Patent Application
20060029017
The invention generally relates to block transmission systems. More specifically, the invention relates to a method and system for transmitting training information in a block transmission system.
In known block transmission systems, the training information such as a preamble includes a cell identifier based on which the receiver identifies a base station. A cell identifier generally refers to a sequence based on which a receiver identifies a base station. The preamble is used for synchronizing the receiver. The method of synchronization involves obtaining timing offset estimate and frequency offset estimate along with the cell identifier, which requires substantial receiver complexity.
The preamble in such systems, is transmitted by a first transmit antenna. Further, some of the control information and/or broadcast information is transmitted by the first transmit antenna alone. This includes the information regarding the number of transmit antennas in a base station.
There is therefore a need for a method and system that reduces the computation involved in synchronizing the receiver. Further, there is a need for a method and system that enables transmission of control information and/or broadcast information through a plurality of transmit antennas.
An objective of the invention is to provide a method and system that simplifies synchronization of a receiver.
Another objective of the invention is to enable the receiver to determine a number of transmit antennas of a base station based on an information bearing unit.
In order to fulfill the above-mentioned objectives, a method for transmitting training information in a block transmission system is provided. The method includes transmitting an information bearing unit. The information bearing unit is transmitted by at least one transmit antenna and can be one of preamble, midamble, or postamble. In an embodiment of the invention, the information bearing unit includes a predetermined identifier and a cell identifier. In an embodiment of the invention, the frequency domain sub-carriers of the predetermined identifier are spaced apart by twice the number of sectors in a cell. Also, the sub-carriers on which the predetermined identifier is transmitted in adjacent sectors are shifted by an even integer. As a result, symbol corresponding to the predetermined identifier as received by a receiver is periodic at each point in the cell.
A more complete appreciation of the invention and the attended advantages will become readily apparent as the same becomes better understood by reference of the following detailed description when considered in conjunction with the accompanying drawings in which reference symbols indicate the same or similar components, wherein:
The invention provides a method and system for transmitting training information in a block transmission system. Examples of block transmission include Orthogonal Frequency-Division Multiplexing (OFDM), Multi-Carrier Code Division Multiple Access (MC-CDMA), Discrete Multi-Tone (DMT) and the like. The IEEE 802.16d and 802.16e wireless Metropolitan Area Network (MAN) standards, which use OFDM-like technology, also fall in this category. In various embodiments of the invention, the block transmission system is a frequency reuse system. In an exemplary embodiment of the invention, the block transmission system is a frequency reuse-1 system.
The method for transmitting training information in the block transmission system includes transmitting an information bearing unit. The information bearing unit is transmitted by at least one transmit antenna of a base station. In an embodiment of the invention, the information bearing unit is a preamble. In another embodiment of the invention, the information bearing unit may be a midamble or postamble.
In an embodiment of the invention, the information bearing unit includes a cell identifier and a predetermined identifier. A cell identifier is generally referred to as a sequence based on which a receiver identifies a base station. In an exemplary embodiment of the invention, the cell identifier is a binary sequence of −1 s and +1 s and has a predefined length. For example, in 802.16e/d standards, the cell identifier of a base station is selected from a set of 114 binary sequences. The predetermined identifier is another sequence like the cell identifier.
In various embodiments of the invention, the information bearing unit further includes at least one training sequence. The training sequence is known to the receiver and may be used for training at least one transmit antenna.
In various embodiments of the invention, a receiver is synchronized based on the information bearing unit. This is further explained in conjunction with
In another embodiment of the invention, if the symbol corresponding to the predetermined identifier as received by the receiver is periodic, periodicity is used in simplifying the method of obtaining the timing offset estimate and the frequency offset estimate. As a result, the periodicity helps in reducing the complexity of the method of synchronization. For example, a coarse estimate of the timing offset can be obtained by using the periodicity, before obtaining the fine estimate of the timing offset. The periodicity may also be used for estimating the fractional frequency offset.
In an embodiment of the invention, timing offset estimate and/or frequency offset estimate is obtained based on the predetermined identifier, if the predetermined identifier is known to the receiver. In another embodiment of the invention, timing offset estimate and/or frequency offset estimate is obtained along with the predetermined identifier, if the predetermined identifier is unknown to the receiver. In various embodiments of the invention, the predetermined identifier is selected by the base station from a predefined set. This is further explained below in conjunction with
At step 115, the cell identifier is identified by the receiver based on the obtained at least one of timing offset estimate and frequency offset estimate. Therefore, the transmission of the predetermined identifier simplifies the method of synchronization.
In various embodiments of the invention, the frequency offset is corrected based on the obtained frequency offset estimates.
At step 220, the cell identifier is identified by the receiver based on the obtained at least of timing offset estimate and frequency offset estimate. In various embodiments of the invention, the information bearing unit can further be used for performing channel estimation for each transmit antenna-receive antenna pair.
In various embodiments of the invention, control information or broadcast information can also be transmitted using a plurality of transmit antennas as the number of transmit antennas of the base station is determined by the receiver based on the predetermined identifier. The control information may be, for example, but not limited to, indicative of a specific mode of transmission, a service provider, or a combination thereof.
