Transmission scheme

In a transmission scheme for transmitting data over a connection connecting a transmitting Main Unit and a receiving Radio Unit of a cellular radio system, where the transmitted data corresponds to user-plane data in the form of digital baseband data for different antenna-carriers, the digital signal is transmitted over the connection, e.g. a fiber, before spreading and combining is provided. The power and weight factor is transmitted separately with a few additional bits. Hereby, the Digital signal before spreading and combining is transmitted in the fiber instead of the combined multi-code signal.

TECHNICAL FIELD

The present invention relates to a method and a device for transmitting data.

BACKGROUND

In the MU (Main Unit)/RRU (Remote Radio Unit) structure of a radio system, such as a cellular radio system and in particular a Time Division Synchronous Code Division Multiple Access (TD-SCDMA) radio system, a fiber is used to connect the MU and RRU. In CPRI (Common Public Radio Interface), it is also called REC (Radio Equipment control)/RE (Radio Equipment). The data transfer in the fiber follows the Ir or the CPRI specification, which includes the user-plane data, i.e., typically the digital baseband (IQ) data for different antenna-carriers (A×C). The IQ data is the chip level symbols after spreading, which is combining multi-code signal including power and weights factor information. The signal is quantized to a few bits, normally 16 bits.

InFIG. 1the structure of the MU/RRU is depicted.

Different antenna-carriers (A×C) transport different data. Due to the limitation of fiber capacity, the 2.5 Gbps fiber can only support the data for 48 A×C. (8 antenna and 6 carriers). Currently China Mobile Communication Corporation CMCC requires that 9 carriers or 12 carriers can be supported, which can not support by 2.5 Gbps fiber.

To solve the limitation of fiber capacity, a few solutions have been proposed:

1. Use a high speed fiber (3.7 G or 5 G)

3. Automatic Gain Control (AGC) is used for the IQ data. (The IQ bit-length can be reduced from 16 to 10).

The first solution is very expensive with high speed fiber. The cost of the second solution will be doubled. Simulation shows that the third solution has little effect in the uplink performance. The down link (DL) performance (such as EVM, PCDE) of the third solution will be affected. The solution can not support 12 carriers.

Hence there exists a need for an improved method and device that can meet the above requirements.

SUMMARY

It is an object of the present invention to provide an improved method and device to address the problems as outlined above.

This object and others are obtained by the method and device as set out in the appended claims.

Thus, in accordance with the present invention, a transmission scheme where the digital signal is transmitted over the connection, e.g. a fiber, before spreading and combining is provided. The power and weight factor is transmitted separately with a few additional bits. Hereby, the Digital signal before spreading and combining is transmitted in the fiber instead of the combined multi-code signal.

In accordance with a first embodiment, the user-plane data, typically IQ data, for each of the downlink codes is transmitted before combining. The factor of the power and weights for each code is also transmitted. After receiving the user-plane data, the receiving RRU sums the received user-plane data of all the codes with the received factor of the power and weights, which can be easily handled by the Field-Programmable Gate Array (FPGA) in the RRU. The user-plane data (IQ data) for each downlink codes are the same for all the antennas. The factor of the power and weights for the antennas can be different. The factor of the power for midamble part and data part can be different.

In accordance with a second embodiment the symbol data for each downlink codes is transmitted before spreading. Also, the factor of the power and weights for each code is transmitted. After receiving the symbol data, the receiving RRU spreads the code and sums the user-plane data (IQ data) of all the code with the factor of the power and weights. The data for each downlink codes are the same for all the antennas. The factor of the power and weights for the antenna can be different. The factor of the power for midamble part and data part can be different. For the midamble part, a midamble identity (ID) can be transmitted instead of the real midmable code.

The invention also extends to a transmitter and a receiver adapted to transmit and to receive data in accordance with the above data transmission schemes. The different transmission schemes can in accordance with one embodiment be software implemented.

DETAILED DESCRIPTION

InFIG. 2, a view of a MU/RRU structure similar to the structure ofFIG. 1is depicted. The MU comprises a module201for transmitting data over a connection connecting a transmitting Main Unit (MU) and a receiving Remote Radio Unit (RRU) of a cellular radio system, where transmitted data corresponds to user-plane data in the form of digital baseband data for different antenna-carriers. The module201is adapted to transmit the digital signal before spreading and combining the digital signal. The module201is further adapted to transmitting a power and a weight factor separately to the receiving RRU with a few additional bits. Hereby, the Digital signal before spreading and combining is transmitted in the fiber instead of the combined multi-code signal.

