Land system for a mobile radio system

(54) NAME Country system for a mobile radio system (56) CALLED PUBLICATIONS: --- (57) SUMMARY:

The invention relates to a land system in which a switch (SWTRA) is located between the radio equipment of the base stations (BS1, BS2) and associated transcoder equipment.

PRV 328 ALLF 138 9 132 AA

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The numbers in brackets indicate the international identification code. INID code. Letters in clamps indicate international document code.

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TECHNICAL FIELD

The present invention relates to an improved land system for a mobile radio system.

BACKGROUND OF THE ART

In mobile radio systems, it is known to use a so-called transcoder for encoding / decoding information between a pulse code modulated form and a more compact pulse code modulated form suitable for radio transmission. Such transcoder equipment is located in the radio system's land system. US patents 4,675,106,83 and 4,777,633 are known to provide each traffic channel with its own transcoder in the land system. This transcoder is usually located in or adjacent to one of the base station's base stations. This is a costly solution since all the traffic channels are rarely occupied at the same time and therefore expensive transcoder equipment is unused for long periods of time.

More recently, it has also been proposed to move the transcoder equipment from the base station to the mobile telephone exchange in order to reduce the transmission rate between the base station and the mobile telephone exchange to the lower transmission rate obtained in the radio connection due to the more compact pulse code modulated form of the signal.

DISCLOSURE OF THE INVENTION

An object of the invention is to provide an improved land system that better utilizes the available transcoder capacity.

This object is solved by a land system for a mobile radio system comprising transcoders for encoding / decoding information between a pulse code modulated form and a more compact pulse code modulated form suitable for radio transmission, and characterized by a switch being positioned between at least one set of transcoders and corresponding radio equipment in the base station of the land system.

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Another object of the invention is such an improved land system in which an improved transcoder is utilized.

This object is solved by a land system for a mobile radio system, which includes means for encoding / decoding information between a pulse code modulated form and a more compact pulse code modulated form suitable for radio transmission, and characterized in that at least some of said means for encoding / decoding information is a transcoder having means for time multiplex coding / decoding of a predetermined number of traffic channels and of a switch being placed between at least one set of such transcoders and corresponding radio equipment in the base system of the land system.

FIGURES

The invention, further objects and advantages obtained by the invention are best understood by reference to the following description and the accompanying drawings, in which:

Fig. 1 shows simplified structure of a known land system for a mobile radio system;

Fig. 2 shows the structure of a land system for a mobile radio system in accordance with the present invention;

Fig. 3 shows the structure of an improved transcoder;

Fig. 4 shows the format for a time-division pulse code modulated signal;

Fig. 5 shows the format of the compressed pulse code modulated signal; and

Figure 6 shows a preferred embodiment of a transcoder block in the land system of Figure 2.

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In the figures, the same reference numerals are used for the corresponding elements throughout.

The conventional land system of a mobile radio system shown in Fig. 1 comprises a mobile telephone exchange MSC to which two base stations BS1 and BS2 are connected. The other side of the mobile telephone exchange MSC is connected to the public telephone network via a switch SW. Each base station is assigned a number of radio frequencies, which in turn are divided into a number of traffic channels. Before information is transmitted via a traffic channel, it is encoded or compressed in a transcoder TRA1-TRA8 in accordance with, for example, the procedure described in GSM Recommendation 06.10.

Information received via the traffic channel is decoded or expanded in the same transcoder. In the conventional land system, each traffic channel is assigned its own transcoder for this encoding / decoding. However, such a solution is expensive as all the traffic channels in all base stations are rarely occupied at the same time and expensive transcoder equipment is therefore often unused.

Figure 2 shows the construction of a land system for a mobile radio system in accordance with the present invention. In accordance with the invention, the transcoders that were previously in the base stations BS1 and BS2 have been moved to a TRABL transcoder block in the mobile telephone exchange MSC and a switch SWTRA has been placed between the base stations BS1, BS2 and the TRABL transcoder block. The other side of the transcoder block TRABL is connected to switch SW, which corresponds to switch SW in fig. 1 and thus is connected to the public telephone network.

