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:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to communication systems. More specifically, the invention relates to a communication system using a code division multiple access air interface between a plurality of individual subscribers distributed within a cellular community and a plurality of small capacity base stations, some colocated per cell to increase operational economy in proportion to the number of subscribers. 
   2. Description of the Prior Art 
   Advanced cellular communication makes use of a state of the art technique known as code division multiplexing, or more commonly, as code divisional multiple access or CDMA. An example prior art communication system is shown in  FIG. 1 . 
   CDMA is a communication technique in which data is transmitted with a broadened band (spread spectrum) by modulating the data to be transmitted with a pseudo-noise signal. The data signal to be transmitted may have a bandwidth of only a few thousand Hertz distributed over a frequency band that may be several million Hertz wide. The communication channel is being used simultaneously by m independent subchannels. For each subchannel, all other subchannels appear as noise. 
   As shown, a single subchannel of a given bandwidth is mixed with a unique spreading code which repeats a predetermined pattern generated by a wide bandwidth, pseudo-noise (pn) sequence generator. These unique user spreading codes are typically orthogonal to one another such that the cross-correlation between the spreading codes is approximately zero. The data signal is modulated with the pn sequence producing a digital spread spectrum signal. A carrier signal is then modulated with the digital spread spectrum signal establishing a forward-link and transmitted. A receiver demodulates the transmission extracting the digital spread spectrum signal. The transmitted data is reproduced after correlation with the matching pn sequence. When the spreading codes are orthogonal to one another, the received signal can be correlated with a particular user signal related to the particular spreading code such that only the desired user signal related to the particular spreading code is enhanced while the other signals for all other users are not enhanced. The same process is repeated to establish a reverse-link. 
   If a coherent modulation technique such as phase shift keying or PSK is used for a plurality of subscribers, whether stationary or mobile, a global pilot is continuously transmitted by the base station for synchronizing with the subscribers. The subscriber units are synchronizing with the base station at all times and use the pilot signal information to estimate channel phase and magnitude parameters. For the reverse-link, a common pilot signal is not feasible. Typically, only non-coherent detection techniques are suitable to establish reverse-link communications. For initial acquisition by the base station to establish a reverse-link, a subscriber transmits a random access packet over a predetermined random access channel (RACH). 
   Most prior art CDMA communications systems employed to date, whether communicating with fixed or mobile subscribers that include personal communication services (PCS), have been designed for immediate large scale traffic considerations. A communication system specification proposed by a service provider establishes a required number of base stations which determine the region of communication coverage. The specification geographically locates each cell and establishes a traffic capacity that determines the number of anticipated subscribers per cell including fixed and mobile. The maximum capacity of communication traffic in each cell is typically fixed by this design. 
   Prior art CDMA communication systems have been designed and sized to immediately handle many simultaneous communications and are therefore costly start-up installations for the service provider. These systems have not addressed the need for a flexible base station architecture that permits a cost effective, small scale initial installation that can accommodate future subscriber growth. 
   Accordingly, the object of the present invention is to decrease the initial installation cost of a CDMA communication system while allowing future expansion when the need arises. 
   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 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. 
   The present invention supports both small and large size sectors or omni-cells with an architecture that allows for easy growth to support expanding traffic capacity without incurring a large initial fixed cost. 
   Accordingly, it is an object of the present invention to allow for easy expansion when subscriber communication traffic increases. 
   Other advantages may become apparent to those skilled in the art after reading the detailed description of the preferred embodiment. 

