NB-IOT Downlink OFDM parameters
1.
downlink based OFDMA, FF = 128 points, baseband sampling rate is 1.92MHz, the sub-carrier spacing of 15kHz, the effective bandwidth of 180kHz = 1PRB
OFDMA:
Orthogonal Frequency Division Multiple Access, OFDMA technique is an evolution of OFDM, the
OFDM
and
FDMA
bonding techniques. After use of the OFDM carriers of the parent channel, transmission technology for transmitting data loaded on the part of subcarriers. OFDM is a modulation scheme; OFDMA is a multiple access technique, the user OFDMA resources shared frequency band, the access system.
OFDMA is divided into sub-channels (Subchannel) OFDMA and frequency hopping OFDMA.
KoNobumichi OFDMA
The OFDMA subchannel of the OFDM system bandwidth into a number of subchannels, each including a plurality of sub-carriers assigned to a user (a user may occupy multiple sub-channels). OFDM subcarriers may be combined into subchannels in two ways: centralized and distributed, as shown below.
Centralized and distributed
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The centralized several contiguous subcarriers assigned to a subchannel (user), the system can in this way select superior transmission subchannel (user) by a frequency domain scheduling (Scheduling), thereby obtaining a multiuser diversity gain. Further, a centralized fashion may also reduce the difficulty of channel estimation. Obtained in this manner but the frequency diversity gain is small, the average user slightly worse performance.
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Distributed system assigns a subchannel to subcarriers dispersed throughout the bandwidth of each subcarrier alternately arranged, to thereby obtain a frequency diversity gain. However, channel estimation is complex in this manner, it can not be frequency-domain scheduling, poor anti-offset ability.
Design should be selected flexibly according to the actual situation in the above-described two ways.
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When the channel estimation accuracy higher low-speed mobile terminal may be assigned a centralized, multi-user diversity gain is obtained.
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When the channel estimation accuracy is not as high as when the terminal moves fast, distributed allocation may be employed to obtain single-user frequency diversity gain.
Hopping OFDMA
OFDMA subchannel relatively fixed subchannel (user) subcarrier allocation, i.e. a user specified subcarrier group (the long period determined by the Frequency Domain Scheduling) is a long length of time.
This is sufficient to achieve multiple access OFDMA system within the district, but realize there is a certain inter-cell multiple access problem. Because if each cell performs scheduling according to the channel variation of the present cell, the cell use respective subcarrier resource conflict inevitable, with consequent inter-cell interference. To avoid such interference, it is necessary to coordinate (joint scheduling) between adjacent cells, but the coordination may need to support the network layer signaling exchanges, greater impact on the network structure.
A good choice is to use frequency hopping OFDMA.
In a frequency hopping OFDMA system, a user assigned to the subcarrier resource rapidly changing, each time slot, the user extracting a plurality of sub-carriers used in all sub-carriers in the same time slot, each user choice of different sets of subcarriers, As shown below.
Users subcarrier
Different subchannels of the frequency domain based scheduling, the selection of this sub-carrier channel generally does not depend on condition, but randomly selected. The next time slot, whether the channel is changed or not, each of the user jump to another set of sub-carrier transmission, the subcarriers used by the user but still conflicts. Hopping period may be shorter than the scheduling period of the multi-OFDMA subchannels, the shortest length of an OFDM symbol. Thus, within the cell, each user remains orthogonal, a diversity gain and a frequency domain utilization. Without coordination between district, sub-carriers used may conflict, but these can be fast frequency hopping mechanism interference in the time domain and frequency domain spread out into white noise interference can be greatly reduced interference hazards. In the system load it is not heavy, a frequency hopping OFDMA can be simply and effectively suppress inter-cell interference.
Fast Fourier Transform FFT points
1, should generally be n-th power of 2, it is easy for more levels of FFT-half, thus speeding up the conversion rate.
2, in order to perform FFT for all the points should be, should be greater than the number of points have been taken to the point. If a conversion points less than the existing number of samples, FFT segment will deal with it?
In python, if the sampling points is 8172, NFFT = 256, then the segmentation process should be performed, then again, different segments averaging, see not understand the code, to be further understood.
3, the more the actual sampling point, the higher the frequency resolution of the FFT.
4, FFT up to zero, the spectral envelope unchanged, but will increase the frequency resolution, from a different point of view there are differences, to be further understood.
2.CP length=10/9 samples