This design has several advantages. An advantage is that the transmission speed between the base stations BS1, BS2 and the mobile telephone exchange MSC is reduced by the information being still compressed during this distance. Another advantage is that the previously necessary assignment of a transcoder to each traffic channel is eliminated. Instead, the TRABL transcoder block now constitutes a common resource shared by all the traffic channels. Therefore, at one point in time via the SWTRA switch, a certain traffic channel can be associated with a transcoder in the TRABL transcoder block while at another time it is associated with another transcoder in the block.

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A significant advantage of the proposed construction is that the number of transcoders in the TRABL block can be reduced to the expected need. Thus, the number of transcoders in the TRABL block may be less than the number of traffic channels available to the base stations BS1, BS2. In other words, the switch SWTRA forms a concentrator

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I associate busy traffic channels with available transcoders in the TRABL transcoder block while unoccupied traffic channels are not associated with any transcoder.

In Fig. 2, the switches SW and SWTRA have been shown as two separate gears for clarity. However, in a preferred embodiment, these gears are integrated into a common gear. In such an embodiment, the signal passes the switch twice, once in compressed form and once in expanded form.

In Fig. 2, the SWTRA switch and the TRABL transcoder block have been placed in the MSC mobile telephone exchange. Another embodiment of the invention consists in placing these elements closer to the base stations or even within them. In such an embodiment, however, one switch is needed for each base station and the advantage of reduced transfer rate between base station and mobile telephone exchange is lost.

Figure 3 shows an embodiment of a transcoder suitable for use in the land system of Figure 2.

The transcoder of Figure 3 will be described with reference to the GSM standard, but the principles also apply to other standards, for example the US standard IS-54.

In the European PCM standard, a transmission link transmits 2048 Mb / s distributed over 32 channels, 30 of which are voice channels, see Fig. 4. Each voice channel transmits 64 kb / s in each direction, corresponding to a voice connection in the public telephone network.

Each such PCM voice channel is compressed in a transcoder to a more compact form by expecting 1280 bits of PCM code (= 20 msec speech) and recoding to a maximum of 320 bits (to 260 bits of the GSM standard). The 320-bit block in which this compressed signal is stored is packaged in a common PCM voice channel along with similar 320-bit blocks corresponding to 3 other compressed

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PCM voice channels. This is done by utilizing 2 bits per PCM time slot in accordance with Figure 5. Channel 1, which is intended for synchronization, and channel 16, which is intended for signaling, are not compressed but are unchanged in the format shown in Figure 5. . The total length of a frame consisting of 32 time slots is 125 ps in both Figures 4 and 5. The effect of the compression is thus that each time slot in Figure 5 intended for voice transmission will contain information from 4 time slots in Figure 4. Otherwise, however, the 32 time slots in Figure 5 are treated in the same way as the 32 time slots in Figure 4, ie they still form a frame in a transmission link containing 32 channels.

In Fig. 2, the transmission links have been designated PCM_TL while the PCM channels are designated PCM_CH. The packing / separating of the compressed code takes place in the PCK unit.

The voice coding / decoding algorithm in the GSM standard consists of a number of blocks as described in GSM Recommendation 06.10. However, these blocks are speech frame oriented, ie each block must perform calculations on a whole or a predetermined part of a speech frame before the next block of the algorithm can take over. This has been utilized in the transcoder of Fig. 3. Here digital signal processors DSP1-DSP6 are used for encoding and digital signal processors DS7-DS9 for decoding the signals.

The pulse code modulated signal PCM of Fig. 4 reaches the coding portion consisting of the digital signal processors DS1-DS6. In this way, the various processors have been assigned the following algorithm blocks in GSM Recommendation 06.10:

DSP1 pretreatment

DSP2 short-term LPC analysis

DSP3 short-term analysis filter

DSP4 RPE grid selection and coding

DSP5 LTP analysis

DSP6 PRE grid decoding and positlonation, long-term analysis flits.

