WO1997001889A2

WO1997001889A2 - Minimizing interference in fringe areas of communication systems

Info

Publication number: WO1997001889A2
Application number: PCT/IL1996/000022
Authority: WO
Inventors: Yoav Yaacoby
Original assignee: Powerspectrum Technology Ltd.
Priority date: 1995-06-27
Filing date: 1996-06-20
Publication date: 1997-01-16
Also published as: IL114366A0; AU6136196A; WO1997001889A3

Abstract

This invention discloses a method of communicating signals in a communication system having a plurality of sectors, including the steps of communicating in a first sector by hopping the signals over a first set of frequency sequences and by using a plurality of time slots, communicating in a second sector by hopping the signals over a second set of frequency sequences and by using the plurality of time slots less a first selected time slot, and communicating in an area where the first sector and the second sector overlap by hopping the signals over the set of frequency sequences used in the first sector and by using the first selected time slot not used in the second sector. Apparatus for communicating signals in a communication system having a plurality of sectors is also disclosed.

Description

MINIMIZING INTERFERENCE IN FRINGE AREAS OF COMMUNICATION SYSTEMS

Background of the Invention

The present invention relates to radio communication systems and methods. More particularly, it relates to system and methods for minimizing or eliminating interference problems in the fringe areas of the communication system where communication cells or sectors intersect.

In cellular or sectorized communication systems, communications in a cell or sector are provided by hopping communication signals over a set of frequency channels in a frequency hopping system and/or by multiplexing communication signals through a plurality of time slots in a time division multiple access (TDMA) communication system. Communications in other cells or sectors are provided by reusing the frequency channels in a frequency hopping system or, in a TDMA system, by reusing the time slots.

In general, interference between communications in adjacent sectors is limited by proper selection of the hopping sequences. For example, sequences can be selected within a sector so that sets of frequency hopping sequences used within that sector are orthogonal, that is, at a given time a frequency channel is only used once. Also, sequences can be selected in adjacent sectors so that utilization of a frequency at the same time in the adjacent sectors is minimized. The selection of sequences is described in United States Patent No. 5,408,496 which is hereby incorporated by reference.

Interference problems, however, arise with communications in the fringe areas, that is, the areas where adjacent cells or sectors overlap. The fringe areas result from imperfect patterns of the antenna used by adjacent sectors. Thus, the pattern from a first sector antenna will normally have some overlap with the pattern from an adjacent sector antenna. These interference problems also occur in cellular type systems where adjacent cells overlap.

Thus, to improve communications in the fringe areas and in adjacent sectors, and to eliminate interference coming from the fringe areas, new systems and methods are needed to minimize the interference problems encountered.

Summary of the Invention

The present invention provides apparatus and method for minimizing the interference that results from communicating in the fringe areas of a sectorized communication system. In accordance with a preferred method of the present invention, communications are established in a first sector by hopping communication signals over a first set of frequency sequences and by hopping the signals over a plurality of time slots. Then, communications are established in a second sector by hopping communication signals over a second set of frequency sequences and by hopping the signals over the same plurality of time slots used in the first sector, less a selected one of the plurality of time slots. In the fringe area - the area where the first sector and the second sector overlap - communications are established by hopping the signals over the set of frequency sequences used in the first sector and by using the one time slot not used in the second sector.

Accordingly, it is an object of the present invention to improve communications in the fringe areas found in sectorized communication systems.

It is another object of the present invention to minimize the

interference caused by the subscribers to the communication system that is sometimes caused when those subscribers attempt to communicate in the fringe areas of a sectorized communication system.

The invention will now be described in connection with certain illustrated embodiments; however, it should be clear to those skilled in the art that various modifications, additions and subtractions can be made without departing from the spirit and scope of the claims.

Description of the Drawings

FIG. 1 illustrates a sectorized communication system;

FIG. 2 illustrates communication signals from several users being frequency and time hopped;

FIG. 3 illustrates a block diagram of a base station;

FIGS. 4 and 5 illustrate a block diagram of a subscriber unit; FIGS. 6 to 1 0 illustrate various partitions of the fringe areas of sectors in accordance with the present invention; and

FIGS. 11A, 11B, 11C and 12 illustrate the method by which the base station determines whether a subscriber unit is located in a fringe area and the resulting assignment of time slots and frequency hopping sequences which are based on a subscriber's location within a communication system.

FIG. 1 3 illustrates the hand off process in accordance with a preferred embodiment of the present invention.

Description of the Preferred Embodiment

FIG. 1 illustrates a communication system 1 in which the present invention can be used. The system 1 includes a base station 2, dispatch stations 4, a plurality of subscriber units 6, and three sectors 8, 9 and 10. The base station 2 includes the communication equipment necessary to provide the multiple access communications for the plurality of subscribers units 6 and for the dispatch stations 4. The base station 2 also includes the communication equipment needed to provide communications through the Public Switched Telephone Network (PSTN).

The dispatch station 4 includes equipment necessary to dispatch communications from the dispatch station 4 to a number of subscriber units 6, commonly referred to as point-to-multipoint communications, and it also includes equipment that provides point-to-point communications. The subscriber units 6 generally consist of mobile or portable equipment necessary to transmit, receive and process communication signals.

The communication signals transmitted over communication system 1 are preferably time multiplexed, often referred to as TDMA communications. The system 1 is also preferably a frequency hopping system. Referring to FIG. 2, the time mulitplexing (or time hopping) and frequency hopping of the communication signals from several users in a single sector by the system 1 is illustrated, in FIG. 2, five users, USER A, USER B, USER C, USER D and USER E are attempting to communicate in a single sector. Associated with each user are four packets of communication signals. For example, signal packets A1 , A2, A3 and A4 are associated with USER A.

