This invention relates to an orthogonal frequency division multiplexing (OFDM) communication system, and more particularly to an OFDM communication system for a multiple access communication network.
Orthogonal frequency division multiplexing (OFDM) is a relatively well known multiplexing technique for communication systems. OFDM communication systems can be used to provide multiple access communication, where different users are allocated different orthogonal tones within a frequency bandwidth to transmit data at the same time. In an OFDM communication system, the entire bandwidth allocated to the system is divided into orthogonal tones. In particular, for a given symbol duration T available for user data transmission, and a given bandwidth W, the number of available orthogonal tones F is given by WT. The spacing between the orthogonal tones Δ is chosen to be 1/T, thereby making the tones orthogonal. In addition to the symbol duration T which is available for user data transmission, an additional period of time Tc can be used for transmission of a cyclic prefix. The cyclic prefix is prepended to each symbol duration T and is used to compensate for the dispersion introduced by the channel response and by the pulse shaping filter used at the transmitter. Thus, although a total symbol duration of T+Tc is employed for transmitting an OFDM symbol, only the symbol duration T is available for user data transmission and is therefore called an OFDM symbol duration.
In prior OFDM techniques, an OFDM signal is first constructed in the frequency domain by mapping symbols of a constellation to prescribed frequency tones. The signal constructed in the frequency domain is then transformed to the time domain by an inverse discrete Fourier transform (IDFT) or inverse fast Fourier transform (IFFT) to obtain the digital signal samples to be transmitted. In general, symbols of the constellation have a relatively low peak-to-average ratio property. For example, symbols of a QPSK constellation all have the same amplitude. However, after being transformed by the IDFT or IFFT, the resultant time domain signal samples are the weighted sum of all the symbols, and therefore generally do not preserve the desirable low peak-to-average ratio property. In particular, the resulting time domain signal typically has a high peak-to-average ratio.
Existing techniques for implementing OFDM communication systems can be highly inefficient due to the relatively high peak-to-average ratio when compared with other signaling schemes, such as single carrier modulation schemes. As a result, existing OFDM techniques are not well suited for a wireless multiple access communication network with highly mobile users because the high peak-to-average ratio of the transmitted signal requires a large amount of power at the base station and at the wireless device. The large power requirements result in short battery life and more expensive power amplifiers for handheld wireless communication devices or terminals. Accordingly, it is desirable to provide an OFDM technique which reduces the peak-to-average ratio of the signal to be transmitted, while simultaneously taking advantage of the larger communication bandwidth offered by an OFDM communication system.
In one aspect of the communication system, power consumption associated with generating and transmitting OFDM signals is reduced as compared to the prior OFDM systems discussed above. The OFDM signaling method includes defining a constellation having a plurality of symbols, defining the symbol duration for the OFDM communication signal, and defining a plurality of time instants in the symbol duration. In a given symbol duration, a plurality of tones in the symbol duration are allocated to a particular transmitter and the signal to be transmitted is represented by a vector of data symbols from the symbol constellation. The symbols are first directly mapped to the prescribed time instants in the symbol duration. A continuous signal is then constructed by applying continuous interpolation functions to the mapped symbols such that the values of the continuous signal at the prescribed time instants are respectively equal to the mapped symbols and the frequency response of the continuous signal only contains sinusoids at the allocated tones. Finally the digital signal, which is to be transmitted, consists of samples of the continuous signal. Alternatively, the digital signal can be generated directly by applying discrete interpolation functions to the mapped symbols. As symbols from the constellation generally have good peak-to-average ratio property, proper choices of allocated frequency tones, prescribed time instants and interpolation functions can result in a minimized peak-to-average ratio of the continuous function and the digital signal samples.
