MXPA97007144A - Method and device for the communication of movib user data - Google Patents

Abstract

The present invention relates to a high-speed wireless transmission system that can be employed in a macrocell environment. In the system employ multiple transmission antennas. Multiple bearer tones are used to transmit the data. The bearer tones can be assigned to the respective transmit antennas in a manner such that each antenna is provided with a subset of bearer tones with each subset that is broadcast over the transmission spectrum. In addition, the operation is improved by providing Reed-Solomon coding of the data through consecutive time intervals.

Description

METHOD AND APPARATUS FOR THE COMMUNICATION OF DATA OF MOBILE USERS BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for facilitating data communication of mobile users, such as high speed data. The invention relates specifically to a new arrangement for assigning bearer tones to a plurality of antennas and to a coding technique for providing reliable, high-speed wireless access to mobile users in macrocells. As more and more people are becoming dependent on wireless communication and as the use of the Internet is also becoming more popular, it is becoming desirable to provide the ability for mobile users using wireless devices to access multiple media such as to Internet. However, effective multi-media access requires a high-speed communication capability such as, for example, a bit rate of 1 to 2 Mbps. Re.025651 It is already commonly known to provide a wireless data system with high data transmission rates over a short distance such as in a wireless LAN environment. A U.S. patent application provisional, co-pending, entitled CLUSTERED OFDM ITH TRANSMITTER DIVERSITY AND CODING, describes a technique for providing such a wireless LAN of high bit rate. In such a technique an input data stream is encoded to allow error correction / erasure in a receiver. Then, a multi-carrier (or multi-tone) signal is formed. For multicarriers, the basic idea is to divide the transmitted bandwidth into many narrow subchannels that are transmitted in parallel. Each subchannel is then modulated at a very slow speed to avoid significant intersymbol interference (ISI). The method described uses Multiplexing Orthogonal Frequency Division (OFDM), a multiplexing technique described for example in, "Data Transmission by Frequency-Division Multiplexing Using the Discrete Fourier Transform" by Weinstein et al., IEEE Trans. Commun. Technol. Vol. COM-19, No. 5, October 1971, pp. 628-634 and "Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come", by Bingham, IEEE Commun. Mag., Vol. 28, No. 5, May 1990, pp. 5-14. In the method described in the provisional application the groups of adjacent tones are grouped together and the separate groups are provided at different tones of a plurality of separate independent antennas. A single reception antenna is then used to demodulate the OFDM signal with conventional techniques. A user data system that uses mobile devices has particular problems which limit the ability to provide access to high-speed multiple media. The main damage encountered in a mobile radio environment is diffusion of the delay, an apparent change in tone and path loss as represented by the reduced power of the received signal. Delayed broadcasting refers to the fact that because the signal will experience a wireless path that will have different impacts on different frequencies, it is likely that the entire signal will not be received at the receiver at the same instant in time. A delay will be introduced. The spread of the delay in the macrocell environment could be as large as 40 μsec, which could limit the data transmission rate to about 50 Kbaud if no action is taken to counteract the resulting ISI. In the 2 GHz PCS bands, the apparent change rate of the tone could be as high as 200 Hz (ie, a mobile unit moving at approximately 107.8 kph (67 mph)). In addition, the power of the received signal is inversely related to the data transmission speed in such a way that, for example, at a data transmission rate of 1 Mbaudio (approximately 50 times that of a typical voice circuit), there is a deficit of at least 15 dB in the received power compared to cellular voice signal services and this creates a connection provision problem. Therefore, without any modification of the system, the coverage and operation of such systems will be severely limited. In fact, in the present wireless systems covering a large area with mobile receivers, bit transmission rates of 10 to 20 Kbps have been reached. Therefore, it is desirable to adapt the wireless transmission systems to facilitate the communications of the data at a high speed.

