METHOD FOR THE REALIZATION OF A QUICK AND COMPLETE PRE-SELECTION OF THE CHANNELS IN A DIGITAL TELECOMMUNICATION SYSTEM

Abstract: Description of a method and a receiver for the realization of the radio access to a channel of a digital telecommunication system, as for example of the type operating according to the DECT standard. The receiver is of the type comprising a frequency synthesizer circuit, a correlation circuit for the capture of the received time slot, a circuit for the clock recovery, a demodulation circuit, a decision circuit, a circuit for the measurement of the value of the received power or RSSI. According to the invention the receiver comprises moreover: means for the quick sampling (SAMPLER) of the RSSI signal made available by the same receiver; means for the analog/digital conversion (A/D CONVERT) of the signal corresponding to the output of the above-mentioned sampling means; means for the processing (DSP) of the signal for the supply of the final value of RSSI. The method enables the execution of a Bearer handover procedure even in the condition of system congestion.

Description: 1 METHOD FOR THE REALIZATION OF A QUICK AND COMPLETE PRESELECTION OF THE CHANNELS IN A DIGITAL TELECOMMUNICATION SYSTEM Technical Field The present invention relates to a digital telecommunication system with time division access technique TDMA (Time Division Multiple Access), and in particular but not exclusively systems operating according to the DECT standard (Digital Enhanced Cordless Telecommunications). In particular the present invention relates to the radio access procedure which in every wireless or radiomobile systems has to maximize the probability to obtain and to maintain the telephone connections. In the following description, reference will be especially made to a DECT system; this is not limiting the invention, which finds general application in systems where the connection is realized between two radio TDMA transceiving units, one fixed and one mobile. In the DECT case the following definitions apply: - Fixed Part (FP): a set composed of Radio Transceiving Units and one Central Controller (CCFP) performing the Radio Access functions - Radio Fixed Part (RFP) or Base Station: a fixed unit containing the radio transceivers) - Portable Part (PP): a mobile radio transceiving unit - Wireless Relay Station (WRS): a radio tranceiver unit, fiexd or mobile, able to perform as a Fixed Part when connected to a Portable Part and as a Portable Part when connected to a RFP. The two connections may be active simultaneously - Fast Hopping equipments: equipments able to change the frequency carrier in one guard time period - Slow Hopping equipments: equipments able to change the frequency carrier in a time comparable to a time slot period As already known, in a DECT system the transmission/reception of the signals between the two radio units, mobile (Portable Part) or PP and fixed (Radio Fixed Part) or RFP, respectively, is organized in frames. The frame is subdivided into two subframes, of which normally the first haifframe is assigned to the transmission in the direction from the fixed unit towards the mobile unit (downlink direction) and the second haifframe to the transmission from a mobile unit towards a fixed unit (uplink direction). Moreover each haifframe is subdivided into 12 time intervals according to a time division access technique TDMA (Time Division Multiple Access). The transmission inside a specific time slot uses one of the 10 radio carriers/frequencies arranged, for instance, in Europe, in the band (1880 - 1900 MHz). To set-up the connections, the DECT system uses the so-called Dynamic Channel Selection (DCS) based on the fact that the channel choice is made in real time following environmental conditions and in that the choice of the channel used for the connection is made by the Portable Part. The fixed part may cooperate in this procedure proposing alternative channels to PPs. The portable part draws up a list of channels and Base Stations (RFPs) using the RSSI measures (RSSI: Received Signal Strength Indication, that is electromagnetic field level values at a radio receiver) and the Base Station identifiers transmitted on air by the RFP on one or two beacon channels or Dummy Bearers. In particular the channel list is divided into bands according to a set of RSSI thresholds. Channels with an RSSI measure inferior to the minimum threshold are considered as "quiet" and they may be selected immediately. On the contrary the channels with an RSSI measure superior to a maximum threshold are considered as "busy" and cannot be selected. The channels with an RSSI measure comprised between the minimum and the maximum limits are ordered in function of the band they belong to and may be selected starting from the lowest band. On the basis of such informations the portable part is therefore able to get access to