Patent Application
20200169447
The invention relates to the implementation of an active distributed antenna system (DAS) transporting digital passband data.
A distributed antenna system (DAS) is a technology for providing radio coverage in an area which cannot be directly served from a radio base station, and is particularly advantageous in applications where multiple wireless service providers need to provide coverage since a single DAS can be used with many radio base stations.
A state of the art DAS 100 uses digital transport for radio signals as shown in
In general, the DAS 100 consists of one or more source units 107, 108, which each interface to one or more base stations 101-104, and one or more remote units 109, no which drive the antennas 109, 110 in the respective coverage area. In between the source units 107, 108 and the remote units 109, no there may be some type of routing unit 111, either as one or more separate units, or integrated into the source and remote units, or some combination thereof. In the example of
In the example DAS of
The bandwidth of the signal into the ADCs 112, 113 is defined by the minimum and maximum frequencies allowed for base station input signals in the band of interest. As an example, the 1800 MHz Digital Cellular System (DCS) cellular band has a downlink frequency range of 1805.2 to 1879.8 MHz. This is typically a much wider band than that of any individual signal from the RBSs 101-104, and the ADC sample rate must be high enough to sample the entire input signal band. As an example, in order to avoid aliasing, the sample rate of the ADCs 112, 113 must exceed twice the total bandwidth, i.e. in this particular example 2*(1879.8−1805.2)=149.2 MHz.
A number of channel filters 114-117 separate the individual channels of the respective base station 101-104 into independent streams of samples. These individual streams of samples corresponding to signals transferred over each channel of the RBSs are scheduled and serialized by a scheduler 118, 119 and a serializer 120, 121 for transmission over high speed digital links 122, 123 such as Common Public Radio Interface (CPRI) links using fiber-optic connections. As can be concluded, this is problematic for the scheduler 118, 119 and the serializer 120, 121, since the ADCs 112, 113 must sample incoming data at a high sample rate, resulting in a great amount of digital data arriving at the scheduler/serializer.
At the routing unit 111, the samples are de-serialized 124, 125 and passed onto a routing function 126 which forwards the samples to the required output ports. At each output port, the set of samples destined for that port are again scheduled and serialized by a scheduler 127, 128 and a serializer 129, 130 for transmission over high speed digital links 131, 132.
Finally, at each remote unit 109, no, the samples are de-serialized 133, 134 and passed to transmit filtering functions 135-138, which are configured to recreate the original radio signals for each channel being transported. The outputs of all the transmit filtering functions 135-138 for a frequency band are summed and passed to a respective digital-to-analogue converter 139, 140 (DAC) to recreate an analogue signal which can be amplified and transmitted over the antennas 105, 106 providing the coverage areas serving the wireless communication devices.
Source units 107, 108 can also be designed which have a purely digital interface to each base station 101-104, in which case the signals to be transmitted are transmitted in the form of digital samples. In this case, the role of the receive filtering 114-117 is to format and convert the digital samples from the base stations 101-104 into a format suitable for transmission through the DAS 100.
Signals in the “real” world exist over a finite range of frequencies, such as radio frequency (RF) signals transmitted by the respective base station 101-104, and when sampled (e.g. by the analogue-to-digital converter 112, 113) can be represented by a sequence of digital samples.
U.S. Pat. No. 8,929,288 discloses a DAS including a host unit and a plurality of remote units. The host unit includes a plurality of base transceiver stations and a switch. Each of the base transceiver stations is configured to provide a downstream baseband digital signal to the switch and to receive an upstream baseband digital signal from the switch, wherein each downstream baseband digital signal and upstream baseband digital signal is a digital representation of the original radio frequency channel at baseband of the respective base transceiver station. The switch is configured to route each of the downstream baseband digital signals to a respective subset of the remote units as one or more downstream serial data streams and to route each of the upstream baseband digital signals from one or more upstream serial data streams to a respective subset of the base transceiver stations.
Further, the standardized Common Public Radio Interface (CPRI) interface specification “Common Public Radio Interface (CPRI), Interface Specification”, currently Version 6.0, advocates serialization of baseband data over a high speed digital link between a baseband unit and a radio head. Digital DAS implementations have followed this approach, such as the system described in U.S. Pat. No. 8,929,288 where baseband samples are serialized over a digital link.
Processing the digital data at baseband in the DAS has its advantages. With reference to
The well-known Nyquist's theorem teaches that as long as the occupied bandwidth of a signal is less than half of the sample rate Fs then the analogue signal can be perfectly reconstructed from the stream of digital samples.
