Patent Application
20150016573
The present disclosure relates generally to signal processing, and particularly to methods and systems for reception and processing of radio signals.
Various systems, such as software radios, comprise a receiver that receives and processes radio signals. Processing of radio signals typically involves down-conversion, digitization and processing of the resulting digital signals. Several off-the-shelf products provide such capabilities. For example, Ettus Research (Mountain View, Calif.) offers a family of Universal Software Radio Peripheral (USRP™) products. A product sheet entitled “USRP N200/N210 Networked Series,” September, 2012, is incorporated herein by reference.
As another example, Pentek Inc. (Upper Saddle River, N.J.) offers a line of multichannel, high-speed data converters called Cobalt®. Two example product sheets of data converters denoted Model 78660 (June, 2012) and Model 78662 (March, 2013) are incorporated herein by reference. As yet another example, Spectrum Signal Processing by Vecima (Burnaby, British Columbia) offers a wideband digital receiver/digitizer module called XMC-1151. A data sheet of this product (September, 2012) is also incorporated herein by reference.
An embodiment that is described herein provides a method, which includes receiving an analog signal including multiple communication channels. Multiple single-channel digital signals are derived from the analog signal, each including a single respective one of the communication channels. Multi-channel digital signals, each including a respective set of the communication channels that are over-sampled and distributed over frequency, are generated from the single-channel digital signals. The multi-channel digital signals are sent to a host over an interface.
In some embodiments, the communication channels include Global System for Mobile telecommunication (GSM) channels. In an embodiment, deriving the single-channel digital signals includes digitizing the analog signal with a predefined over-sampling ratio relative to a symbol rate of the communication channels.
In some embodiments, generating the multi-channel digital signals includes frequency-shifting one or more of the single-channel digital signals in a given set, and summing the single-channel digital signals. Frequency-shifting the single-channel digital signals may include multiplying the single-channel digital signals by respective sequences of complex phase rotation values. Alternatively, frequency-shifting and summing the single-channel digital may include applying a Fourier Transform process.
In a disclosed embodiment, deriving the single-channel digital signals includes filtering each communication channel with a bandwidth that is smaller than a symbol rate of the communication channel, and generating the multi-channel digital signals includes positioning the communication channels in each multi-channel digital signal with a frequency spacing that is equal to the symbol rate.
In some embodiments, the communication channels are extracted in the host from the digital multi-channel signals, and the extracted communication channels are processed. Extracting a communication channel from a multi-channel digital signal may include frequency-shifting the multi-channel digital signal such that the communication channel is shifted to baseband, and filtering the communication channel from the frequency-shifted multi-channel digital signal. In an embodiment, frequency-shifting the multi-channel digital signal includes multiplying the multi-channel digital signal by a sequence of complex phase rotation values. In another embodiment, frequency-shifting and filtering the multi-channel digital signal includes applying a Fourier Transform process.
There is additionally provided, in accordance with an embodiment that is described herein, an apparatus including a host interface and a front-end processor. The host interface is configured for communicating with a host. The front-end processor is configured to receive an analog signal including multiple communication channels, to derive from the analog signal multiple single-channel digital signals each including a single respective one of the communication channels, to generate from the single-channel digital signals multi-channel digital signals, each including a respective set of the communication channels that are over-sampled and distributed over frequency, and to send the multi-channel digital signals to the host over the interface.
There is also provided, in accordance with an embodiment that is described herein, a system including a Radio Frequency (RF) receiver, a front-end processor and a host. The RF receiver is configured to receive an RF signal including multiple communication channels, and to down-convert the RF signal so as to produce an analog signal. The front-end processor is configured to derive from the analog signal multiple single-channel digital signals each including a single respective one of the communication channels, to generate from the single-channel digital signals multi-channel digital signals, each including a respective set of the communication channels that are over-sampled and distributed over frequency, and to send the multi-channel digital signals over the interface. The host is configured to receive the multi-channel digital signals over the interface, to extract the communication channels from the multi-channel digital signals, and to process the extracted communication channels.
The present disclosure will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:
Embodiments that are described herein provide improved methods and systems for receiving and processing multiple radio communication channels. The disclosed techniques can be used, for example, for passive monitoring and analysis of large numbers of cellular communication channels. One example embodiment comprises a passive monitoring system for Global System for Mobile telecommunication (GSM) channels.
In some embodiments, the processing functions of a monitoring system are split between a front-end processor and a host that communicate over an interface. In an example embodiment, the host comprises a personal computer, the front-end processor comprises a signal-processing board installed in the computer, and the interface comprises a Peripheral Component Interconnect Express (PCIe) bus.