BRIEF DESCRIPTION OF THE INVENTION The present invention achieves the wireless transmission at high speed, desired, modifying the system to correct the effects of the diffusion of the delay and the loss of the trajectory. The present invention proposes an asymmetric service: a high-speed connection or downlink (for example, data transmission rates of 1 to 2 Mbps of the maximum point or more) and a higher link or connection of bit rate more slow (for example 50-100 Kbs). This could alleviate the problem of increasing the power consumption in the mobile terminal to overcome the 15 dB deficit in the received power. However, it should still be sufficient for most applications, such as leafing or browsing, access to voice signals, email, and interactive computing. In addition, the present invention provides an Orthogonal Frequency Division Multiplexing (OFDM) system having sufficiently narrow sub-channels and a sufficient protection period to minimize the effects of delay broadcasts as large as 40 μsec. To overcome the 15 dB deficit in the provision of the connection, the present invention provides a variety of transmission antennas and the coding of transverse frequencies. In one example, the base station has four transmit antennas. Each antenna is assigned to transmit a subset of the total number of tones. A particular subset is composed of a plurality of widely spaced tones that cover the bandwidth of the complete transmission. As a consequence, a subset of tones on a second antenna will include tones between those transmitted on the first antenna. Alternatively, each subset of tones for a given transmission antenna may include groups widely spaced in tones, eg, two or three adjacent tones, which cover the bandwidth of the entire transmission. The scattering or diffusion of the tones over the transmit antennas randomly fading across the OFDM bandwidth. The coding is also selected to help reduce the problem of provision of the connection or link. The digital data is encoded using the Reed-Solomon (R-S) coding where the words of the symbols within the codewords are created by grouping modulation symbols in time that are consecutive in time. The coding uses a combination of the erasure correction, based on the strength or intensity of the signal, and a correction of the error. When the technique of assigning the tone antenna and the coding operation are combined, the problem of the provision of the connection or link is substantially alleviated.

In the alternative modalities, the mobile station may include the diversity of the reception antennas. Also, the assignment of tones to the transmit antennas can be arranged in such a way that the same tone is transmitted simultaneously by two or more antennas. In yet another modification, the tones assigned to a given antenna can be changed over time so that the effect of any negative correlation between a given tone and a given transmission pattern from a transmitting antenna to a receiving antenna can be minimized BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 (A) and 1 (B) respectively, illustrate the possible configurations of a transmitter and a reception station in a wireless LAN environment. Figure 2 illustrates a possible frequency characteristic of a given transmission path from a transmission antenna to a reception antenna. Figure 3 illustrates in block diagram form one embodiment of the present invention.

Figure 4 illustrates a graphic representation of a second aspect of the present invention. Figure 5 illustrates, in block diagram form, a more detailed embodiment of the present invention. Figure 6 illustrates how the embodiment of the present invention operates for improvement during the deficits of the provision of the connection or link.

DETAILED DESCRIPTION An example of a wireless transmission system in a LAN environment such as the one described in the provisional application referred to above is illustrated in Figures 1 (A) - (B). A stream of data bits is provided to an encoder 101 which produces a plurality of symbols. In this case the encoder produces N x M symbols. No tones are assigned to each of the M antennas, for example, 104? . . . 104M. The first N tones are provided to the IFFT (Inverted Fast Fourier Transformers) 102? while the Mestimo group of the N tones is provided to the IFFT 102M. Each group of tones is provided to the RF circuits (for example, filter and amplifier), for example, 103? . . . 103M and then they are passed over their respective transmission antenna 104. The total number of tones (N x M) is equal to the total number of carriers in the multi-carrier OFDM configuration. The carriers are scattered over the transmission spectrum. The graphic representations are provided adjacent to each antenna 104? to 104M to illustrate that a given antenna is assigned to a particular group of adjacent tones or carrier frequencies. Each group is part of a very localized portion of the complete transmission spectrum. The groups of the M tones are transmitted from the M transmitting antennas simultaneously and received by the receiving antenna 110. As can be seen from the adjacent graphic representation, the antenna receives all the groupings simultaneously. The antenna provides the received multi-carrier signal to the RF circuits 111 which then provide the processed signal to the FFT 112. The resulting data corresponds to the N x M symbols produced by the encoder 101 in Figure 1 (A) and the decoder 113 receives these symbols and provides the current of the data bits as an output. What is shown in the graphic representation is that the receiving antenna 110 can receive different frequencies at different intensities.