the less interfered channel. It is also able to connect to the best Base Station, that is the RFP received with highest power level. All considerations here reported apply to PP equipments as well as to WRS equipments when communicating with an RFP. Therefore what will be described hereafter for Portable Parts (PP) can also be applied to the Wireless Relay Stations (WRS). For simplicity reasons hereafter reference will only be made to the Portable Parts. Background Art According to the actual DECT standard the initial access procedure follows the steps described in the following text. Initially the RFP is activated and transmits by air all system informations on one or two beacon channels indicated in the particular technology by the term "Dummy Bearer". The RFP transmits also the so-called PSCN information (Primary receiver Scan Carrier Number) which enables the terminal to transmit the first access request when the RFP is receiving. The Portable Part performs the synchronization procedure in two steps: I. Capture of the time slots on air via synchronization, and from the captured packets, derivation of the timing of the RFP. This makes possible to read all system informations. II. Reading of the system informations, such as the identity of the RFP and of the FP and determine the real access possibility in relation to previous subscription. Then the PP performs the following procedures: 1. compilation of channels and Base Stations lists; 2. choice of the best RFP (that is the RFP received with maximum power level); 3. choice of the less interfered channel (that is the one received with the minimum power level) and access request of that channel to the selected RFP. The channel list will be created through a scanning of the time-frequency matrix with a free scanning sequence. Fig. 1 represents the scanning sequence turning out to be the fastest at equal consumption and it requires a period of time equal to 10 frames or 100 ms. In abscissa time slots and in ordinate frequencies are reported, so that each square corresponds to one physical channel, that is a combination of time slot and frequency indicated in the specific technique by the term "bearer" or radio channel. As said before, PP compiles two lists referred to received RFPs and channels in use; RFP and channels are ordered according the RSSI parameter. The RFP to which try the first set-up is chosen according to the maximum RSSI value, the channel according to the minimum RSSI value, starting from the lowest band (presumably it contains the list of unoccupied channels, the so-called "quiet" channels). This criterium assures a minimum interference connection to the "nearest" RFP. Once the channel has been chosen, it is necessary to wait until the RFP will get in reception on the same channel and then forward the access request. According to the scanning position of the RFP, the waiting time ranges from 0 to 100 ms. During the passage from one cell to a neighbouring one (handover) the channel list - in the known systems - has not necessarily to be updated, however it is suggested to carry out a periodical updating, for example every 30 seconds. However this possible updating has some limits: it cannot take into consideration the columns of the channel in use for the so-called "Fast Hopping" equipments (Fig. 2a); for "Slow Hopping" equipments (Fig. 2b) the limitation is extended to the three channel columns including the one with channels in use . In the Figures 2a and 2b the channels represented in dark grey are in use, while the channels which cannot be monitored or which are "blind" are shown in a clear grey. In conclusion the fast hopping equipments have only one blind column, while the slow hopping ones have three blind channels. Therefore there is a limitation due to the fact that the updating of the channel list results incomplete unless not suspending the connection on the channel in use for 10 frames, thus carrying out an exhaustive scanning. The use of the incomplete table may make the access difficult in case no quiet channels are available, for example in a situation of congestion. More precisely there are substantially two limits in the known systems related to the access speed and to the completeness of the scanning, respectively. The access speed is determined by the channel selection time, ranging from 20 ms (with an already updated list 2 frames are sufficient) to 100 ms (complete channel scanning), and by the waiting period to forward the access request which needs from 0 to 100ms. Therefore the gloabl time between the beginning of the research and the forwarding of the first access request reaches from 20 to 200 ms. Regarding the incompleteness of the scanning, there is a missing column for fast hopping equipments (see Fig. 2a) or three columns for slow hopping equipments (see Fig. 2b). This incompleteness can be avoided suspending the communication