VARIABLE DEPTH

Description

Embodiments of the present invention relate to communications devices supporting adaptive OFDM (Orthogonal Frequency Division Multiplexing) using a plurality of different modulation schemes for sub-carriers. Further embodiments relate to a method of operating a communications system supporting adaptive OFDM and using a plurality of different modulation schemes for sub- carriers. OFDM distributes data over a large number of orthogonal sub- carriers. The orthogonality of the sub -carriers allows a per-carrier demodulation at the receiver side, since at the receiver side the demodulators are prevented from considering other sub-carriers than that one they are dedicated to. OFDM provides high spectral efficiency and allows sub-carrier selection and modulation to be adapted to the transmission channel characteristics.

It is an object of the invention to provide communications devices with enhanced transmission performance and an operating method, which provides enhanced transmission performance in a communications system.

The object is achieved by the subject-matters of the independent claims. Further embodiments are specified in the dependent claims respectively. Details of the invention will become more apparent from the following description of embodiments in connection with the accompanying drawings , wherein features of the various embodiments may be combined unless they exclude each other.

Figure 1 is a schematic block diagram illustrating a communications system with a first communications device including a transmitter unit and a second communications device including a receive unit in accordance with an embodiment of the invention. Figure 2 is a schematic block diagram illustrating the transmitter unit of Figure 1 in more detail. Figure 3A is a schematic block diagram illustrating a communications device providing intra-output frequency interleaving at a constellation level in accordance with an embodiment.

Figure 3B is a schematic block diagram illustrating a communica- tions device providing inter-output frequency interleaving at a constellation level in accordance with another embodiment.

Figure 3C is a schematic block diagram illustrating a communications device providing intra-output frequency interleaving at a bit or symbol level in accordance with another embodiment.

Figure 3D is a schematic block diagram illustrating a communications device providing inter-output frequency interleaving at a bit or symbol level in accordance with another embodiment.

Figure 4A is a schematic diagram for illustrating the grouping of sub -carriers to constellation paths in dependence on an SNR (signal-to-noise ratio) determined for the respective sub-carrier and illustrates the mode of operation of a communications system in accordance with another embodiment.

Figure 4B is a schematic diagram illustrating the sub -carriers of Figure 4B being grouped to different constellation paths in dependence on the SNR.

Figure 4C is a schematic diagram illustrating the grouped sub- carriers of Figure 4B frequency-interleaved at constellation level.

Figure 4D is a schematic diagram illustrating the sub -carriers of Figure 4C in an order in which they are provided to an IFFT (inverse fast Fourier transformation) unit recombining the modulated sub-carriers of Figure 4C to generate a signal in the time domain. Figure 5 is a schematic block diagram illustrating a modulation unit for rotated constellations according to another embodiment.

Figure 6A is a schematic diagram illustrating an orthogonal con- stellation map for illustrating effects of embodiments of the invention.

Figure 6B is a schematic diagram illustrating a rotated constellation map for illustrating effects of embodiments of the invention.

Figure 7 is a schematic block diagram of a receive unit according to a further embodiment.

Figure 8 is a simplified flow chart illustrating a method for operat- ing a communications system in accordance with further embodiments.

Figure 1A shows a wired or wireless communications system 199 , which is based on an OFDM modulation scheme, for example a DVB-C2 (digital video broadcasting-cable) system. In accordance with an embodiment, the communications system 199 is a system using power distribution wires for data communication. For example, the communications system 199 is a power line communications (PLC), mains communications, power line telecommunica- tions (PLT), broadband power line (BPL) or power band or power line networking (PLN) system using a modulated carrier superimposed to the power line alternating current, which may have a frequency of 50 or 60 Hz, by way of example. The communications system 199 may be a SISO (single -input- single-output) or a MIMO (multiple-input-multiple-output) system with a first communications device 100, which includes a transmitter unit 1 10 employing one, two or more transmit ports 101 - 103, and a second communications device 200, which includes a receive unit 220 employing at least one, for example two, three or four, receive ports 201 -203. A transmission channel 300 connects the transmitter unit 1 10 and the receive unit 220. The first communications device 100 may be an exclusively transmitting device with transmit ports 101 - 103 only. According to other embodiments, the first communication device 100 is a bidirectional device including, in addition to the transmitter unit 1 10, a receive unit 120 which may be of the type of a receive unit 220 in the second communications device 200, wherein the ports 101 - 103 may be bidirectional ports. The second communications device 200 may be an exclusively receiving device. According to other embodiments, the second communications device 200 is a bidirectional device including, in addition to the receive unit 220, a transmitter unit 210 which may be of the type of the transmitter unit 1 10 in the first communications device 100, wherein the ports 201 -203 may be bidirectional ports. The communications devices 100, 200 may be stand-alone devices or may be integrated in an electronic device for consumer applications, for example a storage unit, a television set, an audio receiver, or a video recorder.