In the propagation of multipaths of mobile environment a significant problem for such a configuration can be raised in such a way that certain of the frequencies can be seriously vanished so that they are separated in an essential way. It is considered that the channel of one of the transmit antennas for the receiving antenna may be different from the channel of another transmit antenna for the receiving antenna. These are considered separate and distinct trajectories. Each trajectory has its own frequency response characteristic. For example, as illustrated in the graphical representation of Figure 2, a first trajectory can more successfully transmit frequencies in the interval of f2 while it has to more difficult transmit frequencies in the interval of fo and fi. Accordingly, the inventors recognized that for a number of tones in the region of f0 (or fi) if these tones are grouped together and are the only group provided along antenna 1, then the signal from antenna 1 either it will be difficult to detect in the receiver or it will probably contain many errors due to the characteristics of the trajectory. To remedy this problem, the inventors propose to provide a diffusion or dispersion outside the carrier tones through the transmission spectrum. This will counteract any dependence on the frequency that a particular trajectory might have, optimizing the probability that each trajectory will substantially transmit correct and useful information. As illustrated in a first embodiment of the present invention shown in Figure 3, a sequence of data bits is provided to a modulator 301 that creates a modulator symbol. The symbols of the modulator are grouped, coded and subsequently decomposed by the elements 302, 303 and 304 as will be described in further detail below. The resulting data stream is converted to a parallel data stream by a serial to parallel converter 305. As an example, 120 data symbols are provided in parallel (Xo to XHT). Where the symbols are modulated by QPSK, each consists of two bits. Other modulators can be used to create either symbols such as 8-PSK symbols. A distributor receives this data block of 120 parallel symbols. Each of the 120 symbols corresponds to one of the 120 bearer tones that are to be used in multi-carrier OFDM configuration. The distributor can send groupings of symbols to each of the M IFFTs (307? To 307M). In the present example, the distributor sends groups of five symbols corresponding to five bearer tones to each of the IFFTs. In the present example, it is proposed that M = 4 so that there are four transmission antennas (309? To 309M) that provide four separate paths to a single receive antenna. The distributor therefore provides thirty symbols of the 120 symbols to each of the trajectories of the transmitting antenna. This is done in the groupings of five symbols that are scattered or spread over the entire transmission spectrum, that is, the IFFT 307i receives the symbols in X0 through X, X2 or a? >; X4o to X «•. . Xioo to X? 0 Similarly, the remaining IFFTs also receive the symbol groups assigned to the bearer tones that are broadcast over the transmission spectrum. Therefore, as shown in the graphic representations associated with the antennas 309? and 309M in Figure 3, the first antenna sends out six groups of tones. These tones are spread over the entire transmission spectrum. As you can see the six groups transmitted by the 309M antenna are interspersed with the groups transmitted by 309 ?.

Although not shown, the groupings for 3092 and 3093 (where M = 4) are also interspersed with the groupings of tones that are going to be transmitted on the other antennas.

