Patent Application
20190182532
The disclosure relates to a method, apparatus and system for fast channel scan for DOCSIS cable modems.
Data Over Cable Service Interface Specification (DOCSIS) is an international telecommunications standard. DOCSIS permits the addition of high-bandwidth data transfer to an existing cable TV (CATV) system and is employed by CATV operators to provide Internet access over the existing hybrid fiber-coaxial (HFC) infrastructure. The DOCSIS standard uses conventional modems to provide gateway access to the internet. The latest version of the standard is DOCSIS 3.1 which was released in October 2013. It purports to support capacities of at least 10 Gbit/s downstream and 1 Gbit/s upstream using 4096 quadrature amplitude modulation (QAM). The new standard uses 6 and 8 MHz wide channel spacing and instead uses smaller (20-50 kHz wide) orthogonal frequency-division multiplexing (OFDM) subcarriers. The OFDM subcarrier can be bonded inside a block spectrum that can be about 200 MHz wide.
The DOCSIS architecture includes two primary components: a cable modem (CM) located at the customer premises, and a cable modem termination system (CMTS) located at the CATV headend. Cable systems supporting on-demand programming use a hybrid fiber-coaxial system. Fiber optic lines bring digital signals to system nodes where they are converted into RF channels and modem signals on coaxial trunk lines.
The conventional DOCSIS channel scan uses the physical layer link channel (PLC). Each 192 MHz DOCSIS channel contains a 400 KHz PLC embedded anywhere within it. The receiver, and once detected, Low Density Parity Check (LDPC) decode the PLC to get information on the main data channels. However, to get at the PLC, the receiver has to first search and detect the FFT size and the cyclic prefix size. Then it has to synchronize to the channel. The bandwidth of a DOCSIS channel may be as large as 192 MHz or as narrow as 24 MHz. Furthermore, there may be legacy single-carrier QAM channels and other video channels embedded within the DOCSIS channel. Searching for the PLC in 1.8 GHz of cable bandwidth can be very time consuming. Therefore, there is a need for quick searching and capturing an OFDM- DOCSIS channel.
These and other embodiments of the disclosure will be discussed with reference to the following exemplary and non-limiting illustrations, in which like elements are numbered similarly, and where:
Certain embodiments may be used in conjunction with various devices and systems, for example, a mobile phone, a smartphone, a laptop computer, a sensor device, a Bluetooth (BT) device, an Ultrabook™, a notebook computer, a tablet computer, a handheld device, a Personal Digital Assistant (PDA) device, a handheld PDA device, an on board device, an off-board device, a hybrid device, a vehicular device, a non-vehicular device, a mobile or portable device, a consumer device, a non-mobile or non-portable device, a wireless communication station, a wireless communication device, a wireless Access Point (AP), a wired or wireless router, a wired or wireless modem, a video device, an audio device, an audio-video (AV) device, a wired or wireless network, a wireless area network, a Wireless Video Area Network (WVAN), a Local Area Network (LAN), a Wireless LAN (WLAN), a Personal Area Network (PAN), a Wireless PAN (WPAN), and the like.
Some embodiments may be used in conjunction with devices and/or networks operating in accordance with existing Institute of Electrical and Electronics Engineers (IEEE) standards (IEEE 802.11-2012, IEEE Standard for Information technology-Telecommunications and information exchange between systems Local and metropolitan area networks—Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, Mar. 29, 2012; IEEE 802.11 task group ac (TGac) (“IEEE 802.11-09/0308r12-TGac Channel Model Addendum Document”); IEEE 802.11 task group ad (TGad) (IEEE 802.11ad-2012, IEEE Standard for Information Technology and brought to market under the WiGig brand—Telecommunications and Information Exchange Between Systems—Local and Metropolitan Area Networks—Specific Requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications—Amendment 3: Enhancements for Very High Throughput in the 60 GHz Band, 28 Dec. 2012)) and/or future versions and/or derivatives thereof, devices and/or networks operating in accordance with existing Wireless Fidelity (Wi-Fi) Alliance (WFA) Peer-to-Peer (P2P) specifications (Wi-Fi P2P technical specification, version 1.2, 2012) and/or future versions and/or derivatives thereof, devices and/or networks operating in accordance with existing cellular specifications and/or protocols, e.g., 3rd Generation Partnership Project (3GPP), 3GPP Long Term Evolution (LTE), and/or future versions and/or derivatives thereof, devices and/or networks operating in accordance with existing Wireless HDTM specifications and/or future versions and/or derivatives thereof, units and/or devices which are part of the above networks, and the like.