In one aspect, a digital processing system is defined that is utilized in a transmitter that generates a transmission signal having a first circuit that receives a discrete signal of complex data symbols, calculates a frequency response of the discrete signal, and outputs a first vector of length M, a second circuit that receives the vector of length M and generates a second vector of length N, wherein the second vector of length N includes elements of the first vector at positions corresponding to tones allocated to the transmitter, and a third circuit that receives the second vector and generates a digital time domain vector therefrom for transmission by the transmitter.
In one aspect, an apparatus that generates a transmission signal is defined which comprises means for receiving a discrete signal of complex data symbols, calculates a frequency response of the discrete signal, and outputs a first vector of length M, means for receiving the vector of length M and generates a second vector of length N, wherein the second vector of length N includes elements of the first vector at positions corresponding to tones allocated to a transmitter, and means for generating a digital time domain vector of the second vector for transmission by the transmitter.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.
Referring to
Next, a time instant mapping circuit 18 maps each complex symbol 16 to a prescribed time instant within a given OFDM symbol duration. The mapping operation is performed in the time domain such that the mapping circuit 18 generates a discrete signal of mapped symbols within the time domain symbol duration. The output of the mapping circuit 18 is provided to an interpolation circuit 20, that produces a series of digital signal samples {Si} 22. The digital signal samples 22 are formed by sampling a continuous signal, which is constructed by applying one or more predetermined continuous interpolation functions to the mapped complex symbols 19. Alternatively, the digital signal samples 22 are formed by directly applying one or more predetermined discrete interpolation functions to the mapped complex symbols 19. When using the technique of applying discrete interpolation functions, no intermediate continuous signal is generated and the step of sampling the continuous signal is not necessary. The operation of the interpolation circuit 20 is described in greater detail below. A cyclic prefix circuit 24 receives the series of digital signal samples 22 from the interpolation circuit 20 and prepends a cyclic prefix to the digital signal samples 22. The cyclic prefix circuit 24 operates to copy and prepend the last portion of the digital signal sample vector S 22 to the beginning of the OFDM symbol duration. The resulting digital signal samples 22 with the prepended cyclic prefix are converted to an analog signal by a digital to analog converter 28. The resulting analog signal is further processed by a pulse shaping filter 30, the output of which is modulated to a carrier frequency, and amplified by a power amplifier unit 32 for transmission through an antenna 34.
In one implementation of the OFDM communication system 10, the symbol-to-symbol mapping circuit 14, the time instant mapping circuit 18, the interpolation circuit 20, and the cyclic prefix circuit 24 are implemented in a digital signal processor (DSP) 26, and may include a combination of hardware modules and/or software modules. These circuits 14, 18, 20, and 24 can also be implemented as separate discrete circuits within the OFDM communication system 10.
The details of the interpolation circuit 20 are shown in
In one implementation, the OFDM communication system 10 is a multiple access communication system where the entire bandwidth available to all transmitters within the system is divided into F orthogonal frequency tones, f1, f2, . . . , fF. In the given symbol duration 40, a particular transmitter operating within a multiple access communication system is allocated M frequency tones fi(1), fi(2), . . . , fi(M), which is a subset of, f1, f2, . . . , fF (the total number of frequency tones) in order to transmit the signal. As part of this implementation, the number of tones allocated to a particular transmitter is equal to the number of symbols to be transmitted by that transmitter. Later in
The complex data symbols C1, C2, C3, . . . , CM 16 are first mapped to t1, t2, t3, . . . , tM, respectively, where t1, t2, t3, . . . , tM are M prescribed time instants within the time domain symbol duration 40. The mapping operation generates a discrete signal of mapped symbols. It should be noted that the number of prescribed time instants is equal to the number of symbols M to be transmitted. As described above, the symbol mapping occurs in the time domain. Continuous interpolation functions 42 are then applied to the discrete signal of mapped symbols 16 to generate a continuous function CF(t) for t in the time interval from 0 to T.