Summarizing then, the problem of the dependence of the frequency of a given path from an antenna to the receiving antenna and the susceptibility of the path to adversely affect the characteristic of the complete transmission when only transmitting a group of the multiple carrier tones, are overcome by providing a subset of the carrier tones to each of the antennas where the subset for a given antenna is broadcast over the entire transmission spectrum. As a consequence, not all tones on a given antenna are adjacent to each other. In effect, where the tones on the antenna 1 are not adjacent to each other (for example, the tone for X and the tone for X2o) there are intermediate tones which are supplied by some different ones of the transmit antennas. Modifications to this arrangement may be desirable. For example, in the example described above the distributor 306 receives 120 symbols and divides them among four transmitters. Each group within a subset of tones can be constituted by a single tone instead of a group of adjacent tones. Therefore, a possible modification to the arrangement of Figure 3 could assign the tones corresponding to the symbols Xo, X, Xß, X12 / Xiß etc., to the antenna and the tones for the symbols Xi, X5, Xg, X13, etc. ., to antenna 2 and etcetera. This arrangement must achieve a substantially similar result because the improvement arises from the external dispersion of the tones for a given antenna over the entire transmission spectrum and interleaving the tones carried by the various tones of the antennas. In another modification to improve the strength of the signal received at the receiver, it might be appropriate to send the same signals on multiple transmitters. In this case, it is conceivable to use, for example, eight transmit antennas in which each antenna is separate and distinct, and therefore provides different trajectories each having its own characteristics. Then, the same configuration that was described with respect to Figure 3 could be used with the change that is that the same output current provided at 309? it could also be provided to another antenna so that two transmit antennas could be responsible for transmitting the symbols Xo to X, X20 to X2 and so on. This could improve the complete or total reception characteristics. In yet another modification to this design it is possible to vary the tone assignment between the transmit antennas. As an example, when the path associated with the transmission antenna 309? have characteristics which are adverse to the tones for symbols Xo to X, this problem can be alleviated by rotating the assignment of the groups of the tones between the various antennas. Therefore, in a first case a first block of 120 symbols must be assigned in the manner illustrated in Figure 3. A second block could be transmitted with a different set of tone assignments, for example, 309? that receive tones for the symbols X15 to X? 9, X35 to X39, etc. This change in the allocation of the tones to a given transmission antenna helps to avoid the potential adverse impacts of an antenna transmission characteristic, given, during any of the tones or groups of carrier tones. This could be done by inserting a switching arrangement between the IFFTs (307? -307M) and the RF circuits so that the IFFts are alternately assigned to the respective antennas. Of course, a person with ordinary skill in the art could recognize that given this description of several modifications to the first embodiment of Figure 3, combinations of these modifications could also be possible. For example, the rotation of the carrier tones between the transmit antennas could also be effected when individual tones are transmitted instead of the tone groups.