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

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention is described with reference to the drawings figures where like numerals represent like elements throughout. 
   A system diagram illustrating a CDMA communication system  15  employing scalable modular base stations is shown in  FIG. 2 . Four cells  17 ,  19 ,  21 ,  23  of a multi-cellular telecommunication system are shown with respect to their base station transceivers  17 N,  19 N,  21 N,  23 N. One subscriber unit  25  is shown within one cell. A plurality of individual forward and reverse signals are transmitted in respective regions of the common CDMA frequency bandwidth between the base station  17 ′ and subscriber unit  25 . 
   The base station units or BSUs employed in the scalable modular base station enable a scalable configuration proportional to the number of subscribers  25 . As an example, 150 subscribers whose average utilization during busy period is less than 10 percent, would require a base station unit with 16 modems supporting up to 15 simultaneous calls. For redundancy in case of a single failure, the scalable modular base station requires two colocated BSUs (having twice the minimum capacity) to serve the same communicating population to provide limited service in the event that one BSU failed. 
   The colocated modular approach supports additional growth, expanding beyond the two BSUs as the need arises. Each BSU is omnidirectional or may be configured with a directional antenna for sectoring. Likewise, as growth in a particular area of the cell arises, BSUs favoring a specified direction would be deployed to service the higher density sector. Each BSU connects to the public switched telephone network or PSTN via any one of several standard or proprietary terrestrial interfaces. 
   To support fault tolerance, it is necessary that each subscriber unit  25  be capable of communicating with a minimum of two BSUs. If 1 to n BSUs share coverage of a given cell area or sector, each subscriber unit  25  can communicate with any one of the n BSUs. In a presently preferred embodiment, n=6. Each subscriber unit  25  with the cell selects the BSU having the smallest path loss. 
   The scalable modular base station for a CDMA air interface requires a set of global channels to support operation. The global pilot supports initial acquisition by the subscriber and provides channel estimation for coherent 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 and is therefore costly to assign a set of full strength global channels for each BSU. 
   The scalable modular base station supports subscriber operation on battery standby during power outages. To do so requires a sleep mode where the subscriber unit  25  wakes up briefly, for example, once per second, to check for paging messages indicating an incoming call. However, when a subscriber&#39;s waking period is short, a base station&#39;s global pilot must be strong. The pilot strength must be greater 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 colocated BSUs and alternately acquires each one in sequence, once per wake up period. The subscriber unit  25  acquires a first BSU on even seconds and a second BSU on odd seconds. If more than two BSUs are deployed, the subscriber acquires each BSU in sequence returning to the first for the next interval. In direct correspondence, each BSU transmits its pilot at alternating high and low power levels in dependence upon how many BSUs are deployed in the particular cell. Only one BSU transmits a high power global pilot at a given time. The BSUs are preprogrammed to specify which BSU is selected to send its pilot at high power and which is selected to send its pilot at low power. 
   All colocated BSUs of the same group are preprogrammed to store two indices; Igroup, which designates the identity of the group and Iunit, which designates the identity of the BSU within the group. Each subscriber unit  25  is assigned to a group, designated by Igroup. For fixed wireless access, this can be designated and entered during registration. For mobile subscribers, this can be derived by the subscriber unit  25  testing the relative strengths of BSU pilots and selecting the strongest as is used for roaming and handoff. 
   Once a subscriber unit  25  is associated with an Igroup, when synchronizing it accesses each member BSU of the group; Igroup, Iunit. Each time a subscriber unit  25  wakes up, it re-synchronizes with the pilot signal of the BSU (Iunit) transmitting the pilot at full power. The subscriber unit  25  derives the identity of the BSU based on time of day. Other subscriber units  25  associated with Igroup use the same method to specify which BSU is transmitting the strong pilot and broadcast channels. The effect is that all subscriber units  25  wake up and listen to the pilot and broadcast channels of the respective BSU transmitting at full power. 
   Each subscriber unit  25  receives the time of day from the PSTN. Network Operations and Maintenance functions provide messages which contain the time of day accurate to within 2 milliseconds. The messages are sent over the terrestrial link from the O&amp;M function to each base station location and on to each BSU. Each BSU sends the time of day once over a slow broadcast channel. The subscriber unit  25  uses the message to synchronize its internal clock. 
   The time of day (tod) is converted to the identity of one BSU by using modular arithmetic
 
 Iunit=tod mod ( n )  Equation 1
 
where n is the stored value of the number of BSUs within Igroup. Both the BSU and all subscribers of Igroup know which BSU will be broadcasting at a specific time. When awakened, the subscriber unit  25  synchronizes time, reads the messages in its assigned time slot and measures the strength of the received pilot signal from the transmitting BSU. The subscriber unit  25  also measures the activity of the transmitting BSU.
 