The transmission channel 300 may be multi-wire connection. In accordance with an embodiment, the transmission channel 300 is a power cable containing two or more electrical conductors used for transmission of AC (alternating current) electric power and installed as permanent wiring within buildings or buried in the ground. For example, the transmitter unit 1 10 may supply two dif- ferential transmit signals using the live or phase wire (L, P), the neutral wire (N), and protective earth (PE), wherein the differential transmit signals are modulated on a carrier superposing the AC frequency of the mains voltage. According to an embodiment, the receive unit 220 receives three differential receive signals between live wire and neutral wire, between neutral wire and protective earth, and between live wire and protective earth. According to another embodiment, the receive unit 220 may receive the three differential receive signals and a common mode signal resulting from a leakage current from the wiring as a fourth receive signal.

Figure 2 shows the transmitter unit 1 10 of Figure 1 in more detail. The transmitter unit 1 10 receives an input data stream d l including payload data to be transmitted over the transmission channel. An FEC (forward error correction) unit 1 15 inserts code redundancy into the input data stream d l according to an error detection scheme for facilitating error correction at the receiver side. The FEC unit 1 15 outputs an encoded data stream d2.

The transmitter unit 1 10 may include a serial-to-parallel multiplexer unit 130 that receives the encoded data stream d2 and splits or multiplies the encoded data stream d2 into a predefined number of parallel data streams d3, wherein each data stream d3 is assigned to one of a plurality of signal paths, wherein each signal path corresponds to one of the transmit ports. In embodiments referring to SISO systems, the multiplexer unit 130 is omitted and the encoded data stream d2 is directly transmitted to a first modulator unit 150.

According to the illustrated embodiment, the parallel data streams d3 are transmitted to a first modulator unit 150. The first modulator unit 150 includes, for each signal path, a plurality of modulations. Each modulation is assigned to a constellation group, wherein each constellation group uses another modulation scheme. The modulation scheme may be any modulation from 1 bit up to x bits per carrier, for example PSK (Phase-Shift Keying), QPSK (Quadrature Phase-Shift Keying), 4 -QAM (Quadrature Amplitude Modulation), 8-QAM, 64-QAM, 256-QAM, 1024-QAM, 4096-QAM, any other 2 ⁿ -QAM, or any 2 ⁿ -PAM (Pulse Amplitude Modulation). In dependence on the SNRs determined for each sub- carrier, for each signal path a control unit 195 assigns the sub- carriers to one of the constellation groups. Each modulation in each signal path combines data bits to symbols for addressing the constellation points of the respective constellation group and outputs modulated sub -carriers carrying the symbol information d4.

According to embodiments of the invention, each constellation group of the first modulator unit 150, or each modulation is com- bined with an interleaving process suitable for performing interleaving at bit level, at symbol level or at constellation level in each modulation path, wherein each interleaving process has a variable size and can be adapted to the number of sub -carriers available for the respective constellation group. For instance, symbols of a constellation group might be mapped to various frequency carriers. A second modulator unit 170 may modulate the sub-carriers carrying the symbol information d4 of the various modulations of each signal path on a main carrier to generate one digital transmission stream d5 for each signal path. These operations may be performed in the frequency domain and an IFT (inverse Fourier transformation) unit 180 may transform the digital transmission streams d5 into the time domain to obtain a digital signal d6 for each signal path. A converter unit 190 may convert the digital signal d6 into analogue transmission signals d7 and couples each analogue transmit signal d7 to one of the transmit ports.

Figure 3A refers to a transmitter unit 1 10 with two signal paths 131 , 132 and shows details of an embodiment of the first modulator unit 150 of Figure 2. In each signal path 131 , 132 a grouping unit 152 may map the data bits, which are contained in the data stream d3 assigned to the signal path 131 , 132, to a plurality of modulation paths, wherein each modulation path is assigned to a constellation group. Each single of a plurality of sub-carriers is assigned to none or one of the different constellation groups. According to an embodiment, the assignment of the sub -carriers to the constellation groups may be performed according to channel state information describing frequency dependent channel characteristics such that the modulation scheme applied to a sub -carrier depends on the current transmission channel characteristic for this sub-carrier on the respective transmission path. The trans- mission channel characteristics may change with time and the assignment of the sub -carriers to the constellation groups may be continuously adapted.

