Locating a mobile subscriber using spare DCC (1997-08-23)

1. Abstract

In the market, a mobile unit can send and receive a short message via a mobile telephony network or satellites, but we will discuss the short message service (SMS) handling between a Message Center (MC) and a mobile unit. Furthermore, we will describe how to locate the exact position of a mobile unit by using a digital control channel (DCC).

2. Marketing Analysis

This system could be used by

·        Police department

·        Adventurer

·        Blind persons

·        Any organization

·        Tourists

·        Adventurous games’ organizers

3. System Analysis

3.1 Determine location of a mobile unit via satellites

·        There is a system available in the market that assists blind persons to find out his position and suggest a path taken to destination. That system combines with an electronic geographical maps and costs about thousands of Canadian dollars. The satellites returns longitude and latitude of user, and system computes the exact location of the user.

·        The system, which includes a laptop and a special alphanumeric keyboard, allows user requesting his exact location. This may require annual subscription to satellite services plus access to an electronic geographical map. Thus, it is an expensive product.

3.2 Determine location of a mobile unit via a mobile telephony network supporting DCC.

·        A mobile unit must register whenever it enters a new cell site or location area. Its position (cell identity) should be reported to the serving Radio Base Station (RBS), this cell identity will be relayed to the serving MSC. Thus, we can determine which cell the mobile unit is in by printing out its cell identity (applicable for both analog and digital control channel), which could be stored in the serving MSC, HLR, or an MC.

·         Remember that a mobile unit is able to register on either an analog or DCC. However, it only receives SMS that is sent from an MC or MXE over the DCC. Therefore, we will consider using DCC of the serving cell and adjacent cells to compute mobile unit’s exact location.

3.3 Short Message Services

·        This feature allows users sending an SMS from a MC to a mobile unit or a cellular phone. In the market, this feature supports mobile unit that is capable of handling messages over DCC. However, when a cellular phone is busy (on voice channel), the MS doesn’t deliver that message. The HLR will inform the MC to delay that message delivery.

·        Options: If the MSC buffered that short message, we would consider cases that the mobile unit “hands off” to another RBS belonging to another MSC, i.e. the anchor exchange no longer has controlled over the mobile unit. Thus, it may be better to get the MC resending the short message. Note that: the HLR will inform the MC when the mobile unit is free.

4. Implementation overview

·        Using the spare DCC (TDMA), we may be able to communicate with the mobile unit roaming in cellular telephony network, and report its exact location toward an HLR, an MC, or serving MSC.

Figure 1. Locating distance of a mobile unit in a location using RBS.

·        As shown in the above diagram, a mobile unit is roaming across the border of 3 cells, which 2 of them belonging to MSC-2, and 1 cell is served by MSC-1.  Thus the mobile unit has been capable of tuning to all 3 DCCs, collecting their cell identities, original transmitting power, and receiving signal strength of each cell. Based on those data, the mobile unit should be able to compute its distance from 3 RBS. It then relays the findings to the RBS that has highest signal strength. That serving MSC will send a report to the HLR or an MC. These operations are carried out on a spare DCC of each cell (each cell site is served by an RBS). This way we could avoid using the principal DCC, which is dedicated for paging mobile station (MS), delivering SMS, etc.

·        An exact location of the MS in terms of building, streets or address will require an electronic geographical map.

Figure 2. Hardware modification of a mobile unit

5. Future enhancement

·        An adaptor connected the mobile unit, e.g. MS, to a laptop computer could be serve as mobidem that allows users connect their laptop to an Internet service over the air.

·        Nokia planned to design a portable unit that looks like a mini laptop when it opens. It functions as a mobile phone if closes.

6. References

·        Mobile Communications Design Fundamentals, Williams C. Y. Lee, Section 6.5.1 on “Conversion between dBu and dBm in power delivery. See also section 9.2.1

7. Update Notes (2018-12-11): back to 1997, I didn’t know that building and construction between an RBS (radio base station) and a mobile phone would change the signal strength measurement that makes computing distance inaccurate.