low noise amplifier for the received frequencies. A separate power supply module capable of supporting two base station units provides continuous service in the event of a shutdown of the main power.