In such a system, a Radio Frequency (RF) receiver receives and down-coverts one or more RF bands of interest, which comprise a large number of communication channels. The front-end processor digitizes the received signals and produces a plurality of single-channel digital signals. This process can be performed using hardware or firmware, as an acceleration procedure for a Software Defined Radio (SDR). Each single-channel digital signal comprises a single communication channel that is over-sampled with a certain predefined over-sampling ratio.
In principle, it is possible for the front-end processor to transfer the single-channel digital signals to the host for subsequent processing (e.g., demodulation and data extraction). This sort of solution, however, is problematic because of the extremely high data rate that is required over the interface between the front-end processor and the host. In some real-life scenarios that are addressed herein, this data rate may be on the order of 30 G, a rate that is far beyond the capabilities of a PCIe bus.
In order to avoid such high data rates, the front-end processor generates and sends to the host multi-channel digital signals instead of single-channel digital signals. Each multi-channel digital signal comprises a respective set of communication channels (e.g., four channels), which are distributed over frequency.
By using this technique, the front-end processor takes advantage of the fact that only a small portion of the spectrum of the single-channel digital signals actually contains signal energy. This property is due to the over-sampling and filtering of the single-channel digital signals, and also because the GSM channel spacing (200 KHz, which determines the filtering bandwidth) is narrower than the symbol rate (˜270.83 KHz, which determines the over-sampling rate).
Exploiting this property, the front-end processor superimposes sets of single-channel digital signals with appropriate frequency shifts, so as to produce the multi-channel digital signals. This signal format uses the over-sampled range much more efficiently. As will be shown below, the multi-channel digital signals can be produced by computationally-simple operations such as sign changes and In-phase/Quadrature (I/Q) swapping, or using a Fourier-Transform process such as Discrete Fourier Transform (DFT), Fast Fourier Transform (FFT) or a technique based on such transforms.
The resulting multi-channel signals have the same sampling rate as the single-channel signals, but their number is considerably smaller. Therefore, the data rate over the interface is reduced by a factor that is equal to the number of channels per multi-channel signal. As a result, the interface data rate no longer limits the maximum number of channels that can be processed simultaneously by the system.
The host typically receives the multi-channel digital signals over the interface, and filters them in order to demodulate the individual communication channels. This filtering operation is computationally simple and can be implemented in software, because the frequency separation between channels in the multi-channel digital signals is relatively large. Parsing the multi-channel signals to derive the individual channels can also be produced by computationally-simple operations such as sign changes and In-phase/Quadrature (I/Q) swapping, or using a Fourier-Transform process.
The property of frequency spacing being smaller than channel bandwidth is particularly noticeable in GSM, which makes the disclosed techniques especially attractive for GSM monitoring. Nevertheless, the methods and systems described herein can be used with various other types of communication channels.
In the present example, system 20 receives, demodulates and extracts data from a large number of Global System for Mobile telecommunication (GSM) channels in multiple bands simultaneously. In an example embodiment, system 20 is able to process all possible uplink and downlink GSM channels in the 900 and 1800 MHz bands simultaneously, totaling over 1000 channels. A system that also covers the 850 and 1900 MHz bands would need to process well over 2000 channels. In alternative embodiments, system 20 can be used for processing communication channels in accordance with any other suitable communication protocol, in any desired number of frequency bands and on any suitable frequency.
In the example of
(The embodiment of
The analog signals are provided to a front-end processor 32. Front-end processor 32 digitizes the analog signals and generates digital signals that are sent to a host 36 for subsequent demodulation and analysis. The host typically demodulates the various communication channels, extracts the data conveyed by the channels, and presents the extracted data and/or additional information to an operator 44 using an operator terminal 40.
Front-end processor 32 sends the digital signals to host 36 over a high-speed interface 48. In the present example, host 36 comprises a personal computer, and interface 48 comprises a Peripheral Component Interconnect Express (PCIe) bus. In this embodiment, front-end processor 32 is implemented on a board that is plugged into a PCIe slot of the personal computer. For example, processor 32 may be implemented using the Pentek Model 78660 module, cited above, or using any other suitable platform.
In the embodiment of
In some embodiments (sometimes referred to as “complex” or I/Q configurations) the analog signals received from RF RX 28 comprise I/Q baseband or low-IF signals, in which case a given band is digitized by a pair of ADCs in a quadrature configuration. In alternative embodiments (sometimes referred to as “real” or “IF sampling” configurations) the analog signals received from RF RX 28 comprise IF signals, in which case a given band is digitized by a single high-speed ADC in an IF sampling configuration.
