DE19705354A1 - Digital signal transmission system for radio communications network - Google Patents

Abstract

The system provides different quality classes (HQRT, LQNT) for the transmission of the digital signals, in which the digital signals are transmitted with different transmission safety and different bit rates. The granularity of the bit rate assigned to an active subscriber is preferably determined according to the compatibility to a corresponding wire-based system. Two quality classes are preferably provided. These include an upper quality class with a real-time transmission and a predetermined, guaranteed transmission quality, and a lower quality class for a non real-time transmission and without a guaranteed quality.

Description

In the subscriber access area of a telecommunications network can un for the connection of the individual subscriber stations Different transmission media such as Cu two-wire line (unshielded symmetrical subscriber connection line), Ko axial cable, fiber or radio can be provided, being in be standing telecommunication networks Cu-double-wire lines play a dominant role. The - late 80s with the introduction of the integrated service digital network (ISDN) with a usable transmission capacity of up to 144 kbit / s on a 6 km long copper two-wire cable setting - trend to higher transmission rates possibly to introduce fiber optic lines up to a switching point close to the subscriber in the subscriber line rich, the so-called Optical Network Unit ONU (Fiber to the Curb); in the branch cable area, d. H. for the rest of the route between the cable distributor and the subscriber station, it remains as a rule, however, with the subscriber installed here Connection cable in the form of a symmetrical copper double wire Management.

For the transmission of digital signals via unshielded symme trical lines (telephone subscriber lines) there there are already different systems:

Narrowband ISDN (N-ISDN) covers two in two-wire mode B channels and a D channel a data rate of 144 kb / s in each of the two directions of transmission. With one - two Using two-wire lines - HDSL (High Speed Digital Subscriber Line) system is the transfer in four-wire operation 1.5 or 2Mb / s over a distance of up to 4 km possible; with one - requiring only a two-wire line - ADSL (Asymmetric Digital Subscriber Line) system can in  Two-wire operation up to 6 Mb / s towards the subscriber and some 100 kb / s towards the subscriber via Ent distances of a few km can be transmitted (IEEE Communica tions Magazine, May 1994, pp. 102-109). With VADSL (Very High Speed ADSL) are asymmetrical systems also for higher ones Data rates in discussion; even higher data rates, esp special of ATM signals with data rates or interfaces according to ATM Forum standard suggestions, can be - in two wired or four-wire operation - with VHDSL (Very High Speed Digital Subscriber Line, also known as VDSL) via kür transfer longer distances (ETSI TM3 Working Doc. No. 91, Vienna, March 18-22, 1996).

The need to transfer such higher data rates arises from the emergence of new multimedia services, which include moving picture transmission, as well as from the Need to have LANs with significant aggregate data rates to connect others. With the growing use of the Internet is also a rapid increase in data rates in the Networks connected.

For a high quality moving picture transmission with a quality corresponding to a PAL signal when using the MPEG2 standards for data compression are about 4 Mb / s compels; a VHS-like quality can be around 1.5 to Achieve 2 Mb / s. Such high data rates can be found today in the Technically master subscriber access network; the one for that necessary effort and the necessary investments for Upgrading the network is significant.

A potentially much cheaper option opens when new multimedia services are successful significantly lower data rates of n × 64kb / s (with n ≦ 30) to realize bidirectionally. Recent knowledge at Data compression indicates that such an expect tion is realistic.  

The previously installed two-wire subscriber lines are generally from one country operated against telecommunications companies; want more network establish operators, they have the choice between an operator rental of such already laid subscriber line gene, the relocation of subscriber lines or bridging the so-called last mile to the participant wirelessly, d. H. the establishment of a radio subscriber connection network. Network concepts (DECT link, CDMA-link) for radio subscriber access networks with low Bandwidths or also for broadband radio subscribers access networks with bidirectional transmission one each Sum bit rate of, for example, 155 Mb / s on an ATM basis.

The invention now shows an advantageous way of transferring supply of digital information in a radio subscriber line network.

The invention relates to a system for transmitting Digi valley signals in a radio subscriber access network, in particular for the bidirectional transmission of digital signals in a broadband RLL (Radio in the Local Loop) subscriber access network; this transmission system is according to the invention characterized in that for the transmission of the digital signals of different quality classes are provided in which the digital signals with different transmission security and different bit rates are transmitted, the granularity of an active participant cash bit rate in an advantageous embodiment of the invention according to the compatibility with a corresponding lei tied system.

