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`1
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`5,020,056
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`the present invention for avoiding synchronous fading
`data loss.
`
`REDUCTION OF SYNCHRONOUS FADING
`EFFECTS BY TIME HOPPING OF USER SLOTS IN
`TDMA FRAMES
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`5
`
`BACKGROUND OF THE INVENTION
`The present application relates to communication
`systems and, more particularly, to a novel method for
`the reduction of synchronous fading effects in a time- 10
`diversity multiple-access (TDMA) communications
`system.
`In a communications system environment, such as a
`mobile radio channel, a fade is said to have occurred if
`the received signal envelope drops below some prede- 15
`fined acceptable level. Synchronous fading is that situa(cid:173)
`tion where a plurality of signal fades occur with a rela(cid:173)
`tively constant time periodicity, as, for example, may be
`related to speed of the vehicle in which the receiving
`radio telephone is carried. In such a synchronous fading 20
`situation, the signals received from a central station
`experience periodic fading at fairly constant time inter(cid:173)
`vals and pose a serious problem if the fading occurs
`synchronously with the occurrence of the particular
`time slot, or an integral multiple of time slot occur- 25
`rences, in which data for that particular channel is trans(cid:173)
`ferred such that the transmissions to/from a particular
`user suffer degradation and cannot be mitigated by
`interpolation techniques,
`techniques using forward
`error correction codes, or other high overhead error 30
`correction techniques. It is therefore highly desirable to
`provide a method for the reduction of synchronous
`fading effects, particularly in a TDMA communication
`system.
`
`DETAILED DESCRIPTION OF THE PRESENT
`INVENTION
`Referring initially to FIG. la, a signal 10, received at
`a mobile station in a time-diversity multiple-access
`(TDMA) system, is graphed, with amplitude indicated
`along ordinate 11, for increasing time along ordinate 12.
`The signal is provided by a stationary main transmitter.
`It is well known that fading occurs due to many differ-
`ent situations, with a fade being defined as the reduction
`of the level of received signal envelope 10 below some
`acceptable level lla, such as occurs at fade troughs 13a,
`13b and 13c. Thus, the transmitted signal, which may be
`reflected from various objects in the terrain and the like,
`may have these reflected signals arrange themselves in
`some form of standing wave pattern, with the standing
`wave pattern comprising the series of troughs 13 in the
`signal level 10. When a mobile receiver passes through
`each of signal troughs 13a, 13b, 13c, ... , the signal falls
`below the minimum signal level lla at which satisfac(cid:173)
`tory data recovery can take place and a fade occurs.
`Therefore, successive fades will occur at each of the
`troughs 13a, 13b, 13c, ... with the average fade duration
`and the interval between successive fades depending on
`factors such as antenna pattern, threshold level, carrier
`frequency, vehicle speed, and the like. Of primary inter(cid:173)
`est are the vehicle-speed-related synchronous fades
`which occur with a relatively constant time periodicity
`T related to the vehicle speed. In such a situation, the
`received signal level 10 experiences each of fades 13
`with fairly constant time intervals T therebetween.
`Referring now to FIGS. la and lb, in the prior art
`35 TDMA system, each user (such as user I) is assigned a
`BRIEF SUMMARY OF THE INVENTION
`time slot during which the user can send or receive
`In accordance with the invention, a method for the
`data. Several users have their time slots arranged in a
`time-aligned pattern which forms a frame (which may
`reduction of synchronous fading effects in a time-diver-
`have, e.g., 3-10 user slots). As illustrated, each of the
`sity multiple-access communications system, in which 40 consecutive frames A, B, C, ... has four users I, II, III
`the data to/from a particular user is to appear in a par-
`and IV, with the first user having the first slot in each of
`ticular assigned slot, utilizes the step of: varying the
`a succession of frames, e.g. the first slot 14a in the first
`position of the time slot, assigned to that particular user,
`frame, the first slot 14b in the second frame, the first slot
`l4c in the third. frame and so on. It will be seen that, if
`to have a different position in each successive frame in
`which that assigned slot appears; the position may be set 45 the first. user always has the first slot in each sequential
`frame, then there will be a problem if a fade 13a, 13b,
`relative to a frame feature, such as its start time and the
`like. Selected-schedule patterns may be used, with the
`13c, . . . occurs substantially simultaneously with the
`pattern either being preset at the various stations or
`occurrence of the particular time slot (e.g. slots la, lb,
`le, ... ) of the particular user (e.g. user I), or at an inte-
`being sent to the stations prior to use, as part of a slot
`50 gral multiple of the time between successive occur-
`preamble and the like.
`rences of the same slot. In fact, it will be seen that syn-
`Accordingly, it is an object of the present invention
`to provide a novel method for reduction of synchro-
`chronous fading of a most severe type occurs if fades
`13a, 13b, 13c, ... each occur with a peri~icity such that
`nous fading effects by time-hopping user slots in succes-
`sive frames of a TDMA communications system.
`the time T therebetween is essentially equal to the peri-
`This and other objects of the present invention will 55 odicity of the frame, which frame periodicity is the time
`between the successive occurrences of each slot in con-
`become apparent upon a reading of the following de-
`tailed description, when considered in conjunction with
`secutive frames. These synchronous mobile channel
`the associated figures.
