Configuration dependent compensation rotation of symbols

#The present disclosure relates to a method, system and wireless device for wireless communications, and in particular to rotation of symbols for transmission.

#To enable inexpensive power amplifiers (PA) in wireless communication devices or user equipment (UE), hereinafter collectively referred to as user equipment, the communication system may allow the option of using signals with small amplitude variations. In such cases, the linear region of the PA can be smaller or non-existent.

#In Single-Carrier Frequency Division Multiple Access (SC-FDMA) systems such as Long-Term Evolution (LTE) or NarrowBand Internet of Things (NB-IoT), a single-subcarrier, or single-tone, transmission is used to achieve a signal with close to unit amplitude. SC-FDMA has two subcarrier spacing options: 15 kHz for single-tone and multi-tone transmissions, and 3.75 kHz for single-tone transmission only. The objective of NB-IoT is to address improved indoor coverage, support for massive number of low throughput devices, low delay sensitivity, ultra-low device cost, low device power consumption and optimized network architecture. In one or more embodiments, two modulation options are considered for NB-IoT uplink which are pi/2 BPSK and pi/4 QPSK. In these predefined modulations, the constellation is rotated pi/2 or pi/4 radians every symbol. In general, these rotations would allow smoother transitions between constellation points, reducing the peak to average power ratio (PAPR). These modulation options are considered for uplink data and control channel transmissions, except for M-PRACH preambles.

#A more complete understanding of the present embodiments, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

#FIG. 1 is a diagram of signal spectrum characteristics of a twelve subcarrier or twelve tone transmission;

#FIG. 2 is a diagram of signal spectrum characteristics of a single-subcarrier transmission of NB-lot with 15 kHz subcarrier spacing;

#It has been determined that if no subcarrier is centered at the DC frequency, as in NB-IoT, the modulation constellation in baseband rotates in the complex plane, and the speed and direction of the rotation is determined by the subcarrier offset, i.e., unwanted rotation is induced. The inventors also discovered that if each OFDM symbol is further extended by the cyclic prefix, the final symbol is somewhat longer than an integer number of rotations. Because of this “induced” rotation or rotation that is caused by the transmission scheme during transmission, the rotation of symbols and transitions between symbols such as π/2 BPSK symbols does not adhere to the predefined modulation rotation of π/2 BPSK, for example, but varies dependent on the one or more configurations used for transmission. Therefore, the transition between symbols will be more abrupt than expected, resulting in greater amplitude variations.

#The disclosure describes the spectral characteristics, the relative cubic metric compared to GMSK, and the performance with a realistic PA model, of NB-IoT uplink with SC-FDMA and a subcarrier spacing of 15 kHz.

#Referring back to FIG. 4, the reason for not being able to fulfill the 23 dBm mask 16 requirements is because the inventors discovered that transitions between π/2 BPSK symbols are not π/2 radians or 90 degrees, but vary depending on the subcarrier position and the cyclic prefix length such that the transition between symbols is more abrupt than expected, resulting in greater amplitude variations. In other words, the induced rotation caused by the one or more configurations used for transmission results in greater amplitude variations even though π/2 BPSK, in this example, is used to help reduce these variations.