Patent Application
20080002581
Each system 24 comprises an indoor unit (IDU) 28, which is located in a building 30 or other indoor installation. The IDU is connected by a cable connection 32 to an outdoor unit (ODU) 36. The outdoor unit is coupled to an antenna 40. The ODU is typically located adjacent to the antenna in order to reduce the signal losses between the ODU and the antenna. Cable connection 32 may comprise any suitable data connection, such as a connection over a coaxial cable, a twisted pair or an optical fiber. The IDU and/or ODU may comprise suitable interface circuitry (not shown), such as line drivers, modems, or optical transceivers, for interfacing with the medium in question.
Consider, for example, transmission of data from a communication system acting as a transmitter to another communication system acting as a receiver. (Typically, each system 24 comprises a bidirectional system that performs both transmission and reception. In other words, each system typically serves both as a transmitter and as a receiver simultaneously.)
The IDU of the transmitter accepts the data for transmission. The IDU sends the data over the cable connection to the ODU. The transmitter ODU encodes the data using an error correcting code (ECC) and modulates the data using a predetermined modulation scheme to produce a modulated radio signal. Typically, the transmitter ODU modulates the data by mapping data bits to transmitted symbols selected from a predetermined signal constellation. The ODU transmits the radio signal via the transmitter antenna over wireless channel 26. The radio signal is received by the receiver antenna and provided to the receiver ODU. The receiver ODU demodulates and decodes the signal to extract the transmitted data, and sends the extracted data over the cable connection of the receiver to the receiver IDU. The IDU outputs the data.
The rate of information transmitted over the wireless link, i.e., the data rate of the transmitted radio signal, is variable. As will be shown below, the ODUs of the transmitter and receiver comprise variable rate modems, which vary the data rate of the radio signal. For example, in some embodiments the receiver estimates the conditions of channel 26, and the data rate of the wireless link is configured so as to suit the estimated channel conditions. Estimating the channel conditions can be based on measurements of signal-to-noise ratio (SNR), bit error rate (BER), and/or received signal level (RSL), or using any other suitable method known in the art.
In some embodiments, a decision to change the data rate of the wireless link may be taken based on a prediction indicating that the channel conditions are expected to deteriorate, even though the current conditions are still acceptable. For example, the receiver may track the reception quality (e.g., SNR, BER or RSL) over time and extrapolate it to predict the future channel conditions. Additionally or alternatively, the receiver modem typically comprises an adaptive equalizer (not shown), which is used, inter alia, for compensating for the channel response. Tracking the values of the equalizer coefficients often provides a good prediction of future channel conditions.
In some embodiments, the transmitter and receiver may use adaptive coding and modulation (ACM). In ACM, the code rate of the ECC and the signal constellation are jointly selected to produce the desired data rate and/or quality of service, given the estimated channel conditions. Typically, two or more combinations of code rate and signal constellation are predefined. Each combination of code rate and signal constellation is referred to as an ACM setting. A suitable ACM setting, often based on reception quality measurements performed at the receiver, is selected. The selection may be performed either by the receiver or by the transmitter, as will be explained below.
For example, the transmitter and receiver may use low density parity check (LDPC) ECC and Quadrature amplitude modulation (QAM). Each ACM setting comprises a particular LDPC code rate and a particular QAM constellation. Alternatively, any other suitable modulation and coding scheme can be used for defining the ACM settings. Often but not necessarily, the symbol rate (and consequently the occupied bandwidth) over the wireless link remains constant, and the ACM settings differ from one another by the code rate and/or the number of constellation symbols.
The data transmitted from the transmitter to the receiver can be viewed as passing through three data links in cascade: a cable link (IDU to ODU) in the transmitter, a wireless link from the transmitter to the receiver (ODU to ODU) and another cable link (ODU to IDU) in the receiver. Since the data rate over the wireless link is variable, the data rate of the two cable data links in the cascade should be configured accordingly. The methods and systems described herein provide such data rate coordination.
When system 24 functions as a transmitter, data for transmission enters IDU 28 and is formatted in data frames by a variable rate data framer 44. The data frames are later used in the ODU, for structuring the data for transmission over the wireless channel. In particular, as will be shown below, each data frame is encoded and modulated in accordance with a particular ACM setting. In order to transmit the data over cable connection 32 to the ODU, an IDU encapsulation module 48 encapsulates the data frames in outer frames, referred to as cable frames.
The cable frames and data frames may differ in size and are generally not synchronized with each other. As part of the encapsulation process, module 48 typically inserts into the cable frames synchronization markers that indicate the location of the data frames. For example, a synchronization marker may comprise a field in the cable frame, whose value indicates a location of the beginning of the data frame within the cable frame. Alternatively, synchronization markers may comprise any suitable predetermined data sequences, such as correlation sequences or other known data patterns inserted into the cable frames to indicate the location of the data frame within the cable frame.