In the example of FIG. 2, the sector uses nine frequencies and three . time slots to transmit the signals from the users. The frequencies used are f₁ to f₉. A tenth frequency, f₁₀, is shown but is used to transmit control information. The time slots used are labeled A, B and C.

The signals from the users are processed by assigning each signal packet from each user to a time slot and to a frequency. The assignment is preferably made from a set of hopping sequences maintained by sector hardware in the base station 1 0. In FIG. 2, USER A's signal packets, A1 , A2, A3 and A4 are assigned to the time slot IB and to frequency f3, to the time slot IIC and to frequency f₆, to the time slot IIIA and to frequency f₉ and to the time slot IVB and to frequency f₂, respectively. Thus, the signals are hopped across the frequency channels f ₁ to f ₉ and across the time slots A, B and C.

The signals from USER B, USER C and USER D are transmitted across the frequencies and the time slots as illustrated. These signals, therefore, are also hopped across a plurality of frequencies and are also hopped across a plurality of time slots. Although FIG. 2 illustrates the system 1 frequency hopping and time hopping the signals, the system 1 can accomplish either independently. That is, the system 1 can either time hop the signals from a user without frequency hopping or can frequency hop the signals from a user without time hopping those signals or can do both. Where no frequency hopping is utilized, the signals from a selected user are assigned to a single frequency and then hopped across the available time slots. Where no time hopping is utilized, the signals from a selected user are assigned to a single time slot and then hopped across the available frequencies.

Referring to FIG. 3, the base station 2 is illustrated. The base station 2 includes a first sector unit 1 6, a second sector unit 1 7, a third sector unit

1 8, a microsector unit 20, a redundant sector 21 , a PABX 22, a voice mail unit 24, a central frequency source unit 326, an administration computer 28, a central controller 30, a database server 32, a local administrative computer 34, a terminal server 36, a local area network 38, a power supply 39, data computers 40 and a modem pool 41 . The base station 2 may also include a billing station, but where the communication system 1 is a regional or national system, it is preferred to provide a central billing system that serves all of the communication systems. The base station 2 of FIG. 3 does not include a billing station.

The sector units 1 6 to 1 8 establish the communication channels illustrated in FIG. 2 in the sectors 8 to 10, respectively, with the subscriber units 6 in the sector. The microsector unit 20 establishes communications with additional communication equipment, called microsites, which provide service to trouble spots in the communication system 1 . The redundant sector 21 provides redundant communication channels for the sector units 1 6 to 1 8. The base station 2 provides communications with the Public Switching Telephone Network (PSTN) via the PABX 22. The PABX 22 also provides three way conferencing, routing, least cost routing of long distance calls, voice mail interfacing, dispatch bridging, user services support and metering functions.

The voice mail unit 24 provides voice mail capability. The central frequency source unit 26 provides timing references throughout the system.

The administrative computer 28 tracks subscriber unit 6 configurations, tracks administrative activities, performs network management, performs built-in-test management and performs system initialization. The central controller 30 provides various functions, including call management, dispatch management, control of the PABX 22, voice mail interfacing, operational mode management, subscribers management, call management, supplies billing information, and generates reports. The data base server 32 stores user data concerning user rights, status, calls and airtime. It also provides basic data base management and services to all data base clients, ouch as the local operator, fleet administrators and remote operators. The local administration computer 34 provides maintenance and operational control of the base station 2. The local area network 38 enables communications between the various components connected to the network.

The subscriber unit 6 is preferably divided into two pieces of

equipment, a subscriber terminal 1 4 illustrated in FIG. 4 and a radio unit 1 2 illustrated in- FIG. 5. In general, the subscriber terminal 1 4 provides an

interface to the subscriber and the radio unit 1 2 transmits and receives

communication signals to and from the base station 2, the dispatch station 4 or other subscriber units 6 under the control of the subscriber terminal 14.

Referring to FIG. 4, the subscriber terminal 14 includes a

microprocessor 50, an oscillator circuit 52, a power supply circuit 54, a serial interface circuit 56, a display driver circuit 58, a memory circuit 60, a

keyboard interface circuit 62, a keyboard 26, an I/O decoder circuit 64, a LED driver 66 and an interface circuit 68. The subscriber terminal 1 4 can also include a real time clock 70 and associated power supplies 72 and 74 or the base station 2 distributes real time clock information during its transmission to each cf the subscriber units 6.

The memory circuit 60 includes a decoder circuit 76, an address latch 7S, a boot ROM S0, a flash memory 82 and a static RAM 84. The boot ROM 80 stores the code necessary to initialize the microprocessor 50 and the circuitry of the subscriber terminal 1 4 as well as code necessary to download future software versions for the subscriber terminal 1 4. The flash memory 82 is non-volatile re-writable memory which is utilized to store information which must be maintained even during a loss of power. The static RAM 84 is utilized as a working memory as needed.

The display driver circuit 58 includes the LCD display 24, a power supply 86, a LCD controller 88, a memory circuit 90 and an address controller 92. The display driver circuit 58 is accessed by the microcontroller 50 via the I/O decoder 64. The information to be displayed is downloaded from the microprocessor 50 to the memory circuit 90. The display of the information on the display 24 is then controlled in a conventional manner by the LCD controller 88.

The microprocessor 50 is responsive to the selection of the keys by the subscriber. When the keys are selected, selection signals are processed through the keyboard interface circuit 62 to the microprocessor 50. The microprocessor 50 also controls LEDs through the LED driver circuit 66. The RS-232 interface 94 provides communications between the subscriber terminal 1 4 and external devices, such as notebook computers, magnetic swipe devices used to read magnetic strips, printers, bar code readers and serial keyboards. The RS-485 interface 96 and the interface circuitry 68 provides communications between the subscriber terminal 1 4 and the receiver and transmitter circuitry of the radio unit 1 2.