The invention enables the establishment of attractive Ge business connections for both CBR (Constant Bit Rate) signal transmission as well as parallel internet access and file Transfer, optimal use of the spectrum, oh ne that possibly an unused part of the disposal  cash transmission capacity would be given away, as well as full Compatibility with corresponding devices and end devices in wired networks.

Further special features of the invention will become apparent from the following ing closer explanation of the invention with reference to the drawing can be seen. It shows

Fig. 1 shows schematically a section of a radio subscriber access network; in

Fig. 2 is a frame structure of a transmission system according to the invention outlined and

Fig. 3 illustrates an implementation of different qualities of service by non-uniform modulation.

In the drawing Fig. 1 is schematically outlined a portion of a radio subscriber network, in which a number of decentralized (subscriber-side) Funkabschlußein devices (transceivers) TRX are each connected via radio to a central base station BS. The base station BS can be connected to a switching center VSt via a (preferably optical) connecting line OF; to the decentralized radio termination devices TRX, end devices on the subscriber side - in the example telephone, set-top box with television and PC - are connected. The BS - TRX radio range may be able to transmit bidirectional digital signals with bit rates of n × 64 kb / s (with n ≦ 30).

For digital signal transmission on the BS - TRX radio links different quality of service classes are now envisaged, in which the digital signals with different transmissions security are transferred. It is in all Re sufficient to provide two classes of service, näm An upper quality class HQRT for real-time transmission in a guaranteed quality of service (QoS - Quality of service) and a lower quality class LQNT without real-time Transmission and without guaranteed quality of service.  

The upper quality class HQRT can, for example, by Digital signal transmission in real time with constant or too Variable bit rate (constant bit rate CBR or variable bit Rate VBR) at a below a certain, possibly also in threshold specified individually by the connected terminal characterized bit error rate (BER) be. This quality of service can be achieved by appropriate Measures such as B. Choosing a sufficiently low data rate or bandwidth, error-securing coding (forward error Correction FEC) in connection with robust modulation or also ensure appropriate equalization.

The lower quality class LQNT can, for example, by Pa ket transmission with the currently available bit rate (Available Bit Rate ABR), automatic inquiry and re fetch (Automatic Repeat Request ARQ) with accordingly unprepared predictable delays. This service quality is for non-real-time services (e.g. file transfer, Internet surfing) suitable and sufficient.

The signals are expediently of different quality to a sum signal with a fixed frame structure summarized, the granularity for the bit rate per (active) subscriber with a gross data rate of, for example 2 Mbit / s per participant is selected so that com compatibility (same framework, gross data rate per participant 2 Mb / s) to corresponding wired systems, as they are known per se (from DE 197 02 142 A1). In order to can be at a total data rate of 155 / Mb / s under Be consideration of statistical traffic behavior approx. 128 Easily operate participants per radio cell. The participant he thus holds a flexible n × 64kb / s connection, which he individually or in parallel for services with different quality classes can use and which is fully compatible to corresponding wired systems and the associated end devices is.  

This represents an attractive multimedia-enabled business connection. The discernible progress in the data Compression for moving picture sequences also leave expect that with the available data rates n × 64 kb / s (at n ≈ 6.. .18 also a high-quality Be in the foreseeable future moving picture retrieval (video-on-demand) can be realized, so that there is also a connection of the type described here partial in so-called residential areas, i.e. for private Home connection, can be used.

The use of ATM is not mandatory and also not in reasonable in all cases; ATM transmission is optional possible, with different classes for ATM (CBR, ABR) are defined.

Because of the rough granularity and the associated relatively high reserved data rate per active subscriber normally remains (n × 64 kb / s with e.g. n <15) if the channel is not fully used by the subscriber te transmission capacity. The free capacity can ge be used, e.g. B. for the quality class HQRT for further Improve quality of service by adding more Redun danz for the information really to be transmitted, with what smaller n a particularly high-quality transmission possible is not quite as good at large n. Okay In many cases, however, pulling will be the alternative that over additional data rate (unused HQRT capacity) to be used for the transmission of LQNT information.