   The BSUs indicate the amount of capacity over the slow or fast broadcast channels. The slow broadcast channel indicates the amount of activity. The fast broadcast channel indicates activity through the use of traffic lights. Each traffic channel has an indicator called a traffic light resident on the fast broadcast channel which tells the subscriber unit  25  availability. Using the traffic lights as capacity indication, the subscriber unit  25  can derive which of the BSUs is least busy. All BSUs send paging messages. Upon identifying a page, the subscriber unit  25  will select the optimal BSU to connect with. The choice is determined on information such as level of usage and signal strength. The subscriber unit  25  will select the BSU which is associated with the strongest received pilot level unless that BSU is near maximum capacity determined by the traffic lights and/or the level of activity. 
   Since a BSU pilot is always programmed to be strong when a subscriber unit  25  wakes-up, the wake up time can be minimized. The strong pilot is required to simplify reacquisition by a subscriber unit  25  after wake-up. Thereafter, the subscriber unit  25  returns to low duty cycle and low battery consumption. The lower level pilot, with a signal power level approximately ½ of a normal traffic channel is transmitted at all times. Since each BSU is transmitting a global pilot at a lower power level when not supporting the wake-up process, each BSU supports coherent demodulation of established traffic channels at all times with a negligible affect on total air capacity. 
   For each wake-up cycle, the subscriber units  25  derive the BSU of choice from the Igroup, based on the time of day, and load the pn spreading codes corresponding to the global pilot and broadcast channels of the BSU chosen. The subscriber unit  25  then measures the relative strength of the received pilot signal, once per wake-up cycle and stores the relative level and performs an average of 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 on the slow broadcast channel or, observes and stores the number of red traffic lights on each BSU maintaining a short term average. 
   The subscriber unit  25  performs a selection process to identify a favored BSU. When a subscriber unit  25  requests an access channel, the preferred BSU is selected loading the appropriate codes and initiating a normal ramp-up process. 
   The BSUs maintain a time of day clock, reading the time at either once per millisecond or once per subepoch. The time of day is used to identify its global channel transmit period. Thereafter, its respective global channels are allocated and the transmit power is set to the desired level. Traffic messages and signals normally sent by the BSU over its broadcast channels proceed. When synchronization between the subscriber unit  25  and a BSU is complete, the subscriber unit  25  transmits symbol length short code while gradually increasing the transmit power level. The subscriber unit  25  monitors the BSU for an acknowledgment signal, which acts as a traffic light to determine if the BSU receives and acknowledges the short code. 
   The subscriber unit  25  process for BSU selection includes keeping a data base in memory with the following information: 
   RelPower(Iunit); where Iunit=1 to n
             where RelPower is the relative power of BSU (Iunit) and there are n units total.       Activity(Iunit); where Iunit=1 to n
 
For each wake up cycle:
   RelPower(Iunit) is maintained as a low pass filtered estimate of the received measured pilot power:
 
 RelPower ( Iunit )= RelPower ( Iunit )+α(measured pilot power− RelPower )  Equation 2
   Activity(Iunit)=level of traffic as sent on broadcast channel, or   Activity(Iunit)=number of red traffic lights counted on current wake up cycle for the BSU
 
When a subscriber unit  25  attempts an access request, the BSU assignment is determined as a function of relative received pilot power level and relative activity. For example, the subscriber unit  25  can select the BSU with the strongest received pilot provided its activity is below a threshold. As one skilled in this art would recognize, other performance criteria could be used.
       