A constellation control unit 195 may receive or derive the channel state information from feedback information that the transmit unit 1 10 may receive from another communications device with which the transmit unit 1 10 communicates. In the case the transmit unit 1 10 is included in a communications device 100 which also includes a receive unit 120 as shown in Figure 1 , the receive unit may be used for receiving the feedback information.

For each modulation path the modulator unit 150 includes a modulation unit 154a-f realizing the respective constellation group and using a configurable number of sub -carriers to generate a modulated output signal, which may be, for example , a PSK, a QPSK, a 16-QAM, 64-QAM, 256-QAM, 1024-QAM, a 4096-QAM any other 2 ⁿ -QAM, or a 2-PAM or any other 2 ⁿ -PAM signal. Each modulation unit 154a-f uses another one of the constellation groups. The outputs of the modulation units I 54a-f represent modulated sub-carriers. The bits contained in the data stream delivered by the grouping unit 152 are mapped to QAM symbols. Dependent on the transmission channel characteristics, each modu- lation unit 154a-f processes a different number of symbols. Each modulation unit 154a-f is adaptive and programmable with regard to the number and frequency of the used sub -carriers. For example, the constellation control unit 195 may be capable of determining the number and the frequencies of the sub -carriers used in each modulation unit 154a-f according to the current transmission channel characteristics.

In accordance with an embodiment, for each modulator path the interleaving unit 156 includes an interleaver unit 156a that inter- leaves the sub-carriers in the respective modulator path on constellation level. The interleaver units 156a may be based on pseudo-random number generators, for example shift registers, where the register input is derived from an exclusive -or function of two register cell outputs. The interleaver units 156a are scal- able such that each interleaver unit 156a is capable of handling different numbers of sub -carriers in the same constellation group. In this way, the interleaver unit 156a can be combined with adaptive modulation units 154a-f and each interleaver unit 156a can be controlled according to the mode of that modulation unit 154a- f it is assigned to.

In each signal path 131 , 132, a degrouping unit 158a may recom- bine the frequency interleaved sub -carrier groups of the QAM modulator paths, where the interleaved sub-carrier groups are reordered at their original positions, but where the sub-carriers in each of the QAM constellation groups are spread due to the interleaving process. As a result, frequency diversity is increased sig- nificantly. Increased frequency diversity improves system bit error performance, especially in case of narrowband interferers. The scalability of the interleaver units 156a allows the combination with an adaptive OFDM system. In accordance with another embodiment, the modulator unit 150 may include one single modulation unit, which can be over sampled to process the group of carriers serially instead of parallelly as in the preceding embodiment. The symbols of each constellation group can be mapped to various frequency carriers. In a MIMO system, the various carriers of a constellation group can be split to different communication paths .

The transmitter unit 1 10 of Figure 3B exclusively refers to MIMO systems and differs from that illustrated in Figure 3A in that one single degrouping unit 158b combines the frequency interleaved groups of at least two signal paths 13 1 , 132. According to an embodiment, one single combined degrouping unit 158b combines the frequency interleaved groups of all, for example three or four, signal paths 131 , 132 of the transmitter unit 1 10. This allows a further degree of diversity since information which otherwise would be encoded in one transmission path can be transmitted via more than one transmission path such that transmission becomes more robust against interferences on one transmission path. Figure 3C refers to another embodiment of a transmitter unit 1 10 with at least two signal paths 131 , 132. In each signal path 131 , 132 a grouping unit 152 maps data bits contained in the respective data stream d3 to one of a plurality of modulation paths , wherein each modulation path is assigned to another, different constellation group.

Each single of a plurality of sub -carriers is assigned to none or one of the constellation groups. According to an embodiment, the assignment of sub-carriers to constellation groups is performed in accordance with tonemap information describing frequency dependent characteristics of the current transmission channel. A constellation control unit 195 may receive and manage the tone- map information.

In each modulator path an interleaver unit 156b may interleave the data bits in the respective modulator path before the data bits are grouped to symbols, or may interleave the symbols before or when they are used to address the various constellation points of the respective constellation group.