When combining the signals from two different quality classes in the example to form a sum signal with a fixed frame structure, a frame can contain a frame code word RK marking the beginning of the frame, a subsequent information field of the upper quality class HQRT, an information field of the lower quality class LQNT and the boundary between the two Include information fields marking sub-frame identifier SRK. Such a frame structure is also illustrated in FIG. 2. The frame code word RK is selected so that it corresponds to the HQRT requirements and, due to the periodic return, can be recognized with a high degree of certainty even with a bad BER value or occasionally falsified individual bits. Quality monitoring of the HQRT signal can be carried out in the form of monitoring the bit error rate of the frame identifier word RK. The same applies to a subframe identifier word SRK, which separates the areas HQRT and LQNT. The allocation of the available transmission capacity to the two quality classes HQRT and LQRT is either fixed or can be set variably according to the respective needs of the user.

In a first scenario, the HQRT data rate is fixed give, such that with the addition of an appropriate redundancy for error protection for the vast majority of all Radio connections of a radio cell the desired quality of service is secured.

In this first exemplary embodiment, the subframe identifier SRK is fixed in the subframe in the main frame identifier RK, HQRT information field, subframe identifier SRK and LQNT information field according to FIG. 2; the relation of HQRT bits and LQNT bits in the frame is fixed. Frame code word RK, HQRT signal and sub-frame code word SRK can be adequately secured by appropriate channel coding and robust modulation, so that the required quality of service is guaranteed.

In a second scenario, error rates occur measurement at different transmission speeds determined the maximum possible data rate for the HQRT signal and thus adaptively adapted to the radio field properties. The Location of the subframe identifier SRK in the frame - and thus also the ratio of HQRT information to LQNT information - is variable and adaptively adjustable.  

In both scenarios there is a reduction to a robust one Fallback mode with lower net data rate possible for LQNT Lich, if the ARQ (Automatic Repeat Request) requirements exceed a predetermined limit per unit of time, the transmission properties of the radio field are bad are. This fallback mode is described in more detail below will.

When realizing the different quality of service can class different transmission bit rates for the individual quality classes in the same frequency band or in separate frequency bands can be provided, the data advise together in a simple manner to ensure easy filling into the - fixed - frame to ensure.

For the individual quality classes, in an expedient embodiment of the invention, a non-uniform signal modulation is provided, as is in principle from R. Kays: television transmission - system concepts and opportunities for introduction; 15th annual conference of the FKTG, Berlin, 1-5. June 1992, conference volume p. 74-89 is known and so far requires no further explanation. So z. B. at 16 QAM, the respective distance between the points describing the modulation in the modulation plane can be selected differently, as is indicated in FIG. 3. The evaluation is done e.g. B. such that first write the first two bits of the quadrant ( 1 ) ( 2 ), ( 3 ) or ( 4 ) be, in which the modulation vector shows; the fol lowing 2 bits then describe the exact position of the respective vector (a1 or a2 in Fig. 3) in the relevant quadrant. Since in the two cases the signal-to-noise ratio is different due to the accumulation of the individual vector points in certain areas of the modulation level ("clusters"), the first two bits are transmitted relatively reliably, the last two bits are more uncertain. This follows from the fact that the difference between the vectors a1 and b1 (in FIG. 3) or between the vectors a2 and b1 (in FIG. 3) is substantially greater than the difference between the vectors a1 and a2. The quadrant can therefore be determined with greater certainty or more easily than the exact position of the vector point in the quadrant.

For the individual quality classes is in expedient design of the invention also a non-uniform signal co dation provided by the effort (additional redundant Bits) for additional error protection is distributed to the individual parts of the information. A such non-uniform signal coding is also in itself (e.g. from data compression for moving images where critical cal information (sync) should be transmitted more securely than less critical information, which only the image content falsify insignificantly) known, so that it is so far also requires no further explanation. That prin zip leads to different efforts in the present case for error assurance in HQRT quality and LQNT quality; in the LQNT quality class can possibly be a fail safe tion can also be dispensed with.