   The architecture and physical implementation for an example scalable modular base station  61  is shown in  FIGS. 3 ,  4  and  5 . The physical configuration for a base station  61  includes four separate enclosures: 1) a digital base station cabinet (DBC)  63 ; 2) a base station power supply module (BSPM)  65 ; and 3&amp;4) two amplified antenna modules (AAM)  67   1 ,  67   2 . 
   The base station cabinet  63  is an environmental enclosure which supports indoor or outdoor installation. The DBC  63  houses BSUs  69 . The AAMs  67   1 ,  67   2  are mounted remote from the BSU  69 , at a high elevation  71 . Each BSU  69  requires two AAMs  67 . 
   The BSPM  65  is shown in  FIG. 4  and includes storage batteries  73 , an ac/dc rectifier/inverter  75  and active voltage regulation  77 . The BSPM  65  receives external power  79  from a 120/220 Vac mains power supply (not shown) and provides an isolated filtered output  81  to a DBC  63 . Operation is similar to an uninterruptable power supply commonly known in the electronic arts. The batteries  73  provide up to four hours of continuous operation for one DBC  63  (two BSUs  69 ) configured for maximum capacity upon a mains power supply fault. Power is coupled via an umbilical to the respective BSU(s)  69 . Since a DBC  63  may be located outdoors, the BPSM  65  is remote and environmentally sealed as well. 
   As shown in  FIG. 5 , the BSU  69  is a card rack  83  assembly having a common communication backplane  85  using a high speed parallel data bus  87  and a power distribution bus  89 . The removable card complement for a base station  61  requires: 1) one system control module (SCM)  91 ; 2) one baseband transceiver module (BTM)  93 ; 3) one 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 (not shown). The PSM  95  couples the external BSPM  65  with a BSU  69  via male/female connectors (not shown) and provides local power supply regulation and filtering. 
   The SCM  91  contains a systems level microprocessor with collateral memory for controlling transmit/receive modem selection and coordinating component failure with another colocated BSU  69 . Each SCM  91  includes a communication bus port  105  to allow communication over a data transport such as Ethernet® E1 line between colocated BSUs  69 . The communication bus also allows external interrogation of each SCM  91  for up-loading or down-loading operational software or operation parameters. SCM  91  identification is accomplished via DIP switches or the like. External connections to the modular base station are made via F-ports  109  on this module and can support copper HDLC lines or fiber optic lines for receiving a POTS E1 line  111  which may carry up to 60 EDPCM calls. 
   The BTM  93  coordinates transmission by combining the analog baseband signals from active transmit AIMs  99  and distributes received communication signals to active receive AIMs  99 . If the required capacity of an installation requires two BSUs  69 , each BTM  93  per BSU  69  is coupled with each other. 
   The RFC  97  accepts the signal from a BTM  93  and upconverts  113  for transmission L 0 , L 1 . Likewise, the RFC  97  downconverts  115  received signals A, B for the BTM  93 . Digital to analog conversion along with transmit  117  and receive 119 selectable digital delays take place in the RFC  97 . 
   The AAM  67  encloses an omnidirectional printed circuit antenna  121  for transmission L 0, L   1  and reception A, B of communication signals. A directional antenna may be employed if cell sectorization is a design requirement. The A directional antenna may be configured to support three and six sector operation. High  125  and low  127  power duplexers separate the transmitting L 0 , L 1  and receiving A, B frequencies with separate amplifiers  129 ,  131  located in between for each respective frequency direction. Remote location of the transmitting  129  and receiving  131  amplifiers allow the use of low cost coaxial cable  133  between a RFC  97  and an AAM  67 . A dc potential is impressed by the BTM  93  on the coaxial cable to power both amplifiers  129 ,  131 . 
   Each AIM  99  includes up to 16 individual modems (not shown) for either transmission L 0 , L 1  or reception A, B depending on assignment. A BSU  69  can be configured with a minimum of one up to a complement of six AIMs  99 . Each AIM  99  contains 16 modems (15 simultaneous calls plus one broadcast modem). Depending upon traffic need, a maximum of six AIMs  99  can support up to 98 PCM or 180 LD-CELP calls. 
   The modular architecture  61  can support both small and large size sectors in a cell or an omni cell. Each BSU  69  is initially configured to support the number of calls and the specific type of service required depending upon the number of modems  135  (AIMs  99 ) installed. A minimum of two colocated BSUs  69  are required for redundant operation at a designated cell location. Since each BSU  69  has no internal redundancy if a single failure occurs, redundancy is achieved by allowing any fixed or mobile subscriber unit  25  to communicate with a colocated BSU  69  at the cell base station site. Redundancy is achieved by allowing any subscriber  25  to associate with any BSU  69  in a sector. If a BSU  69  should fail, capacity is lost, but a subscriber  25  can access another colocated BSU  69 . A BSU  69  in a sector can be configured with excess capacity thereby providing a cushion in the unlikely event of a failure in that sector. 