DDC unit 56 processes the digitized signals so as to produce digital single-channel signals, each comprising a single respective communication channel from among the multiple communication channels received in the RF signals. Unit 56 then combines sets of the single-channel signals, using techniques that will be described in detail below.
The process above produces multi-channel signals, each comprising a respective set of communication channels that are over-sampled and distributed in frequency. Processor 32 sends the multi-channel signals to host 36 over interface 48 for subsequent processing. As will be shown below, transmitting the multi-channel signals instead of the single-channel signals to the host reduces the data volume on interface 48 considerably.
The configurations of system 20 and front-end processor 32 shown in
Typically, host 36 and/or processor 32 comprises one or more general-purpose processors, which are programmed in software to carry out the functions described herein. The software may be downloaded to the processors in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory.
to 270 KHz, for the sake of clarity.)
As noted above, DDC unit 56 of front-end processor produces single-channel digital signals, such that each single-channel digital signal comprises a single GSM channel. In some embodiments, the front-end processor digitizes the GSM channels with a certain over-sampling ratio, i.e., with a sampling rate that produces multiple samples per symbol. The over-sampling ratio is defined as the ratio between the sampling rate and the symbol rate of a GSM channel.
Over-sampling is advantageous for subsequent processing of the signal, e.g., for accurate timing and frequency synchronization, channel estimation and/or equalization. In the present example, the over-sampling ratio is four, i.e., each GSM channel is sampled with four complex samples per symbol. Alternatively, however, any other suitable over-sampling ratio can be used.
Although beneficial for performance, over-sampling causes a considerable increase in data volume. In particular, the bandwidth required for sending the single-channel digital signals over interface 48 to host 36 becomes prohibitive. Consider, for example, a scenario in which system 20 monitors a total of 1100 GSM channels (the total number of uplink and downlink channels in a pair of 850 & 1900 MHz, or 900 & 1800 MHz, GSM bands). Assuming an over-sampling ratio of four and a DDC output resolution of twenty-four bits per sample, the required bandwidth of interface 48 is 32 Gbps.
This sort of bandwidth is beyond the capabilities of the PCIe interface. Therefore, unless accounted for, the bandwidth of interface 48 becomes the bottleneck that limits the number of channels that can be processed by system 20. The techniques described herein reduce the required bandwidth on interface 48, with little or no degradation in demodulation performance.
The GSM symbol rate of 270⅚ KHz is denoted fsym. In the present example, the single-channel digital signal has an over-sampling ratio of four, i.e., a sampling rate of fsym×4≅1083.33 KHz. The signal is represented by a complex representation, i.e., by an in-phase component and a quadrature component. The spectrum of such a signal is defined in the range [−2fsym,2fsym]≅[−541.66 KHz, 541.66 KHz] on the frequency axis.
In a typical embodiment, DDC unit 56 produces each single-channel signal by digitally shifting the center frequency of the desired GSM channel to baseband (i.e., to zero Hz), resulting in a signal 74. The DDC unit filters signal 74 with a sharp digital Low-Pass Filter (LPF) to produce signal 76. Any suitable digital down-conversion and filtering schemes can be used for this purpose.
As can be seen in the figure, a large portion of the spectral range [−2fsym,2fsym] has no signal energy, due to the high over-sampling ratio. This is especially true when the signal is filtered sharply, in which case most of the signal energy is confined to the central 200 KHz (curve 76). This representation demonstrates that sending the single-channel signals in this format to host 36 is extremely redundant and inefficient in terms of bandwidth.
In order to reduce the required bandwidth on interface 48, in some embodiments DDC unit 56 converts the single-channel digital signals into multi-channel digital signals, such that each multi-channel digital signal comprises a respective set of single-channel digital signals that are distributed in frequency over the over-sampled range. Front-end processor 32 sends the multi-channel signals, and not the single-channel signals, over interface 48 to host 36. As a result, the data rate over interface 48 is reduced by a factor that is equal to the number of GSM channels per multi-channel digital signal.
Typically, DDC unit 56 produces each multi-channel digital signal of this sort from a respective set of four single-channel digital signals. The DDC unit may produce such a signal, for example, by:
The frequency shifting and summing procedure can be also performed by a Fourier Transform process, e.g., FFT, DFT or a technique based on such transforms. In some embodiments, the frequency shifts of signals 84B, 84C and 84D can be implemented efficiently in DDC unit 56, by using the fact that these frequency shifts are equivalent to ±¼ and ½ of the sampling rate.