As can be seen in the left-hand power spectral density (PSD) illustration of
From a signal processing perspective such a representation is commonly used because, among other reasons, as shown in the right-hand time domain illustration, it allows the real and imaginary components to be processed in parallel. Digital baseband signals are commonly referred to as IQ data, i.e. user plane information in the form of in-phase and quadrature modulation data. This IQ modulation enables the digital data to be represented by the real and imaginary components shown in the right-hand time domain illustration of
A DAS needs to deal with a wide range of signals corresponding to different cellular operators and mobile transmission standards. This means that a wide range of different signal bandwidths may be presented, for example within the total ADC input bandwidth. The minimum sample rate for a wideband signal is larger than that for a narrowband signal, and so the range of bandwidths which must be supported leads to the need to support a wide range of different sample rates to make efficient use of the available capacity of the digital interconnection. This means that the scheduling process located before each serializer has a challenging task; it is necessary to find a schedule for transmitting data samples for each channel while adding a minimum amount of delay. At each step through the DAS where it is necessary to carry a different mix of samples (for example at each intermediate routing step) it is necessary to provide further buffering to compensate for the extra scheduling delay introduced. The total delay can be critical to the performance of a DAS since there are limits to how much the base stations can be adjusted to compensate for the delay through the DAS.
The flexibility of the schedulers 118, 119 of the DAS 100 in
A drawback with the digital baseband representation discussed in the above is that there is delay associated with the processing of each time domain digital data component.
An object of the present invention is to solve, or at least mitigate, this problem in the art and to provide an improved DAS and a method of transporting digital data in the DAS.
This object is attained in a first aspect of the invention by a method of transporting digital data in a DAS. The method comprises receiving data from at least one data source, processing the received data, and providing the processed data as digital real-valued passband data for further transport within the DAS.
This object is attained in a second aspect of the invention by a device configured to transport digital data in a DAS, the device comprising a processing unit and a memory, said memory containing instructions executable by said processing unit, whereby said device is operative to receive data from at least one data source, process the received data, and provide the processed data as digital real-valued passband data for further transport within the DAS.
As previously has been discussed, when the real and imaginary component of a complex digital signal is scheduled and serialized in a DAS, a processing delay occurs at the processing of the respective signal components.
Hence if the delay for processing a single one of the real and the imaginary signal component is denoted D, is follows that the delay for processing both components of each sample is 2*D. Inevitably, both components must be processed in order to be transmitted over the high speed links of the DAS.
This problem is advantageously overcome by a method of transporting digital data in a DAS using a passband representation according to the invention. In contrast to a baseband representation, a passband representation does not consist of independent real and imaginary components, but rather can be represented by a single component, such as a purely real-valued signal.
In the DAS proposed with the embodiments of the invention described herein transporting a real-valued digital passband signal, the processing delay of the schedulers and serializers for preparing transport of each data sample over the high speed data link is reduced by 50%, and hence amounts to a processing delay of D for each digital passband data sample as compared to a delay of 2*D as would be the case for each digital baseband data sample consisting of a real and an imaginary component.
In an embodiment, the processing of the received signal includes filtering, in what is referred to as an RX filter, each channel provided by the base stations (or in case of uplink communication; provided by wireless communication terminals). Hence, each respective channel of the base stations is processed by a corresponding frequency-selective RX filter. In this context, a channel should be construed as a set of signals occupying a range of frequencies which the operator wishes to transfer together through the DAS.
Thereafter, the signals of each filtered channel is re-sampled or decimated in the respective RX filter in order to reduce the sample rate Fs of the signal, where the sample rate Fs is adapted to the bandwidth of the signals transported over the respective channel. As can be concluded, the sample rate Fs applied in the RX filters must be at least twice the bandwidth of the highest-bandwidth signal of the filtered channel.
This is highly advantageous, as the resulting sample rate Fs of the real-valued digital passband data provided by each RX filter for further transport within the DAS is adapted to the actual bandwidth of the filtered channel, rather than the total bandwidth of the signals coming into the ADCs 112, 113.
The filtered base station signals have a narrower bandwidth, and can be represented with a lower sample rate than the ADC sampling the incoming composite signal of the base station. For efficient transfer over the digital link, the channel filtering thus also in an embodiment includes one or more stages of decimation or resampling to reduce the sample rate of each base station channel.
