The underlying technology that opens the LPWAN 2.0 era: Advanced M-FSK

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The underlying technology that opens the LPWAN 2.0 era: Advanced M-FSK [Copy link]

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In response to the problems of low LPWAN communication rate, difficulty in coverage and monitoring of moving objects, ZETA LPWAN has recently developed the Advanced M-FSK modulation method, which improves and optimizes the physical layer of ZETA's wireless communication modulation/demodulation processing, allowing ZETA to adapt according to the different rate requirements of various application scenarios. At the same time, it can fully draw on 5G's advanced receiver technology to improve sensitivity and break through the upper limit of the existing LPWAN technology receiving sensitivity, thereby providing new ideas for the evolution of the next generation of LPWAN2.0 technology.

There is a new technology that can solve the problems of low LPWAN communication rate, difficulty in coverage and monitoring of moving objects. Compared with the LPWAN technology currently on the market, in typical application scenarios, its transmission rate has increased by more than 3 times, the sensitivity has increased by more than 5dB, and the maximum receiving sensitivity can reach -150dbm.

This technology is the latest Advanced M-FSK modulation method proposed by ZETA LPWAN. It improves and optimizes the physical layer of ZETA's wireless communication modulation/demodulation processing, enabling ZETA to adapt to the different rate requirements of various application scenarios. At the same time, it can fully draw on 5G's advanced receiver technology to improve sensitivity, thereby providing new ideas for the evolution of the next-generation LPWAN2.0 technology.

1. LPWAN technology needs to find the best solution with low power consumption, long distance and adaptability to various scenarios

According to Shannon's theorem,

C is the transmission rate, B is the transmission bandwidth, η=C/B is the spectrum efficiency,

The red line in the figure below is the line of Shannon's theorem. Through coding modulation, we can get closer to this line, but we can never cross this line.

5G and other eMBB technologies focus on the spectrum efficiency area, focusing on the transmission rate within a limited bandwidth. LPWAN technology requires a long battery life (3-5 years), so the focus is on the energy efficiency area, that is, each bit needs to transmit as little energy as possible. In other words, in the process of pursuing extreme energy efficiency, LPWAN can sacrifice a certain spectrum efficiency.

Signal transmission distance:

Where Pt is the transmit power, Pr is the receive sensitivity, Gt is the transmit antenna gain, and Gr is the receive antenna gain, which is related to the radiation pattern. In LPWAN, a single antenna is often used for transmission and reception, and the transmission power is limited.

Therefore, effectively utilizing the transmission power and improving the sensitivity of the receiver have become the main goals of developing LPWAN physical layer technology.

1. Maximize the use of transmission power to ensure that LPWAN communication does not have instantaneous high power while maximizing power consumption energy efficiency (i.e. PAPR). Linearity is a very important indicator in power amplifier design. Since the signal has instantaneous high power, in order to ensure the linearity indicator at the instantaneous high power point, power back-off technology is usually used to ensure linearity so that the signal is not distorted after passing through the power amplifier. Power back-off technology reduces the efficiency of power consumption, so it is necessary to find a way to reduce the average power ratio (PAPR—Peak to Average Power Ratio).

2. Improve the sensitivity of the receiver and increase the coverage distance. If the receiver sensitivity is increased by 6dB, that is, 4 times, the coverage distance can be doubled.

3. Meet the specific data monitoring needs of different industries. For example, with the vigorous development of the logistics industry, IoT technology not only needs to support the access of a large number of static sensors, but also needs to support the access of a large number of mobile packages, that is, support the access of objects in complex Doppler and multipath wireless environments.

2. Implementation Path: Proposing Adaptive Advanced M-FSK Technology

In order to solve the three major pain points of "low power consumption, long distance, and diversified scenarios require completely different performance indicators" (for example, logistics needs to support mobility, and industrial scenarios need to pay more attention to communication rate and latency on the basis of meeting a certain coverage distance), ZETA has innovated the traditional LPWAN technology and proposed the latest Advanced M-FSK modulation technology, making the physical underlying technology of ZETA LPWAN better: 1. PAPR is zero; 2. By utilizing more bandwidth, more bits of information can be transmitted, thereby reducing the energy transmitted per bit; 3. Better receiving sensitivity.

1) Key parameter design can be adaptive to maximize energy efficiency

Generally, M-FSK modulation is to modulate and send a signal with a time domain value of 1 at one of M orthogonal frequency points in the frequency domain. As shown in the figure below, M=8, and each symbol at each frequency point can modulate 3 bits of information. The frequency point interval is 2kHz.