The interpolation functions 42 are constructed such that the values of the continuous function CF(t) at time instants t1, t2, t3, . . . , tM are respectively equal to C1, C2, C3, . . . , CM and the frequency response of the continuous function CF(t) contains only sinusoids at the allocated tones. Therefore, CF(t) is constructed as
where J=√{square root over (−1)} and coefficients Ak are given by
Thus, each coefficient Ak is generated by multiplying a matrix of predetermined sinusoids with the single column of data symbols C1, C2, C3, . . . , CM 16.
The output of the DSP 26 is a vector of digital signal samples S 22, which are the samples of the continuous function CF(t) at discrete time instants 0, T/N, 2T/N, T(N−1)/N, that is, S1=CF(0), S2=CF(T/N), S3=CF(2T/N), . . . , SN=CF(T(N−1)/N), where N is the number of discrete time instants in the vector of digital signal samples 22. In a general form, t1, . . . , tM may not necessarily be equal to any of the time instants 0, T/N, 2T/N T(N−1)/N. Therefore, while the digital signal samples S 22 may occur at the time instants, t1, . . . , tM, the OFDM communication system 10 does not require that the time instants 0, T/N, 2T/N . . . , T(N−1)/N be equal to t1, . . . , tM.
In another implementation of OFDM communication system 10, the digital signal samples S 22 may be generated by the DSP 26 by directly multiplying a matrix of precomputed sinusoidal waveforms Z, operating as discrete interpolation functions, with the discrete signal of mapped symbols C in order to satisfy the transformation function S=ZC according to the following:
where C represents the symbol vector, and the matrix Z represents the product of the two matrices in the second line of the above equation. Each column (i) of matrix Z represents the interpolation function 42 of a corresponding symbol Ci to generate the digital signal samples S 22. As such, the matrix Z can be pre-computed and stored in the interpolation function memory 23 of the interpolation circuit 20 (
The purpose of constructing the signal in the time domain is to directly map the symbols 16, which have a desirable low peak-to-average ratio property, to the prescribed time instants within the symbol duration 40. Appropriate interpolation functions 42 are selected to obtain the continuous function CF(t) and the digital signal samples 22 such that the desirable low peak-to-average ratio property of the symbols 16 is substantially preserved for the continuous function and for the digital signal samples 22. The peak-to-average ratio property of the resulting (interpolated) continuous function CF(t) and the digital signal samples 22 is dependent upon the interpolation functions 42, the choice of allocated frequency tones fi(1), fi(2), . . . , fi(M) from the set of tones, and the prescribed time instants t1, . . . , tM.
Referring to
Complex symbols C1, . . . , CM 16 are mapped in the time domain to the following time instants tk=(k−1)T/M, for k=1, . . . , M. As part of this implementation, the prescribed time instants ti, . . . , tM are equally-spaced time instants uniformly distributed over the entire OFDM symbol duration 40 as shown in the first time domain graph of
The second time domain graph of
Turning to
Turning to
The complex symbols C1, . . . , CM are first mapped in the time domain to time instants t1, . . . , tM respectively. Next, the mapped symbols C1, . . . , CM are leftwards and rightwards shifted and replicated to an expanded set of prescribed time instants, which is a superset of t1, . . . , tM and consists of an infinite number of equally-spaced time instants covering the time interval from −∞ to +∞. This technique creates an infinite series of mapped symbols C. The continuous function CF(t) is then constructed by interpolating the infinite series of mapped symbols using a sinc interpolation function 60. Mathematically, the above steps construct the continuous function CF(t) as
where sinc(a,b)=sin(πa/b)/(πa/b). The sinc interpolation function 60 can also be precomputed and stored in the interpolation function memory 23. As discussed in
For comparison purposes,
In this case where the allocated tones are equally-spaced tones, the constructed continuous function CF(t) is identical in each of the L time intervals, [0,T/L), [T/L,2T/L), . . . , and [(L−1)T/L, T/L). As part of this technique, symbols C1, . . . , CM 16 are mapped to the following time instants tk=(k−1)T/M/L, for k=1, . . . , M. In this implementation, the prescribed time instants t1, . . . , tM are equally-spaced time instants uniformly distributed over a fraction (1/L) of the symbol duration 70. As a comparison, in the case of allocated contiguous tones (
The procedure for obtaining the digital signal samples S 22 described in
In
With continued reference to
Referring now to
With reference to
At an OFDM symbol duration, if the digital signal samples S1, S2, . . . , SN have been obtained using the method as described in
In one implementation, the value of d is determined by
where L is discussed in
For example, d can be the integer closest to
the largest integer not greater than
or the smallest integer not smaller than
In one example, d is chosen to be the largest integer not greater than
This example can be easily extended for other choices of d.