Some embodiments may be implemented in conjunction with the BT and/or Bluetooth low energy (BLE) standard. As briefly discussed, BT and BLE are wireless technology standard for exchanging data over short distances using short-wavelength UHF radio waves in the industrial, scientific and medical (ISM) radio bands (i.e., bands from 2400-2483.5 MHz). BT connects fixed and mobile devices by building personal area networks (PANs). Bluetooth uses frequency-hopping spread spectrum. The transmitted data are divided into packets and each packet is transmitted on one of the 79 designated BT channels. Each channel has a bandwidth of 1 MHz. A recently developed BT implementation, Bluetooth 4.0, uses 2 MHz spacing which allows for 40 channels.
Some embodiments may be used in conjunction with one way and/or two-way radio communication systems, a BT device, a BLE device, cellular radio-telephone communication systems, a mobile phone, a cellular telephone, a wireless telephone, a Personal Communication Systems (PCS) device, a PDA device which incorporates a wireless communication device, a mobile or portable Global Positioning System (GPS) device, a device which incorporates a GPS receiver or transceiver or chip, a device which incorporates an RFID element or chip, a Multiple Input Multiple Output (MIMO) transceiver or device, a Single Input Multiple Output (SIMO) transceiver or device, a Multiple Input Single Output (MISO) transceiver or device, a device having one or more internal antennas and/or external antennas, Digital Video Broadcast (DVB) devices or systems, multi-standard radio devices or systems, a wired or wireless handheld device, e.g., a Smartphone, a Wireless Application Protocol (WAP) device, or the like. Some demonstrative embodiments may be used in conjunction with a WLAN. Other embodiments may be used in conjunction with any other suitable wireless communication network, for example, a wireless area network, a “piconet”, a WPAN, a WVAN and the like.
Certain embodiments of the disclosure may be implemented at a subscriber's premises using one or more of the disclosure implementations.
CMTS 140 communicates with DOCSIS 150 as an interface between CMTS 140 and home subscriber 160. DOCSIS 150 permits the addition of high-bandwidth data transfer to an existing cable TV system for subscriber 160. As shown in
Operator core backbone 214 provides packet data to operator aggregation network 216, which relays data to one or more CMTS 218. The data may include both packet data from IP network 212 and cable data from MPEG network 210. Each CMTS communicates with one or more subscriber access network though subscriber modem 226. The operator aggregation network 216 and a portion of the CMTS define Aggregation Network 242. Cable modems 226 may be installed at the subscriber's residence 230 or office. The backend of the CMTS, the CM and the subscriber's access point 230 define Access Network 244. The distribution network of
In order to provide high speed data services, a cable company will connect its headend 252 to the Internet via very high capacity data links to a network service provider (e.g., IP 212). On CPE 254 side of headend 252, the CMTS enables the communication with CM 226 of subscriber 230. Different CMTSs are capable of serving different cable modem population sizes. A given headend may have between 1-12 CMTSs 218 to service the cable modem population served by that headend.
Conventional CMTSs may carry only IP 212 traffic. Traffic destined for the cable modem from the Internet, known as downstream traffic may be carried in IP packets encapsulated according to DOCSIS standard. The packets are mapped into bit streams, encoded and quadrature amplitude modulated on to OFDM subcarriers. Given the volume of incoming data to the customer's premises 230, locating the PLC in a 1.8 GHz bandwidth is time consuming and inefficient. As stated, conventional channel scan of DOCSIS modems use the PLC. Each 192 MHz DOCSIS channel contains a 400 KHz PLC embedded therein. The receiver (or CM 226) will have to detect and LDPC decode the PLC to obtain information on the main data channels.
To identify the PLC, conventional DOCSIS receivers first search and detect Fast Fourier Transform (FFT) size and the cyclic prefix size. The cyclic prefix refers to the prefixing of a symbol with a repetition of the end. Although the receiver is typically configured to discard the cyclic prefix samples, the cyclic prefixes serve two purposes. First, they act as a guard interval to eliminate the intersymbol interference from the previous symbol. Second, they act as a repetition of the end of the symbol to allow the linear convolution of a frequency-selective multipath channel to be modelled as circular convolution which may be transformed to the frequency domain using a discrete Fourier transform. This approach allows for simple frequency-domain processing, such as channel estimation and equalization.