As shown in the figure above, the general M-FSK modulation technology has the following characteristics: 1. The modulation information only changes in phase, and the PAPR is zero when the amplitude remains unchanged, thus maintaining low power consumption characteristics; 2. When the transmission power remains unchanged, the bandwidth increases and the number of modulation bits increases (log2(M)). 3. In order to reduce spectrum leakage, the phase continuity between symbols must be maintained.

Advanced M-FSK proposed by ZETA has been deeply explored and designed. The most important parameters are as follows:

Frequency points:

Where K is the number of information bits that can be modulated by this symbol, the frequency point spacing (SubCarrier space), and the code rate (code rate).

The total bandwidth/rate can be calculated based on these parameters:

The symbol duration is:

The bit rate is:

The more M there is under a certain bandwidth, the smaller the SCS is, that is:

Taking LoRa as an example, the comparison between Advanced M-FSK and its several parameters is as follows:

As can be seen from the table above, compared with LoRa technology, Advanced M-FSK technology is simpler to send signals, and is based on the mainstream 4G/5G modulation technology, namely frequency domain modulation. It can fully draw on the advanced receiver technology of 5G, making the Advanced M-FSK receiver technology simpler and the performance better.

2) Advanced M-FSK frame structure can meet various application scenarios of LPWAN

Advanced M-FSK optimizes the frame structure design, taking into account national regulatory requirements while adapting to the rate requirements (0.02-20kbps) of various application scenarios.

According to the relevant regulations of the Radio and Television Commission, when a terminal sends a signal on an unlicensed spectrum, the bandwidth cannot exceed 200KHz and the duration cannot exceed 1 second. The frame structure design must also meet these requirements.

The Advanced M-FSK frame structure consists of three parts: a preamble frame, a SYNC frame, and a data transmission body.

The main function of the preamble frame is to detect the receiver and synchronize the time and frequency of the receiver (i.e., channel estimation), making the receiving sensitivity lower. When the signal is low, data demodulation is particularly sensitive to the accuracy of time and frequency estimation, so the pilot design also takes this into consideration, mainly based on narrowband design, and improves the time and frequency estimation accuracy based on the preamble pilot through a low-pass filter at the receiving end. In addition, the use of 2FSK technology also makes it compatible with existing 2FSK chips. The maximum transmission time of each burst is limited to 1 second, and the time allocation of the pilot and data frames must also be balanced. When designing, while meeting the time and frequency estimation performance, try to make the preamble take up as little time as possible to increase the data transmission time.

The SYNC frame carries the data modulation format information transmission. AdvancedM-FSK is designed to meet a variety of application scenarios, such as high-rate good signal scenarios, coverage-based extremely low signal scenarios, and compatibility with existing 2FSK products (such as Silicon Labs SI4463, STMicro stm32WL). Different scenarios will inevitably lead to different data frame formats. The SYNC frame completes the transmission of data modulation format information through a special encoding method. At the same time, while being compatible with existing 2FSK products, the SYNC frame retains the original format and information.

The data frame is the valid information transmitted from the MAC layer to the physical layer. Advanced M-FSK can support data transmission at various rates. The entire transmission process is as follows, namely data encoding, whitening, interleaving, and finally mapping and sending. Several key information in this process: encoding rate, number of repetitions, and number of transmission frequencies determine the data and the corresponding receiver sensitivity. These key information are sent through SYNC frames.

3) Advanced M-FSK improves sensitivity through advanced receiver technology

Advanced M-FSK has better sensitivity at very low signals through the design of transmitter technology and advanced receiver technology at the same time. Compared with other LPWAN technologies: at the same rate, it has lower sensitivity. At the same sensitivity, it has higher rate. Through field tests, the sensitivity can reach -144.7dBm at a data rate of 100bps. At a rate of 30bps, the sensitivity can reach -149.2dBm.

The three key technologies of the receiver are: time-frequency synchronization, data demodulation, and terminal mobile speed support. For example, the time-frequency synchronization performance under extremely low signal-noise conditions: compare the two scenarios of large terminal frequency deviation and small frequency deviation. In the scenario of small frequency deviation, assuming that the residual frequency deviation error is within 200Hz as the standard, the sensitivity can reach -150dBm. If some low-cost terminals, that is, when there is a large frequency deviation, the sensitivity can also reach -145dBm.

For more details on key technologies, please stay tuned for the next issue.

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