Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J3/00—Time-division multiplex systems
  • H04J3/02—Details
  • H04J3/06—Synchronising arrangements
  • H04J3/0602—Systems characterised by the synchronising information used
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W88/00—Devices specially adapted for wireless communication networks, e.g. terminals, base stations or access point devices
  • H04W88/08—Access point devices

Definitions

• the present invention relates to a method for connecting base stations in a radiotelephone communication system, and more particularly, to a method for cascade connecting a plurality of base stations or icrobases in a cellular telephone system.
• microcells which include a base station and which cover an area of 100-500 meters from the base station.
• the typical vertical height of an antenna for a microcell base station is approximately the same as street lighting.
• Microcells are particularly well suited for urban areas which have a large amount of traffic and considerable amounts of interference.
• Picocells are primarily intended for indoor use.
• the base station of a picocell can cover an area of approximately 20-30 meters.
• microcells and picocells may be established within overlying macrocells to handle areas with relatively dense concentrations of mobile users, sometimes referred to as "hot spots".
• microcells may be established for thoroughfares such as crossroads or streets, and a series of microcells may provide coverage of major traffic arteries such as highways.
• Microcells may also be assigned to large buildings, airports, and shopping malls.
• Picocells are similar to microcells, but normally cover an office corridor or a floor of a high- rise building. Microcells allow additional communication channels to be located in the vicinity of actual need, thereby increasing cell capacity while maintaining low levels of interference.
• Macrocell umbrella sites typically cover radii in excess of 1 kilometer and serve rapidly moving users, for example people in automobiles.
• Microcell sites are usually low power, small radio base stations or microcells, which primarily handle slow moving users such as pedestrians.
• Each microba ⁇ e of a microcell may be connected to a macrocell base station through digital radio transmissions, cables or optical fibers.
• the microbases of the microcells may be connected directly to a MSC (mobile switching center) via a PCM (pulse code modulated) link that transmits time division multiplexed (TDM) signals having time slots which correspond to control time slots and voice channels for each base station or microbase.
• MSC mobile switching center
• PCM pulse code modulated
• microbases of microcells When the microbases of microcells are connected to a MSC via a PCM link, they typically are in turn cascade connected to one another.
• This cascade connection requires a relatively complex switching facility at each node or microbase for modifying the positions of control time slots of the TDM signals transmitted over the PCM link. Accordingly, there is a need for an improved method for cascade connecting microbases which can eliminate the relatively complex switching facilities.
• the present invention provides a novel method for cascade connecting a plurality of base stations or microbases via a PCM link that does not require a relatively complex switching facility for modifying the position of control time slots of the TDM signals.
• the base stations of the present invention are of a type that receive over the PCM link a TDM signal having a plurality of frames that are divided into time slots that correspond to control time slots and voice channels.
• the TDM signal is transmitted to a first or source base station from a MSC.
• the frame synchronization position of the TDM signal is preferably moved to a time slot that is the intended control time slot of a second or destination base station.
• the TDM signal is then transmitted to the destination base station, and the received TDM signal at the destination base station includes a new frame structure having consecutively renumbered time slots.
• FIG. 1 illustrates a typical multi-layered cellular system employing umbrella macrocells, microcells and picocells;
• Fig. 2 is a diagram illustrating two microcell base stations cascade connected to a MSC
• Fig. 3 is a diagram of a TDM signal used for communications between a MSC and base stations or microbases of a conventional system
• Figs. 4a and 4b are block diagrams of two different embodiments of the present invention having cascaded microbases; and Fig. 5 is a diagram of a TDM signal used for communications between a MSC and base stations or microbases of the present invention.
• Fig. 1 is an exemplary multi-layered cellular system.
• An umbrella macrocell 10 represented by a hexagonal shape makes up an overlying cellular structure.
• Each umbrella cell may contain an underlying microcell structure.
• the radio coverage of the umbrella cell and an underlying microcell may overlap or may be substantially non- overlapping.
• the umbrella cell 10 includes microcells 20 represented by the area enclosed within the dashed line and microcells 30 represented by the area enclosed within the dotted line corresponding to areas along city streets, and microcells 40, 50, and 60, which cover individual floors of a building.
• the intersection of the two city streets covered by the microcells 30 and 40 may be an area of dense traffic concentration, and thus might represent a hot spot.
• control channels are used for setting up calls, informing the base stations about location and parameters associated with mobile stations, and informing the mobile stations about location and parameters associated with the base stations.