Referring to FIG. 5, the radio unit 1 2 includes antennas 102 and 104, a radio board 1 06, a baseband unit 1 08, a service board 110, and a GPS interface 111 . The radio unit 1 2 is housed separately from the subscriber terminal 1 4, for example, inside the trunk of an automobile. The antennas

102 and 104 are preferably mounted externally on the automobile.

The radio board 1 06 includes transmitter and receiver circuitry. More particularly, it includes a transmitter 112, two receiver channels 114 and 116, a duplexer 118, a frequency synthesizer 1 20, gain and frequency control circuitry 122 and transmitter gain control circuitry 1 24. As previously describer, the circuitry of the radio board 1 06 provides

communication via frequency hopping, that is, a communication signal is transmitted and received by hopping the signal over several different frequency channels.

The radio board 106 is interfaced with the baseband unit 108. The baseband unit 108 includes a modem 1 26, a controller 1 28, a voice processing package 130 and an interface circuit 1 32 to the subscriber terminal's 1 4 circuitry. This baseband unit 1 08 receives the signals to be transmitted to the base station 2 from the subscriber terminal 14 and sends the signals to the transmitter 112 in the radio board 1 06 for transmission.

This baseband unit 108 also receives the signals that the radio board 1 06 receives from the base station 2, performs some processing on the received signals and sends them to the subscriber terminal 1 4 through the interface

1 32.

Referring to FIG. 6, a first embodiment of the system 1 in accordance with the present invention is illustrated. In FIG. 6, there are two sectors, SECTOR I and SECTOR II, in the communication system 1 . The sectors can also be adjacent cells in a cellular type system. Communications in each sector are performed via frequency hopping across a plurality of frequency channels and/or via time hopping over a plurality of time slots, as illustrated in FIG. 2. There is also a fringe area 200 which is defined by the overlapping areas from SECTOR I and from SECTOR II. The method of providing communications in the sectors and in the fringe area 200 will now be described.

Referring to FIG. 6, in SECTOR I, three time slots are utilized, t₀, t₁ and t2to provide communications. The signals in SECTOR I are preferably, but not necessarily, time hopped, that is, the signals are preferably transmitted in various sequences of the time slots t₀, t₁ and to. The signals in SECTOR I are also hopped over a plurality of frequency channels in accordance with a set of frequency hopping sequences. In the case of FIG. 6, there are nine frequency channels utilized and the set of hopping sequences over these frequency channels is designated as f_A(f ₁ . . . .₉). The time and frequency hopping sequences that are assigned to users in SECTOR I are preferably orthogonal to one another, that is, there is no time in which one user transmits in a time slot t_x and on a frequency f_y and another user also transmits in the time slot t_x and on the frequency f_y. This also applies to the other sectors and is described in United States Patent No. 5,403,496.

In SECTOR II, communications are established by utilizing the plurality of time slots used in SECTOR I, less a selected time slot. In the particular example of the present invention shown in FIG. 6, two of the time slots used in SECTOR I, t₁ and t₂, are used in SECTOR II and the signals are preferably, but not necessarily, time hopped in various sequences of the time slots t₁ and t₂. Thus, the selected time slot which is not used in communications in SECTOR II is t₀. The signals in SECTOR II are also hopped over a plurality of frequency channels, preferably the same plurality of frequency channels used in SECTOR I, in accordance with a new set of frequency hopping sequences, designated f_B(f₁ . . . f₉), which are preferably selected so as to minimize interference in adjacent sectors. The selection of sequences is explained in greater detail in United States Patent No. 5,408,496, which is hereby incorporated by reference.

In the fringe area 200, communications are provided by using the time slot t₀. Therefore, the time slots t₁ and t₂ which are used in both SECTOR I and in SECTOR II are not used in the fringe area 200. It is preferred to hop the communication signals in the fringe area 200 over the plurality of frequency channels using the same frequency hopping sequences used in SECTOR I, that is by using the set of hopping sequences f _A(f₁ . . . f₉) . By communicating in the fringe area 200 in this manner the interference between the sectors or between the fringe area 200 and the sectors is minimized. For example, since time slot t₀ is used in the fringe area 200, but is not used in SECTOR II, the interference between communications in the fringe area 200 and communications in SECTOR II is minimized. Also, since communications in the fringe area 200 are provided using the hopping sequences of SECTOR I which are selected to provide minimum interference within a sector, as described in United States Patent No. 5,408,496, interference within the fringe area 200 is minimized.

As mentioned earlier, the system 1 can accomplish either time hopping or frequency hopping independently. Thus, the system 1 , in any sector, can either hop the signals being communicated over a plurality of time slots without frequency hopping. Conversely, the system 1 , in any sector can hop the signals being communicated over a plurality of frequency channels without time hopping.

Thus, the present invention is applicable to systems that only frequency hop, to systems that only time hop and to systems that do both. In the general case, there are a plurality of sequences, s_l . . . s_k, that hop in time and/or in frequency and that are used in SECTOR !. There is another set of time and/or frequency hopping sequences, s_l' . . . s_k' , that are used in

SECTOR II. The sequences s_l' . . . s_k' are chosen to minimize interference in SECTOR II and also to minimize interference between the two sectors. In SECTOR I, there is a subset of the sequences s_l . . . s_k that are orthogonal to all the sequences in SECTOR II, which are designated s_l . . . s_c. This subset of sequences, s_l . . . s_c, is used in the fringe area. They can also be used in other parts of SECTOR I. The sequences s_d . . . s_k are not used in the fringe area.

