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LORA设计指导

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标签: lora射频

lora技术设计经验

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WIRELESS SENSING SX12723678 LoRa Modem Design Guide SX12723678 LoRa Modem Designers Guide AN120013 TCo Revision 1 July 2013 2013 Semtech Corporation 1 WIRELESS SENSING SX12723678 LoRa Modem Design Guide Table of Contents Overview 3 Principles of LoRa Design 3 LoRa Modulation 3 Receiver Sensitivity 3 SNR and Spreading Factor 3 BW and Chip Rate 4 Advanced LoRa Design Parameters 5 Forward Error Correction 5 Hard......

WIRELESS & SENSING SX1272/3/6/7/8 LoRa Modem Design Guide SX1272/3/6/7/8: LoRa Modem Designer’s Guide AN1200.13 TCo Revision 1 July 2013 © 2013 Semtech Corporation 1 WIRELESS & SENSING SX1272/3/6/7/8 LoRa Modem Design Guide Table of Contents Overview ........................................................................................................................................................ 3 Principles of LoRa Design ............................................................................................................................ 3 LoRa Modulation ............................................................................................................................................. 3 Receiver Sensitivity ......................................................................................................................................... 3 SNR and Spreading Factor ............................................................................................................................. 3 BW and Chip Rate........................................................................................................................................... 4 Advanced LoRa Design Parameters ........................................................................................................... 5 Forward Error Correction ................................................................................................................................ 5 Hardware Implementation ............................................................................................................................... 6 Low Data Rate Optimisation Mode & Header Mode ....................................................................................... 6 The LoRa Packet Format & Time On Air ..................................................................................................... 7 LoRa Calculator ............................................................................................................................................. 8 1. 2 2.1 2.2 2.3 2.4 3 3.1 3.2 3.3 4 5 Table of Figures Figure 1. The LoRa Bandwidth Corresponds to the Double Sided Transmit Spectrum Bandwidth .............................. 4 Figure 2. Influence of Coding Rate on Sensitivity (SF = 7, BW = 125 kHz, 13 Byte Payload) ...................................... 5 Figure 3. Individual RF transmit and receive paths (left) provides better sensitivity than the single shared TRx path (right). ...................................................................................................................................................................... 6 Figure 4. LoRa Modem Packet formatting. .................................................................................................................... 7 Figure 5. The LoRa Calculator Interface. ....................................................................................................................... 8 DISCLAIMER The performance figures are for indication only. For definitive product performance data please refer to the datasheet. Revision 1 July 2013 © 2013 Semtech Corporation 2 WIRELESS & SENSING SX1272/3/6/7/8 LoRa Modem Design Guide 1. Overview This guide provides the basic information necessary for the designer to evaluate the suitability of the LoRa modem for their radio application. The design is split into two sections covering basic and advanced design topics. 2 Principles of LoRa Design 2.1 LoRa Modulation LoRa is a spread spectrum modulation scheme that that uses wideband linear frequency modulated pulses whose frequency increases or decreases over a certain amount of time to encode information. The main advantages of this approach are twofold: a substantial increase in receiver sensitivity due to the processing gain of the spread spectrum technique and a high tolerance to frequency misalignment between receiver and transmitter. To better understand how to implement a radio design using the LoRa modulation format it is necessary to briefly examine the factors influencing radio receiver sensitivity. 