on the channel in use for at least 10 frames, completing the scanning on the channels which were in the "blind" positions. These limitations may lead to a situation where, owing to congestion, the Bearer Handover procedure may fail despite the existence of suitable channels (this means not interfered), in the blind columns. Objects of the Invention The main object of the present invention is to overcome the drawbacks and limitations mentioned before, and in particular to overcome the above-mentioned limit concerning the Bearer Handover procedure. This improvement regards both Fixed and Mobile equipments (RFPs, WRSs and PPs) and both fast hopping equipments (with one blind column) and slow hopping equipments (with three blind columns). Typically, but not necessarily, RFPs are fast hopping and PPs are slow hopping. Brief Description of the Drawings The invention achieves these scopes through a method with the characteristics illustrated in claim 1 and through a receiver according to claim 8. Further advantageous characteristics are subject of the following claims. According to the method proposed by the invention, the channel list will be updated within a preset time, in particular 30 seconds, and if there are no "quiet" channels, a quick scanning (within one time slot) of the blind channels will be carried out which enables a handover procedure if such quick scanning supplies indications about the presence of a "quiet" or low interfered channel among the blind ones. Now the invention will be described in more details with reference to a preferred but not limiting embodiment, illustrated with reference to the annexed drawings, where: Fig. 1 (already described) illustrates the scanning sequence which needs a period of time equal to 10 frames; Figures 2a and 2b (already described) show the blind columns in the case of fast hopping and slow hopping equipment, respectively. Fig. 3 illustrates the quick scanning according to the method of the present invention, and Fig. 4 shows the block scheme of a device for the implementation of the method according to the present invention. Detailed Description of a Preferred Embodiment of the Invention Hereafter the implementation case of the fast and complete selection method will be at first taken into consideration on the Portable Part (or WRS). In the case of a connection in progress in which, using the "incomplete table", the portable part is able to identify alternative "quiet" channels, a "quiet" channel is chosen and it will be controlled for two following frames. If its RSSI value is not changed for more than 12 dB, the terminal forwards the handover request. In that particular situation it is already possible to obtain the maximum performances, if the table will be kept updated. If on the other hand the available incomplete table does not present "quiet" channels, according to the invention an accelerated procedure will be applied suitable to provide a complete table in a very reduced time. The new scanning will be carried out on the missing column(s) according to the cycle described hereafter with reference to Fig. 3. With reference to Fig. 3 the method according to the invention imply to carry out the scanning of the channels of a blind column within the duration of a single time slot. The figure shows time slot N, and the dark squares indicate how the 10 channels of the blind column will be scanned in sequence, each time for a period of time much shorter compared to a time slot, with an interval (D3) between two adjacent scannings and with an initial interval which will be illustrated below. More precisely a first interval of the time slot itself is indicated by DO, and it represents the delay of the signal compared to the reference timing. This parameter influences the size of the correlation window (the window where a correlation is expected), which consists of a maximum of 14 bits according to the standard and 22 bits for a preferential embodiment adopted by the Applicant. The following interval D1 consists in the length of the preamble (32 bits) at the end of which the correlation will start. The remaining portion of the time slot will be subdivided in 10 intervals D2, each one corresponding to the measure time of the RSSI value on different channels and intervals D3 corresponding to the switching time of the synthesizer from frequency J to frequency J+1 (33 bits for the above-mentioned preferential implementation adopted by the applicant). Although the value of D2 is imposed by the other restraints, it is anyhow necessary to push it to the maximum in order to get a significant RSSI measure. Therefore the relation (in bits) is: Ltot = D1 + 9 * (D2 + D3) + D2 where Ltot is the length of the time slot. Then the maximum time D2 for the definition of the RSSI for each channel is given by: D2 = [Ltθt -D1 -9 * D3]/10 In the above-mentioned suppositions we obtain D2 = 9.5 