The conventional scanning method is slow and inefficient because it has to first identify the FFT size and the cyclic prefix size. This is achieved through cyclic prefix correlation and spectral analysis. Since the OFDM bandwidth is variable and because there are so many non-OFDM signals in the channel, this correlation has to be done over small bandwidths. Hence the search frequency has to be moved in small steps, and as a result the search is time consuming.
After searching and detecting the FFT size and the cyclic prefix size, the conventional receiver has to synchronize to the PLC channel. The bandwidth of the DOCSIS channel may take any value up to 192 MHz or it may be as narrow as 24 MHz. In addition, there may be legacy single carrier QAM channels and other video channels embedded within the DOCSIS band. Therefore, searching for the PLC in 1.8 GHz of cable bandwidth is time consuming and inefficient.
The disclosed embodiments enable DOCSIS channel scan without searching for FFT or cyclic prefix. The disclosed embodiments do not require synchronization or equalization. Accordingly, the disclosed embodiment are significantly faster and more efficient than the conventional methods and systems.
In one embodiment of the disclosure, a reference or a beacon channel is provided as a pointer to the PLC location. The beacon channel may be configured at a known frequency offset from the PLC.
In certain embodiments, the CMTS may be configured to place PLC 314 at a desired location on the spectrum and use beacon 312 channels subcarriers (not shown) to indicate the presence of the PLC. The receiver can easily and quickly locate beacon 312.
The beacon channel is formed of OFDM subcarriers. There are two possible FFT sizes for the main channel: 8K and 4K, with subcarrier spacing of 25 and 50 kHz, respectively. However, the beacon channel will use one subcarrier spacing irrespective of the subcarrier spacing of the main OFDM channel. This allows the beacon to be detected without a search for the FFT size. Furthermore, the beacon channel will use the smaller subcarrier spacing of 25 kHz, since with this spacing it is possible to make the beacon subcarriers orthogonal to the subcarriers of both 4K and 8K FFTs as explained below. The FFT of an OFDM subcarrier is a sinc function. If a beacon subcarrier frequency is fc then the zeroes of the sinc function centered at fc are at frequencies (fc±25 kHz). The zeroes of this function will coincide with the OFDM subcarriers of a 4K FFT that are spaced 50 kHz apart and also with OFDM subcarriers of an 8K FFT that are spaced 25 kHz. As a result, the beacon channel will not introduce any interference to the data channel, irrespective of the size of the FFT used for the data channel.
In one embodiment, the beacon channel may be formed of eight subcarriers that are spaced 25 kHz. Each subcarrier may be BPSK modulated. The BPSK modulation of all eight subcarriers need not be the same. An 8-point pseudo-random sequence may be used to BPSK-modulate the eight subcarriers.
The time domain signal for the beacon channel is formed by taking the 8K-point inverse FFT. Since there are only 8 subcarriers, the other 8184 inputs to the IFFT are set to zero. In the time domain the 8192 samples of the beacon signal are repeated periodically. As far as the main data channel is concerned, the 8 subcarriers of the beacon channel are excluded subcarriers. DOCSIS 3.1 standard specifies a subset of subcarriers as excluded. Thus, the data channel will not attempt to decode the excluded subcarriers. Therefore, the beacon channel may be introduced while complying with the DOCSIS 3.1 specification.
At the receiver an 8K-point FFT of the broadband 192 MHz wide input signal is computed. The DOCISY 3.1 channel may only be 24 MHz wide and the channel may only be partially within the 192 MHz bandwidth. Therefore the 192 MHz signal may contain a beacon signal as well as many other OFDM and non-OFDM signals.
Assume that Y1(k) and Y2(k), k=0, 1, . . . , 8191 may be the FFTs of two successive 8192-point samples. Although the transmitter signals are BPSK, the received signals will be equal to the transmitted signal convolved with the channel impulse response which is a function of the micro-reflections in the channel. In the frequency domain, this is equivalent to multiplying the transmitted signal by the channel frequency response. Hence, for beacon subcarriers:
Y
1(k)=H(k)X(k) (1)
Y
2(k)=H(k)X(k) (2)
The channel frequency response does not change rapidly with time and hence is taken to be the same for both FFTs. Similarly, the data sequence is also periodic at the transmitter and hence the left hand sides of both Equations (1) and (2) are the same.