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The function is as follows: The PCM signal reaches the first digital signal processor DSP1. The first channel's speech frame, ie 1280 bits of PCM code, is read via bus B into a memory RAM. The first speech frame is processed in the digital signal processor DSP1 for performing the first algorithm block preprocessing. Data being processed is extracted from RAM and the end result is saved in the same RAM.

At the same time, data for the next speech frame is loaded into RAM. DSP1 then performs the same operations on this speech frame. In parallel, DSP2 retrieves DSP1's pre-processed data from RAM via bus B and performs the algorithm block short-term LPC analysis on them. The result is again transferred to RAM.

In this way, the first speech frame of all digital signal processors DSP1-DSP6 is processed in turn. After each step, the result is transferred to RAM for retrieval of the next digital signal processor in the chain. At the same time, the subsequent speech frames are processed in the same manner in sequence by the other digital signal processors. After implementing the entire coding algorithm for the eight speech frames, the compressed pulse code modulated code CPCM can be retrieved from RAM by the interface unit I / O for routing to a base station.

Decoding of encoded information is similarly done in digital signal processors DSP7-DSP9. These perform the following algorithm blocks:

DSP7 RPE grid decoding and positioning, long-term synthesis25 filters

DSP8 short-term synthesis filter

DSP9 finishing.

The embodiment of a transcoder described in Figure 3 is characterized by parallel processing of multi-channel speech frames, each digital signal processor executing a specific algorithm block on each, channel's speech frame. However, if an algorithm block is particularly time-consuming, it is possible to have two or more digital signal processors perform each part of the algorithm block. Another possibility is to use, for example, two * 35 similar signal processors which perform the same algorithm block but on

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different speech frames. For example, the first processor can process odd speech frames while the second processes even speech frames. This allows time-critical algorithm blocks to be accelerated. Combinations of these variants are also possible.

A significant advantage of the proposed transcoder is that the input digital signal processors can be optimized for the algorithm block they are intended to implement. For example, some processors may be 8 bit processors while others may be 16 bit processors. In known transcoders for processing only one traffic channel, there is no such flexibility, but a compromise must be made regarding the desired processing speed and word length for the transcoder's digital signal processor.

Since the various digital signal processors DSP1-DSP9 need not be general processors but are intended only to perform very specific algorithm blocks, it is also possible to customize each processor for each algorithm block. This means that the processors can be significantly simplified compared to general processors. This feature can in turn be utilized to integrate the entire transcoder for, for example, eight traffic channels into a VLSI circuit.

A further advantage of the transcoder lies in a reduced delay. A conventional transcoder for the GSM system, which uses 8 traffic channels per radio frequency, causes a delay of approximately 7 ms when encoding to compact form. The corresponding delay at the described transcoder is about 2 ms.

Figure 6 shows a preferred embodiment of a TRABL transcoder block suitable for use in the land system of Figure 2. The TRABL transcoder block contains a suitable number of groups of multi-transcoder. In the example, each group contains three transcoders. For example, the first group contains transcoders MTRA1, MTRA2, MTRA3. Here, the letter M in the reference numeral denotes that it is a multi-transcoder according to FIG

3rd The right side of each multi-transcoder outputs / receives compressed pulse code modulated CPCM code, which in the present case consists of eight 2-bit channels packed in two 64 kb / s PCM channels.

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The output and input signals on the left side of each multi-transcoder consist of eight 64 kb / s PCM channels.

In a preferred embodiment of the land system of the invention, in which a transcoder block of Fig. 6 is used, the concentration of genon described in Fig. 2 is obtained that the number of multitranscoders is less than the number of available radio frequencies, or the number of traffic channels divided by 8, the transcoder block connected to the base stations.

Those skilled in the art will recognize that various changes and modifications to the invention are possible without departing from the scope of the invention as defined by the appended claims.

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