The digital signal sample vector S′ is then passed to the cyclic prefix prepender circuit 24, as shown in
Not specifically shown in
which is a fraction of a sample period T/N.
As a variation of the technique shown in
A first series of digital signal samples 22 are generated using the technique of
is changed to
In the block diagram method described with regard to
Referring to
For example, if the communication system 10 is allocated M+Mex contiguous frequency tones, fi(1), fi(2), . . . fi(M+Mex), and M symbols C1, . . . , CM of the constellation are to be transmitted at an OFDM symbol duration, from the comparison of
for k=1, . . . , M+Mex. For purposes of description, let f0=0. The construction for the other cases where f0≠0 can be similarly obtained.
As with the technique described with respect to
As part of this technique shown in
for any frequency f, where Δ is the spacing between adjacent tones.
A′k=Ag(k)*P((k−1)Δ+f1)
where index g(k)=mod(k−i(1)−Mex/2, M)+1, for k=1, . . . , N.
At block 88, the digital signal sample vector S is obtained by taking an N-point inverse discrete Fourier transform (IDFT) of the new vector A′. Finally, the cyclic prefix is added by cyclic prefix circuit 24 as described above with regard to
To provide additional insight to the above signal construction technique, assume that the allocated tones fi(1), fi(2), . . . , fi(M+Mex) are centered at frequency 0. In
CF(t)=Σi=1MCiΣk=−∞∞p(t−tikT)
where p(t) 92 is the time domain response of P(t) 90. As with
In one exemplary aspect of this technique, if a raised cosine windowing function is used, i.e.,
where β=(Mex+2)/M represents the percentage of excess tone overhead, then, the interpolation function p(t) 92 is given by
As β increases, the interpolation function p(t) 92 decays faster, thereby reducing the probability of having large peak at samples between ti.
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.
The present application is a continuation of U.S. patent application Ser. No. 12/429,896, entitled “SIGNALING METHOD IN AN OFDM MULTIPLE ACCESS SYSTEM”, filed on Apr. 24, 2009, now pending, which is a continuation of U.S. patent application Ser. No. 11/186,490, entitled “SIGNALING METHOD IN AN OFDM MULTIPLE ACCESS SYSTEM”, filed on Jul. 20, 2005, now U.S. Pat. No. 7,623,442, which is a divisional of U.S. patent application Ser. No. 09/805,887, entitled “SIGNALING METHOD IN AN OFDM MULTIPLE ACCESS SYSTEM”, filed on Mar. 15, 2001, now U.S. Pat. No. 7,295,509, which claims priority to U.S. Provisional Application No. 60/230,937, entitled “SIGNALING METHOD IN AN OFDM MULTIPLE ACCESS WIRELESS SYSTEM”, filed on Sep. 13, 2000, all of which are assigned to the assignee hereof, and expressly incorporated herein by reference.
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Number | Date | Country | |
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Number | Date | Country | |
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Parent | 09805887 | Mar 2001 | US |
Child | 11186490 | US |
Number | Date | Country | |
---|---|---|---|
Parent | 12429896 | Apr 2009 | US |
Child | 12759630 | US | |
Parent | 11186490 | Jul 2005 | US |
Child | 12429896 | US |