A METHOD FOR ACCESSING A MICROCELL USING ANALOG CONTROL CHANNELS

Abstract: The process of the present invention monitors a macrocell's control channel using a microcell having a control channel tuned to the same frequency and digital color code. Both the microcell and the macrocell have the same controller that waits for an access message. Once the access message is received, the controller waits for a duplicate access message within a predetermined time. If the duplicate access message is received from the microcell, the microcell's control channel RSSI of the radiotelephone is measured (RSSIm) to determine if it is greater than or equal to the threshold required for adequate communication with the microcell. If the RSSIm is greater than or equal to the threshold, the radiotelephone is assigned to the microcell. Otherwise, the radiotelephone is assigned to the macrocell.

Description: A METHOD FOR ACCESSING A MICROCELL USING ANALOG CONTROL CHANNELS BACKGROUND OF THE INVENTION I. FIELD OF THE INVENTION The present invention relates to radio communications. More particularly, the present invention relates to cellular radiotelephone systems. II. DESCRIPTION OF THE RELATED ART A cellular radio communication system is typically comprised of a number of cells covering a geographic region. Each cell is comprised of a base station that is allocated a number of radio channels for transmitting from a number of directional antennas. The cell may also be divided up into sectors, each sector having a number of different channels. The cell shapes are determined by both the radiation pattern of the antennas and the local conditions at the cell site. Cells, however, are typically idealized as hexagonal patterns since such a pattern closely approximates the ideal antenna radiation pattern. Cells are typically organized in clusters. Each cluster has a predetermined number of cells (e.g., an N=7 system refers to each cluster in the system having 7 cells) with each cell being assigned different frequency groups. The same frequency groups are reused in corresponding cells of different clusters. These clusters are repeated as needed to cover a geographic area. An example of this frequency layout concept is illustrated in FIG. 1. The numbers illustrated in the cells of FIG. 1 indicate the frequency groups assigned to each 120 sector of the cell. In this example, there are 21 different frequency groups that are used in a cluster. The cluster is reused multiple times within the geographic region depicted in FIG. 1. FIG. 2 illustrates a prior art cell site connected to the mobile telephone exchange (MTX). The antenna (200) at the cell site is connected to the base station (210) that contains the receivers and transmitters for the cell site to communicate with the mobile radiotelephone (230). The base station is connected to and controlled by a base station controller (BSC) (215). The BSC (215) is connected to the MTX (220) that is then connected to the public switched telephone network (PSTN). The MTX handles all of the switching for a number of cell sites and BSCs, routing calls from the PSTN to the appropriate cell site and routing calls from the cell site to the PSTN. In a typical radiotelephone system, a subscriber is not configured to a specific cell. A radio channel is allocated to the radiotelephone for call initiation based on a number of factors including: the Received Signal Strength Indicator (RSSI) (measured during access), the type of radiotelephone (analog or digital), interference conditions, and the availability of radio channels. As the radiotelephone moves through different cells of the cellular system, the radio channels are dynamically allocated to the radiotelephone in response to changes in these factors (see Electronic Industry Association Recommended Standard-553 (EIA RS-533) for a detailed explanation of the RSSI). Microcells are lower power cells that cover a smaller area than macrocells, as illustrated in FIG. 3. FIG. 3 shows a microcell (300) that is located within the bounds of a macrocell (305). In a typical urban setting, the radius of the macrocell (305) is approximately 500 meters while the radius of a microcell (300) is typically less than 200 meters. Microcells are typically deployed in a cellular system as a cost effective * solution for at least two applications: cold and hot spot coverage. Cold spot coverage occurs in a macrocell system that has radio signal coverage holes. In this situation, the radiotelephone either cannot communicate with any base station or the quality of the communication signal is substantially reduced due to lack of coverage. In a hot spot coverage situation, a macrocell or macrocells experience a high concentration of traffic. In this case, the base station may run out of available frequencies and not be able to handle additional radiotelephone traffic. In both situations, microcells are typically added as underlay cells to the existing macrocell system. Once underlaid, the microcells require a frequency assignment. The typical methods for assigning frequency groups to microcells are reusing frequencies and reserving or segregating frequencies from the macrocell frequency groups. In the frequency reuse scheme, a microcell reuses the frequencies assigned to the macrocell system. The frequency reservation or segregation scheme reserves a block of frequencies normally used by the macrocell system. In theory, a microcell handles any traffic within its defined boundaries. This includes calls originating or