Patent Application
20080088518
Wireless communication links often use directional antennas having narrow beam widths. In many cases, the ability to establish and maintain communication over the link is highly sensitive to the alignment of the antennas, i.e., to the accuracy with which the antenna at one end of the link (the transmitter or receiver) points toward the antenna at the other end. Millimeter-wave links having highly directional antennas are particularly sensitive to alignment errors.
The gain difference between the antenna's main lobe and side lobes is significant, often on the order of 20 dB or more. As a result, the signal used for normal communication can usually be detected only via the antenna main lobe and not via its side lobes. When initially setting up a communication link, adjusting the main lobe of the antenna to point in the right direction by attempting to receive the signal used for normal communication is difficult, because of the narrow angular range in which this signal can be detected.
Embodiments of the present invention provide improved methods and systems for aligning directional antennas in wireless communication links. As will be described in detail hereinbelow, the methods and systems described herein enable the transmitter and receiver to communicate via the antenna side lobes during antenna alignment. The transmitter and receiver modems used in the communication link are capable of switching between two operational modes. A normal operational mode is used for normal communication when the antennas are aligned. An alignment operational mode, having an improved link budget with respect to the normal mode, is used during antenna alignment.
A link budget is commonly defined as the sum of all gains and losses applied to the communicated signal along the link. Gains and losses may comprise, for example, analog gains or losses (e.g., an antenna gain or a filter insertion loss) and processing-related gains or losses (e.g., a coding gain of a particular error correction code or the modulation gain of a particular modulation scheme). The term “improved link budget” is used to describe a link budget that enables the transmitter and receiver to communicate in the presence of the higher path attenuation encountered when the antennas are misaligned. Improving the link budget often involves improving the sensitivity of the receiver.
Because of the improved link budget in the alignment mode, the receiver is able to reliably receive and measure the signal transmitted by the transmitter over a relatively wide range of angles, i.e., over a wide angular skew relative to optimal alignment of the antenna and not only via the antenna main lobe. Thus, the received signal can be used as a sensitive and reliable indication for antenna alignment.
In some embodiments, the improved link budget in the antenna alignment operational mode may be achieved, for example, by using a lower symbol rate, a signal constellation having fewer symbols, a higher density of pilot symbols, longer synchronization sequences and/or a lower forward error correction (FEC) code rate, than in the normal operational mode.
In some embodiments, the antenna alignment procedure begins with a relatively coarse alignment in which the main lobe is brought to cover the distant end of the link, and a finer alignment in which the antenna orientation is fine-tuned within the angular range of the main lobe. The methods and systems described herein are particularly suitable for carrying out the coarse alignment, although they can also be used to carry out the fine alignment, as well as the entire procedure.
Unlike some known antenna alignment methods and systems, in the methods and systems described herein the alignment procedure uses the same transmitter and receiver as for normal communication, thus eliminating the need for installing and operating additional or alternative alignment-related equipment. In addition, the antenna alignment can be corrected or refined as needed by switching back to the alignment operational mode during the life cycle of the link, with only minor interruption to the link operation, and not only during initial link installation.
Link 20 may comprise a standalone point-to-point link or may be part of a point-to-multipoint communication system. For the sake of conceptual clarity, the description that follows refers to a unidirectional link. Typically, however, link 20 is part of a bidirectional link between two communication systems, wherein each system comprises a transmitter similar to transmitter 24 and a receiver similar to receiver 28.
The data input to transmitter 24 is formatted and encapsulated in data frames by a framer 30. The data frames are encoded and modulated by a transmit (TX) modem 32. In some embodiments, the TX modem encodes the input data with a forward error correction (FEC) code. Any suitable FEC code can be used. The TX modem modulates the encoded data in accordance with a particular modulation scheme, typically by mapping bits or groups of bits to symbols selected from a particular signal constellation. For example, modem 32 may use quaternary phase shift keying (QPSK), 16-symbol quadrature-amplitude modulation (16-QAM), 64-QAM, or any other suitable modulation scheme. As noted above, the TX modem is capable of switching between a normal operational mode used for communication when the antennas are aligned, and an alignment operational mode used for antenna alignment.
The modulated symbols produced by TX modem 32 are converted to an analog signal using a digital-to-analog (D/A) converter 36. The analog signal is filtered, amplified and up-converted to a suitable radio frequency by a transmitter front-end (TX FE) 40. The radio signal is amplified by a power amplifier (PA) 44 and transmitted to receiver 28 via a transmit (TX) antenna 48.
The signal transmitted by transmitter 24 is received by a receive (RX) antenna 52. A receiver front end (RX FE) 56 down-converts the signal to a suitable intermediate frequency (IF) or to baseband. The RX FE may also perform functions such as low-noise amplification, filtering, gain control, equalization, synchronization and carrier recovery. The signal produced by the RX FE is digitized by an analog-to-digital (A/D) converter 60. The digitized signal is provided to a receive (RX) modem 64. The RX modem demodulates the received symbols and decodes the FEC, so as to reconstruct the data frames. A de-framer 66 extracts the data from the data fames and provides the extracted data as output.