`signal fades produce burst errors which, while normally
`mitigated by use of high overhead forward error cor(cid:173)
`BRIEF DESCRIPTION OF THE DRAWINGS
`rective code and other fade-combatant interpolation
`FIG. la is a graph illustrating the received signal
`techniques, will have a loss of data if every user slot in
`level versus time, at a mobile receiver, in a typical
`a succession of frames is affected by a fade.
`TDMA radio communications system;
`Referring now to FIGS. la and le, in accordance
`FIG. lb is a time-synchronized graph of user slot
`with my invention, synchronous fading is avoided if the
`assignments in a prior art system, illustrating the prob- 65
`time slots in each frame are sent in a non-fixed user
`Jem to be solved; and
`order, and are switched according to some predet~r-
`FIG. le is a time-synchronized graph of a like plural(cid:173)
`mined, i.e. non-random, pattern so that the probability
`ity of frames of a TDMA system utilizing the method of
`of the same slot being subjected to a synchronous fade
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`Ericsson Exhibit 1006
`Page 3
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`5,020,056
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`in successive frames is lowered. Thus, a particular user
`(e.g. user I) is assigned the first user slot 16a in the first
`frame A, but is not placed in the same first slot in each
`of the subsequent frames. Instead, in frame B, the first
`user slot lb is the last slot 16b and is some other slot in 5
`subsequent frames, such as the third slot 16c in frame C,
`the second slot 16d in the next frame, and so forth, in
`accordance with some predetermined pattern. Here,
`this pattern is the cyclic shifting of the first user slot by
`one slot to the left in each successive frame, with wrap- 10
`around to the last slot in a next frame, after a frame in
`which the first slot is used. The effective result is that
`only in one time slot, i.e. slot 16a, is the first user sub(cid:173)
`jected to one of fades 13 (e.g. fade 13a). Utilizing this
`time-slot-hopping method of slot variation to switched 15
`slot assignments will tend to break down any synchro(cid:173)
`nous fade pattern which might be present and turns that
`pattern into an asynchronous pattern with reference to
`a particular user slot over a relatively short time inter(cid:173)
`val. It will be seen that there is, in the example, still 20
`some degree of periodicity in the sequence of the sec(cid:173)
`ond slot 16b, third slot 16c, fourth slot 16d, and so forth,
`until the user attains the first slot in a frame, and then
`rolls to the last slot in the next subsequent frame to
`break the pattern. Even this degree of periodicity can be 25
`removed by generating a preselected pattern which is
`deliberately designed to remove synchronisity. It will
`be understood that this pattern is predetermined, for the
`particular communication system, so that the base sta(cid:173)
`tion and the mobile stations are all pre-informed of the 30
`time-slot-hopping pattern to be utilized and are thus
`maintained in time alignment during the data exchange,
`so that the time hopping procedure is user transparent.
`The particular pattern to be utilized and/or frame time
`synchronization information for carrying out framing 35
`procedures, can be provided in a data preamble trans(cid:173)
`mission 18a at the start of each of a plurality of super(cid:173)
`frames 18 in each slot or as signaling information trans(cid:173)
`mitted in either slow or fast associated control channel
`modes. It will be seen that the exact manner of hopping 4-0
`between slots will depend upon the particular TDMA
`channel structure utilized, which will include such fac(cid:173)
`tors as: frame length; data rate; slot quantities, number
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`4
`of users per frame; and system factors, such as antenna
`diversity switching ability, mobile-assisted hand-off and
`time alignment, presence of techniques such as synthetic
`regeneration, automatic repeat request, and the like.
`While one presently preferred embodiment of my
`novel method for reduction of synchronous fading ef(cid:173)
`fects by time-hopping user slot assignments in time(cid:173)
`diversity, multiple-access framed communication sys(cid:173)
`tems, is described in detail herein, many modifications
`and variations will now become apparent to those
`skilled in the art. It is my intent, therefore, to be limited
`only by the scope of the appending claims, and not by
`the specific details provided by way of explanation of
`the this presently preferred embodiment.
`What is claimed is:
`1. A method for reduction of synchronous fading in a
`TDMA communications system, comprising the steps
`of:
`assigning each user to a time slot in which data to or
`from that user appears; and
`varying the position of the time slot assigned to that
`particular user to be in a different part of each
`successive frame.
`2. The method of claim 1, wherein the part of the
`frame in which the particular user time slot appears is
`varied according to a predetermined pattern.
`3. The method of claim 2, wherein the pattern is a
`selected schedule pattern.
`4. The method of claim 2, wherein the pattern is
`preset into the user station.
`5. The method of claim 2, wherein the pattern is sent
`to the user station as slow associated control channel
`data prior to use.
`6. The method of claim 2, wherein the pattern is sent
`as part of a slot preamble.
`7. The method of claim 2, wherein the pattern is sent
`to the user station as fast associated control channel
`data.
`8. The method of claim 2, wherein the pattern is
`selected to reduce the probability of the same time slot
`being subjected to synchronous fading in successive
`frames.
`
`* * * * *
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`Ericsson Exhibit 1006
`Page 4
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