terminating within its range as well as hand- offs of radiotelephones passing through its coverage area. In reality, however, this operation is difficult to achieve. A problem comes from the limitations of the cellular air interface standards for analog control channels (EIA RS-553, IS-91, and IS-54). These standards were not designed to work with overlay/underlay schemes. For example, they lack the ability to steer radiotelephones to the desired control channel. As a consequence, the cellular system must attempt to compensate for the behavior of the radiotelephones, as dictated by the standards, and must make fast and accurate decisions on the dispositions of calls based on inadequate, and potentially inaccurate, information. Analog radiotelephones are required to lock onto the strongest control channel received. In an underlay /overlay arrangement, that control channel is almost always the macrocell control channel. Even if the mobile is physically closer to the microcell, the macrocell transmits at a higher power. There is no information in the mobile access messaging that tells the system how close the radiotelephone is to any underlaid microcells and, hence, no effective method for redirecting the access from the macrocell. Additionally, since there is little reliable information available from the voice channel serving a radiotelephone as to its distance and direction from the macrocell, there is no way of directly determining that the mobile has entered the coverage area of the microcell. Most systems rely on continuous polling of the radiotelephones in the macrocell to evaluate which ones may be candidates for handing down to the microcell. This ties up the base station's resources that could be better used servicing additional traffic. There is a resulting need for a more efficient and inexpensive analog cellular system and process that increases call capacity with a minimal impact on call quality SUMMARY OF THE INVENTION The present invention encompasses a process for accessing a microcell by a radiotelephone. In the preferred embodiment, the microcell is underlaid to a macrocell. The process first determines the proximity of the radiotelephone to the microcell. If the proximity is less than or equal to a predetermined distance, the radiotelephone is assigned to the microcell. If the proximity is greater than a predetermined distance, the radiotelephone is assigned to the overlying macrocell. In the preferred embodiment, the proximity of the radiotelephone to the microcell is determined by the microcell using a control channel with the same frequency and digital color code as the macrocell's control channel. This allows the microcell to monitor the RSSI .of the radiotelephone. If the RSSI is above a predetermined threshold, and the microcell has available voice channels, the radiotelephone is assigned to the microcell. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a typical prior art cellular system frequency layout. FIG. 2 shows a typical prior art cell site. FIG. 3 shows a macrocell with an underlaid microcell. FIG. 4 shows a flowchart of the process of the present invention. FIG. 5 shows a block diagram of a typical prior art radiotelephone of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The process of the present invention determines the radiotelephone's proximity to the microcell using the microcell's control channel. If the RSSI of the radiotelephone is greater than or equal to a predetermined threshold and a voice channel is available from the microcell, the radiotelephone is assigned to the microcell. A typical prior art analog radiotelephone of the present invention is illustrated in FIG. 5. This radiotelephone is comprised of a microphone (510) that takes the voice signal from the user and converts it to an analog signal. The transmitter (520) modulates the voice signal to the proper frequency assigned by the system and transmits it, through the dup lexer (530) to the antenna (535) for radiation to the cell site. The antenna (535) also receives radiotelephone signals from the cell site. The dup lexer couples the antenna (535) to the receiver (525). The receiver (525) then demodulates the received signal into an analog signal for conversion to a sound signal by the speaker (505). The radiotelephone user inputs telephone numbers and generally controls the operation of the radiotelephone through the keypad and display (500). This information is used by the radiotelephone controller (515) to control transmitter (520) and receiver (525) and to determine what telephone number is transmitted. As is well known in the art, the radiotelephone accesses a cell's control channel when it sends an origination message to a cell or responds to a page from a cell. The cell measures the radiotelephone's RSSI during the access to determine if it is strong enough for communication with that particular cell. Control channels are typically reused within a cellular system requiring that the radiotelephone identify itself with the cell since it cannot be identified on the basis of frequency. This is accomplished by the radiotelephone sending its electronic serial number and assigned mobile identification number over the control channel. In the preferred embodiment, the microcell of the present invention uses a receive-only control channel tuned to the same frequency as the overlaid macrocell. If the microcell were allowed to transmit on this frequency, the macrocell coverage area would be subjected to intolerable levels of interference. The