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Ofdm pre-equalizingRelated Patent Categories: Pulse Or Digital Communications, Transmitters, Plural DiversityOfdm pre-equalizing description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060109932, Ofdm pre-equalizing. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The present invention relates to a method for adjusting the transmission characteristics of subcarriers of a multicarrier transmission system using a plurality of antenna elements, to a computer software program product for implementing such a method when run on a computing device of a transmitting device, a transmission diversity device as well as to a base station comprising such a transmission diversity device. [0002] To reduce interference from other transmitters, the transmission power should be as small as possible in any wireless transmission system. The background of the present invention is the reduction of transmission power in multicarrier systems and particularly the reduction of transmission power of OFDM wireless (LAN) systems. According to these systems a plurality of antenna elements share the transmission power. The transmission power of each subcarrier of the multicarrier transmission system can be adjusted such that the receiver can get a flat spectrum. In case sub-carriers are fading at all the antenna elements, these subcarriers should be transmitted with less power. [0003] From EP 0 932 285 A2 a technique for the reduction of instantaneous maximum power in multicarrier signals is known. According to this technique from a serial-to-parallel converting part, complex information subcarrier signals based on an information bit sequence are outputted to a fast inverse Fourier transforming part. The fast inverse Fourier transforming part gives fast inverse Fourier transformation to inputted sub-carrier signals to obtain a complex baseband time waveform of OFDM symbols. A complex redundant subcarrier signal generating part generates complex redundant sub-carrier signals to reduce amplitude of a complex baseband time waveform of OFDM symbols, and makes them undergo fast inverse Fourier transformation together with complex information subcarrier signals. With these processes, the amplitude of a complex baseband time waveform of OFDM symbols is reduced and instantaneous maximum power is also reduced. [0004] From WO 97/26743 a multicarrier communication system and method for peak power control is known. According to this technique a communication device for simultaneously transmitting information on multiple sub-channels encodes information for each of the multiple sub-channels with a coding scheme to produce channel encoded information. A mask vector derived from a redundancy in the coding scheme encodes the channel encoded information to transform the channel encoded information into codewords having pairwise Euclidean distance properties identical to those of the channel encoded information. Modulation of the sub-channels in accordance with the codewords in a modulators then produces a composite signal envelope having a peak-to-mean envelope power ratio reduced relative to a power ratio for correspondingly modulated channel encoded information. [0005] From EP-0 881 782 A2 a single carrier maximum-ration synthetic transmission diversity device as shown in FIG. 8 is known. According to this known transmission diversity device antenna elements are arranged at intervals greater than .lamda./2. A signal received by an antenna element is sent by way of an antenna multiplexer to a receiver, where the signal is demodulated. The thus-demodulated signal is sent to a phase- and power detection section, where a phase and power of the signal are detected. On the basis of the result of such detection, a control section calculated the phase and power of a transmission signal. On the basis of the result of the calculation, a transmission signal generation circuit transmits a transmission signal to each of the antenna elements by way of the antenna multiplexer. [0006] Note that the technique of EP 0 881 782 A2 claiming the calculation of the phase of a signal of each antenna cannot be applied to the multicarrier case, but only to a single carrier case, as it is impossible to measure phases of received signals if there are more than two carriers. [0007] In single carrier applications the phase of the signal changes frequently as the symbols are transmitted serially. Therefore it is difficult to compare phases between different antennas, as the phase is not varying uniformly. Therefore in singles carrier applications a phase comparison is preferably done using pilot symbols which phases are varying uniformly or which are known. [0008] From U.S. Pat. No. 5,973,642 adaptive antenna arrays for orthogonal frequency division multiplexing systems (OFDM systems) with co-channel interference is known. According to this known technique parameters for adaptive antenna arrays in OFDM systems with co-channel interference are estimated. The channel parameter estimation is performed using a two pass process that advantageously expands the temporal scope and considers past, present and future temporal channel estimations during parameter estimation. Channel parameters are estimated by processing the signals through fast Fourier transforms, temporal filters and inverse fast Fourier transforms. The temporal filters optimize parameters estimation based upon instantaneous correlation of the received signals. This all takes place on the receiver's side of the OFDM system. [0009] The technique of transmission antenna selection of OFDM subcarrier using power measurement of received subcarriers in a TDD system is known of the Japanese patent 11(1999)-205205 of NTT Docomo. [0010] High speed radio systems use a very wide radio bandwidth. Therefore, the channel characteristics cannot be flat for all subcarriers over the wide radio band even in short delay environment. Therefore, usually it is not possible to use a complicated modulation scheme such as 16 or 64 QAM for all subcarriers. On the other hand, any radio communication system suffers from interference from other communication transmitters. [0011] Principally a subcarrier which is in fading can be transmitted with stronger power. This solves the bit error problems, but causes interference with other receivers and therefore reduces the total capacity of the system. [0012] In view of the above-captioned prior art and problems it is the object of the present invention to propose a technique which is particularly adapted for lowering the transmission power in a multicarrier system such as the OFDM system without decreasing the overall bit rate. [0013] This object is achieved by means of the features of the independent claims. The dependent claims develop further the central idea of the present invention. [0014] According to a first aspect of the present invention therefore a method for adjusting the transmission characteristics of subcarriers of a multicarrier transmission system using a plurality of antenna elements is proposed. On the transmitting side the subcarrier frequency channel characteristics of the multicarrier transmission is detected. Then the power of each subcarrier is distributed to the antenna elements by multiplying (weighting) it (by) the complex conjugate of the corresponding subcarrier frequency channel or [0, 1] value according to the decision which antenna is selected for the corresponding subcarrier to make the communication more efficient. Therefore depending on the detected frequency channel characteristics at each antenna element for each subcarrier signal the antenna element having the best channel characteristics for said subcarrier signal can be used for transmission. [0015] The power of the subcarriers can furthermore be pre-equalized on the transmitter side by dividing the sub-carrier signals respectively by the sum of the squared magnitude of the frequency channel characteristics of all antenna elements or by the magnitude of the frequency channel characteristics of the selected antenna. [0016] Along with the pre-equalizing furthermore the phase of the subcarrier signals can be respectively compensated for at the transmission side according to the detected frequency channel characteristics. [0017] Alternatively or additionally, the phase can be compensated for at the receiving side. [0018] The pre-equalization of the power of the subcarrier signal can be limited to an upper threshold value. [0019] In case along with the pre-equalization the upper threshold is reached for a subcarrier signal, the transmission power of the corresponding subcarrier signal is fixed at the upper threshold value. Instead of further raising the transmission power the modulation scheme for said subcarrier is adapted. [0020] The adaptation of the modulation scheme of a subcarrier signal can be signaled to the receiving side. [0021] To adapt the modulation scheme of a subcarrier signal, the modulation scheme can be simplified or the subcarrier signal can even not be modulated at all. [0022] In case in a modulation scheme a subcarrier signal is adapted such that the bit rate of the subcarrier signal is reduced, the modulation scheme of at least one other sub-carrier signal can be changed to a more complex modulation scheme. [0023] The detection of the frequency channel characteristics can be performed on the basis of received pilot symbols. [0024] According to a further aspect of the present invention a computer software program product implementing a method as set forth above when run on a computing device of a transmitter is proposed. Continue reading about Ofdm pre-equalizing... Full patent description for Ofdm pre-equalizing Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Ofdm pre-equalizing patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. 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