FFT numerology for an OFDM transmission system
View Patent ↗An exemplary fast Fourier transform (FFT) numerology for an orthogonal frequency division multiple access (OFDMA) downlink transmission system is described. The exemplary FFT numerology reduces the FFT sampling rate for a given transmission bandwidth, thereby increasing the battery life of a UE. The FFT numerology increases robustness against Doppler spread, phase noise, and frequency offset, enabling operation in channels with high delay spread, such as occurs in mountainous regions. The described numerology might provide the following without altering standard sub-frame duration: increased intercarrier spacing; reduced FFT sampling frequency across the transmission bandwidths; reduced FFT size across all transmission bandwidths; increased number of OFDM symbols per sub-frame; and/or increased cyclic prefix length choices.
1. A method of receiving data in an orthogonal frequency division multiplex (OFDM) system operating in accordance with an FFT numerology and having one or more channels each having a corresponding transmission bandwidth, the method comprising the steps of:
a) demodulating a received signal to generate an analog time domain signal;
b) converting, at an FFT sampling rate, the analog time domain signal to a time-domain sample sequence;
c) applying, at the FFT sampling rate, an N-point FFT algorithm to the time-domain sample sequence; and
d) generating one or more data tones in combination with one or more reference pilot tones from the time-domain sample sequence based on the N-point FFT algorithm, wherein the FFT numerology defines:
i) N as an integer representing a size of the N-point FFT algorithm determined by the FFT sampling rate, wherein a value for N is defined for each transmission bandwidth, and
ii) the FFT sampling rate based upon a rational multiple of a base clock signal of the OFDM system so as to be less than 1.2 times the corresponding transmission bandwidth.
2. The invention of claim 1 , wherein the one or more transmission bandwidths comprise 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz.
3. The invention of claim 2 , wherein the FFT sizes N corresponding to the one or more transmission bandwidths are 64, 128, 256, 512, 768, and 1024, respectively.
4. The invention of claim 2 , wherein the FFT sampling rates corresponding to the one or more transmission bandwidths are 1.28 MHz, 2.56 MHz, 5.12 MHz, 10.24 MHz, 15.36 MHz, 15 MHz, and 20.48 MHz, respectively.
5. The invention of claim 2 , wherein numbers of occupied sub-carriers of the OFDM system corresponding to the one or more transmission bandwidths are 56, 112, 224, 448, 672, and 896, respectively.
6. The invention of claim 1 , wherein:
the FFT numerology includes at least one cyclic prefix (CP); and
the method generates at least one CP time-domain sample representing each CP into the time-domain signal.
7. The invention of claim 6 , wherein the at least one CP comprises a short CP, a medium CP, and a long CP.
8. The invention of claim 7 , wherein a number of OFDM symbols per frame for a short CP is 9, a number of OFDM symbols per frame for a medium CP is 8, and a number of OFDM symbols per frame for a long CP is 7.
9. The invention of claim 7 , wherein, for the transmission bandwidth of 1.25 MHz, the short CP is at least one of ((5.47/7)×8)μs/samples and ((6.25/8)×1)μs/samples; the medium CP is ((12.5/16)×8)μs/samples, and the long CP is at least one of ((21.1/27)×6)μs/samples and ((23.44/30)×1)μs/samples.
10. The invention of claim 7 , wherein, for the transmission bandwidth of 2.5 MHz, the short CP is at least one of ((5.47/14)×8)μs/samples and ((6.25/16)×1)μs/samples; the medium CP is ((12.5/16)×8)μs/samples, and the long CP is at least one of ((21.1/547)×6)μs/samples and ((23.44/60)×1)μs/samples.
11. The invention of claim 7 , wherein, for the transmission bandwidth of 5 MHz, the short CP is at least one of ((5.47/28)×8)μs/samples and ((6.25/32)×1)μs/samples; the medium CP is ((12.5/16)×8)μs/samples, and the long CP is at least one of ((21.1/108)×6)μs/samples and ((23.44/120)×1)μs/samples.
12. The invention of claim 7 , wherein, for the transmission bandwidth of 10 MHz, the short CP is at least one of ((5.47/56)×8)μs/samples and ((6.25/64)×1)μs/samples; the medium CP is ((12.5/16)×8)μs/samples, and the long CP is at least one of ((21.1/216)×6)μs/samples and ((23.44/240)×1)μs/samples.
13. The invention of claim 7 , wherein, for the transmission bandwidth of 15 MHz, the short CP is at least one of ((5.47/84)×8)μs/samples and ((6.25/96)×1 )μs/samples; the medium CP is ((12.5/16)×8)μs/samples, and the long CP is at least one of ((21.1/324)×6)μs/samples and ((23.44/360)×1)μs/samples.
14. The invention of claim 7 wherein, for the transmission bandwidth of 20 MHz, the short CP is at least one of ((5.47/112)8)μs/samples and ((6.25/128)×1)μs/samples; the medium CP is ((12.5/16)×8)μs/samples, and the long CP is at least one of ((21.1/432)×6)μs/samples and ((23.44/480)×1)μs/samples.
15. The invention of claim 1 , wherein the FFT numerology sets a sub-carrier spacing for the OFDM system as 20 kHz with a sub-frame duration of 0.5 ms.
16. The invention of claim 1 , wherein the FFT numerology defines an oversampling ratio as a ratio of the FFT sampling rate to an occupied RF bandwidth for each transmission bandwidth and the oversampling ratio is about 1.14.
17. The invention of claim 1 , wherein the method operates in accordance with a 3GPP-LTE standard.