The inventors also recognize that the way in which the coding is done, can have a positive influence on the speed of error of the words, to solve by means of this additionally the problem of the provision of the link or connection. In particular, the inventors have selected the Reed-Solomon (R-S) coding. In an example of such coding scheme each RS symbol is constituted by six information bits and the RS block is constituted by a predetermined number of RS symbols with a certain subset of these symbols which is addressed to the data symbols and the remaining ones are directed to parity symbols. As it is known, if there is a bit of an RS symbol which is erroneous or if there are multiple bits of the erroneous RS symbols, two parity symbols are taken to correct each erroneous RS symbol unless the location of the RS symbol is is wrong is known. In this last circumstance such an R-S symbol is considered an erasure and only a parity symbol is necessary to correct such an error. To improve the performance of the data it is desirable to keep the number of parity symbols low. However, in order to achieve this goal it is beneficial to construct the RS data symbols in a way that maximizes the concentration of the errors, that is, instead of externally spreading the errors of the data bit over the multiple symbols, it is desirable to increase the probability that these bits that are erroneous will be in the same RS symbol. The inventors have determined that the optimal way to concentrate these bit errors is to group the modulator symbols in time instead of by frequency. As shown in the example of Figure 4, there are three blocks of multi-carrier signals displayed at different times, each with a time width of 200 microseconds. The frequencies fi a f3 correspond to the multicarriers on the transmission spectrum. The inventors discovered that it is beneficial to construct a given R-S symbol, for example R-Sx of the three symbols of the same frequency over the three consecutive periods of time. Accordingly, for example, R-Sx could be comprised of the frequency fx at time tx, fx at time t2 and fx at time t3. The actual construction of these Reed-Solomon symbols and the code words are described in relation to Figures 3 and 5. As illustrated in Figure 3, the modulator symbols at the output of the modulator are provided to a symbol grouper 302 An example of a sequence of data bits is shown at 51 in Figure 5. In a more specific embodiment of the present invention, modulator 301 is a QPSK modulator (Key or Shift Key of the Quadrature Phase). In this mode the modulator converts a block of 360 bits of data into 180 2-bit symbols (do to d? G). Each R-S symbol is six bits long, therefore three QPSK symbols can be grouped to form a single R-S symbol. According to the discovery of the inventors who consider the grouping in time of the symbols as illustrated in figure 4, three consecutive symbols in time instead of frequency can be grouped together to form the symbol R-S. For example, where there are 180 two-bit symbols, there are three blocks of sixty two-bit symbols: d0 to ds9 at the time of transmission ti, d or to dng at the time of transmission t2, and? 2o to d? in the transmission time t3. Therefore, a grouping in time of three 2-bit symbols to create an R-S symbol could be done by grouping the symbols d0, d6o and d? 2o. The R-S coding shown in Figure 3 could lead to three sets of forty R-S symbols with each set containing 20 data symbols and 20 parity symbols. The decomposition device 304 could then reconfigure the QPSK symbols within the R-S words in time, to create blocks of time of the transmission symbols, for example, z0 to Z59, z6o to Z119, and Z120 to z? g. A serial to parallel converter 305 takes the stream of symbols and creates a parallel configuration of 120 symbols at a moment in time. A distributor 306 then divides the 120 symbols between the multiple transmit antennas according to the assignment of the bearer tone for that given antenna according to the above descriptions. Accordingly, in this exemplary arrangement there are 120 tones with a block size of 160 μsec and a protection of 40 μsec. This leads to subchannels that are spaced by 6.25 kHz, block rates of 5 kbaud, and a total speed of 600 kbaud or equivalent bit rates of 1.2 Mbps in the channel for QPSK. The combination of the coding technique with the assignment of tones to the various transmission antennas has shown an ability to substantially overcome the problem of provision of the connection or link described above. As shown in Figure 6 where the RS encoder provides words of 40 symbols, each word includes 20 parity symbols with 20 data symbols grouped in time, a desired error rate of the WER words, of 1%, requires a signal-to-noise ratio of less than 8.5 dB instead of 17 to 20dB which is typically required for cellular systems. This represents a reduction of approximately 9dB in the deficit of the provision of the connection or link described above. This significantly improves the ability to transmit high-speed data in the wireless environment. In relation to the actual detection of the error at the receiving end, and with the aim in mind of maximizing the use of the parity symbols, it is possible to designate a percentage of the parity symbols such as the one related to the erasures for correction. and the rest that are aimed at correcting errors. For example, where there are 20 parity symbols it is possible to correct ten erasures (one symbol per erasure) and five errors (two symbols per error). To carry out this purpose in a given R-S word, the algorithm may designate that the ten least powerful R-S symbols may be treated as erasures and corrected as such. Then five additional errors could be corrected if they existed in any of the remaining R-S symbols. Another criterion for estimating that an erasure has occurred can be employed, such as measuring the speed of bit errors or using an "internal code" (an error detection code) to detect where errors occur. The exemplary embodiments illustrated in Figures 3 and 5 can be constructed using well-known components. First, the R-S encoder may be replaced by the Reed-Solomon Error Correction Device sold by Advanced Hardware Architectures of the State of Washington. The functions of signal processing can be implemented in a DSP which provides the IFFTs as is well known. The grouping and the decomposition in time can be implemented using separating elements or dampers. For example, to group a buffer or separator element in time it could store 120 2-bit symbols and then these symbols could be read externally in a predetermined time order. Similarly for the decomposition element, the words of the R-S code could be stored in a separating element and the individual 2-bit symbols within the codeword could be read in a predetermined order. Also, the distributor can take the form of a demultiplexer in which it takes the symbols from the Serial / Parallel converter and passes the selected symbols to the selected IFFTs which is simply the division of the information between the channels, a common demultiplexing function. To further improve the reception characteristics in the mobile station, it is possible to use multiple antennas, for example, two. The signals from two antennas can then be combined to further reduce the probability that any significant number of carrier tones will be received insufficiently. In what has been given above, the applicants have described two techniques that can be employed in relation to the wireless transmission of the data to increase the transmission speed of the bits, especially the assignment of carrier tones to multiple transmission antennas with the tones carriers assigned to any antenna, which are broadcast over the transmission spectrum and a particular type of coding technique. These two aspects can be used separately or they can be combined together to further improve the bit rate that can be achieved.