The receiving RRU is provided with a corresponding module203to receive the signal transmitted in accordance with the above. Hence, the module203is adapted to receive an un-spread and un-combined digital signal from the transmitter, and to separately receive a power and a weight factor. The module203can also be adapted to, after receiving the user plane data, summing the received user plane data of all the codes with the received factor of the power and weights.

The modules201and203can advantageously be software implemented, whereby no physical alteration of an existing structure is required.

InFIG. 3, some procedural steps performed when transmitting/receiving user-plane data in accordance with the above are illustrated. Thus, in a method of transmitting data over a connection connecting a transmitting Main Unit and a receiving Radio Unit of a cellular radio system, where the transmitted data corresponds to user-plane data in the form of digital baseband data for different antenna-carriers, the following steps can be performed. First, in a step301, the digital signal before spreading and combining the digital signal is transmitted. Next, in a step303a power and a weight factor are transmitted separately to the receiver. The corresponding steps can be performed by the receiver.

In accordance with a first embodiment, the IQ data for each of the downlink codes is transmitted before combining. The factor of the power and weights for each code is also transmitted. After receiving the IQ data, RRU sums the received IQ data of all the codes with the received factor of the power and weights, which can be easily handled by the FPGA in the RRU. The IQ data for each downlink codes are the same for all the antennas.

The factor of the power and weights for the antennas can be different. The factor of the power for midamble part and data part can be different.

In accordance with a second embodiment the symbol data for each downlink codes is transmitted before spreading. Also, the factor of the power and weights for each code is transmitted. After receiving the symbol data, RRU spreads the code and sums the IQ data of all the code with the factor of the power and weights. The data for each downlink codes are the same for all the antennas. The factor of the power and weights for the antenna can be different. The factor of the power for midamble part and data part can be different. For the midamble part, a midamble ID can be transmitted instead of the real midamble code.

Below a comparison between different approaches is performed.

Conventional Method

In the traditional method without AGC, the number of bits of 48 A×C (8 antenna and 6 carriers) in one slot is: 16*2*848*8*6=1302528 Each IQ symbol have 16*2 bits. There are 848 symbols in one slot. There are 8 antenna and 6 carriers

Conventional Method with AGC

In the conventional method with AGC, the number of bits is: 10*2*848*8*6+6=814080 Each IQ symbol has 10*2 bits. There are additional 6 bits for AGC factor.

First Embodiment

The number of bits for the transmission scheme in accordance with the first embodiment is: (2*848+16*2*8+16*2*8)*6*16=211968 for QPSK (4*848+16*2*8+16*2*8)*6*16=374784 for 16QAM (6*848+16*2*8+16*2*8)*6*16=537600 for 64QAM Each IQ symbol has 2 bits for QPSK, 4 bits for 16QAM, 6 bits for 64QAM. There are additional 16*2 bits for power factor. There are additional 16*2 bits for beamforming weights. There are 16 spreading codes and each code has different value.

Second Embodiment

The number of bits for the transmission scheme in accordance with the second embodiment in one slot is: (2*44+16*2*8+16*2*8+5)*6*16=58080 for QPSK (4*44+16*2*8+16*2*8+5)*6*16=66528 for 16QAM (6*44+16*2*8+16*2*8+5)*6*16=74976 for 64QAM Each symbol has 2 bits for QPSK, 4 bits for 16QAM, 6 bits for 64QAM. There are 44 symbols for spreading 16. There are additional 16*2 bits for power factor. There are additional 16*2 bits for beamforming weights. Additional 5 bits are used to identify the midamble shifting. There are 16 spreading codes and each code has different value.

Using the transmission method and the sender and the receiver as described herein will significantly reduce the amount of data between MU and RRU. The only alteration needed in a conventional system is the use of different software in the transmitter and receiver. The method, sender and receiver can advantageously be used in a TD-SCDMA system and also other cellular radio systems such as a Time Division Duplex—Long Term Evolution (TDD-LTE) system.