A cable transmitter 52 encodes and modulates the data in the cable frames for transmission over the cable connection. The cable transmitter encodes the data of the cable frames with a cable ECC. The cable ECC is used only for communicating over cable connection 32, and should not be confused with the ECC used over the wireless link. In some embodiments, cable transmitter 52 uses a Reed-Solomon code. Alternatively, any other suitable ECC can be used as a cable ECC. The encoded cable frames are transmitted to the ODU over cable connection 32.
At the ODU, the data is decoded and demodulated by a cable receiver 56. In particular, cable receiver 56 decodes the cable ECC. An ODU de-capsulation module 60 extracts the data frames from the outer cable frames. The output of module 60 comprises the data frames, as produced by framer 44, together with the synchronization markers that mark the location of the data frames. The data frames are provided to a variable rate modem 64. The modem modulates and encodes the data frames using the currently-used ACM setting, to produce a modulated radio signal. The radio signal is up-converted, filtered and amplified by a radio frequency (RF) front end (FE) 66. The signal is then fed to antenna 40 and transmitted over wireless channel 26 to another system 24 serving as a receiver.
An ODU encapsulation module 68 encapsulates the data received over the wireless link in cable frames. A cable transmitter 72, similar in structure to transmitter 52, modulates and encodes the data. The cable frames are then transmitted over connection 32 to the IDU.
A cable receiver 76, similar in structure to receiver 56, decodes and demodulates the data transmitted over the cable. An IDU de-capsulation module 80 extracts the data received over the wireless link from the cable frames using synchronization markers indicating the location of the data frames in the cable frames. A variable rate data de-framer 84 extracts the information or data from the data frames produced by framer 44 of the remote system, and the information is provided as output.
IDU 28 comprises an IDU controller 88, which performs the various management functions of the IDU. ODU 36 comprises an ODU controller 92, which performs the various management functions of the ODU. The ODU and IDU controllers can exchange management data and messages with one another over cable connection 32.
In some embodiments, the controllers insert management data into the cable frames, as part of the encapsulation of the data frames. IDU controller 88 provides management data to IDU encapsulation module 48. Module 48 inserts the management data into the cable frames. At the ODU, de-capsulation module 60 extracts the management data from the cable frames as part of the de-capsulation process, and provides the management data to ODU controller 92. Similarly, management data can be transmitted from controller 92 to controller 88 over cable connection 32 using modules 68 and 80. In general, the four controllers of link 20 (i.e., the IDU and ODU controllers of the two communication systems) can communicate with one another either directly or indirectly using the framers and encapsulators.
In some embodiments, direct communication is possible only between the IDU controller and ODU controller of a particular system, or between the IDU controllers of the transmitter and the receiver. In these embodiments, other communication paths are relayed. For example, the ODU controller of the receiver and the IDU controller of the transmitter may communicate via the IDU controller of the receiver.
Note that the communication system configuration of
Since modems 64 of the transmitter and receiver communicate using ACM, the data rate over the wireless link is variable. Switching from one ACM setting to another is performed in a coordinated and synchronized manner across network 20, so that all three cascaded data links modify their data rates without losing data.
As noted above, changing the ACM setting is usually determined based on reception quality measurements performed by the receiver. Therefore, the information on which the decision is based is produced by modem 64 of the receiver, located in the receiver ODU. The decision as to which ACM setting to use in the next data frame, based on the measurements of the receiver modem, may be carried out either in the receiver or in the transmitter, as desired.
On the other hand, the process of switching to a different ACM setting is typically initiated by data framer 44 of the transmitter on the remote side of the link. Thus, regardless of the system element that determines the next ACM setting, this decision should be communicated to the data framer in the transmitter IDU, which initiates the switching process. The information is typically communicated using management data exchanged among the IDU and ODU controllers of the transmitter and receiver. Generally, a change in ACM setting involves and affects the data framer and de-framer, the IDU and ODU encapsulators and de-capsulators, the cable transmitters and receivers, as well as both modems.
In some embodiments, each data frame comprises a field indicating the ACM setting selected for the next data frame. In alternative embodiments, each data frame may comprise an indication of the ACM setting to be used in the current frame, or in any other frame offset agreed between the transmitter and receiver.
When the wireless link is about to switch to a different ACM setting, the data rates of cable connections 32 in the transmitter and receiver should be changed accordingly.
In some embodiments, the cable connection is operated at a constant physical data rate, which is sufficiently high to support the highest possible data rate of the wireless link, plus overheads such as the cable ECC and synchronization markers. When the wireless link operates at a lower data rate, the encapsulation module (48 and/or 68, depending on the direction of transmission) inserts dummy data into the cable frames between successive data frames to compensate for the difference in data rate. In other words, inserting the dummy data modifies the effective data rate of the cable connection without modifying its physical data rate. The amount of dummy data inserted by the encapsulation module depends on the difference between the data rates of the cable connection and the wireless link. This process is referred to as byte stuffing.
