Patent Application
20160006594
The present invention relates to a phase reference symbol format used for phase synchronization in a multi-carrier communication system.
OFDM is a powerful modulation format over frequency-selective radio channels. Low complexity equalization, robustness for frequency selective channels, and the ability to use different data modulations on different sub-carriers are the main advantages of OFDM. OFDM systems are, however, also known to be sensitive to phase-noise; a radio-hardware imperfection that causes the OFDM sub-carriers to become nonorthogonal. Due to the resulting inter-sub-carrier interference, ICI, the demodulator signal-to-noise plus interference ratio, SINR, is reduced. If the phase noise process can be estimated, its negative effect on receiver performance can partly be compensated for by digital signal processing at the receiver.
In principle, phase noise estimation is performed at the receiver by observing the change in carrier-phase between the received symbols and a sequence of known phase reference symbols. Various phase estimation techniques exist which can be divided into two main categories. On the one hand, there are reference symbol-based estimators where reference symbols are known a-priori, and on the other hand, decision feedback estimators where tentative decisions on data symbols are made and used as reference symbols in an iterative fashion. It is also possible to combine reference symbol-based and decision feedback based estimators.
In single-carrier schemes it is often a relatively easy task to estimate and compensate for phase noise, as the phase noise process typically is narrowband compared to the modulation bandwidth. In other words, the change in phase from one known reference symbol to the next is typically small enough to be accurately estimated and compensated for. The situation is, however, different in OFDM systems. An OFDM system with N sub-carriers has an N-fold increase in symbol time, compared to a single-carrier system with the same modulation symbol rate1. This can be seen in
In the OFDM literature, most phase compensation schemes are targeting estimators and compensation algorithms for standardized OFDM systems, such as IEEE 802.11 and 3GPP LTE. Very few, if any, are targeting the design of reference signals for phase noise estimation in applications where phase noise is the limiting factor. Reference R1 provides an overview of state-of-the-art phase noise estimation and mitigation techniques. The most basic techniques are based on common phase error, CPE, estimation and compensation for all sub-carriers. However, CPE-based compensation algorithms have the same effect on all sub-carriers inside one OFDM symbol and will therefore not mitigate ICI. More advanced ICI reduction techniques range from fairly simple interpolation between consecutive CPE-estimates to more advanced MMSE estimators or iterative methods.
However, the above methods do not provide sufficient ICI mitigation in OFDM systems operating under severe phase noise. For example, OFDM has previously not been suitable for applications such as microwave radio backhaul where phase noise is often the most performance-limiting factor. Hence, there is a need to design a phase reference symbol format to be used for phase synchronization in OFDM systems where phase noise is the limiting factor.
It is an object of the present invention to remedy, or at least alleviate, some of these drawbacks for OFDM systems operating under severe phase noise. This is provided in a number of aspects of the present invention described below.
In a first aspect the invention describes an OFDM transmitter arranged to embed single-carrier phase reference symbols into an OFDM symbol, wherein the OFDM transmitter comprises a single-carrier pre-processing unit arranged to receive phase reference symbols as input and provide pre-processed phase reference samples as output. The OFDM transmitter further comprises an OFDM modulator arranged to receive data symbols and the pre-processed phase reference samples as input and map the data symbols to sub-carriers and embed the single-carrier samples into a frequency sub-band of the OFDM symbol.
The pre-processed phase reference samples may be frequency-domain pre-processed phase reference samples in which case the single-carrier pre-processing unit comprises a K-point discrete Fourier transformation (DFT) unit arranged to receive K phase reference symbols as input and provide K DFT-precoded frequency domain samples as output, and an insert prefix postfix unit arranged to insert P prefix and P postfix samples to the DFT-precoded frequency domain samples to obtain the frequency-domain pre-processed phase reference samples. Furthermore, in the case that the pre-processed phase reference samples are frequency-domain pre-processed phase reference samples, the OFDM modulator comprises an N-point inverse discrete Fourier transformation (IDFT) unit arranged to receive the data symbols and the frequency-domain pre-processed phase reference samples as input, and map the data symbols onto sub-carriers and the frequency domain pre-processed phase reference samples onto a single-carrier signal by inputting the frequency-domain pre-processed phase reference samples to K+2P adjacent inputs of an N-point IDFT unit to obtain an OFDM symbol with embedded single-carrier phase reference symbols from the N-point IDFT output.