microcell also uses the same digital color code (DCC) value as the macrocell. As is well known in the AMPS art, up to three different DCC's are used to differentiate between reuses of the control channel frequencies. For example, if a radiotelephone accesses a cell having control channel fl and DCCl, then another cell having the same control channel fl but DCC2 will ignore the access. In a typical frequency plan, 21 different control channel frequencies are used. Using the same control channel and DCC as the overlaid macrocell enables the microcell to monitor the macrocell control channel for access attempts by a radiotelephone. When the access attempt is detected, the microcell measures the RSSI of the radiotelephone, thus enabling the BSC to determine whether the access should be served by the macrocell or the microcell. In the preferred embodiment, the macrocell and microcell of the present invention are both controlled by the same BSC. This enables the system to better coordinate channel allocations between the two cells, thus minimizing delays and race conditions between cells. Alternate embodiments could be controlled by multiple BSCs. A flowchart of the process of the present invention is illustrated in FIG. 4. The process begins with the macrocell and microcell monitoring their control channels (401). If an access attempt is detected on either control channel (402), the control channel measures the radiotelephone's signal strength (RSSI m ) during the access (403) and reports the access to the BSC (405). The BSC then determines if the access was reported by the macrocell or the microcell (440). If the access was reported by the macrocell, the BSC waits for a duplicate access message from the microcell (410). In the preferred embodiment, this message must be received in the range of 20 - 200 ms to be considered a valid message. Alternate embodiments use other time thresholds for determining that a valid access message exists. If the BSC receives a duplicate access message from the microcell within the range of time allowed (415), the radiotelephone's RSSI, as measured by the microcell's control channel (RSSI m ), is compared to a predetermined RSSI threshold (425). If the RSSI m is greater than or equal to the RSSI threshold (425) and the microcell has available voice channels, the radiotelephone is assigned to the microcell (430). This predetermined threshold, as well known in the art, is determined during RF planning of the cellular system. It is the threshold below which a quality communication cannot take place between the radiotelephone and the microcell. If the BSC does not receive a duplicate message within the required time (415), the radiotelephone is assigned to the macrocell (435), assuming the macrocell has available voice channels. If there are no available voice channels, the radiotelephone is not allowed access to the macrocell. If the BSC does not receive a duplicate message, this implies that the radiotelephone is outside the microcell's coverage area and could not be heard by the microcell. If an access is detected and it is not from the macrocell (440), then the access messaεre is from the microcell. The BSC waits for 20 - 200 ms for a duplicate message from the macrocell (450). If the duplicate message is received in time (455), then the radiotelephone's RSSI, as measured by the microcell's control channel (RSSI m ) is compared to a predetermined RSSI threshold (460). If the RSSI m is greater than or equal to the RSSI threshold and the microcell has available voice channels, the radiotelephone is assigned to the microcell (430). Otherwise, the radiotelephone is assigned to the macrocell (435), assuming it has available voice channels. If the macrocell does not have voice channels available, the radiotelephone is denied access to the macrocell. If the BSC does not receive a duplicate access message in the allotted time, the access attempt is discarded (470). This may have been caused by a collision on the macrocell control channel due to more than one radiotelephone attempting to access the control channel at the same time. This cannot be detected by the microcell. The goal of the process of the present invention is to use the macrocell control channel to manage the communications between the radiotelephone and the MTX. The microcell's control channel is used to assess the radiotelephone's proximity to the microcell. In general, the radiotelephone is assigned to the microcell only if the access is seen by both the macrocell and the microcell, and the microcell RSSI reading is high enough to indicate that the radiotelephone is within the microcell's coverage area. In the preferred embodiment, the microcell's coverage area is 200 meters from the microcell. Alternate embodiments have other coverage areas. The above described embodiment assumes there is only one microcell in a given macrocell. However, alternate embodiments use multiple microcells deployed in a single macrocell's coverage area. With several control channels tuned to the same frequency and DCC, the BSC collects all of the duplicate accesses received within the delay period and determines to which cell the radiotelephone will be assigned. The above described embodiment also assumes that the microcell and the macrocell are controlled by the same BSC. However, alternate embodiments use different BSCs to control the cells. If the radiotelephone is going to be assigned to a microcell, the macrocell BSC requests the microcell BSC to identify the microcell voice channel to be allocated to the radiotelephone. Other alternate embodiments associate a microcell with more than one macrocell. In this case, the microcell must be equipped to monitor multiple control channels. The preferred embodiment was discussed using AMPS as an example. It should be clear that the present invention can be used with other cellular systems that use analog access channels. WE CLAIM:

