Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-08-31
2001-02-13
Trost, William G. (Department: 2744)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S437000
Reexamination Certificate
active
06188900
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and circuitry for a mobile device assisted handoff system for code division multiple access and wideband code division multiple access networks.
BRIEF DESCRIPTION OF THE PRIOR ART
Code division multiple access (CDMA) networks are full duplex systems which are capable of simultaneous transmission and reception. CDMA mobile systems transmit to and receive from different base stations as they move about, the particular base station utilized at any point in time generally being related to signal strength and/or signal integrity from the base station, this generally being related to geography (i.e., the closest base station to the mobile unit at a given point in time). In CDMA, the delta f or frequency difference between the transmit frequency and the receive frequency is always the same constant for a given mobile device. As the mobile unit moves away from one base station and toward other base stations, the mobile unit will ultimately change base stations with which it is in contact, the change again generally being related to signal strength and/or signal integrity of the base station signal. This base station change requires a change in frequency of transmission and/or reception and is referred to in the art as “hard handoff”. CDMA radio networks of the prior art have placed the burden of coordinating hard handoff (switching radio frequencies as the mobile device moves from base station to base station or from cell to cell) on the base station alone. At hard handoff, the frequency synthesizer in the mobile device tunes to the new frequency and the data transmission/telephone call resumes only after the synthesizer in the mobile device settles at the new frequency. This frequency tuning and locking processing, based upon present synthesizer topologies, requires on the order of 5 milliseconds in CDMA mobile devices. The result of failure to provide proper handoff is often a dropped call with the apparent consequences thereof, both when voice information as well as data are being transmitted.
In an attempt to alleviate this problem, new CDMA and wideband CDMA (W-CDMA) networks are being standardized to require that the mobile device participate actively along with the base station in the hard handoff process to simultaneously reduce the settling time of the synthesizer in the mobile device providing the operating frequency by up to an order of magnitude. This requires that the mobile unit continue to operate at the serving frequency while constantly being on the lookout by sampling for superior candidate channels which operate at a different frequency so that handoff can take place, when required, to the best available base station. The problem arises that the presently used integer-N synthesizer topology is incapable of simultaneously providing the required very fast frequency settling time while meeting increasingly stringent phase noise requirements. This problem increases in the case of W-CDMA networks which operate at even higher data rates since even the faster state of the art fractional-N synthesizer topology is inadequate for the same reasons as in CDMA.
SUMMARY OF THE INVENTION
In accordance with the present invention, the above described problems inherent in the prior art is minimized.
Briefly, several frequency synthesis techniques are utilized to achieve a “best of both worlds” synthesizer lock time, phase noise, and power consumption performance. The synthesizer topology in accordance with the present invention (see
FIG. 2
) takes advantage of the classic integer-N synthesizer in combination with a direct digital synthesizer (DDS) which is clocked from the output frequency of another of the synthesizers in the CDMA mobile device. Programming of the DDS output frequency and integer-N synthesizer is achieved using information provided by the CDMA base station as part of the mobile assisted handoff (MAHO). As part of MAHO, the DDS is powered up and tuned to the candidate frequency instead of the output from the integer-N synthesizer. Then, a two input selector toggles to select the mixed output from the DDS at the target frequency. The sub 100 microsecond tuning time of the DDS allows the CDMA receiver to begin its time-consuming pilot search as well as data and clock resynchronization almost immediately. Signal strength measurements are made at the “candidate frequency” provided by the DDS. If MAHO occurs, the DDS maintains its frequency while the synthesizer is programmed and begins to tune to the candidate frequency. As the integer-N synthesizer begins to settle at the target frequency a short time later (several milliseconds), the out of lock indicator is disabled, toggling the two input selector and selecting the low phase noise output from the integer-N oscillator again, while powering down the DDS.
A benefit derived from the invention are significantly reduced probability of “call dropping” at CDMA hard handoff “in network” and especially during hard handoff between cells operated by different network providers (due to inadequate time for hard hand off tuning and call processing). In addition, high speed tuning is achieved without consuming large amounts of power (the DDS is powered up only briefly prior to hard handoff). Also, the DDS technology can be integrated directly into semiconductor integrated circuits. Existing CDMA mobile radio architectures can easily adapt to the above described technology by adding minimal components to the mobile device. This approach is directly scalable to W-CDMA networks (e.g., future CDMA networks, such as 3rd generation GSM).
REFERENCES:
patent: 5504783 (1996-04-01), Tomisato et al.
patent: 5701598 (1997-12-01), Atkinson
patent: 5999816 (1999-12-01), Tiedermann, Jr. et al.
patent: 6075989 (2000-06-01), Moore et al.
patent: 6078570 (2000-06-01), Czaja et al.
Henwood Andrew M.
Ruiz Everardo D.
Brady III Wade James
Gelin Jean A
Neerings Ronald O.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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