Alternatively, the pre-processed phase reference samples may be time-domain pre-processed phase reference samples in which case the single-carrier pre-processing unit comprises a single-carrier modulator arranged to receive K phase reference symbols as input and provide time-domain pre-processed phase reference samples as output.
Furthermore, in the case that the pre-processed phase reference samples time-domain pre-processed phase reference samples, the OFDM modulator comprises an N-point IDFT arranged to receive data symbols as input, map the data symbols onto sub-carriers and allocate a phase reference frequency sub-band by inputting 0 to K+2P adjacent inputs of the N-point IDFT to obtain time-domain OFDM samples from the N-point IDFT output, and an adder arranged to add the time domain OFDM samples and the time-domain pre-processed phase reference samples to obtain an OFDM symbol with embedded single-carrier phase reference symbols.
In another aspect the present invention describes a method in an OFDM transmitter for embedding single-carrier phase reference symbols into an OFDM symbol. The method comprising the steps of allocating a phase reference frequency sub-band within the OFDM symbol for single-carrier phase reference symbols and embedding the single-carrier phase reference symbols into the phase reference frequency sub-band. The step of embedding may further comprise the additional steps of pre-processing the phase reference symbols in a single-carrier pre-processing unit arranged to provide pre-processed phase reference samples to an OFDM modulator and mapping data symbols to sub-carriers and the pre-processed phase reference samples onto a single-carrier signal in an OFDM modulator.
The above OFDM transmitter and method will provide a phase reference symbol format that can be used in an OFDM receiver to remedy, or at least alleviate, the impact of severe phase noise in an OFDM system.
The drawings are not necessarily to scale and the dimensions of certain features may have been exaggerated for the sake of clarity, emphasize is instead being placed upon illustrating the principle of the embodiments herein.
The following abbreviations are used throughout the text and the drawings:
ADC Analog-to-digital converter
DAC Digital-to-analog converter
DFT Discrete Fourier transform
IDFT Inverse discrete Fourier transform
LP Low-pass filter
OFDM Orthogonal frequency division multiplexing
P/S Parallel-to-serial converter
S/P Serial-to-parallel converter
Four embodiments of the present invention are described in detail below with reference to
The IDFT 112 of the transmitter and the DFT 212 of the receiver are key components in an OFDM communication system. The N-point IDFT and DFT transforms are respectively given by
In practice, the IDFT 112 and DFT 212 are efficiently implemented using an inverse fast Fourier transform (IFFT) and a fast Fourier transform (FFT). It should be noted that it is well-known to a person skilled in the art that the IDFTs and DFTs can be implemented using FFTs and IFFTs. The present invention is therefore not restricted to any specific algorithmic implementation of the DFT and IDFT.
The main concept of the present invention is schematically illustrated in
The first and the second embodiment of the present invention relate to an OFDM transmitter 400 arranged for embedding single-carrier phase reference symbols into an OFDM symbol. A block diagram of an OFDM transmitter solution common for both embodiments is schematically illustrated in
The single-carrier pre-processing unit 420, 520, 620 has phase reference symbols 101 as input and is configured to provide pre-processed phase reference samples 430, 530, 630 to the OFDM modulator 410, 510, 610 as output. The OFDM modulator has data symbols 100 and pre-processed phase reference samples 430, 530, 630 as input and is configured to output an OFDM symbol with an embedded single-carrier phase reference signal 471 in the manner schematically illustrated in
In a first embodiment of the present invention, shown in
The frequency-domain pre-processed phase reference samples 530 are obtained by inputting K phase reference symbols to the single-carrier pre-processing unit 520. The phase reference symbols 101 are parallelized in an S/P unit 521 and then precoded by a K-point DFT 522 whose frequency-domain output 530′ is cyclically extended in an insert prefix postfix unit 523 by assigning a prefix of P sub-carriers and a postfix of P sub-carriers to the K DFT-precoded frequency-domain samples 530′. The prefix is obtained by taking the last P samples of the K-point DFT output and the postfix by taking the first P samples of the K-point DFT, as schematically illustrated in