In the first scenario mentioned, one can e.g. B. about 50% of Transmission capacity for the transmission of HQRT information use and securely with 4QAM, d. H. with only the qua transmitting two bits; the other 50% of the Transmission capacity can be used to transmit LQNT-In use formation, which is then transmitted more securely with 16 QAM becomes. The HQRT information can be added by additional errors encoding (Forward Error Correction FEC) protected during the LQNT information Automatic Repeat Request (ARQ) and possibly by a lower one valuable error protection can be secured. Such a thing The procedure is analogous to other types of modulation, such as for example m-PSK, can be combined. If you want the LQNT rate further reduce due to frequent ARQ requests in the company (relapse mode mentioned above), for example: B. in the Modula  tion also reduced the number of points per quadrant from 4 to 2 be decorated and / or the step duration per LQNT bit doubled be pelt. In a corresponding way can be too bad Bit error rate (BER) for HQRT bits e.g. B. the stride duration ver double, d. H. the data rate will be halved, and / or it will when modulating only between the left and right half-planes distinguished, whereby the position of the vector points makes sense is to increase the signal-to-noise ratio (S / N). This would only need one bit per current step. Well Of course, with these measures, there is a reduction in the disposal net data rate.

In the second scenario mentioned, one can e.g. B. the modulation onshub or the number of modulation levels for the two quality classes adaptively to the respective radio field properties, expediently for reasons of simplicity, the frame length and thus the sum of HQRT and LQNT bits per frame including added redundancy for error protection is kept constant. An optimal utilization of the transmission capacity of the respective radio connection can thus be achieved. The subframe identifier word SRK (cf. FIG. 2), which indicates the start of the LQNT range, now "floats" depending on the transmission mode selected in the frame, but usually not continuously, but in several discrete stages, through the modulation and error protection modes selected are determined. The better the radio connection, the higher the HQRT share can be.

It should also be mentioned that the (HQRT or LQNT) bits of the ver different quality classes an intermediate storage via egg ne frame duration and before the modulation or after the demodula undergo a rearrangement. This can be convenient be interleaved to avoid bundle errors in break up individual errors easier to correct.

The type of modulation and, if applicable, the error-securing coding (FEC) can be selected for the HQRT signal in accordance with the current BER (bit error rate) measurement in the (sub) frame identifier words RK or SRK (see FIG. 2) or in the first scenario mentioned; for the LQNT signal, the type of modulation and, if necessary, coding (FEC) can be determined according to the frequency of the ARQ requirements. The information about the bit error rate (BER) must be transferred back from the receiver to the transmitter. Additional "housekeeping bits" and bits for the automatic repeat request (ARQ) are also required, and information about the transmission and coding modes selected in the transmitter must be transmitted to the receiver (transceiver) on the other side of the transmission path. For this purpose, some bits of the frame identifier RK and / or subframe identifier SRK (in FIG. 2) can be rededicated to form a signaling channel. But you can also provide a full 64kb / s channel for this purpose and otherwise maintain the structure of a 2 Mbit / s PCM signal, channel 1 possibly containing the frame identifier RK and channel 16 the subframe identifier SRK . The signaling can then take place in a reserved additional channel (e.g. channel 2). The redundancy in the signal due to the frame structure is then 3/32 = 9.4%. However, a longer frame can also be selected. Since the RK and SRK words u. U. must be longer than a standard 2 Mbit / s signal and since HQRT information in particular also requires a corresponding additional redundancy, the total available net bit rate is less than 29 × 64 kbit / s, which, however is of minor importance. The fail-safe coding (FEC) can take place together or separately for frame identifier RK, subframe identifier SRK and HQRT signal. The exact data must be determined on the basis of practical examinations and the data of real networks.

In the subscriber-side radio termination device TRX (in FIG. 1), the HQRT signal and the LQNT signal can be made available at two different interfaces or also at a common (for example 2 Mbit / s) interface with a corresponding frame structure.

The signals of the two transmission directions can be in Fre quenz separated or (with echo canceling) in frequency be transferred equally.

Because RLL (Radio in the Loop) systems are point-to-multipoint systems are, the use of a suitable multiple access ver driving (e.g. TDMA) in the upstream direction away from the subscriber required. Downstream towards the participants can each a multiplex signal from the 2 Mb / s signals of the connected active participants are formed. Every ak Tive participants will have a corresponding transmission capacity or space in the multiplex superframe thus formed pointed. In the upstream direction, the TDMA operation Bursts of the active participants each from a 2 Mb / s frame with the required to form the TDMA signal (preamble, etc.) additional information built up.