In FIG. 7, another embodiment of the present invention is illustrated. Three sectors, SECTOR 1, SECTOR II and SECTOR III, are illustrated. In SECTOR I communications are provided by frequency hopping over a plurality of frequency channels (nine in this case) in accordance with a set of frequency hopping sequences, the set being designated as f_A(f ₁ . . f₉). The signals are also transmitted over a plurality of time slots, t₀, t₁ and t₂, preferably in a time hopping fashion as previously described.

In SECTOR II, communications are provided by frequency hopping over a plurality of frequency channels (again nine) in accordance with a new set of frequency hopping sequences, which sequence is designated as fg(f₁ . . . f₉).

Once again the sets of hopping sequences f_A(f ₁. . . f₉) and f_B(f₁ . . . f₉) are selected to provide no interference within a sector and to provide minimum interference between adjacent sectors, as explained in United States Patent No. 5,408,496. The signals in SECTOR li are transmitted over the plurality of time slots used in SECTOR I, less one of the time slots, preferably although not necessarily, in a time hopping fashion. In this case, time slots t ₁ and t₂ are used in SECTOR II. In the fringe area 202, defined by the area where SECTOR I and

SECTOR II overlap, communications are provided by using the time slot that is not used in SECTOR II, t₀, and by using the set of frequency hopping sequences f_A(f₁ . . . f₉) used in SECTOR I. Thus, interference between SECTOR I, SECTOR II and the fringe area 202 is minimized.

In SECTOR HI, communications are provided by frequency hopping over a plurality of frequency channels (again nine) in accordance with a new set of frequency hopping sequences, which sequence is designated as f_C(f₁ . . . f₉) . Once again, the sets of hopping sequences f_A(f₁ . . . f₉) and f_C(f₁ . . . f₉) are selected to provide no interference within a sector and to provide minimum interference between adjacent sectors, as explained in United States Patent No. 5,408,496. The signals in SECTOR III are transmitted over the plurality of time slots used in SECTOR I, less one of the time slots, _. preferably although not necessarily, in a time hopping fashion. In this case, time slots t₀anc t ₂are used in SECTOR III.

In the fringe area 204, defined by the area where SECTOR I and SECTOR HI overlap, communications are provided by using the time slot that is not used in SECTOR III, t₁, and by using the set of frequency hopping sequences f_A(f₁ . . . f₉) used in SECTOR I. Thus, interference between SECTOR I, SECTOR III and the fringe area 204 is minimized.

Referring to FIG. 8, yet another embodiment of the present invention is illustrated. In FIG. S, SECTO R I uses the set of frequency hopping sequences f_A(f₁ . . . f₉) and three time slots t₀ , t₁ and t₂, preferably in a time hopping fashion, to provide communications. SECTOR II uses the set of frequency hopping sequences f_B(f ₁ . . . f_g) and two time slots, t₀ and t₁, preferably in a time hopping fashion, to provide communications. SECTOR III uses the set of frequency hopping sequences f_C(f ₁. . . f₉) and two time slots, t₀ and t₂, to provide communications. Communications in the fringe area 206, defined by the overlapping areas of SECTOR II and SECTOR III, are provided by using the set of frequency happing sequences of SECTOR II, f_B(f₁ . . . f₉) and the time slot which is not used in SECTOR III, which is t₁.

Communications in the fringe area 208, defined by the overlapping areas of SECTOR I and SECTOR II, are provided by using the set of frequency hopping sequences of SECTOR I, and the time slot which is not used in SECTOR II, which is t₂. Once again, this arrangement minimizes the interference in the fringe areas 206 and 208 and in sectors adjacent to the fringe areas 206 and 208.

FiG. 9 illustrates another embodiment of the present invention. In SECTORS I and III, communications are provided by time hopping over ail three time slots t₀. t ₁ and t ₂and by frequency hopping over the sets of frequency hopping sequences f_A( f ₁ . . . f₉) and f_C(f ₁ . . . f₉), respectively.

Communications in SECTOR II are provided by time hopping over two time slots t₁ and t ₂ and by frequency hopping over the sets of frequency hopping sequences f_B(f₁ . . . f₉) . Communications in the fringe area 209A between SECTORS I and II are provided by frequency hopping over the sequences used in SECTOR I, f_A(f₁ . . . f₉) and by using the time slot t₀.

Communications in the fringe area 2093 between SECTORS II and III are provided by frequency hopping over the sequences used in SECTOR III, f _C(f₁ . . . f₉) and by using the time slot to ;

In FIG. 9, two of the three sectors utilize the full range of available time slots whereas in FIGS. 7 and 8 only one of the three sectors utilize the full range. Accordingly, the embodiment of FIG. 9 yields greater system capacity than the embodiments of FIGS. 7 and 8 and, for that reason, is generally preferred.

Referring to FIG. 10, a communication system having a four sector arrangement that uses the present invention is illustrated. The four sectors are SECTOR I, SECTOR II, SECTOR III and SECTOR IV. Located between each sector are fringe areas 21 0, 21 2, 21 4 and 21 6 which are defined by the overlapping areas of adjacent sectors. Communications in SECTOR I and in SECTOR III are provided utilizing three time slots, t₀, t_j and t₂, preferably in a time hopping fashion, and by utilizing the sets of frequency hopping sequences f_A(f₁ . . .f₉) and f_C(f₁ . . . f₉), respectively.

Communications in SECTOR II, which has fringe areas 21 0 and 21 2 between SECTOR I and SECTOR HI, respectively, are provided utilizing the time slots of SECTOR I and SECTOR III, less a selected one of the time slots. In this example, time slots t ₁ and t₂ are selected for use in SECTOR II and time slot t₀ is reserved for use in the fringe areas 210 and 21 2.

Communications in SECTOR II are also provided by utilizing the set of frequency hopping sequences f_B(f₁ . . . f₉).