As a result, by re-sampling/decimating the signal of the respective filtered base station channel at a lower sample rate Fs, using only as high sample rate as necessary taking into account the bandwidth of the filtered channel, the amount of digital passband data provided to the schedulers is greatly reduced while still enabling subsequent reconstruction of the original signal at the remote units for transmission over the antennas. This greatly mitigates the processing burden on the schedulers/serializers.
Further embodiments of the invention will be discussed in the detailed description.
Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the element, apparatus, component, means, step, etc.” are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.
The invention is now described, by way of example, with reference to the accompanying drawings, in which:
The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.
As previously has been discussed, when the respective component of the digital signal is scheduled and serialized by the schedulers 118, 119, and the serializers 120, 121 of the source units 107, 108 and subsequently by the schedulers 127, 128 and serializers 129, 130 of the routing unit 111, a processing delay occurs at the processing of the respective signal components.
Hence if the delay for processing a single one of the two digital signal components is denoted D, is follows that the delay for processing both components of each digital sample is 2*D. Inevitably, both components must be processed in order to be transmitted over the high speed links 122, 123 and 131, 132.
This problem is advantageously overcome by a method of transporting digital data in a DAS using a passband representation. In contrast to a baseband representation, a passband representation does not consist of independent real and imaginary components, but rather can be represented by a single component, such as a purely real-valued signal.
A passband signal occupies a band of frequencies which are only positive (or, equivalently, only negative). It is not necessary that negative and positive frequency components are independent from one another, and therefore the signal does not need both real and imaginary components in order to be represented. A real-valued digital passband signal is defined as being a signal where each sample is represented by a single coordinate. This implies that the possible values for samples of the signal map to a line in the complex plane. This line is typically the real axis in the complex plane, but could equally be the imaginary axis or a straight line at any other position.
The method of the invention comprises, with reference to the flowchart of
In the following, it is assumed that any mixing of an RF signal down to a lower-frequency IF signal is performed by an RF mixer/demodulator located between the respective RBS 101-104 and the DAS 100, or within the DAS 100 as illustrated with the mixer/demodulator 141, 142.
The RF signal is typically a passband analogue signal, which when supplied to a DAS (via any mixer device) commonly is a composite of signals from one or more RBSs 101-104, even though a DAS may receive signals from a single RBS. However, in practice, a DAS typically receives signals from numerous different RBSs, which may be operated by various operators each operating at different frequencies. These different channels must be separated by the respective channel RX filter 114-117, and subsequently reconstructed by the respective channel TX filter 135-138 at the remote units 109, 110.
Thereafter, the received data is processed in step S102 accordingly, depending on which type of data it comprises—analogue, digital, RF, IF, baseband, etc.—and the processed data is provided in step S103 to the respective scheduler 118, 119 as real-valued digital passband data for further transport within the DAS 100.
In the downlink, the processing as illustrated in steps S101-S103 is in an embodiment performed in the source units 107, 108 close to the RBSs 101-104 for further downlink transport, while in the uplink the processing of steps S101-S103 is performed in the remote units 109, 110 for further uplink transport. Hence, when data enters the DAS 100—either in an uplink or downlink direction—the data is processed such that it can be provided as real-valued digital passband data for further transport within the DAS 100.
With reference to
Thereafter, in step S102b″, the signals of each filtered channel is re-sampled or decimated in the respective RX filters 114-117 in order to reduce the sample rate Fs of the signal, where the sample rate Fs is adapted to the bandwidth of the signals transported over the respective channel. As can be concluded, the sample rate Fs applied in the RX filters 114-117 must be at least twice the bandwidth of the highest-bandwidth signal of the filtered channel.
This is highly advantageous, as the resulting sample rate Fs of the real-valued digital passband data provided by each RX filter 114-117 for further transport within the DAS 100 is adapted to the actual bandwidth of the filtered channel, rather than the total bandwidth of the signals coming into the ADCs 112, 113.
The filtered RBS signals have a narrower bandwidth, and can be represented with a lower sample rate than the ADC 112, 113 sample rate. For efficient transfer over the digital link, the channel filtering thus also in an embodiment includes one or more stages of decimation or resampling to reduce the sample rate of each RBS channel.
As a result, by re-sampling/decimating the signal of the respective filtered RBS channel at a lower sample rate Fs, using only as high sample rate as possible taking into account the bandwidth of the filtered channel, the amount of digital passband data provided to the schedulers 118, 119 is greatly reduced while still enabling subsequent reconstruction of the original signal at the remote units 109, 110 for transmission over the antennas 105, 106. This greatly mitigates the processing burden on the schedulers/serializers.