In various embodiments of the invention, the sub-carriers of a symbol on which a transmit antenna transmits is shifted by a predefined number with respect to the sub-carriers of the symbol on which a remaining transmit antenna transmits if the number of transmit antennas is at least two. The predefined number is equal to the number of sectors in the cell. For example, the sub-carriers of a symbol on which a second transmit antenna transmits is shifted by three (if the number of sectors is equal to three) with respect to the sub-carriers of the symbol on which a first transmit antenna transmits. Similarly, the sub-carriers of a symbol on which a third transmit antenna transmits is shifted by three with respect to the sub-carriers of the symbol on which the second transmit antenna transmits, if the number of transmit antennas transmitting on the symbol is at least three.
In an embodiment of the invention, the frequency domain sub-carriers of the predetermined identifier are spaced apart by twice the number of sectors in the cell. Further, the sub-carriers on which the predetermined identifier is transmitted in adjacent sectors are shifted by an even integer with respect to each other. Therefore, the symbol corresponding to the predetermined identifier as received by a receiver is periodic at each point in the cell even in the presence of interference from other base stations. This embodiment is herein after referred to as a periodic-predetermined-identifier-embodiment. This is explained in conjunction with
In the periodic-predetermined-identifier-embodiment, the information bearing unit is transmitted on a first symbol and a second symbol. The first symbol is used to transmit only the predetermined identifier from the first transmit antenna. The cell identifier is transmitted in the second symbol by one of the transmit antennas.
In the periodic-predetermined-identifier-embodiment, if the number of transmit antennas in a particular sector is equal to one and the number of sectors in a cell is equal to three, the sub-carriers on which the transmission occurs on the second symbol are spaced apart by six and shifted by three with respect to the first symbol. The pilot spacing of the sub-carriers for the transmit antenna is therefore three.
In the periodic-predetermined-identifier-embodiment, if the number of transmit antennas in a particular sector is equal to two and the number of sectors in a cell is equal to three, the sub-carriers on which the transmission occurs from a given transmit antenna in the second symbol are spaced apart by six. Further, the sub-carriers on which the transmission occurs from the first transmit antenna on the second symbol is shifted by three with respect to the sub-carriers on which the predetermined identifier is transmitted. Also, the sub-carriers of the second symbol on which a second transmit antenna transmits is shifted by three with respect to the sub-carriers of the second symbol on which the first transmit antenna transmits. Therefore, the pilot spacing of the sub-carriers for the first transmit antenna is three and for the second transmit antenna is six.
In the periodic-predetermined-identifier-embodiment, if the number of transmit antennas in a particular sector is equal to three and the number of sectors in a cell is equal to three, only the second transmit antenna and a third transmit antenna transmit on the second symbol. Further, the sub-carriers on which the transmission occurs from a given transmit antenna on the second symbol are spaced apart by six. Also, the sub-carriers of the second symbol on which the second transmit antenna transmits are shifted by three with respected to the sub-carriers on which the third transmit antenna transmits. The pilot spacing for each of the three transmit antennas in the sector is therefore six.
In the periodic-predetermined-identifier-embodiment, if the number of transmit antennas in a particular sector is equal to four and the number of sectors in a cell is equal to three, only the second transmit antenna, the third transmit antenna and a fourth transmit antenna transmit on the second symbol. Further, the sub-carriers on which the transmission occurs from a given transmit antenna on the second symbol are spaced apart by nine. Also, the sub-carriers of the second symbol on which the second transmit antenna transmits are shifted by three with respected to the sub-carriers on which the third transmit antenna transmits. Also, the sub-carriers of the second symbol on which the third transmit antenna transmits are shifted by three with respected to the sub-carriers on which the fourth transmit antenna transmits. The pilot spacing for the first transmit antenna of this sector is therefore six, and for the remaining transmit antennas of this sector is therefore nine.
In various embodiments of the invention, the length of the cell identifier is dependent on the maximum number of transmit antennas per sector of the block transmission system. In the periodic-predetermined-identifier-embodiment, if the number of transmit antennas in a particular sector is less than the maximum number of transmit antennas, the transmit antenna that transmits the cell identifier can also further transmit a training sequence in the second symbol.
In various embodiments of the invention, the information bearing unit is transmitted in at least one symbol. If the number of transmit antennas is equal to one, the predetermined identifier and the cell identifier are transmitted simultaneously in one symbol using a first transmit antenna. The sub-carriers on which the predetermined identifier is transmitted is shifted with respect to the sub-carriers on which the cell identifier is transmitted by a number equal to the number of sectors in a cell. In an exemplary embodiment of the invention, if the number of the sectors is equal to three, the sub-carriers of the predetermined identifier are spaced apart by six and the sub-carriers of the cell identifier are spaced apart by six on the first transmit antenna. The pilot spacing for the first transmit antenna is therefore three sub-carriers.