Communications in the fringe area 21 0 are provided by using the selected time slot t₀and by using the set of frequency hopping sequences, f _A(f₁ . . . f₉), of SECTOR I. Communications in the fringe area 21 2 are provided by also using the selected time slot t₀and by using the set of frequency hopping sequences, f_C(f₁ . . . f₉), of SECTOR III. Thus,

communications are provided in a minimally interfering manner in the fringe areas 210 and 212.

Communications in SECTOR IV, which has fringe areas 214 and 21 6 between SECTOR III and SECTOR I, respectively, are provided utilizing thetime slots of SECTOR I and SECTOR 111, less a selected one of the time slots. In this example, time slots t₀and t₂ are selected for use in SECTOR IV and time slot t ₁ is reserved for use in the fringe areas 21 4 and 21 6.

Communications in SECTOR IV are also provided by utilizing the set of frequency hopping sequences f_D(f₁ . . . f₉).

Communications in the fringe area 21 4 are provided by using the selected time slot t₁ and by using the set of frequency hopping sequences, f_C(f ₁. . . f₉), of SECTOR III. Communications in the fringe area 21 6 are provided by also using the selected time slot t₁ and by using the set of frequency hopping sequences, f_A(f₁ . . . f₉), of SECTOR I. Thus, communications are provided in a minimally interfering manner in the fringe areas 21 4 and 21 6.

Referring to FIGS. 11 and 1 2, the method by which the base station 2 and the subscriber units 6 determine whether a subscriber unit 6 is located within a sector 8, 9 or 1 0 or whether the subscriber unit is located in a fringe area 21 8, 21 9 or 220, is illustrated. As previously described, there are nine frequency channels that are used to transmit communications between the base station 2 and the subscriber units 6. In a preferred communication system, an additional frequency channel is dedicated to transmitting control information from the base station 2 to the subscriber units 6. This frequency channel is preferably referred to as the Control Channel ("CCH").

Referring to FIGS. 1 and 3, the sector hardware 1 6, 1 7 and 1 8 in the base station 2 transmits CCH signals, CCH8, CCH9 and CCH 11, respectively to the sectors 8, 9 and 10, respectively. The control information is transmitted from the base station 2 to the subscriber unit 6 in a non-hopping fashion, that is, the control information is continuously transmitted on a single frequency channel, which is referred to as the CCH. Also, each sector 8 to 1 0 preferably transmits the signal in a dedicated time slot, that is, the CCH is net time hopped either. Thus, the CCH signals might be transmitted as follows: CCH8 from sector hardware 1 6 into sector 8 during time slct t₀;

CCH9 from sector hardware 1 7 into sector 9 during time slot t₁; and CCH10 from sector hardware 18 into sector 1 0 during time slot t₂. Referring to FIG. 11 , the signal strengths of CCHs transmitted into three sectors are illustrated. Each operational subscriber unit 6 receives the CCHs with the receiver circuitry illustrated in FIG. 5 and, with a digital signal processor (DSP) in the modem 1 26, monitors the signal strength of the CCH transmitted into the sector in which the subscriber unit 6 is located and the signal strength of the CCH transmitted into the sectors adjacent to the sector in which the subscriber unit 6 is located, referred to as adjacent sectors. Thus, in the communication system illustrated in FIG. 1 2, the subscriber unit 6 receives three signals: CCH8, CCH9 and CCH10.

When the subscriber unit 6 is located within a sector, but not within a fringe area, the signal strength of the CCH transmitted within the sector will be higher than any other CCH transmitted from another sector by more than a threshold, th. As the subscriber unit 6 approaches a fringe area, however, the signal strength of the CCHs transmitted from adjacent sectors will approach the signal strength of the CCH transmitted from the sector in which the subscriber unit 6 is located. Eventually, the difference between the signal strength of the CCHs transmitted from adjacent sectors and the signal strength of the CCH transmitted from the sector in which the subscriber unit 6 is located will be less than some threshold, th. When this happens, DSP in the modem 1 26 in the subscriber unit 6 determines that the subscriber unit

6 is in a fringe area.

For example, if the subscriber unit 6 is in the sector 8, but not in any fringe area, then the strength difference between the signal CCH8 and each of the signals CCH9 or CCH10 will be above the threshold th. If the subscriber unit 6 is in the sector 9, but not in any fringe area, then the strength difference between the signal CCH9 and each of the signals CCH8 and CCH10 will be above the threshold th. If the subscriber unit 6 is in the sector 1 0, but not in any fringe area, then the strength difference between the signal CCH10 and each of the signals CCH8 and CCH9 will be above the threshold th. In each of these cases, the DSP 1 26 in the subscriber unit 6 determines which sector it is located in based on the strength of the CCH signals and, with the transmitter circuitry of FIG. 5, reports this information to the base station 2, either over one of the time-frequency hopping channels or over a dedicated access frequency channel, depending on whether the subscriber unit 6 is in a call session and whether there is an uplink or downlink assigned. The central controller 30 and the sector hardware 1 6, 1 7, 1 8, 20 or 21 in the base station 2 then registers the subscriber unit 6 in the sector that the subscriber unit 6 is located in. If the subscriber unit 6 is in a call session, it also assigns a frequency hopping sequence and/or a time hopping sequence to the subscriber unit 6 based on which sector the subscriber unit 6 is located in.

As stated before, if the subscriber unit 6 determines that the difference in signal strengths between the sector CCH and the adjacent sector CCH falls below the threshold, th, then the subscriber unit 6 determines that it is located in the fringe area between the sector and the adjacent sector. In accordance with a preferred embodiment, this process is only performed during a call session or during a call session setup, that is, when the subscriber is communicating over the subscriber unit 6, or in the process of setting up the call.