It is noted that in relation to this date the best method known by the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Having described the invention as above, property is claimed as contained in the following

Claims (20)

1. A method for high-speed wireless transmission of data, over a transmission spectrum, characterized in that it comprises the steps of: creating a data stream; encoding the stream of the data to create a plurality of symbols; assigning each of the plurality of symbols to one of a plurality of bearer tones; providing each of the bearer tones to one of a plurality of transmit antennas, in such a way that each antenna receives a subset of the plurality of bearer tones and each subset of the plurality of bearer tones includes at least two non-adjacent bearer tones each other in the transmission spectrum and having at least one carrier tone therebetween which is provided to the other of the plurality of transmit antennas; and simultaneously transmitting the subsets of bearer tones from the plurality of transmit antennas.

2. The method according to claim 1, characterized in that the step of coding comprises the substeps of: grouping the data in the stream to create multiple bit coding symbols, each coding symbol contains a plurality of modulation symbols grouped in the weather; and generating a codeword from a plurality of coding symbols.

3. The method according to claim 2, characterized in that the generation step generates a Reed-Solomon code.

4. The method according to claim 1, characterized in that in a subset of the plurality of the carrier tones, the carrier tones are distributed uniformly over the transmission spectrum.

5. The method according to claim 4, characterized in that the coding step comprises the substeps of: grouping the data in the stream to create multiple bit coding symbols, each coding symbol contains a plurality of modulation symbols grouped in the weather; and generating a codeword from a plurality of coding symbols.

6. The method according to claim 4, characterized in that the generation step generates a Reed-Solomon code.

7. The method according to claim 1, characterized in that in a subset of the plurality of the carrier tones, the carrier tones are distributed over the transmission spectrum in a non-uniform manner.

8. The method according to claim 7, characterized in that the step of coding comprises the substeps of: grouping the data in the stream to create multiple bit coding symbols, each coding symbol contains a plurality of modulation symbols grouped in the weather; and generating a codeword from a plurality of coding symbols.

9. The method according to claim 8, characterized in that the generation step generates a Reed-Solomon code.

10. The method according to claim 1, characterized in that at least two subsets of the plurality of bearer tones on the separated antennas, include the same bearer tones.

11. The method according to claim 1, characterized in that for a given of the plurality of transmission antennas, a first subset of bearer tones in a first time and a second subset of bearer tones in a second time, include different bearer tones.

12. A method for wireless transmission of data at high speed, over a transmission spectrum, characterized in that it comprises the steps of: creating a digital data stream; grouping the digital data into the stream to create multiple bit coding symbols, each coding symbol contains a plurality of modulation symbols grouped in time; generating a codeword from a plurality of coding symbols; assigning each of the modulation symbols to one of a plurality of bearer tones; and providing each of the bearer tones to one of a plurality of transmit antennas; and transmitting the plurality of bearer tones from the plurality of transmit antennas.

13. The method according to claim 12, characterized in that each of the plurality of transmit antennas is provided with a plurality of bearer tones.

14. The method according to claim 12, characterized in that the generation step generates a Reed-Solomon code.

15. The method according to claim 13, characterized in that the generation step generates a Reed-Solomon code.

16. A high-speed wireless transmission system that includes a plurality of transmit antennas, the system is characterized in that it comprises: a modulator that receives a stream of data and creates a plurality of modulated symbols; an encoder coupled to the modulator and receiving the plurality of the modulated symbols and outputting the encoded words; a separator coupled to the encoder that receives the codewords as an input and assigns modulated symbols in the codewords to the plurality of transmit antennas; a transmitter coupled to the encoder and receiving the coded symbols assigned to an antenna associated with the transmitter and providing a data transmission signal that includes a plurality of bearer tones not adjacent to the associated transmit antenna.

17. The system according to claim 16, characterized in that the plurality of non-adjacent bearer tones comprises: a subset of a plurality of bearer tones and wherein at least one bearer tone that is between non-adjacent bearer tones of the subset is assigned to another one of a plurality of transmission antennas.

18. The system according to claim 16, characterized in that the plurality of non-adjacent bearer tones comprises a subset of bearer tones that includes two groups of adjacent bearer tones.

19. The system according to claim 16, characterized in that the modulated symbols are assigned to the plurality of transmitters in a manner that provides a uniform distribution of symbols over the bandwidth of the transmission.

20. The system according to claim 16, characterized in that the modulated symbols are assigned to the plurality of the transmitters in a manner that provides a random distribution of the symbols over the bandwidth of the transmission.