The OFDM modulator 510 has data symbols 100 and frequency-domain pre-processed phase reference samples 530 as input. The frequency-domain pre-processed phase reference samples 530 are fed to K+2P adjacent inputs of the N-point IDFT 112 and the remaining N-K-2P inputs of the N-Point IDFT 112 are typically allocated for data symbols 100, where the data symbols 100 have first been parallelized in an S/P block 511 before being fed to the N-point IDFT 112. It should be noted that adjacent inputs here refers to adjacency in frequency-domain, i.e. inputs corresponding to adjacent OFDM sub-carriers. The outputs of the N-Point IDFT 112 are serialized in a P/S block 113 before a time-domain cyclic prefix is inserted in an insert prefix unit 114 to obtain the OFDM symbol 560 with embedded single-carrier phase reference symbols. The time domain cyclic prefix is obtained by taking the last samples of the N-point IDFT output and inserting them in front of the OFDM symbol, as schematically illustrated in
In a second embodiment of the present invention, shown in
The time-domain pre-processed phase reference samples 630 are obtained by inputting K phase reference symbols 101 to the single-carrier pre-processing unit 520, where the single-carrier pre-processing unit comprises a single-carrier (SC) modulator 621. The SC modulator 621 being arranged to map the K phase reference symbols 101 to N time-domain pre-processed phase reference samples 630 that represents the single-carrier signal 471.
The OFDM modulator 610 has data symbols 100 and time-domain pre-processed phase reference samples 630 as input and is configured such that K+2P adjacent inputs of the N-point IDFT 112 are allocated for the phase reference frequency sub-band 470 by having the input 0. Again, it should be noted that adjacent inputs refers to adjacency in frequency-domain, i.e. inputs corresponding to adjacent OFDM sub-carriers. The remaining N-K-2P inputs of the N-Point IDFT 112 are typically allocated for data symbols, where the data symbols 100 have first been parallelized in an S/P block 511 before being fed to the N-point IDFT 112. The outputs of the N-Point IDFT 112 are serialized in a P/S block 113 before a time-domain cyclic prefix is added in insert prefix unit 114 to obtain an OFDM symbol with empty phase reference sub-band 470. To embed the time-domain pre-processed phase reference samples 630 into the phase reference frequency sub-band 470, the time-domain pre-processed phase reference samples 630 are added to the N-point IDFT outputs 650′. It should be noted that the N-point IDFT outputs are the time-domain OFDM samples 650′ with a frequency sub-band 470 allocated for phase reference symbols. It should also be noted that the time-domain pre-processed phase reference samples 630 can be added at any point after the N-point IDFT 112. Hence, the addition can be performed before the P/S block 113, after the P/S block or after the insert prefix block 114. Alternatively, the addition can be performed after DAC.
The third and the fourth embodiment of the present invention relates to a method in an OFDM transmitter 400 for embedding single-carrier phase reference symbols into an OFDM symbol. Common steps in the two embodiments are the steps of allocating 800 a phase reference sub-band 470 within the OFDM symbol for single-carrier phase reference symbols, and embedding 810 the single-carrier phase reference symbols into the phase reference frequency sub-band 470. Common for the two embodiments is also that the step of embedding 810 may further comprise pre-processing 811 the phase reference symbols 101 in a single-carrier pre-processing unit 420, 520, 620 configured to provide pre-processed phase reference samples 430, 530, 630 to the OFDM modulator 410, 510, 610, and mapping 812 data symbols 100 onto sub-carriers and the pre-processed phase reference samples 430, 530, 630 onto a single-carrier phase reference signal in an OFDM modulator 410, 510, 610. A flowchart illustrating the common steps of the method of the third and fourth embodiments is shown in
The third embodiment of the present invention comprises the common steps described above, wherein the pre-processed phase reference samples are the frequency-domain pre-processed phase reference samples 530 obtained by inputting K phase reference symbols 101 to a K-point DFT 522 and by inserting P prefix 472 and P postfix samples 473 to the K-point DFT output, and the step of mapping 812 comprises inputting the DFT-precoded frequency-domain samples 530 to K+2P adjacent inputs of an N-point IDFT 112.