Transmitter 24 comprises a TX controller 68, and receiver 28 comprises an RX controller 80. The TX and RX controllers respectively manage the operation of the transmitter and receiver, and in particular coordinate the switching between the normal communication and antenna alignment operational modes. Controllers 68 and 80 can be jointly viewed as an alignment processor, which carries out the antenna alignment methods described herein. The different alignment functions can be partitioned between controllers 68 and 80 as desired.
In some embodiments, the TX and RX controllers coordinate the mode changes, and otherwise communicate with one another, by exchanging management information over a management channel 84. For example, the TX controller may send information to the RX controller by embedding management information in the data frames produced by framer 30. When link 20 is part of a bidirectional communication link, the RX channel may send information to the TX controller by embedding management information in data frames of the opposite link direction.
In some embodiments, transmitter 24 comprises a TX technician interface 70. Additionally or alternatively, receiver 28 comprises an RX technician interface 74. The TX and RX technician interfaces serve as user input devices, using which a technician can control the operation of link 20. In particular, the technician may switch between the normal and alignment operational modes.
In some embodiments, the TX and/or RX antennas comprise highly-directional antennas. For example, the antenna main lobe may have a 3dB beamwidth narrower than 1° in both azimuth and elevation. Outside the main lobe, the antenna gain drops rapidly. The average side lobe level of the antennas is often on the order of 20-30 dB below the main lobe gain. In some cases, an antenna may have a narrow beamwidth in one dimension and a wider beamwidth in the other dimension.
The description that follows assumes that both the TX and RX antennas comprise directional antennas. The methods and systems described herein can similarly be. used in links in which only one of the antennas, either the TX or the RX antenna, is directional and requires accurate alignment. Configurations having one directional antenna and one wide-angle antenna are commonly used, for example, in point-to-multipoint systems.
Depending on the azimuth and elevation beamwidths of the TX and RX antennas used, the antennas may be aligned in azimuth, elevation or both. In some cases, the antenna orientation is adjusted manually by a technician. Alternatively, the antennas can be rotated and adjusted by suitable antenna rotators. In some embodiments, transmitter 24 comprises a TX antenna rotator 76, which controls the angular orientation of TX antenna 48. Rotator 76 may rotate the antenna in one dimension (e.g., azimuth only) or in both azimuth and elevation. Rotator 76 is controlled by TX controller 68. Additionally or alternatively, receiver 28 may comprise an RX antenna rotator 82, which is controlled by RX controller 80 and adjusts the angular orientation of RX antenna 52.
As can be appreciated from the typical antenna characteristics described above, when the link antennas are misaligned, the signal level received by receiver 28 may drop significantly with respect to the signal level during normal operation (i.e., when the antennas are aligned). When only one antenna is misaligned, the difference in signal level may be on the order of 20-30 dB. When both antennas are misaligned, the signal level may drop by 40-60 dB or more.
This 20-60 dB drop in signal level is usually far below the sensitivity of the receiver when it is configured for communication via aligned antennas. When attempting to align the antennas, a sufficiently strong signal is received only when the antennas point to one another with an accuracy that is better than the width of the main lobe. In the normal operational mode, the receiver is practically blind and cannot measure signal quality metrics at other angular orientations of the antennas. In most cases, particularly when attempting to point two narrow beam antennas toward one another, the alignment procedure using the normal link budget is all but impossible.
In order to enable communication between transmitter 24 and receiver 28 when antennas 48 and 52 are misaligned, the TX and RX modems support an antenna alignment operational mode, which provides a significantly improved link budget with respect to the normal communication mode. The alignment mode enables the RX modem to operate reliably at significantly lower signal to noise ratios (SNR). In other words, the alignment mode increases the bit energy to noise density ratio (Ebb/No0) at a given signal level, thus improving the receiver sensitivity. The link budget in the alignment mode is typically 20-25 dB better than the link budget of the normal mode.
In some embodiments, the TX and RX modems may use a reduced symbol rate in the alignment mode, in comparison with the normal mode. For example, if the normal symbol rate is 100 million symbols per second (Msps) and the symbol rate in the alignment mode is 2 Msps, the receiver sensitivity is improved by 17 dB.
Additionally or alternatively, the TX and RX modems may use a signal constellation having fewer symbols in the alignment mode, in comparison with the normal mode. Using a smaller signal constellation increases the Euclidean distances between constellation symbols and improves the receiver sensitivity. For example, if the normal mode uses 64-QAM, which modulates six bits per symbols, using BPSK having one bit per symbol in the alignment mode improves the receiver sensitivity by 15 dB.
Further additionally or alternatively, the TX and RX modems may use a reduced FEC code rate in the alignment mode, in comparison with the normal mode. A lower code rate typically provides a higher coding gain, which improves the receiver sensitivity. Lowering the code rate may enable sensitivity improvements on the order of 5-10 dB with respect to the normal mode.
In some embodiments, the TX modem may transmit pilot symbols to the RX modem in order to perform synchronization. In these embodiments, the density of pilot symbols (i.e., the fraction of time allocated to the transmission of pilot symbols) may be increased in the alignment mode, in order to increase the synchronization robustness under low SNR conditions.