bits. Therefore in the method according to the invention the measure of RSSI of each one of the channels of the blind column takes 9.5 bits. In this condition, as the measure is only realized on a limited portion of the time slot, "quiet" channel indication could turn out as not completely reliable, however there is a very high probability to justify the suspension of the communication in progress for only one frame in order to make a standard RSSI measure (that is, on the complete slot) on the identified channel in order to obtain a suitable channel candidate. It has to be noted that quiet channels are preferred, but it is possible to select channels belonging to upper bands, excluding only the bands defined as "busy". So, wherever "quiet" channels are mentioned, it is implicit to refer to an iterative procedure starting from the lowest band and progressing towards the upper bands, until the "busy" band is reached. If no "quiet" channels nor channels belonging to other usable bands are detected by the fast scanning, the system can be considered congested, with no channels available. Therefore thanks to the method according to the invention the scanning will be completed within the duration of a frame per blind column instead of 10 frames per blind column. Another frame will be necessary to confirm RSSI measure for every suitable channel identified. As will be described hereafter, the method according to the invention may be also applied at RFP side with even greater advantages since the RFPs in general are Fast Hopping, and the addition of such procedure requires only minimum modifications, even with the disadvantage that the RFP does not know the exact moment when the Portable Part needs to carry out a handover procedure. Supposing a scenario with Fast Hopping RFP and Slow Hopping PP the incompleteness of the table is equal to one column for the RFP and to three columns for the portable part for each channel in use. The completion of the table could therefore be assigned to the RFP only which should carry out in sequence the following operations: keep the table of the channels updated using the fast method with a high frequency, using for example the lost time slots (a time slot is lost when, during a connection, the correlation operation on that time slot is not successful). In every case the already existing timer, equal to 30 s must not be overcome; communicate to the portable part, with a suitable frequency, the "quiet" channels placed in its blind column and in the two adjacent ones. For this communication it is possible to use the quality messages foreseen by the standard. In this way the portable part may overcome the actual limitations. In the case the RFP carried out a Fast Setup procedure, which is not the typical case but anyhow foreseen by the DECT standard, it is up to the RFP itself and not to the Portable Part to choose and activate the radio bearer. In this case it is not necessary to send these informations to the portable part, but the list updated with the presented method is directly used by the RFP. With reference to Fig. 4 a HW device will now be illustrated to implement the method according to the invention. The HW device supporting the access strategy described before will be applied to a DECT receiver with the following characteristics: 1) single transceiver 2) fast hopping (the synthesizer is suitable to switch in a period of time comparable to 33 bits, less than the guard period) with a real time synthesizer programming. With reference to the example reported in Fig. 3 the programming time must be less than 9.5 bits; 3) presence of a measure circuit suitable to extract an analog signal with amplitude proportional to the value of the received field. It is necessary to remember that such receiver has access to all 120 DECT physical channels. The DECT RECEIVER described before must be also provided with the following parts: a fast sampler SAMPLER of the RSSI signal. The speed of this sampler may be for example equal to 20 Msample/s corresponding to 20 samples per microsecond (or per bit, given that one bit lasts 868 ns); an anolog/digital converter A/D CONVERTER suitable to digitalise said signal for example at 6 bits; a digital processor DSP suitable to implement a calculation algorithm in order to supply a final RSSI value (for example averaging the values of the samples acquired during the measure time); an appropriate module TIME BASE GENERATOR managing all timings. Although the invention has been described with particular reference to some preferred forms of realization, it will be evident to those skilled in the art, that the present invention is not limited thereto, but further variations and modifications may be applied without departing from the scope thereof. It is thus contemplated that the present invention encompasses any and all such embodiments covered by the following claims.