The fourth embodiment of the present invention comprises the common steps described above, wherein the pre-processed phase reference samples are the time-domain samples 630 obtained by inputting K phase reference symbols 101 to a single-carrier modulator 621, the step of allocating 800 comprises inputting zeros to K+2P adjacent inputs of an N-point IDFT 112 to obtain an OFDM symbol with allocated sub-band, and the step of mapping 812 comprises adding the time-domain pre-processed phase reference samples 630 and the OFDM symbol 650′, 660 with allocated sub-band to obtain an OFDM symbol with embedded single-carrier phase reference symbols 660.
The four embodiments described above combine the advantages of OFDM for data transmission over frequency selective channels with the advantage of a single-carrier signal for phase noise tracking. The present invention allows for a flexible way of allocating the phase reference sub-band, as well as the rate of the phase reference symbols. Moreover, the single-carrier reference symbols ensures the possibility of using equal energy phase reference symbols (e.g. phase shift keying); an often desired property in phase noise estimation.
Practical aspects of the present invention that needs to be considered are the phase reference symbol bandwidth selection and sub-band scheduling. The phase reference symbol bandwidth, i.e. the time-domain resolution of the reference symbols, is determined by the number of allocated sub-carriers K, Sampling rate Fs, IDFT/DFT size N, and cyclic prefix length Ng as
Thus, by increasing the number of sub-carriers K allocated to phase reference symbols 101, the time-domain resolution of the phase noise estimate can be increased. The bandwidth of the phase reference sub-band 470 should preferably be high enough for the resulting time-domain symbols to capture the change in phase noise during the OFDM symbol. The bandwidth of the phase reference sub-band 470 is preferably narrow enough to be subject to essentially frequency flat fading over the radio channel and thus avoiding inter-reference symbol interference. Moreover, to avoid interference from data symbols, and to preserve the cyclic property of the K-point DFT 522, the number of sub-carriers P allocated to the prefix 472 and postfix 473 should preferably be high enough to capture most of the energy in the phase-noise process.
Regarding sub-band scheduling, the phase reference sub-band 470 is preferably positioned so that enough reference symbol energy is received, i.e. deep fading dips on the radio channel are avoided. In systems with low Doppler spread, such as point-to-point and multi-point-to-point backhaul systems, this can be accomplished by sending an indicator from the receiver back to the transmitter containing the information of which sub-band that currently is preferred. By adjusting the number of phase reference symbols K and which frequency indices of the N-point IDFT 112 that carry the phase reference symbols, it is possible to adjust in a flexible way both the bandwidth and the position of the phase reference sub-band. In systems with high Doppler spread, feedback from the transmitter to the receiver might be a challenge due to the inherent latency between transmitter and receiver. In this case, to gain diversity, multiple phase reference subbands can be distributed over the total systems bandwidth at the cost of increased computational complexity and higher reference symbol overhead.
It should be noted that the present invention may further comprise the cases when the phase reference symbols 101 are either a-priori known reference symbols (i.e. pilot symbols) or data symbols that are detected and used as known reference symbol (i.e. decision feedback). In a product implementation, it is likely that a combination of pilot symbols and data symbols are used as phase reference symbols. Moreover, channel coding, e.g. a block code, can be used on the single-carrier signal to reduce the number of detection feedback errors.
It should be noted that the S/P 111, 213, 511, 521 and P/S 113, 211 blocks are simply serial-to-parallel and parallel-to-serial conversions. Hence, the inputs of each S/P 111, 213, 511, 521 and P/S 113, 211 block are the same as its outputs. Note also that some of the S/P blocks 111, 311A, 311B and P/S 211 blocks may be omitted if data symbols 100 and phase reference symbols 101 are inputted and outputted in parallel.
It should also be noted that the insert prefix postfix unit 523 and insert prefix unit 114 are simply inserting a prefix (and a postfix) as illustrated in
The embodiments are not limited to the above described embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the embodiments.
It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components, but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. It should also be noted that the words “a” or “an” preceding an element do not exclude the presence of a plurality of such elements.
It should also be emphasized that the steps of the method may, without departing from the embodiments herein, be performed in another order than the order in which they appear.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/056754 | 3/28/2013 | WO | 00 |