In some embodiments, the TX modem transmits known synchronization symbol sequences, such as preambles, to the RX modem, and the RX modem uses the sequences to synchronize the receiver with the transmitter. In these embodiments, the TX and RX modems may improve the robustness of the synchronization under low SNR conditions by using longer synchronization sequences in the alignment mode. In some cases, the transmitter and receiver may switch between two or more modulation schemes, such as when using adaptive coding and modulation (ACM). In such cases, the synchronization sequences typically use the most robust modulation scheme supported by the link, i.e., the scheme having the best noise performance.
Additional link budget improvements can be achieved, for example, by using a modulation scheme having a low peak to average power ratio (PAR), such as a constant-envelope modulation scheme, in the antenna alignment mode. Such schemes may comprise, for example, binary phase shift keying (BPSK) or QPSK. Using a low PAR signal enables driving PA 44 with a higher average power (i.e., smaller back-off) in comparison with the normal mode, thus improving the link budget.
In some embodiments, the type of signal used in the alignment mode can differ from the signal used in the normal mode. For example, the signal transmitted in the alignment mode may comprise an unmodulated carrier. The receiver in this case typically measures the power of the carrier as an alignment indication. As another example, the signal used for alignment may consist entirely of known waveforms, such as pilot symbols or high processing gain sequences. Demodulating only known waveforms significantly improves the robustness of the receiver, and in particular the robustness of the receiver's synchronization mechanism.
Additionally or alternatively, the receiver may function differently in the normal and alignment modes. For example, the receiver may measure the power of the received signal without performing demodulation in the alignment mode. As another example, the receiver may use different demodulation methods in the normal and alignment modes. For example, the receiver may perform symbol-by-symbol demodulation in the normal mode, and batch demodulation of multiple symbols in the alignment mode.
The antenna alignment mode may comprise any combination of one or more of the link budget improvement measures described above. When link 20 operates in the alignment mode, its data throughput may be decreased. Lowering the symbol rate, reducing the constellation size, reducing the code rate, increasing the density of pilot symbols and/or increasing the preamble length all reduce the net data throughput of the link. This data rate reduction is usually tolerable in the antenna alignment mode, since the transmission is mainly used for signal strength measurements and not for transferring user data. In some embodiments, however, the link may still transfer useful data during antenna alignment. This data may comprise user data provided to transmitter 24, or internal management data.
In the normal operational mode, it is assumed that the link is designed with an SNR margin of 5 dB. Thus, a sufficiently strong signal is received over a narrow angular range 92 of only approximately 0.7°. Assuming the antenna alignment operational mode improves the link budget by 25 dB, a sufficiently strong signal is received in this mode over a much wider angular range 96 of approximately 4.5°.
The antenna alignment process usually comprises scanning the antenna over a certain angular range and performing signal measurements at different antenna orientations. Receiving a reliably-detectable signal over a wider angular range significantly shortens the duration and improves the quality of the antenna alignment process. For example, the resolution of the scanning process, i.e., the number of angles at which signal measurements are performed, can be significantly reduced.
Transmitter 24 transmits an alignment signal, at a transmission step 102. The alignment signal may convey real data or may comprise dummy data used only for signal measurements. Receiver 28 receives the alignment signal, at a reception step 104. During signal transmission and reception, the antenna being aligned is scanned through a range of angular orientations. As noted above, TX antenna 48, RX antenna 52 or both may be scanned and adjusted. Scanning may be performed manually by a technician or using antenna rotators 76 and/or 82.
RX modem 64 measures the received signal quality during the antenna scanning, at a signal measurement step 106. The signal quality measurements serve as alignment indications, which are used for aligning the antenna. In some embodiments, the RX modem measures the received signal level (RSL) as a function of the scanning angle. Alternatively, other signal quality metrics such as SNR, mean square error (MSE) or bit error rate (BER) can also be used. Because of the improved link budget provided by the antenna alignment mode, the RX modem can reliably receive and measure the received signal over a relatively wide angular range, as demonstrated by
In some embodiments, the antenna may be scanned through its entire angular range for determining the best-performing angle. Alternatively, the antenna can be scanned only until a peak is found in the signal quality measurements. Determining the best-performing angle can be carried out automatically by RX controller 80, or manually by a technician. For example, the RX controller may output a real-time indication of the received signal quality using technician interface 70 and/or 74, or using a suitable analog or digital display in receiver 28 and/or transmitter 24. The receiver and/or transmitter may also produce an analog voltage that is measured by the technician during the alignment procedure. Generally, any information or indication can be transferred to interface 70 and/or 74 using management channel 84.
The aligned antenna is oriented in the direction that corresponds to the best signal quality, at an antenna setting step 108. Further alternatively, any other suitable scanning method can be used. The transmitter and receiver exit the antenna alignment mode in a coordinated manner, at an exit step 110. Switching from the alignment mode to the normal mode may be performed automatically, such as by automatically determining that the antenna is sufficiently aligned, or manually by a technician.
Although the embodiments described herein mainly address antenna alignment in wireless communication links, the principles of the present invention can also be used for other applications, such as in satellite communication systems.
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.