As previously has been described, at each remote unit 109, no, the samples are de-serialized 133, 134 and passed to transmit filtering functions 135-138, which are configured to regenerate the original radio signals for each channel being transported, including interpolation of the sample rate Fs of each signal to a high enough sample rate such that the entire output frequency band corresponding to the ADC input frequency band can be recreated. The outputs of all the transmit filtering functions 135-138 for a frequency band are summed and passed to a respective digital-to-analogue converter 139, 140 (DAC) to recreate an analogue signal which can be amplified and transmitted over the antennas 105, 106 providing the coverage areas serving the wireless communication devices.
As shown in the right-hand time domain illustration, the digital signal can be represented by a sequence of real-valued samples, as shown in the right-hand illustration. Yr(2n) denotes the real component of the digital signal in the time domain at discrete time sample index 2n.
Again, according to Nyquist's theorem; as long as the sampling rate Fs is at least twice the bandwidth of the sampled analogue signal, the analogue signal can be perfectly reconstructed from the stream of digital samples. The factor 2 for the sample index indicates that the sample rate for a passband representation must be twice that of a baseband representation with the same information bandwidth.
When comparing the real-valued digital passband signal of
However, in the DAS proposed with the embodiments of the invention described herein transporting a real-valued digital passband signal, the processing delay of the scheduler 118, 119 and serializer 120, 121 for preparing transport of each data sample over the high speed data link 122, 123 is reduced by 50%, and hence amounts to a processing delay of D for each digital passband data sample as compared to a delay of 2*D as would be the case for each digital baseband data sample consisting of a real and an imaginary component.
An embodiment of a method of transporting digital data in a DAS according to the invention will now be described with reference to the flowchart of
According to the embodiment, real-valued digital passband data is to be transported by the serializers 120, 121. In this particular embodiment, with reference to the flowchart of
In a quadrature mixer/demodulator, two mixers are utilized which mix the same input signal with two different versions of a local oscillator signal which are 90 degrees offset from one another, which produces an output signal which can be treated as a complex representation.
The ADCs 112, 113 will thus sample the analogue IF signal as a complex baseband signal in step S102b.
Once the IF signal has been digitized, the processing consists of separating the different signals that make up the composite input signal by frequency-selective filtering as has been discussed hereinabove with reference to
As has been previously described, this adaption of the signal bandwidth is performed by re-sampling or decimating the signals of the filtered channels, (step S102b″ of
Now, after this filtering, a digital baseband signal—as illustrated in
This digital baseband signal is in this particular embodiment then transformed into a real-valued digital passband signal using a 3-step approach starting with the step of performing upsampling in step S102c. This step can be omitted if the digital baseband signal already has a sample rate that is at least twice the bandwidth of the information contained in the signal.
In this example, upsampling is performed with a factor 2, i.e. the digital baseband signal is interpolated by a factor 2, thereby doubling the sample rate and increasing the Nyquist bandwidth to 2*Fs.
Effectively, when performing upsampling, zeros are inserted between the original samples to increase the sampling rate, followed by lowpass filtering to smooth the resulting upsampled digital signal, thereby reconstructing the wanted signal across the inserted zeros.
After the upsampling of step S102c, as is illustrated in the time domain representation in the right-hand side of
Thereafter, a frequency shift of the upsampled digital baseband signal is performed in step S102d by a frequency Fs/4 (of the upsampled frequency, corresponding to Fs/2 with respect to the original sample rate applied by the ADCs 112, 113), thereby moving the lower edge of the signal to a positive frequency, in order to create a complex digital passband signal. The result of this operation is illustrated in
Finally, the imaginary component of the complex digital passband signal illustrated in
The result is a signal with the same average data rate—i.e. half the amount of data, twice as often—but with a scheduling granularity that advantageously is half that of a baseband representation, since each of the data samples is real-valued and can be scheduled and routed independently.
Hence, as compared to the digital baseband representation, where one real and one imaginary component must be processed in sequence for each data sample with the corresponding delay, the processing delay for each data sample of the real-valued digital passband signal when scheduling and serializing the data samples is reduced by 50%.