• the base stations listen for call access requests by mobile stations and the mobile stations in turn listen for paging messages. Once a call access message has been received, it must be determined which cell should be responsible for the call. Generally, this is determined by the signal strength of the mobile station received at the nearby cells.
• the assigned cell is ordered, by the mobile switching center (MSC) for example, to tune to an available voice channel which is allocated from the set of voice or traffic channels accessible to the assigned cell.
• MSC mobile switching center
• a diagram illustrates two microcell base stations BSI and BS2 that are cascaded connected to a mobile switching center MSC via a PCM trunk or link Tl.
• the PCM link Tl is preferably of a type that operates at either 1.5 or 2.0 Mbits. American systems typically operate at 1.5 Mbits, and European systems typically operate at 2.0 Mbits.
• the TDM signal transmitted on the PCM link Tl is also of a type which is divided into a plurality of frames and time slots with control time slots and voice channels assigned to predetermined time slots. When more than one base station is connected via the same connection Tl, each base station requires its own time slot for control.
• a TDM signal of a type that is conventional in the art is illustrated as being divided into a plurality of frames and time slots #1-#18.
• base station BSI uses time slot #9 for a control time slot and time slots #l-#8 for voice channels.
• the time slot #9 cannot be used as the control time slot for the base station BS2.
• Time slot #10 is used as the control time slot for the base station BS2, and time slots #11-#18 are used as voice channels for base station BS2.
• Conventional base stations BSI may include a relatively complex switching facility which is used to rearrange the position of time slot #10 to the position of time slot #9.
• the rearrangement of the time slot #10 to the standard position of time slot #9 is indicated by the arrow A of Fig. 3.
• the relatively complex switching facility which performs the rearrangement of time slot #10 represents a significant problem in the design of a compact microbase.
• the PCM link Tl connects a MSC to a source base station BSI which includes a frame synchronization means FS1.
• the source base station is in turn connected to a destination base station BS2 which includes frame synchronization means FS2.
• the destination base station BS2 is in turn connected to a third base station BS3, then the third base station BS3 could include frame synchronization means, but the frame synchronization means does not have to be activated.
• a block diagram illustrates a second embodiment of the present invention.
• the PCM link Tl connects a MSC to a source base station BSI.
• the source base station is in turn connected to a destination base station BS2 which includes frame synchronization means FS2.
• the destination base station BS2 may be in turn connected to a third base station BS3 that includes frame synchronization means FS3.
• a diagram illustrates a TDM signal transmitted over a PCM link to the microbases of the present invention that avoid the problem associated with the TDM signal of a conventional PCM link.
• the solution provided by the method of the present invention is to always use the time slot #0 as the control time slot for each cascade connected base station.
• time slots #0-#23 there are twenty four time slots #0-#23.
• time slot #0 is used for control of source base station BSI
• time slots #l-#6 are used for voice channels of source base station BSI
• time slot #7 is used for control of destination base station BS2
• time slots #8-#13 are used for voice channels of destination base station BS2.
• the voice channels of the present invention are capable of transmitting either voice or data communications.
• the synch position of the TDM signal transmitted over the PCM link is preferably moved at source base station BSI from its original position at time slot #0 and repositioned at time-slot #7.
• the synch position of the TDM signal is moved at the destination base station BS2.
• the first base station BSI never moves the synch position upon reception of the TDM signal from the MSC.
• This arrangement results in the same TDM signal structure being used for both base stations BSI and BS2, and this arrangement permits standardized equipment to be used at both base stations. It should be noted that the present invention is not limited to the use of two base stations and that additional base stations, such as base station BS3, may be cascade connected in accordance with the teachings of the present invention.
• Moving the synch position of the TDM signal of the present invention is a relatively easy procedure compared to the movement of time slots as described above in connection with the TDM signal of Fig. 3.
• the synch position of the TDM signal is moved at either the source base station BSI or the destination base station BS2 by using the frame synchronization means FS1 or FS2.
• the frame synchronization means FS1, FS2 preferably include a counter to delay the signal or suitable software routines for the base stations BSI or BS2 which introduce an appropriate delay into the TDM signal. Accordingly, moving the synch position of the TDM signal in accordance with the present invention advantageously eliminates the need for the complicated switching facility that is required for rearrangement of the position of a control time slot of the TDM signal of Fig. 3.

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• 1994
  • 1994-03-15 EP EP94911336A patent/EP0641504A1/de not_active Withdrawn
  • 1994-03-15 AU AU63880/94A patent/AU6388094A/en not_active Abandoned
  • 1994-03-15 WO PCT/SE1994/000224 patent/WO1994022245A1/en not_active Application Discontinuation
  • 1994-03-22 TW TW83102476A patent/TW232106B/zh active

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