Claims: CLAIMS 1. A method for accessing a microcell by a radio unit, the microcell underlaying a macrocell, the method comprising the steps of: determining a proximity of the radio unit to the microcell; if the proximity is less than or equal to a predetermined distance, assigning the radio unit to the microcell; and if the proximity is greater than a predetermined distance, assigning the radio unit to the overlaying macrocell. 2. A method for accessing a microcell by a radio unit, the microcell underlaying a macrocell, the method comprising the steps of: monitoring a signal strength of the radio unit as received by the microcell; if the signal strength is greater than or equal to a predetermined threshold, assigning the radio unit to the microcell; and if the signal strength is less than the predetermined threshold, assigning the radio unit to the overlaid macrocell. 3. The method of claim 2 wherein the step of monitoring includes monitoring, on a control channel of the microcell, a radio signal strength indicator of the radio unit. 4. The method of claim 3 and further including the macrocell operating a control channel on a macrocell control channel frequency and digital color code. 5. The method of step 4 wherein the step of monitoring includes the microcell control channel monitoring the radio signal strength indicator on the macrocell control channel frequency. 6. A method for accessing a microcell by a radiotelephone, the microcell underlaying a macrocell, the microcell and the macrocell each having a control channel operating on a first frequency and both being controlled by a controller, the method comprising the steps of: the microcell using the microcell control channel to monitor accesses over the macrocell control channel; measuring a signal strength of the radiotelephone over the microcell control channel during an access; if the macrocell control channel indicates an access by the radiotelephone, the controller waiting for a duphcate access message from the microcell; if the controller receives a duplicate message from the microcell, comparing the measured signal strength to a predetermined threshold; and if the measured signal strength is greater than or equal to the predetermined threshold, assigning the radiotelephone to the microcell. 7. The method of claim 6 wherein the step of waiting for a duplicate access message includes waiting for a predetermined length of time. 8. The method of claim 6 and further including the step of if the measured signal strength is less than the predetermined threshold, assigning the radio unit to the macrocell. 9. The method of claim 6 and further including the steps of: if the access is from the microcell, waiting for the duplicate access message from the macrocell; if the controller receives the duplicate access message, comparing the measured signal strength with the predetermined threshold; if the measured signal strength is greater or equal to the predetermined threshold, assigning the radiotelephone to the microcell; and if the measured signal strength is less than the predetermined threshold, assigning the radiotelephone to the macrocell. 10. The method of claim 6 and further including the steps of: checking for availability of a voice channel in the microcell; and if a voice channel is not available in the microcell, assigning the radiotelephone to the macrocell. 11. The method of claim 7 and further including the step of if the duphcate access message from the microcell is not received within the predetermined length of time, assigning the radiotelephone to the macrocell. 12. A method for accessing a microcell by a radio unit, the microcell underlaying a macrocell, the method comprising the steps of: determining if both the macrocell and the microcell are receiving communications from the radio unit; determining if a signal strength signal indicates that the radio unit is within a coverage area of the microcell; and if the radio unit is within the coverage area, assigning the radio unit to the microcell. 13. The method of claim 12 wherein the signal strength signal is a received signal strength indicator. 14. A method for accessing a microcell by a radio unit, the microcell being one of a plurality of microcells underlaying a macrocell, the macrocell having a control channel operating on a frequency and a digital color code, each microcell having a control channel operating on the same frequency and digital color code as the macrocell, the method comprising the steps of: determining a proximity of the radio unit to each microcell of the plurality of microcells; if the proximity is less than or equal to a predetermined distance associated with each microcell, assigning the radio unit to the microcell having the closest proximity; and if the proximity is greater than the predetermined distance associated with each of the plurality of microcells, assigning the radio unit to the overlaying macrocell. 