If the number of transmit antennas is at least two, the predetermined identifier is transmitted by a first transmit antenna and the cell identifier is transmitted by a second transmit antenna on one symbol. In an exemplary embodiment of the invention, if the number of the sectors is equal to three, the sub-carriers of the predetermined identifier are spaced apart by six on a first transmit antenna and the sub-carriers of the cell identifier are spaced apart by six on a second transmit antenna, if the number of antennas is at least two. Also, the sub-carriers on which the second transmit antenna transmits is shifted by three with respect to the sub-carriers on which the first transmit antenna transmits. The pilot spacing for the first transmit antenna and the second transmit antenna is therefore six sub-carriers.
In an embodiment of the invention, the information bearing unit further includes at least one training sequence, if the number of transmit antennas is at least three. In this embodiment of the invention, the predetermined identifier and the cell identifier are transmitted on a first symbol and at least one training sequence is transmitted on a second symbol. For example, the information bearing unit includes a first training sequence and a second training sequence. If the number of transmit antennas is equal to three, the first training sequence helps obtain the channel estimates corresponding to the first transmit antenna and the second training sequence is used to obtain the channel estimates corresponding to a third transmit antenna. Further, the sub-carriers on which the first sequence is transmitted is shifted by three (if the number of the sectors is equal to three) with respect to the sub-carriers on which the predetermined identifier is transmitted. Also, the sub-carriers of the second symbol on which the third transmit antenna transmits are shifted by three with respect to the sub-carriers of the second symbol on which the first transmit antenna transmits. For this case, the pilot spacing for the first transmit antenna is three, and for the second transmit antenna and third transmit antenna is six. If the number of transmit antennas is equal to four, the first training sequence is used to obtain the channel estimates corresponding to a third transmit antenna and the second training sequence is used to obtain the channel estimates corresponding to a fourth transmit antenna. In this case, each transmit antenna has a pilot spacing of six sub-carriers.
In various embodiments of the invention, the pilot spacing of the transmit antennas is good and there is no interference from the neighboring sectors or from different transmit antennas of the same sector. Therefore the quality of the channel estimation is good.
In various embodiments of the invention, each identifier within the predefined set is selected such that the Peak-to-Peak Average Ratio (PAPR) of the signal of the transmit antenna from which it is transmitted is low. This allows the transmit power on the transmit antenna corresponding to the symbol on which the predetermined identifier is transmitted, to be increased, thereby improving coverage and performance. For example, when the number of sectors is equal to three, and the Fast Fourier Transform (FFT) size is 1024, and the left and the right guard band size is 87, the predetermined identifier can be chosen to have a PAPR for the transmit signal corresponding to the symbol as low as 3.5. Similarly, the other sequences such as the cell identifier and training sequences can also be chosen to have a low PAPR.
Each base station includes at least one transmit antenna 515. The system for synchronizing receiver 505 includes at least one transmit antenna 515, an offset estimator 520, an identifier module 525 and a transmit antenna determining module 530.
At least one transmit antenna 515 transmits an information bearing unit. The information bearing unit includes a cell identifier and a predetermined identifier. Offset estimator 520 obtains at least one of timing offset estimate and frequency offset estimate using the predetermined identifier. In an embodiment of the invention, offset estimator 520 obtains at least one of timing offset estimate and frequency offset estimate based on the predetermined identifier, if the predetermined identifier is known to the receiver.
In an embodiment of the invention, offset estimator 420 also corrects for obtained offset estimates.
In an embodiment of the invention, offset estimator 520 obtains at least one of timing offset estimate and frequency offset estimate along with the predetermined identifier. The predetermined identifier is selected from a predefined set. Transmit antenna determining module 530 determines the number of transmit antennas 515 of a base station 510 based on the predetermined identifier. This has been explained in detail in conjunction with
Identifier module 525 identifies the cell identifier based on the estimated at least one of timing offset estimate and frequency offset estimate.
In various embodiments of the invention, offset estimator 520, identifier module 525 and transmit antenna determining module 530 reside on receiver 505. In an embodiment of the invention, these modules may interact with one another. In an embodiment of the invention, offset estimator 520, identifier module 525 and transmit antenna determining module 530 can be integrated into a single module.
The various embodiments of the invention provide a method and system that simplifes synchronization of a receiver. Further, the various embodiments of the invention provide a method and system in which the symbol corresponding to the predetermined identifier is periodic in time domain at each point in the cell as received by a receiver. This further simplifies the method of synchronization.
The various embodiments of the invention provide a method and system in which both the synchronization of the receiver is simplified and good quality channel estimates can be obtained even in the presence of multiple transmit antennas and inter-cell interference.
The various embodiments of the invention provide a method and system that enable a receiver to determine a number of transmit antennas of a base station based on a predetermined identifier. As a result, the control information can be transmitted using a plurality of transmit antennas. Further, this enables hand-off between base stations comprising different number of transmit antennas.
This application claims the priority date of U.S. Provisional Ser. No. 60/591,318 filed Jul. 26, 2004, entitled “PREAMBLE STRUCTURES FOR MOBILE AND FIXED CELLULAR SYSTEMS”.
| Number | Date | Country | |
|---|---|---|---|
| 60591318 | Jul 2004 | US |