2.2 Receiver Sensitivity The sensitivity of a radio receiver at room temperature is given by: Eqn. 1 The first term is due to thermal noise in 1 Hz of bandwidth and can only be influenced by changing the temperature of the receiver. The second term, BW, is the receiver bandwidth. NF Is the receiver noise figure and is fixed for a given hardware implementation. Finally, SNR represents the signal to noise ratio required by the underling modulation scheme. It is the signal to noise ratio and bandwidth that are available as design variables to the LoRa designer. 2.3 SNR and Spreading Factor The basic premise of spread spectrum is that each bit of information is encoded as multiple chips. The relationship between the bit and chip rate for LoRa modulation, and respectively, is given by: Eqn. 2 where SF is the spreading factor. SNR Is the minimum ratio of wanted signal power to noise that can be demodulated. The performance of the LoRa modulation itself, forward error correction (FEC) techniques and the spread spectrum processing gain combine to allow significant SNR improvements. Some example SNRs for both conventional and LoRa modulation formats are shown in the table below. The lower this number the more sensitive the receiver will be. Negative numbers indicate the ability to receive signal powers below the receiver noise floor: Table 1. SNR for Various Modulation Configurations Modulation LoRa SF12 LoRa SF10 GMSK Typical SNR -20 dB -15 dB 9 dB Revision 1 July 2013 © 2013 Semtech Corporation 3 WIRELESS & SENSING SX1272/3/6/7/8 LoRa Modem Design Guide The substitution of one bit for multiple chips of information means that the spreading factor has a direct influence on the duration of the LoRa packet. The influence of the spreading factor on the sensitivity and the time on air are shown below for a fixed bandwidth of 250 kHz. Table 2. Influence of SF on Time on Air and Sensitivity (CR=2, BW=250) SF 12 10 8 Time on air [ms] Sensitivity [dBm] 528.4 132.1 39.2 -134 -129 -124 2.4 BW and Chip Rate One of the principle design compromises that the designer must manage in the selection of spreading factor is that of time on air (packet duration) versus occupied bandwidth. The representation of a single bit by many chips, implies that the chips must either be sent faster than the original bitrate – increasing the occupied bandwidth of the signal, or in the same bandwidth – increasing the time taken to transmit the information. LoRa modulation sends the spread data stream at a chip rate equal to the programmed bandwidth in chips- per-second-per-Hertz. So a LoRa bandwidth of 125 kHz corresponds to a chip rate of 125 kcps. Equation 1 shows us that an increase in bandwidth (BW) due to the integration of additional noise power in the channel, will desensitize the receiver. Meaning that for a given spreading factor the designer can either elect to use a narrow bandwidth, maximizing sensitivity but increasing time on air or increasing the bandwidth for faster transmission but reducing sensitivity. Here we take the example of the SX1272, which has three programmable bandwidth settings 500 kHz, 250 kHz and 125 kHz (as shown below). (The SX1276 has bandwidths from 500 kHz to as low as 7.8 kHz). Figure 1. The LoRa Bandwidth Corresponds to the Double Sided Transmit Spectrum Bandwidth Revision 1 July 2013 © 2013 Semtech Corporation 4 WIRELESS & SENSING SX1272/3/6/7/8 LoRa Modem Design Guide For a fixed spreading factor the influence of bandwidth on the resulting time on air and sensitivity are shown in the table below for a 10 byte payload packet: Table 3. Influence of BW on Time on Air and Sensitivity (CR=2, SF=10) BW 125 250 500 Time on air [ms] Sensitivity [dBm] 264.2 132.1 66 -132 -129 -126 Examination of the basic design criterion of bandwidth and spreading factor allow quick evaluation of the suitability of LoRa for a given application. However, to optimize design performance there are other design criteria that must also be considered. 3 Advanced LoRa Design Parameters In addition to the use of spreading factor and bandwidth there are other design variables that the designer must consider when implementing a LoRa radio link. These are of particular importance when optimizing the robustness to interference and time on air of the LoRa transmission. 3.1 Forward Error Correction The LoRa modem also employs a form of Forward Error Correction (FEC) that permits the recovery of bits of information due to corruption by interference. This requires a small overhead of additional encoding of the data in the transmitted packet. Depending upon the coding rate selected, the additional robustness attained in the presence of thermal noise alone is shown in the family of curves below. Sensitivity as a Function of Code Rate ) % ( R E P 35.00% 30.00% 25.00% 20.00% 15.00% 10.00% 5.00% 0.00% CR = 4/5 CR = 4/6 CR = 4/7 CR = 4/8 -127 -126 -125 -124 -123 -122 -121 -120 Indicated Input Power (dBm) Figure 2. Influence of Coding Rate on Sensitivity (SF = 7, BW = 125 kHz, 13 Byte Payload) Revision 1 July 2013 © 2013 Semtech Corporation 5
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