15. A method for accessing a microcell by a radio unit, the microcell underlaying a plurahty of macrocells, each macrocell having a control channel operating on a frequency and a digital color code, the microcell having a set of control channels operating on the same frequencies and digital color codes as the plurality of macrocells, the method comprising the steps of: determining a proximity of the radio unit to the microcell; if the proximity is less than or equal to a predetermined distance, assigning the radio unit to the microcell; and if the proximity is greater than the predetermined distance, assigning the radio unit to a first overlaying macrocell, of the plurahty of macrocells, that is receiving communications from the radio unit. 16. A method for accessing a microcell by a radiotelephone, the microcell underlaying a macrocell, the microcell and the macrocell each having a control channel operating on a first frequenoy and a first digital color code, the microcell and the macrocell being controlled by different controllers, the method comprising the steps of: monitoring radiotelephone accesses to the macrocell control channel; if the microcell control channel indicates an access by the radiotelephone, measuring a signal strength of the radiotelephone access over the microcell control channel; * if the microcell control channel indicates an access by the radiotelephone, reporting the access and the measured signal strength to the macrocell controller via the microcell controller; if the macrocell control channel indicates an access by the radiotelephone, the macrocell controller waiting for a duphcate access message from the microcell via the microcell controller; if the macrocell controller receives a duplicate message from the microcell controller, comparing the measured signal strength to a predetermined threshold; and if the measured signal strength is greater than the predetermined threshold, assigning the radiotelephone to the microcell. 17. The method of claim 16 and further including the step of assigning the radiotelephone to the microcell if the measured signal strength is equal to the predetermined threshold. 18. A method for accessing a microcell by a radiotelephone, the microcell underlaying a macrocell, the microcell and the macrocell each having a control channel operating on a first frequency and a first digital color code, the microcell and the macrocell being controlled by different controllers, the method comprising the steps of: monitoring radiotelephone accesses to the microcell control channel; if the microcell control channel indicates an access by the radiotelephone, measuring a signal strength of the radiotelephone access over the microcell control channel; if the microcell control channel indicates an access by the radiotelephone, reporting the access and the measured signal strength to the macrocell controller via the microcell controller; if the microcell control channel indicates an access by the radiotelephone, the macrocell controller waiting for a duphcate access message; if the macrocell controller receives a duplicate message, comparing the measured signal strength to a predetermined threshold; and if the measured signal strength is greater than the predetermined threshold, assigning the radiotelephone to the microcell. 19. The method of claim 18 and further including the step of assigning the radiotelephone to the microcell if the measured signal strength is equal to the predetermined threshold. 20. A method for accessing a microcell by a radiotelephone, the microcell underlaying a macrocell, the microcell and the macrocell each having a control channel operating on a first frequency and a first digital color code, the microcell and the macrocell being controlled by different controllers, the method comprising the steps of: monitoring radiotelephone accesses to the macrocell and microcell control channels; measuring a signal strength of the radiotelephone access over the micro- cell control channel; the microcell controller forwarding all microcell access messages to the macrocell controller; and if the radiotelephone is to be assigned to the microcell, the macrocell controller requesting the microcell controller to identify a microcell voice channel to be allocated to the radiotelephone. 21. A method for accessing a microcell by a radiotelephone, the microcell underlaying a macrocell, the microcell and the macrocell each having a control channel operating on a first frequency and a first digital color code, the microcell and the macrocell being controlled by different controllers, the method comprising the steps of: monitoring radiotelephone accesses to the microcell control channel; measuring a signal strength of the radiotelephone access over the microcell control channel; reporting the access message and measured signal strength to the macrocell controller via the microcell controller; if the macrocell controller receives an access message from the microcell controller, the macrocell controller waiting for a duplicate access message from the macrocell control channel; if the macrocell controller receives a duphcate message, comparing the measured signal strength reported by the microcell to a predetermined threshold; and if the measured signal strength is greater than or equal to the predetermined threshold, assigning the radiotelephone to the microcell.

Inventors: CHRISTIE EARL, HANLEY DONALD, MCCARTHY MICHAEL, CHRISTIE, EARL, HANLEY, DONALD, MCCARTHY, MICHAEL
Applicants: CHRISTIE EARL, HANLEY DONALD, MCCARTHY MICHAEL, NORTHERN TELECOM LTD, CHRISTIE, EARL, HANLEY, DONALD, MCCARTHY, MICHAEL, NORTHERN TELECOM LIMITED
Application year: 1997
Main classification: H04Q7
IPC codes: H04W16, H04W64, H04W72