MAHO METHOD FOR SFH-CDMA/TDMA unit relative to the base site. As the communication unit
USING PUNCTURED FRAMES moves out of the coverage area of a serving base site such
movement must be detected and another base site selected
This is a continuation of application Ser. No. 07/970,213, for handoff before signal loss occurs,
filed Nov. 2, 1992 and now abandoned. 5 The need for handoff in some systems may be detected by
FIELD OF THE INVENTION measurement of a signal quality factor (such a received
. . signal strength indication (RSSI)) of the served communi
The field of the invention relates to communication sys- catk)n unft b bage siteg surrounding me served communi.
t£TL lnl.?L!?^li! ... commumcatlon systems cation unit. In other systems the served communication unit
BACKGROUND OF THE INVENTION referred to as mobile assisted handoff (MAHO). In MAHO Cellular communication systems are known. Such systems the most rehable signal for signal quaUty measuresystems, typically, provide communication services ments is the control channel used by the measured base site, throughout a broad geographic area through a number of MAHO signal measurements are transferred to the servbase sites distributed throughout the geographic area. The 15 ing base site by the communication unit on a slow associated base sites of the system, and associated coverage areas, are control channel (SACCH) used in conjunction with a traffic arranged within the geographic area in such a manner to channel. Such signal measurements may be, in turn, transprovide substantially seamless coverage for communication ferred to a mobile switching center (MSC) for a determinaunits passing through the geographic area by being passed tion of a handoff target. Handoff, in such case, is accomfrom one base site to another. 20 plished by the serving base site transmitting a handoff
Base sites within the cellular system offer communication command to the served communication unit, services to communication units within their service cover- While handoff, under MAHO, has worked well in the age areas on a number of communications frequencies past, such success has been based upon readily available (communication resources) (fL-fn) reserved for such use. control signals of surrounding base sites located on fixed Such communication services are facilitated by the trans- frequencies, identifiable through a minimum of scanning or mission of an identification signal by a base site. The information transfer from a serving base site. Such availidentification signal is used by communication units as a ability is necessary because of the limited time that a means of identifying proximate base sites through which communication unit has between communicated frames for service may be requested. 30 scanning for nearby base sites.
Service within the communication system is provided to In MAHO systems using frequency hopping, such hoprequesting communication units through a proximate base ping has been confined to traffic channels, with control site upon receipt of an access request and upon verification channels existing on fixed frequencies. Because of the that the communication unit is registered within the system. importance of frequency hopping in reducing interference, a Notification of service may be provided to the communica- 35 need exists for a method of using MAHO in connection with tion unit by the base site transmitting a resource allocation frequency hopping control channels.
to the communication unit. The base site may also allocate CTT,„.. ~c -r^m rxnm^m^r
. . , , , '.. ... SUMMARY OF THE INVENTION a transceiver and voice path through the base site providing
access to a communication target. In a frequency hopping cellular communication system
The communication resource allocated for use by the 40 having a number of base sites and using mobile assisted
communication unit is typically in the nature of a duplex handoff, a method is offered of providing base, site identi
traffic channel that, in the case of a frequency division fication signals. The method includes the steps of assigning
multiple access system, may be a voice channel comprised eacn base site of me number of base sites to a slot of a
of a frequency pair (transmit and receive). In other systems intersite control frame; and, periodically transmitting, in the
the duplex voice channel may exist within a common 45 assigned slot of the intersite control frame, the identification
spectrum using spreading codes (as with a code division signal of the base site.
multiple access (CDMA) communication system). In still BRIEF DESCRIPTION OF THE DRAWINGS other systems the duplex voice channel may be comprised of
slots on a transmit and receive frequency of a time division mG- 1 comprises a block diagram of a base site in
multiple access (TDMA/TDM) system such as, for example, 50 accordance with the invention.
the Global System for Mobile Communications (GSM) FIG. 2 comprises a block diagram of a mobile commu
Pan-European digital cellular system, as specified in GSM nication unit in accordance with the invention,
recommendations available from the European Telecommu- FIG. 3 depicts a table of hopping codes in accordance
nications Standards Institute (ETSI). Under GSM a number with the invention.
of voice channels may exist within the slots of a repeating 55 FIG. 4 depicts a communication system in accordance
GSM frame. with the invention.
As a means of reducing in reference within a GSM-like FIG. 5 depicts a multiframe including the intersite control
system the frequency of the traffic channel, over which the frame jn accordance with the invention,
duplex voice signal is being exchanged with the base site, pi(j_ g comprises a common channel reuse plan for the
may be changed after each exchanged frame. Operating eo intersite control frequency in accordance with the invention.
frequenciesforeachframe(oversuccessivehoppedframes), mQ 7 comprises a reuse ^ of me intaatB
in such case, may be generated by a hopping algorithm such frame in accordance with me mvention. as, for example, GSM recommendation 6.2.3. The use of
frequency hopping within a GSM system brings such a DETAILED DESCRIPTION OF THE
system within the definition of CDMA. 65 PREFERRED EMBODIMENT
As the duplex voice signal is maintained, signal measure- The solution to the problem of making MAHO signals
ments are used to detect movement of the communication readily available to communication units within a frequency