As an example of this process, consider the situation where the subscriber unit 6 is in sector 1 0 and is receiving the CCH signals illustrated in FIG. 11A. When the DSP in the modem 126 of the subscriber unit 6 compares the CCH signal 223 from adjacent sector 9 to the CCH signal 224 from sector 1 0, it determines that the difference in signal strengths is greater than the threshold, th, so that the subscriber unit 6 is not near the fringe area 21 9 between sectors 1 0 and 9 (FIG. 12). On the other hand, when the DSP compares the CCH signal 222 from adjacent sector 8 to the CCH signal 224 from sector 1 0, it determines that the difference in signal strengths is less than the threshold, th, so that the subscriber unit 6 is in the fringe area 220 between the sectors 1 0 and 8 (FIG. 12).

If the subscriber unit 6 is located in sector 9 and receives the CCH signals 225 to 227 from sectors 8 to 1 0, respectively, as shown in FIG . 11 B, the DSP in the subscriber unit 6 measures the difference between the signal strengths of the signal CCH9 226 and the signals CCH8 225 and CCH10 227. This analysis shows that CCH9 minus CCH10 is greater than the threshold th so that the subscriber unit 6 is not in the fringe area 21 9. However, CCH9 minus CCH8 is less than the threshold th so that the DSP determines that the subscriber unit 6 is within the fringe area 21 8 between sectors 8 and 9 (FIG. 12).

If the subscriber unit 6 is located in sector 1 0 and receives the CCH signals 228 to 230 sectors 8 to 1 0, respectively, as shown in FIG. 11C, the

DSP in the subscriber unit 6 measures the difference between the signal strengths of the signal CCH10 230 and the signals CCH8 228 and CCH9 229. This analysis shows that CCH10 minus CCH8 is greater than the threshold th so that the subscriber unit 6 is not in the fringe area 220.

However, CCH10 minus CCH9 is less than the threshold th so that the DSP determines that the subscriber unit 6 is within the fringe area 21 9 between sectors 9 and 1 0.

In case the subscriber unit 6 finds out more than one difference between its CCH and another CCH is less than the threshold th, the subscriber unit 6 takes the highest CCH except its own to be the one to consider as the adjacent sector it is entering.

Whichever fringe area the subscriber unit 6 determines it is in, that location information is reported back to the base station 2, either over one of the time-frequency hopping channels or over a dedicated access channel, as before. The base station 2, using the location of the subscriber unit 6, then assigns frequency hopping and/or time hopping sequences to the subscriber unit 6 whenever communications with the subscriber unit 6 are needed. In FIG. 1 2, the frequency hopping sequences and the time slot assignments that are assigned by the controller 30 and by the sector hardware 1 6 to 18 or 20 to 21 in the base station 2 in one embodiment of the present invention, are illustrated. If the subscriber unit 6 determines that it is in the sector 8 and reports this information to the base station 2, the base staticn 2 then assigns a frequency hopping sequence from a first set of sequences, f _A(f ₁ . . . f₉) and also assigns the time slots t₀, t ₁ and t₂, preferably in a time hopping fashion, for communications with that subscriber unit. If the subscriber unit 6 determines that it is in the sector 9 and reports that information to the base station 2, the base station 2 assigns a frequency hopping sequence from a second set of sequences, f_B(f₁ . . . f₉) and also assigns the time slots t₀and t₁, preferably in a time hopping fashion, for communications with that subscriber unit. If the subscriber unit 6 .

determines that it is operating in the sector 1 0 and reports that information to the base station 2, the base station 2 assigns a frequency hopping sequence from a third set of sequences, f_C(f₁ . . . f₉) and also assigns the time slots t ₀ and t₂, preferably in a time hopping fashion, for communications with that subscriber unit. The sets of sequences are preferably selected in accordance with the teachings of United States Patent No. 5,408,496, as previously described, to minimize interference.

If the subscriber unit 6 determines that it is operating in the fringe area 21 8 between sectors 8 and 9, the base station 2 assigns a frequency hopping sequence from the first set of sequences, f_A(f₁ . . . f₉) used in sector 8 and also assigns the time slot t₂ for communications with that subscriber unit 6. If the subscriber unit 6 determines that it is operating in the fringe area 21 9 between the sectors 9 and 1 0, the base station 2 assigns a frequency hopping sequence from the second set of sequences, f_B(f₁ . . . f₉) used in sector 9 and also assigns the time slot t₁, for communications with that subscriber unit 6. If the subscriber unit 6 determines that it is operating in the fringe area 220 between the sectors 8 and 1 0, the base station 2 assigns a frequency hopping sequence from the first set of sequences, f_A(f₁ . . f₉) used in sector 8 and also assigns the time slot t₁, for communications with that subscriber unit 6.

It should be noted that many different assignments of hopping sequences and of time slots can be made by the base station 2. In general, it is preferred to make the assignments to maximize the numbers of frequency channels and time slots in use, thereby maximizing the capacity of the communication system.

It should be noted that the CCH signal strengths illustrated in FIGS . 11A-11C are preferably generated by keeping a running average of a plurality of time slots of the CCHs received by the subscriber unit 6. This averaging provides improved determination of subscriber unit 6 location. Also, a hysteresis over the threshold th is preferably inserted so as to prevent the determination of location from jumping quickly from sector to fringe area and back to the sector as described hereafter.

Under normal operation, the base station 2 would effectuate a hand off when a subscriber unit 6 moves from one sector to another. When the fringe area handling process of the present invention is utilized, however, the base station 2, preferably hands off when the subscriber unit 6 moves from a sector into a fringe area and then hands off again when the subscriber unit 6 moves from the fringe area to a sector. The handoff process is not done when the subscriber unit 6 uses one of the fringe area sequences, and it crosses to or from the sector that these sequences belong to.