Claims: CLAIMS 1. Method for the realization of the access to a radio channel in a digital telecommunication system comprising a plurality of radiomobile units or Portable Parts (PP) and/or Wireless Relay Stations (WRS) and at least a fixed unit or Radio Fixed Part (RFP), said units being suitable to carry out the following steps of a procedure: scanning of the time-frequency matrix of the channels according to a preset modality; reading of the detected power values or RSSI (Received Signal Strength Indication) of each channel and the IDENTITY data on channels transmitted by the corresponding Base Stations or RFPs; compilation of a Base Station list using RSSI values and the correspondent Base Station identity compilation of a channel list using the RSSI measures; organization of the channel list into bands determined by suitable thresholds; selection of a channel of the lowest power band; follow-up of an access procedure on the chosen channel, characterized in that it foresees the following preliminary steps: updating the channel list within a preset interval; scanning in sequence - within the duration of a time slot per column - all channels of the blind columns, measuring the power level or RSSI; suspending of the communication in progress for one frame whenever the RSSI values of at least one channel of the column of the channel in use indicate a power lower than a preset threshold, and measuring the RSSI on this time slot in a conventional way thus completing the channel list updating. 2. Method according to claim 1, characterized in that said "complete" channel table will be drawn up by the radiomobile unit PP (or WRS). 3. Method according to claim 1 , characterized in that said complete table of channels will be drawn up by the fixed unit RFP. 4. Method according to claim 2 or 3, characterized in that said measuring cycle of the RSSI value for every blind column channel takes place in an time interval equal to the duration of a time slot (Ltot) and the individual RSSI measures take place in a period of time D2 equal to: D2 = [Ltot -D1 -9 * D3]/ 10 where: 10 D1 = length of the preamble D2 = measuring time of the RSSI value for each channel D3 = switching time of the synthesizer from the freq. J to the freq. J+1 Ltot = duration of the time slot. 5. Method according to claim 3, characterized in that said fixed unit (RFP) drawing up the complete table of channels is of the fast hopping type and in that said measuring cycle of the RSSI value of the blind column coincides with the scanning of a single blind column if fast hopping terminals are used, or with the scanning of three blind columns if slow hopping terminals are used. 6. Method according to claim 5, characterized in that said fixed unit (RFP) communicates the scanning result to the portable part or it starts off a Fast Setup procedure which consists in the choice and in the activation of specific radio bearers. 7. Method according to claim 6, characterized in that said communication to the portable part (PP) of the scanning results by the fixed unit (RFP) will be carried out using the so-called "quality messages". 8. Method according to claim 1 characterized in that said blind column is in number of one column for fast hopping equipment and are in number of three columns for slow hopping equipments. 9. Receiver for a digital telecommunication system of the type which comprises a plurality of radiomobile units or Portable Parts (PP and WRS) and at least a fixed unit or Radio Fixed Part (RFP), and said unit comprises: a frequency synthesizer circuit suitable to determine the generation of a preset number of carrier frequencies; a circuit for the recovery of the clock; a correlation circuit for the recognition of the received time slot; a demodulation circuit; a decision circuit; a circuit for the measuring of the received power value or RSSI, characterized in that it comprises the following additional means: fast sampling means (SAMPLER) for the RSSI signal made available by the receiver with the above-specified Radio Fixed Part topology; analog/digital conversion means (A/D CONVERT) suitable to digitalise the signal corresponding to the output of said sampling means; processing means (DSP) of the signal suitable to implement a calculation algorithm in order to supply the final RSSI value; 11 means (TIME BASE GENERATOR) for the generation of timing signals. 10. Receiver according to claim 9, characterized in that said additional means are placed in the radiomobile unit (PP or WRS). 11. Receiver according to claim 9, characterized in that said additional means are placed in the fixed radio unit (RFP). 12. Receiver according to the claims from 9 to 11 , characterized in that said fast sampling means (SAMPLER) are structured in such a way to carry out the measuring cycle of the RSSI value on all channels belonging to the blind columns in a time interval equal to the duration of a time slot (Ltot), and the single RSSI measurings take place during a period of time D2 equal to: D2 = [Ltot -D1 -9*D3]/ 10 where: D1 = length of the preamble D2 = measuring time of the RSSI value for each channel D3 = switching time of the synthesizer from freq. J to Freq. J+1 Ltot = duration of the time slot. 13. Receiver according to claim 11, characterized in that said fixed unit (RFP) which draws up the complete table of the channels is of the fast hopping type, and in that said measuring cycle of the RSSI value on blind columns coincides with the scanning of a single blind column if fast hopping terminals are used, or with the scanning of three blind columns if slow hopping terminals are used. 14. Receiver according to claim 13, characterized in that said fixed unit (RFP) communicates the scanning result to the portable part (PP), or it starts a Fast Setup procedure which consists in the choice and in the activation of the specific radio bearer. 15. Receiver according to claim 14, characterized in that said communication to the portable part (PP) of the scanning results by the fixed unit (RFP) will be carried out by using the so-called "quality messages".

Inventors: ROSINA GIANCARLO, ROSINA, GIANCARLO
Applicants: ITALTEL SPA, ROSINA GIANCARLO, ITALTEL S.P.A., ROSINA, GIANCARLO
Application year: 1999
Main classification: H04Q7
IPC codes: H04W36, H04W76