   Each BSU  69  communicates independently with an assigned subscriber. As previously described, to accomplish this function each BSU  69  must have unique global channels for the global pilot, the fast broadcast channel and the slow broadcast channel. 
   The unique global pilot allows each subscriber  25  to synchronize with an individual BSU  69 . The fast broadcast channel provides a traffic light function to the subscriber  25  informing him on BSU  69  availability and power ramp-up status from the respective BSU  69 . The slow broadcast channel transports activity and paging information from the BSU  69  to the subscriber  25  for personal communication services (PCS). 
   As discussed above, if each BSU  69  global pilot signal is transmitted as in the prior art, sector or cell capacity availability would be severely affected due to the effect on air capacity. Unlike the prior art, each BSU  69  continuously transmits a weak global pilot signal approximately one half of the signal strength of a standard 32 kbps POTS traffic channel. 
   Each colocated BSU  69  recognizes and handshakes with other colocated BSUs  69  via the external system communication E1 line, coupling each BSU  69  BTM  93 /SCM  91  with each other to coordinate the transmitting of the global pilot signals from one base station location. The E1 line interrogates each of the colocated BSUs  69  to coordinate the transmission of each of their unique global pilot signals. Each BSU  69  increases its global pilot signal level to a normal traffic channel level for a finite period of time. Each other BSU  69  continues transmitting their respective global pilot signals but at the weaker power level. This method insures that only one BSU  69  is transmitting its respective global pilot signal at a high power level. 
   The fast and slow  44  broadcast channels are transmitted from each BSU  69  at a nominal power level. If many BSUs  69  are colocated, the total air capacity overhead required to transmit the fast and slow broadcast channels, global pilot signals  137  and one strong global pilot signal  137  is increased when compared to one base station. However, the maximum capacity of 98 PCM calls per sector or cell is not affected since the overhead occurs only in the forward-link. The reverse-link is more problematic because of the assigned pilots from each subscriber limiting air capacity. 
   The power modulation of each pilot signal from a BSU  69  benefits the acquisition of subscribers  25 . Since each BSU  69  broadcasts its pilot signal at the normal power level for a finite period of time, a subscriber  25  will most likely acquire the strongest pilot signal. If the BSU  69  at maximum power has all of its modems active (either transmitting or receiving), the subscriber unit  25  will pass over and attempt to acquire the next consecutive full power pilot signal. 
   Each BSU  69  requires unique codes to transmit the unique global pilot signals. A common seed is provided to all BSUs  69  for the each pilot signal, but unique identities are manufactured by offsetting the code by z-thousand chips to effectively produce a unique code for each BSU  69 . From a single, common global pilot seed, a plurality of unique codes will be produced for each BSU  69 . 
   Referring to  FIGS. 6 and 7A  through  7 D, a scalable modular base station  61  installation includes at least one, two (as-shown), or a plurality of BSUs if required. 
   The adjustable receive delay units  119  located in associated with each AAM  67  shift the time-of-arrival for the received signals A, B, C, D. A single BSU  69  installation processes two adjustable time of arrivals  119  where each is summed  145  yielding a signal  147  that will have 2 copies of the received signal with different time delays. 
   A modular base station  61  that is sectorized or is configured for a large number of subscribers  25  will have a plurality of BSUs  69 . All AAMs  67  associated with this installation will share their received signals with each BSU  69 . The individual antenna  121  output are coupled to summers  145 ,  149  located on each respective BTM  93  of a BSU  69 . 
   All adjustable  119  time of arrivals are summed and input into each BSU  69  yielding a signal that will have y copies of the received signal with different time delays where y is an integer. Each AAM  67  receive delay unit  119  has a different predetermined delay. Preferably, each delay unit  119  imparts a delay of at least two chips which enables further processing to achieve a net increase in signal strength. 
   Each CDMA communication is associated with a unique code. The AIM  99  modems allow simultaneous processing of multiple CDMA communications, each processing a communication associated with a different CDMA code. Combining x signals with a known distortion enables the lowering of the transmit power required, 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 trunk configuration is shown in  FIG. 8 . A standby relationship is formed between the BSUs inside the DBCs  63  in the event of a single failure. From a radio distribution unit (RDU)  153 , a single E1 line  111  carrying up to 68 PCM calls is coupled to the BSUs. The topology also eliminates single mode failures while increasing signal throughput between modules. 
   While the present invention has been described in terms of the preferred embodiment, other variations which are within the scope of the invention as outlined in the claims below will be apparent to those skilled in the art.