This process is illustrated in FIG. 1 3. The hopping sequences in the fringe area belong to the set of sequences which belong to SECTOR A.

When a subscriber unit 6 is in SECTOR A, the signal strength of the CCH from SECTOR A exceeds the signal strength of the CCH from SECTOR B by the threshold th: CCHA - CCHB > th. As the subscriber unit 6 enters the fringe area, the difference between the signal strength of the CCH from

SECTOR A and the signal strength of the CCH from SECTOR B falls below the threshold th: CCHA - CCHB < th. When this occurs, the base station 2 causes the communications with the subscriber unit 6 to be handed off from the SECTOR A to the fringe area if needed. Thus, the subscriber unit 6 receives new hopping sequences from the base station 2 that are assigned to the fringe area unless the subscriber unit 6 already uses one of these sequences. If the subscriber unit 6 moves back into SECTOR A, then the signal strength of the CCH from SECTOR A will again exceed the signal strength of the CCH from SECTOR B by the threshold th: CCHA - CCHB > th. If this occurs, there is no handoff process that takes place and the subscriber unit 6 continues to use the same sequences. In this case, the base station 2 assigns hopping sequences from SECTOR A to handle the communications with the subscriber unit 6.

If the subscriber unit 6 moves into SECTOR B, however, the difference between the signal strength of the CCH from SECTOR B and the signal strength of the CCH from SECTOR A exceeds the threshold th: CCHB -

CCHA > th. When this occurs, the base station 2 causes the

communications with the subscriber unit 6 to be handed off from the fringe area to the SECTOR B. Thus, the subscriber unit 6 receives new hopping sequences from the base station 2 that are assigned to SECTOR B. If the subscriber unit 6 moves from SECTOR B into the fringe area, the difference between the signal strength of the CCH from SECTOR B and the signal strength from SECTOR A falls below the threshold th': CCHB-CCHA < th' where th' < th. When this occurs, the base station 2 causes the

communications with the subscriber unit 6 to be handed off from SECTOR B to the fringe area. Thus, the subscriber unit 6 receives new hopping sequences from the base station 2 that are assigned to the fringe area.

In accordance with a preferred embodiment, the threshold th is selected to be 6db and th' to be 3 dB. It should be noted that these thresholds th and th' will vary from system to system and is preferably selected empirically.

Any of hand off processes known in the art can be used to effectuate these hand offs. The following, however, describes a preferred hand off process. It is preferred that the hand off be accomplished during a time when there is no communication activity. The lack of activity is detected by a voice activity detector in the vocoder in the base station 2 and in the subscriber unit. When the above-described analysis indicates that a hand off is needed and when the voice activity detector detects no activity, the hand off occurs.

The present invention, while described with respect to sectorized systems, is applicable to any communication system having overlapping communication areas. Thus, the system and method of the present invention is applicable to cellular communication systems. It is also applicable to cellular systems that have sectorized cells.

It is understood that changes may be made in the above description without departing from the scope of the invention. It is accordingly intended that all matter contained in the above description and in the drawings be interpreted as illustrative rather than limiting.

Claims

We claim:

1 . A method of communicating signals in a communication system having a plurality of sectors, comprising the steps of:

communicating in a first sector by hopping the signals over a first set of frequency sequences and by using a plurality of time slots;

communicating in a second sector by hopping the signals over a second set of frequency sequences and by using the plurality of time slots less a first selected time slot; and

communicating in an area where the first sector and the second sector overlap by hopping the signals over the set of frequency sequences used in the first sector and by using the first selected time slot not used in the second sector.

2. The method of claim 1 , further comprising the steps of:

communicating in the first sector by hopping the signals over the plurality of time slots; and

communicating in the second sector by hopping the signals over the plurality of time slots less the first selected time slot.

3. The method of claim 1 , wherein the first set of frequency sequences are orthogonal and wherein the second set of frequency sequences are orthogonal.

4. The method of claim 1 , wherein the first and second sets of frequency sequences are selected such that any one of the sequences from the set of sequences of the first sector and any one of the sequences from the set of sequences of the second sector use the frequencies at the same time a minimum number of times in an allocated period of time.

5. The method of claim 1 , further comprising the steps of:

communicating in a third sector by hopping the signals over a third set of frequency sequences and by using the plurality of time slots less a second selected time slot; and

communicating in an area where the-first sector and the third sector overlap by hopping the signals over the set of frequency channel sequences used in the first sector and by using the second selected time slot.

6. The method of claim 5, wherein the second selected time slot is the same as the first selected time slot.

7. The method of claim 6, further comprising the step of:

communicating in the first sector by hopping the signals over the plurality of time slots.

8. The method of claim 7, further comprising the steps of

communicating in the second sector by hopping the signals over the plurality of time slots less the first selected time slot; and

communicating in the third sector by hopping the signals over the plurality of time slots less the second selected time slot.

9. The method of claim 5, further comprising the steps of:

communicating in a fourth sector by hopping the signals over a fourth set of hopping sequences and by using the plurality of time slots;

communicating in an area where the third sector and the fourth sector overlap by hopping the signals over the set of frequency channel sequences used in the fourth sector and by using the second selected time slot.

1 0. The method of claim 9, further comprising the step of:

communicating in an area where the fourth sector and the second sector overlap by hopping the signals over the set of frequency channel sequences used in the fourth sector and by using the first selected time slot.

11 . The method of claim 1 0, wherein the second selected time slot is the same as the first selected time slot.

1 2. Apparatus for communicating signals in a communication system having a plurality of sectors, comprising:

means for communicating in a first sector by hopping the signals over a first set of frequency sequences and by using a plurality of time slots; means for communicating in a second sector by hopping the signals over a second set of frequency sequences and by using the plurality of time slots less a first selected time slot; and

means for communicating in an area where the first sector and the second sector overlap by hopping the signals over the set of frequency sequences used in the first sector and by using the first selected time slot not used in the second sector.