The encapsulation module marks the location of the data frames within the cable frames using synchronization markers, or using any other suitable mechanism. The de-capsulation module (60 and/or 80) uses the synchronization markers to extract the meaningful information from the cable frames and ignore or discard the dummy data.
As another example, the data rate of the cable link can be adapted by modifying the code rate used by the cable transmitters (52 and/or 72) and receivers (56 and/or 76). When the data rate over the wireless link is low, the code rate of the cable ECC may be decreased, thus producing and sending a higher number of redundancy bits over the cable connection. When the data rate over the wireless link increases, the code rate of the cable ECC is increased.
Additionally or alternatively, the difference in data rates between the wireless link and the cable connection can be compensated for by adapting the physical data rate of the cable to the data rate of the wireless link. For example, the physical data rate of the cable link can be adapted by modifying the symbol constellation used by the cable transmitters (52 and/or 72) and receivers (56 and/or 76). A smaller or larger constellation may be used in the cable link when the data rate over the wireless link decreases or increases, respectively.
Further additionally or alternatively, the physical data rate of the cable connection can be modified by modifying the baud rate, so that the data rate of the cable connection matches the data rate of the wireless link. Modifying the baud rate can be carried out by varying the physical clock rate of the cable connection. Alternatively, the physical clock rate can be kept constant, and a re-sampling or other signal processing method applied to the data transmitted over the cable connection. Any suitable combination of byte stuffing, constellation modification, cable ECC code rate modification and/or baud rate modification can be used to coordinate the data rate of the cable connections and the wireless link.
In some embodiments, communication over the cable connection is implemented using a pair of wireline modems similar to modems 64, for example over a coaxial cable. The wireline modems use ACM, which enables the data rate over the cable connection to vary. In these embodiments, the data rate of the cable connection is modified by changing the ACM setting of the wireline modems, so as to match the data rate of the wireless link. In general, the signal constellations and/or type of ECC used in the wireline modems may differ from those used in modems 64 of the wireless link, since the characteristics of the coaxial cable are generally different from the characteristics of wireless channel 26.
Depending on the assessed channel conditions, the ACM setting to be used in the next data frame is selected, at an ACM setting selection step 102. In general, the selection can be performed by either the ODU or IDU controller of either the transmitter or the receiver.
The next ACM setting is indicated to data framer 44 of the transmitter, at a setting communication step 104. In some embodiments, for example when the selection of ACM setting is performed at the transmitter, the assessed channel condition is sent from the receiver ODU to the transmitter. The transmitter uses the assessed channel condition to select the ACM setting and indicate the setting to its data framer. Alternatively, for example when the ACM setting selection is performed at the receiver, the ACM setting itself is sent from the receiver to the transmitter IDU.
The information is sent using management information. For example, when the decision to change ACM setting is performed at the ODU of the receiver, the ODU controller of the receiver sends the indication of the new ACM setting to the IDU of the receiver using management information inserted into the cable frames. The IDU of the receiver sends the indication to the IDU of the transmitter using the data framer.
Assuming a change of ACM setting is desired, data framer 44 of the transmitter switches to the selected ACM setting, at a setting switching step 106. The new ACM setting is propagated, using management information, across the elements of link 20, at a setting propagation step 108. The actual change in ACM setting is performed in a synchronized manner across the link. In other words, the change is carried out in all link elements at a particular data frame, but not necessarily at the same time. At the appropriate data frame, the different elements of link 20 modify their operation to match the new ACM setting. For example, the transmitter data framer begins to accept and format data at a rate that matches the new ACM setting. The transmitter encapsulator and de-capsulator configure the data rate of the cable connection to match the new ACM setting. The modems of the transmitter and receiver change the ACM setting accordingly. Similarly, the receiver encapsulator and de-capsulator adapt to the new ACM setting. Finally, the receiver de-framer extracts the data in accordance with the new ACM setting.
In particular, the data rates of the cable connections in the transmitter and the receiver are reconfigured to match the new ACM setting (i.e., to match the data rate of the wireless link). The data rate of the cable connection can be modified using any of the methods described above
Link 20 now begins to communicate using the newly selected ACM setting, at a communication step 110. The method then loops back to channel assessment step 100 above, to continue monitoring the channel conditions.
Although the embodiments described herein mainly address point-to-point systems comprising indoor and outdoor units, the methods and systems described herein can be used with any other configuration in which a variable rate data link is cascaded with one or more additional data links whose data rates should be coordinated. In particular, the methods and systems described herein can be used with any communication system, which is partitioned into two communication units connected by a data link. The communication units may be indoor or outdoor, and may be located at any desirable distance from one another. For example, the methods and systems described herein can be used to coordinate the data rates of relayed links, i.e., two or more communication links connected in cascade, of which at least one link has a variable data rate.
It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention 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.