Shifting by ¼ of the sampling rate can be implemented by multiplication by the complex sequence +1, +j, −1, −j, +1, +j, −1, −j, . . . . This multiplication can be performed by sign changes and/or I/Q swapping operations that are simple to implement. Shifting by −¼ of the sampling rate can be implemented by multiplication by the complex sequence +1, −j, −1, +j, +1, −j, −1, +j, . . . . This multiplication, too, can be performed by sign changes and/or I/Q swapping. Shifting by ½ of the sampling rate can be implemented by multiplication by the real sequence +1, −1, +1, −1, +1, . . . that can be performed by sign changes. Sequences of this sort are referred to herein as sequences of complex phase rotation values.
In alternative embodiments, the frequency shifting and summing procedure above can be performed using a Fourier Transform process, as explained above. As will be described below, Fourier Transform processes can also be used in the host for extracting the individual GSM channels from the multi-channel signals.
Consider the multi-channel signal of
Using this technique, the bandwidth of interface 48 no longer limits the number of GSM channels that can be processed simultaneously by system 20. For example, the desired capacity of 1100 GSM channels can be supported with a conventional PCIe interface.
As noted above, an over-sampling ratio of four was chosen purely by way of example. In alternative embodiments, the single-channel and multi-channel digital signals in system 20 may be sampled with any other suitable over-sampling ratio. Additionally or alternatively, DDC unit 56 may combine any desired number of single-channel signals into the respective multi-channel signal.
The multi-channel digital signals, of the form shown in
In some embodiments, in order to extract a certain GSM channel from a multi-channel digital signal, host 36 shifts the frequency of the multi-channel digital signal such that the desired GSM channel is centered at baseband, and filters the frequency-shifted signal with a suitable LPF. (If the desired GSM channel is already centered at baseband, e.g., signal 84A, the host applies filtering without frequency shifting.). Alternatively, this process can be carried out by a Fourier Transform process.
As can be seen in the figure, adjacent GSM channels are separated by a ˜70 KHz transition band, which is sufficient for filtering out the adjacent channels with a modest-complexity LPF. An LPF of this sort may be implemented in software running on host 36. Channel spacing of ¼ and ½ over-sampling frequency also enables using Fourier Transform processes efficiently.
The above-described scheme is particularly suitable for GSM, because in GSM each channel is filtered with a bandwidth that is smaller than the symbol rate (200 KHz filtering for a ˜270 KHz symbol rate). Then, the individual GSM channels are positioned in the multi-channel digital signal with a frequency spacing that is equal to the symbol rate. This difference produces the 70 KHz transition region that enables the host to filter-out the individual GSM channels from the multi-channel digital signals.
In some embodiments, the host may shift the frequency of the multi-channel digital signal by multiplication with the phase rotation sequences described above, since these frequency shifts are also by ½ and ±¼ of the sampling frequency. This process can alternatively be performed by a Fourier Transform process.
Front-end processor 32 converts the analog signal into a plurality of single-channel digital signals, at a single-channel generation step 94. Each single-channel digital signal comprises a single GSM channel, selected from the multiple GSM channels in the analog signal.
Typically, for a Direct Conversion Receiver (DCR), ADCs 52 digitize the analog signal with an ADC sampling frequency of at least twice the GSM band bandwidth for a real ADC configuration, and at least the GSM band bandwidth for a complex ADC configuration. In case of low-IF and/or IF receiver, the ADC sampling frequency should typically be much higher.
DDC unit 56 performs sampling reduction to a target over-sampling rate (e.g., four), and the appropriate frequency shifting and filtering operations to produce the single-channel digital signals. The resulting single-channel digital signals have a spectrum such as shown in
Front-end processor 32 combines sets of single-channel digital signals to produce multi-channel digital signals, at a multi-channel generation step 98. As explained above, DDC unit 56 produces each multi-channel digital signal from a set of (typically adjacent) single-channel digital signals by applying appropriate frequency shifts. Each multi-channel digital signal comprises multiple (e.g., four) GSM channels that are distributed over the over-sampled range. The resulting multi-channel digital signals have a spectrum such as shown in
Front-end processor 32 sends the multi-channel digital signals to host 36 over interface 48, at a signal transfer step 102. Host 36 filters the individual GSM channels from the multi-channel digital signals, at a host filtering step 106. The host demodulates the GSM channels so as to extract the data, at a data extraction step, and outputs the data to operator 44 using terminal 40.
Although the embodiments described herein mainly address GSM channels, the principles of the present disclosure can also be used for other communication protocols, such as General Packet Radio Service (GPRS) or Enhanced Data rates for GSM Evolution (EDGE).
It will thus be appreciated that the embodiments described above are cited by way of example, and that the present disclosure is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present disclosure includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.
| Number | Date | Country | Kind |
|---|---|---|---|
| 227401 | Jul 2013 | IL | national |