In each signal path 131 , 132 a modulation units 154a-f is provided for each constellation group. Each modulation unit 154a-f uses a configurable number of sub-carriers and the interleaved data bits or symbols to generate modulated sub -carriers carrying the symbol information, and may be capable of using a PSK, QPSK, a 16-QAM, 64-QAM, 256-QAM, 1024-QAM, a 4096-QAM, any other 2 ⁿ -QAM or any 2 ⁿ -PAM scheme, by way of example. Each modulation unit 154a-f uses another one of the constellation groups. Each modulation unit 154a-f is adaptive and programmable with regard to the number and frequencies of the used sub- carriers. For example, the constellation control unit 195 may be capable of determining the number and the frequencies of the sub-carriers used in each modulation unit 154a-f according to the current transmission channel characteristics.

The interleaver units 156b are scalable such that each interleaver unit 156b is capable of handling different number of sub -carriers. Therefore the interleaver units 156b can be combined with the adaptive modulation units 154-f, wherein each interleaver unit 156b is controlled in accordance with the mode of the respectively assigned modulation unit 154a-f. In each signal path 131 , 132, a degrouping unit 158a may recom- bine the interleaved sub -carrier groups of the modulator paths of each signal path 131 , 132, wherein the interleaved sub-carrier groups are reordered into their original positions, but wherein the sub -carriers of the different constellation groups are spread due to the frequency interleaving.

The transmitter unit 1 10 of Figure 3D exclusively refers to MIMO systems and differs from that illustrated in Figure 3C in a single combined degrouping unit 158b that combines the frequency interleaved groups of at least two signal paths 131 , 132. According to an embodiment, the combined degrouping unit 158b combines the frequency interleaved groups of three, four or all signal paths of the transmitter unit 1 10.

Each sub-unit of the transmit unit 1 10 may be realized in hardware, software or as a combination thereof. In accordance with embodiments, at least some of the sub-units of the transmit unit 1 10 are realized as digital signal processing circuits integrated in an integrated circuit. According to other embodiments the transmit unit is a software module of a simulation program.

The diagrams of Figures 4A to 4D illustrate the signal processing method accomplished by the transmission unit 1 10 of Figure 3A by means of an illustrating example. The channel characteristics of the current transmission channel are evaluated to generate channel state information describing, for example, the signal-to- noise ratio for each sub-carrier frequency used for transmission.

Figure 4A shows an illustrative tonemap 400 as a diagram where the SNR is plotted against the discrete frequencies of the sub- carriers. Based on the tonemap information a constellation control unit may assign each sub -carrier to none or one of a plurality of modulation paths, wherein each modulation path corresponds to another constellation group. For example, the total SNR range is divided into a plurality of consecutive ranges 401 -406. Sub- carriers with an SNR falling in the same range are assigned to the same modulation path. Sub-carriers falling in a range of high SNR may be assigned to a modulation path using a modulation scheme with a small distance between the constellation points, for example 4096-QAM. Sub-carriers falling in a range of low SNR may be assigned to a modulation path using a modulation scheme with a high distance between the constellation points, for example QPSK, and sub -carriers with an SNR falling below a predefined threshold may be excluded from being used. The tonemaps of the transmission paths of a transmission channel may differ from each other.

In accordance with the illustrative example, sub-carriers whose SNR fall in a first range 401 are assigned to a 4096-QAM modulation path, sub-carriers whose SNR fall in a second range 402 are assigned to a 1024-QAM modulation path, sub-carriers whose SNR fall in a third range 403 are assigned to a 256-QAM modulation path, sub -carriers whose SNR fall in a fourth range 404 are assigned to a 64-QAM modulation path, sub -carriers whose SNR fall in a fifth range 405 are assigned to a 16-QAM modulation path, and sub-carriers whose SNR fall in a sixth range 406 may be assigned to a QPSK modulation path or may be omitted for transmission.

Figure 4B shows the sub-carrier groups used in the respective modulation path. For example, a first sub-carrier group 41 1 is used in the 4096-QAM modulation path, a second sub-carrier group 412 is used in the 1024-QAM modulation path, a third sub- carrier group 413 is used in the 256-QAM modulation path, and a fourth sub-carrier group 414 is used in the 16-QAM modulation path. In Figure 4B the height of an arrow assigned to a sub- carrier corresponds to that in Figure 4A for the only purpose of identifying the sub -carrier and visualizing the effects of grouping and interleaving and does not relate to an amplitude of the sub- carrier. Figure 4C shows the effect of the interleaver units 156a of Figure 3A. Each interleaver unit 156a interleaves the sub-carriers such that their sequence within the group is changed. As a result, successive data bits or symbols use spaced sub-carriers, whereas without interleaving, sub-carriers carrying symbol information re- ferring to successive data bits or symbols would often use directly adjoining sub-carriers.