When transmitting ATM signals to the subscriber the ATM cells to the two quality of service classes HQRT and LQNT divided accordingly: CBR (Constant Bit Rate) -ATM-Zel len with high service quality requirements run in HQRT-In formation field, ABR (Available Bit Rate) ATM cells with never Other service quality requirements run in the LQNT section. The ATM cells can be continuously nested in the frame be, which may be filled with empty cells. The Re redundancy is around the headers of the ATM cells (by 5/53 = 9.4%) elevated.

If you want to avoid the additional redundancy by the cell headers, a purely cell-based solution can be provided as an alternative, similar to ATM-UNI, which avoids the need for explicit frame identifiers (RK, SRK in Fig. 2). There is then no fixed frame structure, but only an ATM cell sequence in which the cells of the quality of service classes in different formats - with different added redundancy and differently modulated - occur with different frequencies. The percentage of the two cell types and thus the respectively possible maximum data rates are either predefined or adjusted adaptively and continuously monitored. The bit error rate (BER) for HQRT cells is determined, for example, using special nested measuring cells. The transmission of the necessary additional information about modulation, coding, ARQ requests, etc. and the transmission of housekeeping information is also carried out in special cells. In order to be able to reliably detect on the receiving side whether the respective received cell is an HQRT, an LQNT or a housekeeping cell, which are to be demodulated, decoded and evaluated if necessary, can e.g. B. the internal header can be slightly extended by an addition, which is strongly secured and easily identifiable. Due to the longer length of an ATM cell compared to the length of a 2 Mbit / s frame according to CCITT Rec. G.703, the intermediate storage of several ATM cells necessary for the transmission-side coding leads to a greater delay. At a bit rate of 2 Mbit / s, an ATM cell (without additional redundancy) has a duration of approximately 200 μs, so that the additional average delay to be expected does not play a significant role in this respect.

Claims (13)

1. System for the transmission of digital signals in a radio subscriber access network, in particular for the bidirectional transmission of digital signals in a broadband RLL (radio in the local loop) subscriber access network, characterized in that different quality classes (HQRT, LQNT) are provided for the transmission of the digital signals in which the digital signals are transmitted with different transmission security and different bit rates. 2. Transmission system according to claim 1, characterized, that the granularity of an active participant bitrate according to the compatibility with an ent speaking wired system is set. 3. Transmission system according to claim 1 or 2, characterized, that two quality classes, namely an upper quality class (HQRT) with real-time transmission in a fixed guaranteed Quality and a lower quality class (LQNT) with non- Real-time transmission provided with no guaranteed quality are. 4. Transmission system according to claim 1, 2 or 3, characterized, that the signals of different quality classes (HQRT, LQNT) combined into a sum signal with a fixed frame structure are. 5. Transmission system according to claim 4, characterized, that a frame marks the beginning of the frame nungswort (RK), an information field of the highest quality class (HQRT) and an information field of lower quality  class (LQNT) as well as the boundary between the two informatics contains sub-frame identifier (SRK). 6. Transmission system according to claim 4 or 5, characterized, that the data rate of the digital signal of the upper quality class (HQRT) is fixed. 7. Transmission system according to claim 5 and 6, marked by a fixed position of the subframe identifier (SRK) in the Frame. 8. Transmission system according to claim 4 or 5, characterized, that the maximum possible data rate of the digital signal upper quality class (HQRT) adaptive to the line's own is adapted. 9. Transmission system according to claim 5 and 8, marked by a variable position of the subframe identifier (SRK) in the Frame. 10. Transmission system according to one of claims 1 to 9, marked by different transmission bit rates of the signals below different quality class (HQRT, LQNT). 11. Transmission system according to one of claims 1 to 10, marked by non-uniform modulation of the signals different Quality class (HQRT, LQNT). 12. Transmission system according to claim 11, marked by  adaptive adjustment of the modulation stroke or modulation Number of levels for the different quality classes (HQRT, LQNT). 13. Transmission system according to one of claims 1 to 12, marked by non-uniform coding of the signals different Quality class (HQRT, LQNT).