1 3. The apparatus of claim 1 2, further comprising:

means for communicating in the first sector by hopping the signals over the plurality of time slots; and

means for communicating in the second sector by hopping the signals over the plurality of time slots less the first selected time slot.

1 4. The apparatus of claim 1 2, wherein the first set of frequency sequences are orthogonal and wherein the second set of frequency sequences are orthogonal.

1 5. The apparatus of claim 1 2, wherein the first and second sets of frequency sequences are selected such that any one of the sequences from the set of sequences of the first sector and any one of the sequences from the set of sequences of the second sector use the frequencies at the same time a minimum number of times in an allocated period of time.

1 6. The apparatus of claim 1 2, further comprising:

means for communicating in a third sector by hopping the signals over a third set of frequency sequences and by using the plurality of time slots less a second selected time slot; and

means for communicating in an area where the first sector and the third sector overlap by hopping the signals over the set of frequency channel sequences used in the first sector and by using the second selected time slot.

1 7. The apparatus of claim 1 6, wherein the second selected time slot is the same as the first selected time slot.

1 8. The apparatus of claim 1 7, further comprising:

means for communicating in the first sector by hopping the signals over the plurality of time slots.

1 9. The apparatus of claim 1 8, further comprising:

means for communicating in the second sector by hopping the signals over the plurality of time slots less the first selected time slot; and

means for communicating in the third sector by hopping the signals over the plurality of time slots less the second selected time slot.

20. The apparatus of claim 1 6, further comprising:

means for communicating in a fourth sector by hopping the signals over a fourth set of hopping sequences and by using the plurality of time slots;

means for communicating in an area where the third sector and the fourth sector overlap by hopping the signals over the set of frequency channel sequences used in the fourth sector and by using the second selected time slot.

21 . The apparatus of claim 20, further comprising:

means for communicating in an area where the fourth sector and the second sector overlap by hopping the signals over the set of frequency channel sequences used in the fourth sector and by using the first selected time slot.

22. The apparatus of claim 20, wherein the second selected time slot is the same as the first selected time slot.

23. A method of communicating signals in a communication system having a plurality of sectors, comprising the steps of:

communicating in an area where the first sector and the second sector overlap by hopping the signals over the set of frequency sequences used in the second sector and by using the first selected time slot not used in the second sector.

24. The method of claim 23, further comprising the steps of:

communicating in an area where the first sector and the third sector overlap by hopping the signals over the set of frequency channel sequences used in the third sector and by using the second selected time slot.

25. Apparatus for communicating signals in a communication system having a plurality of sectors, comprising:

means for communicating in an area where the first sector and the second sector overlap by hopping the signals over the set of frequency sequences used in the second sector and by using the first selected time slot not used in the second sector.

26. The method of claim 25, further comprising:

means for communicating in an area where the first sector and the third sector overlap by hopping the signals over the set of frequency channel sequences used in the third sector and by using the second selected time slot.

27. A method of communicating signals in a communication system having a plurality of sectors, each sector having a plurality of frequency and time hopping sequences over which communications take place, comprising the steps of:

communicating in a first sector by frequency and time hopping the signals over a first set of hopping sequences;

communicating in a second sector by frequency and time hopping the signals over a second set of hopping sequences, the second set of hopping sequences not using a first selected time slot; and

communicating in an area where the first sector and the second sector overlap by hopping the signals over a subset of the set of hopping sequences used in the first sector, the subset only using the first selected time slot not used in the second sector.

28. The method of claim 27, wherein the first set of hopping sequences are orthogonal and wherein the second set of hopping sequences are orthogonal.

29. The method of claim 27, wherein the first and second sets of hopping sequences are selected such that any one of the sequences from the set of sequences of the first sector and any one of the sequences from the set of sequecnes of the second sector use same frequencies and the same time slot at the same time a minimum number of times in an allocated period of time.

30. The method of claim 27, further comprising the steps of:

communicating in a third sector by frequency and time hopping the signals over a third set of hopping sequences, the third set of hopping sequences not using a second selected time slot; and

communicating in an area where the first sector and the third sector overlap by hopping the signals over a subset of the set of hopping sequences used in the first sector and by using the second selected time slot.

31 . The method of claim 30, wherein the first and second selected time slot are the same time slot.

32. Apparatus for communicating signals in a communication system having a plurality of sectors, each sector having a plurality of frequency and time hopping sequences over which communications take place, comprising:

means for communicating in a first sector by frequency and time hopping the signals over a first set of hopping sequences;

means for communicating in a second sector by frequency and time hopping the signals over a second set of hopping sequences, the second set of hopping sequences not using a first selected time slot; and

means for communicating in an area where the first sector and the second sector overlap by hopping the signals over a subset of the set of hopping sequences used in the first sector, the subset only using the first selected time slot not used in the second sector.

33. The apparatus of claim 32, wherein the first set of hopping sequences are orthogonal and wherein the second set of hopping sequences are orthogonal.

34. The apparatus of claim 32, wherein the first and second sets of hopping sequences are selected such that any one of the sequences from the set of sequences of the first sector and any one of the sequences from the set of sequecnes of the second sector use same frequencies and the same time slot at the same time a minimum number of times in an allocated period of time.

35. The apparatus of claim 32, further comprising:

means for communicating in a third sector by frequency and time hopping the signals over a third set of hopping sequences, the third set of hopping sequences not using a second selected time slot; and

means for communicating in an area where the first sector and the third sector overlap by hopping the signals over a subset of the set of hopping sequences used in the first sector and by using the second selected time slot.

36. The apparatus of claim 35, wherein the first and second selected time slot are the same time slot.