At the receiver individual subcarriers successive FFTs are differentially demodulated. This gives:
Y
1(k)Y*2(k)=|H(k)X(k)|2 (3)
Hence, for each subcarrier differential demodulation eliminates the phase component and yields only a magnitude. The differentially demodulated subcarriers are accumulated over time. Since they do not have a phase component, they are added coherently, resulting in relatively large amplitudes at the locations of the preamble subcarriers. After this accumulation along the time dimension, the frequency dimension is scanned to detect eight successive subcarriers with large amplitude. This may be achieved using 8-point moving average filter.
The above description assumes that the preamble subcarriers are aligned with the frequencies of the 8K-point FFT. This will not necessarily be the case because there is no frequency synchronization at this time. For example, the beacon subcarriers may be in the middle of two successive 8K FFT frequencies.
To overcome this problem, in one embodiment a poly-phase filter may be used to interpolate between successive 8K FFT frequencies. For example, the frequency range between two successive 8K FFT frequencies may be subdivided into 16 frequencies through the process of interpolation. Then the differential demodulation and subsequent accumulation may be done individually on these sub-divided frequencies. If the preamble exists in the 192 MHz band, then one of these frequencies will be close enough to the preamble frequency to give relatively large amplitude. The moving average filter may be applied, but now accumulating samples that are 16 points apart in the frequency domain. Thus, the moving average takes the form a comb filter.
The application of the interpolating filter may be computationally complex. The complexity may be reduced by computing a 128K-point FFT instead of an 8K FFT. This will automatically implement the interpolation by 16 along the frequency dimension. In certain embodiments, the longer FFT approach may be appropriate if the algorithm is implemented using a software processor.
Referring again to
In one embodiment of the disclosure, beacon signal 312 may include a few OFDM subcarriers occupying a bandwidth of about 200 KHz. In an exemplary embodiment, eight OFDM subcarriers may be selected. The assigned subcarriers may be excluded so that other modems do not use the same subcarriers. In certain embodiments, the subcarriers are designed such that a simple receiver can be configured detect the subcarriers quickly using a wideband FFT search. The CMTS (See
The subcarriers may be BPSK modulated by a known sequence such as a pseudo random binary sequence (PRBS). The modulation from subcarrier to subcarrier may be different. In one embodiment, the modulation may be the same for every symbol. Therefore, symbol timing synchronization may not be needed to detect the subcarriers. It should be noted that in certain embodiments, the cyclic prefix is also not needed. Finally, the need for channel equalization is removed by differential demodulation at the receiver.
The DOCSIS standard includes two subcarrier spacing at 25 and 50 kHz. The exemplary eight reference subcarriers may use one of the two spacing. In one embodiment, the reference subcarriers use a smaller spacing of 25 kHz and thereby remove the need to search for the FFT size.
The disclosed embodiments obviate the need for symbol timing synchronization or cyclic prefix because symbol modulations in successive symbols are the same. Any phase slope due to symbol triggering as well as any channel frequency responses is removed by differential demodulation (along time axis) because these are approximately the same in successive symbols. In one implementation, the receiver (e.g., CM 226,
The CMTS (218,
Therefore, the location of the PLC may be devised on a case-by-case basis. The receiver (CM 226,
It should be noted that every OFDM channel has a PLC. However, the beacon is required only to point to one of the PLCs because it is possible to get information about all the channels in the system from one PLC. In certain embodiments, the overhead is small, for example, around 200 kHz. It can be implemented without affecting DOCSIS-compliant transmissions.
In another exemplary embodiment, different modulations schemes are applied to different beacon signals to provide multiple types of beacon signals. Thus, the receiver may also detects the type of beacon in addition to detecting the beacon signal. In certain embodiments, beacon type may be used indicate FFT size of the actual channel containing the PLC and the length of its cyclic prefix. This may not be necessary, however, because once the PLC location is detected it is relatively easy to detect the FFT size and cyclic prefix length using cyclic prefix auto-correlation.
The flow diagram of
At step 420, the beacon signal may be detected. Detecting the beacon signal may include known detection techniques. In one exemplary implementation, signal detection may include differential demodulation along the time axis (see, e.g.,
If the beacon signal is detected at step 420, at step 430 the PLC location may be readily determined. Consistent with an embodiment of the disclosure, the PLC location may be at a fixed offset from the beacon. Thus, once the beacon location is determined, the offset may be retrieved from an associated memory circuit to calculate the PLC location. In an exemplary embodiment where side information (e.g., beacon type, modulation type, etc.) is applied, other parameters of the main PLC channel such as FFT size and cyclic prefix may also be obtained at this step.