At the remote units 109, 110, before transmitting the original RF signal received from the RBSs 101-104 via the antennas 105, 106 to any wireless communication devices, the inverse of the above described 3-step approach is performed at each of the channel TX filters 135-138; a baseband signal can be recreated from the passband signal of
Finally, after having transformed the digital signal of each respective channel back to the baseband representation shown in
In a practical implementation, it is possible to integrate some or all of the conversion steps into existing signal processing circuitry (since the receive path typically involves stages of frequency shifts and decimations, with the opposite steps in the transmit path) if use of baseband data representation is desired for internal processing. Alternatively, an implementation could choose to use a passband representation throughout.
A series of transformations to passband representation then back to baseband representation does not provide an identical baseband signal at the receiving end (since the filtering steps involved are non-ideal, and the process is not time-invariant). However, Nyquist's criterion is never violated so the essential properties of the signal are preserved and it is possible to recreate a RF signal with arbitrarily high fidelity at the output.
In an alternative embodiment, described with reference to the flowchart of
The ADCs 112, 113 will thus sample the analogue IF signal as a complex passband signal in step S102b.
Again, once the IF signal has been digitized, the processing consists of separating the different signals that make up the composite input signal by frequency-selective filtering in RX filters 114-117 (step S102b′ of
Further, the signals of the filtered channels are re-sampled/decimated at a reduced sample rate Fs as previously discussed in step S102b″ of
Now, after this filtering, a complex digital signal comprising both real and imaginary components—as illustrated in
This complex digital passband signal is in this particular embodiment then transformed into a real-valued digital passband signal by shifting the signal in frequency to be centered at half the sample rate Fs in step S102d′ and then discarding, in step S102e, the imaginary component of the signal illustrated in
Again, as compared with a digital baseband representation, the result is a signal with the same average data rate—i.e. half the amount of data, twice as often—but with a scheduling granularity that advantageously is half that of the baseband representation, since each of the data samples is real-valued and can be scheduled and routed independently.
Hence, as compared to the digital baseband representation, where one real and one imaginary component must be processed in sequence for each data sample with the corresponding delay, the processing delay for each data sample of the real-valued digital passband signal when scheduling and serializing the data samples is reduced by 50%.
In yet another embodiment, described with reference to the flowchart of
The ADCs 112, 113 will thus sample the analogue IF signal as a real-valued passband signal.
Again, once the IF signal has been digitized, the processing consists of separating the different signals that make up the composite input signal by frequency-selective filtering applied by the RX filters 114-117 as shown in step S102b′ of
Thereafter, the signals of the filtered channels are re-sampled or decimated with a lower sample rate Fs to adapt to the bandwidth of the signals of the filtered channels, as has been discussed with reference to step S102b″ of
Now, after this filtering, since the IF signal is sampled as a real passband signal, a real-valued digital passband signal—as illustrated in
In an alternative embodiment, described with reference to the flowchart of
The digital real-valued passband signal attained after the ADC 112, 113 is processed for each channel with a digital quadrature demodulator in step S102b1 (implemented as a part of the RX filters 114-117) which shifts the real-valued passband signal for each channel to zero IF, by multiplying the received samples with a complex phasor with a frequency equal to the channel center frequency and where the real and imaginary components are 90 degrees out of phase, in a manner identical to the processing in an analogue quadrature demodulator. The frequency-selective RX filters 114-117 can then filter the real and imaginary components with low-pass filters adapted to the bandwidth of the wanted channel, as has been described in detail with reference to step S102b′ of
In still another embodiment, the input to the source units 107, 108 of the DAS 100 is an RF signal which is not mixed down but only passed through a non-quadrature demodulator (not shown). This demodulator may be located within or outside the source units 107, 108.
The ADCs 112, 113 will thus sample the analogue RF signal as a real-valued passband signal.
Again, once the IF signal has been digitized, the processing consists of separating the different signals that make up the composite input signal by frequency-selective filtering. Hence, each channel has a frequency-selective digital filter 114-117 whose bandwidth is adapted to the characteristics of the respective signal source, i.e. each filter is tuned to the operating frequency of the corresponding RBS.
Now, after this filtering, since the RF signal is sampled as a real passband signal, a real-valued digital passband signal—as illustrated in
In still a further embodiment, the input to the source units 107, 108 of the DAS 100 is an already digitized signal, i.e. a digital baseband signal.
In such scenario, there is no need to demodulate or digitize the input signal, even though channel filtering still is necessary. The 3-step approach already described with reference to
As previously has been discussed, the DAS 100 illustrated in
Even though the DAS 100 of
With reference to
The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1651561-1 | Nov 2016 | SE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/080840 | 11/29/2017 | WO | 00 |