Figure 4D shows the effect of the degrouping unit 158a of Figure 3A. The degrouping unit 158a combines the sub-carriers of all modulation paths, wherein it uses the inverse of the grouping scheme used by the grouping unit 152 and reverses the grouping performed by the grouping unit 152. As illustrated in Figure 4D, with respect to Figure 4A, each sub-carrier has changed its position within its sub -carrier group.

The modulation units 154a-f may use orthogonal constellations. According to other embodiments, each of the modulation units 154a-f may use a rotated constellation. Rotated constellations allow two different PAM (phase amplitude modulation) decoders instead of a single QAM modulator.

Figure 5 shows a modulation unit 554g for rotated constellation. A rotation constellation unit 554a generates the orthogonal I and Q components of a rotated constellation. A delay unit 554b delays one of the components, for example the Q component, by one cell/symbol. Each modulation unit of a modulator unit may delay the same component by the same amount. According to an em- bodiment, subsequent interleaver units then ensure that these related components travel in separated sub-carriers. In accordance to an embodiment, constellation rotation and a delay of the Q component is performed in all constellation groups of a signal path separately. Otherwise, with mixed QAM constellations, the output of the QAM constellation stage would include I and Q components from different constellation sizes that do not match with the SNR characteristics of the final chosen sub-carrier.

Use of rotated constellations in connection with frequency inter- leaving allows significant performance improvement for multipath and so-called eraser channels.

Figure 6A shows an orthogonal constellation map for 16-QAM. In each case, four constellation points 601 are projected to the same projection points 61 1 on the I and Q axes.

In contrast, with the rotated constellation of Figure 6B, each constellation point 621 is projected onto another projection point 631 with reference to both axes I and Q, such that it is possible, for example, to omit the evaluation of one of the axes, i.e. one of the carriers at the receiver side. Figure 7 refers to details of the receive unit 220 of the communications device 200 of Figure 1 . A converter unit 230 samples one or more, for example two, three, or four analogue receive signals r l to generate digital receive signals r2. A Fourier transformation unit 240 may transform the digital receive signals r2 into the fre- quency domain, wherein for each digital receive signal a digital data stream r3 may be generated. A first demodulator unit 250 may decompose, in the frequency domain, each data stream into two orthogonally modulated components. A second demodulator unit 260 includes a plurality of QAM demodulators using different constellation groups for each digital data stream r3. The second demodulator unit 260 includes for each digital data stream r3 and for each constellation group a scalable de-interleaver unit capable of de -interleaving the data streams r3 in accordance with an interleaving mechanism used on the transmission side. A demultiplexer unit 270 may recombine the two or more demodulated signals r5 to a combined receive signal r6 and a forward error correction unit 280 may use code redundancy included in the signal on the receiver side for detecting and correcting data errors in a receive signal r7.

According to an embodiment, each de-interleaver unit reverses an interleaving process performed on the transmitter side on constellation level. In accordance with other embodiments, each de- interleaver unit reverses an interleaving process performed on the transmitter side on bit or symbol level. According to further embodiments the receive unit 220 comprises a further mapping unit for reversing an inter-output diversity in the transmission signal as discussed above with regard to the combined degrouping units 158a in Figures 3B and 3D.

Each sub-unit of the receive unit 220 may be realized in hardware, software or as a combination thereof. In accordance with embodiments, at least some of the sub-units of the receive unit 220 are realized as digital signal processing circuits integrated in an integrated circuit. Figure 8 refers to a method of operating a communications device. Data bits are mapped to a plurality of constellation groups according to channel state information (802). Each sub-carrier of a plurality of sub-carriers is assigned to none or one of the constellation groups according to the channel state information (804). Symbols derived from groups of data bits are mapped to the respective constellation group and modulated onto the sub -carriers assigned to the respective constellation group. The assignment of data bits mapped to each constellation group to the sub -carriers that carry the symbol information is interleaved, wherein a range of interleaving, for example the number of concerned sub-carriers, is selected according to the channel state information (806).

In accordance with an embodiment, the modulated sub -carriers carrying the symbol information are interleaved at constellation level. According to another embodiment the data bits mapped to the respective constellation group are interleaved at bit level before the data bits are grouped to symbols used for addressing constellation points of the respective constellation group. According to further embodiments, symbols formed by groups of data bits are interleaved before or when the symbols are mapped to the sub-carriers assigned to the respective constellation group.