At step 440, the CM is tuned to PLC to detect the FFT and cyclic prefix size through auto-correlation. As part of step 440, the FFT may be computed and the PLC may be demodulated and decoded to get the main channel characteristics. In addition, frequency plan of the network may also be determined at step 440.
In one embodiment of the disclosure, apparatus 500 communicates with a headend device (not shown). Receiver 510 receives information signaling information from the headend and designate a wideband FFT search covering the entire channel. The search may be organized by one or more of the processors 530, 540 or 5540. The FFT search may include wideband 8K FFT search covering the entire channel. Once a beacon is detected, information may be decoded or determined from the beacon signal's type and characteristics as discussed above. The processor circuitry may retrieve information about the location of PLC with respect to the beacon channel from Memory circuitry 560. This information may be then be processed to receiver 510 which may use this information to tune to the PLC and detect the FFT and cyclic prefix size. Receiver 510 may also decode the PLC to get the main channel characteristics as well as extract the frequency plan of the network from the PLC. In one embodiment, the
PLC will contain sufficient information to decode the data channel containing PLC. Once the receiver is locked onto the main data channel, it can obtain the frequency plan through the main channel or the PLC channel.
Example 1 is directed to an apparatus, comprising: a transceiver; and a logic, at least a portion of which is in hardware, the logic configured to conduct a wideband FFT search of the communication channel to identify frequency location of a set of beacon reference subcarriers, and to detect a Physical Layer Channel (PLC) location as a predefined frequency offset from the set of beacon reference subcarriers.
Example 2 is directed to the apparatus of example 1, wherein the PLC contains frequency information for all Data Over Cable Service Interface Specification (“DOCSIS”) channels in the spectrum.
Example 3 is directed to the apparatus of example 1, wherein the beacon reference subcarrier occupy about 200 kHz overhead.
Example 4 is directed to the apparatus of example 1, wherein the PLC defines a DOCSIS 3.1 channel.
Example 5 is directed to the apparatus of example 1, wherein the logic operates on a DOCSIS-compliant transmission.
Example 6 is directed to the apparatus of example 1, wherein the logic is further configured to detect a plurality of beacon channel reference subcarriers.
Example 7 is directed to the apparatus of example 6, wherein the beacon channel reference subcarriers are spaced 25 KHz apart.
Example 8 is directed to the apparatus of example 6, wherein the logic further detects FFT size of the PLC and the length of its cyclic prefix as a function of the beacon type.
Example 9 is directed to the apparatus of example 1, wherein the logic is further configured to detect the cyclic prefix length using cyclic prefix auto-correlation.
Example 10 is directed to a computer-readable storage device containing a set of instructions to cause a computer to perform a process comprising: conducting a wideband FFT search of the communication channel to identify frequency location of a set of beacon reference subcarriers; detecting a Physical Layer Channel (PLC) location at a predefined frequency offset from the set of beacon reference subcarriers; accessing the PLC at the predefined frequency offset to obtain information transmitted through the PLC.
Example 11 is directed to the computer-readable storage device of example 10, wherein the PLC contains frequency information for Data Over Cable Service Interface Specification (“DOCSIS”) channels in the spectrum.
Example 12 is directed to the computer-readable storage device of example 10, wherein the beacon reference subcarrier occupy a about 200 kHz overhead.
Example 13 is directed to the computer-readable storage device of example 10, wherein the PLC defines a DOCSIS 3.1 channel.
Example 14 is directed to the computer-readable storage device of example 10, wherein the PLC defines a DOCSIS-compliant channel.
Example 15 is directed to the computer-readable storage device of example 10, wherein the instructions further comprise detecting type of beacon reference subcarriers.
Example 16 is directed to the computer-readable storage device of example 15, wherein the instructions further cause the processor to detect FFT size of the PLC and the length of its cyclic prefix as a function of the beacon type.
Example 17 is directed to the computer-readable storage device of example 10, wherein the instructions further cause the computer to detect the FFT size and cyclic prefix length using cyclic prefix auto-correlation.
While the principles of the disclosure have been illustrated in relation to the exemplary embodiments shown herein, the principles of the disclosure are not limited thereto and include any modification, variation or permutation thereof.
This application is a continuation of U.S. patent application Ser. No. 14/592,512, filed on Jan. 8, 2015, and entitled METHOD AND SYSTEM FOR FAST CHANNEL SCAN FOR DOCSIS CABLE MODEM. The disclosure of the prior application is considered part of and is hereby incorporated by reference in its entirety in the disclosure of this application.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14592512 | Jan 2015 | US |
| Child | 16185465 | US |
