Bluetooth 5.0 changes make physical layer testing more complicated

Bluetooth 5.0 adds speed and flexibility to low energy (LE) solutions. Its data throughput is twice that of version 4.2, and the maximum burst rate jumps from 1Mb/s to 2Mb/s. To increase its versatility, it is now possible to reduce bandwidth to increase the distance by a factor of four while maintaining similar power requirements. Because the distance between devices sending and receiving data is increased by a factor of four, home automation and information security product designers are expected to cover the entire home, the entire building, or the entire community in product design. While the functionality has increased, it has also brought new test requirements, especially at the physical layer.

Bluetooth 5.0 is more efficient in utilizing broadcast channels in the increasingly crowded 2.4 GHz band, requiring less broadcast time to complete tasks. With improved broadcast channels, developers can create experience-based applications that bridge the gap between the physical world and the virtual world.

According to data released by the Bluetooth SIG, Bluetooth 5.0 will add much more capacity to advertising transmissions. This means it can transfer more information to other compatible devices without actually connecting, thus speeding up the interaction. It expands the ad and offloads ad data from three traditional ad channels to a full set of data channels for more frequency diversity, as shown in Figure 1. The larger 255-byte packet enables new thresholding features, such as asset tracking, while being backward compatible with products developed for the previous Bluetooth specification.

Figure 1: In the 2.4GHz band, the Bluetooth 5.0 advertising channel falls between Wi-Fi channels.

Not every application requires the same distance, speed, or broadcast capabilities. Bluetooth 5.0 gives product developers the best choice for their implementation. Since the broadcast message capacity has increased to 8 times that of v4.2 and supports more packets (from 31 bytes to 255 bytes), the Bluetooth SIG estimates that Bluetooth 5.0 can now be used in more than one room or even beyond a house. Internet of Things (IoT) connection. They predict that by 2020, more than 33% of all IoT devices will have Bluetooth built-in.

Bluetooth physical layer change

Bluetooth 5.0 adds two new modes to the LE standard. The first mode has twice the symbol rate of the existing 1Msps low energy standard and is called the LE 2M PHY (formerly known as the LE 1M PHY). Both the LE 1M and LE 2M PHYs belong to the so-called LE uncoded physical layer standard because they have no error correction coding stage internally.

The second mode is called the LE coding physical layer standard. The LE-encoded physical layer standard has two encoding modes: S=8 and S=2, where S is the number of symbols per bit. In addition to the Cyclic Redundancy Check (CRC), there are convolutional coding and mapping, which increases redundancy and reduces the chance of errors. As a result, the encoded information can be transmitted over longer distances because detection and correction can be performed as needed. Table 1 summarizes the different modulation and coding methods and the resulting data rates.

Table 1: Bluetooth 5.0 physical layer modulation and coding methods and the resulting data rate

Figures 2 and 3 show how the low-energy encoding method differs from the unencoded mode in processing the data payload, both of which require CRC generation and whitening. For LE-encoded physical layer standards, the payload undergoes forward error correction (FEC) and pattern mapping. The convolutional FEC encoder uses a non-systematic, non-recursive rate? code with a defined length of K=4. The encoder generates two output bits for each input bit and passes through a convolutional FEC encoder. The two output bits generated by the encoder are further mapped. If S=2, then they will not change, but for S=8, 0 maps to 0011 and 1 maps to 1100. This is the way to create 8 bits for each input bit in the LE-coded physical layer standard S=8.

Figure 2: LE Load Processing for LE Uncoded Physical Layer

Figure 3: Code Stream Processing in the LE Coded Physical Layer Standard adds many steps not required in the LE uncoded physical layer standard.

The packet format specified by the LE-coded physical layer standard is also used for the advertisement channel package and the data channel package. The entire packet is transmitted using a symbol rate of 1 Msym/s. Each packet consists of a preamble, an FEC code group 1 and an FEC code group 2, as shown in FIG.

Figure 4: Contents of the Bluetooth 5.0 LE Encoding Package

The preamble is not encoded. The FEC code group 1 consists of three fields: an access address, a coding indicator (CI), and TERM1. The code group adopts S=8 coding mode, and the final number of symbols is always the same.

The CI field determines which encoding method is used by the FEC code group 2. FEC code group 2 consists of three fields: PDU, CRC, and TERM2. They use S=2 or S=8 encoding, depending on the CI field value. The CI field is just a two-digit field that is used to distinguish between the S=2 mode and the S=8 mode.

Protocol Data Units (PDUs) are between 2 and 256 bytes in length. Therefore, the minimum packet length is 462 μs (if all the values ​​in the last row of S=2 are added, then the PDU is only 2 16-bit bytes), and the maximum packet length is 17040 μs (obtained by S=8, PDU is 257 words) Section).

Bluetooth 5.0 test

The device under test requires a large number of measurements to determine that it meets the Bluetooth specification on the transmitting side, as described in more detail below. These tests can be performed using a mid-range spectrum analyzer equipped with Bluetooth 5.0 analysis software.

In-band Radiation: This test verifies that the in-band spectral emissions of Bluetooth transmission fall within the limits. The limits have been modified to accommodate the LE 2M PHY. The limit of the LE-coded physical layer standard operates at 1 Ms/s with the same limit line as the LE 1M PHY. The 80MHz entire Bluetooth band is divided into 80 channels, each channel is 1MHz wide, and then the integrated power in each band is calculated. The device transmits information on the RF channel with a center frequency of M, and the center frequency of the adjacent channel of the 1 MHz bandwidth is denoted by N. For LE 1M, the integrated power in the 2MHz bias band should be less than -20dBm, and the power in the 3MHz or higher band should be less than -30dBm. For LE 2M, the limit comparison starts from the 4MHz frequency offset on either side (instead of 2MHz). For biased 4MHz and 5MHz bands, the integrated power is expected to be less than -20dBm; only for offsets above 6MHz, a more stringent requirement of <-30dBm will be set.

In Figure 5, you can see that LE 2M power is calculated every 1 MHz, indicated by the blue line. You will also notice that the standard recommends three limits: ±4MHz, ±5MHz, and ±≥6MHz.

Figure 5: Calculating LE 2M power per 1MHz, represented by blue lines

Modulation characteristics: The modulation method used by Bluetooth is Gaussian Frequency Shift Keying (GFSK), and the bandwidth bit period product BT=0.5. The modulation index must be between 0.45 and 0.55. This test verifies that the frequency domain of the known test pattern is within the specified limits. The measurement uses a specific test pattern. In previous versions of Bluetooth, the patterns used were 0 x 0F (00001111) and 0 x 55 (01010101), and then the frequency deviation in each bit interval was calculated in the manner specified by the standard. In Bluetooth 5.0, the pass/fail limit of the LE 2M PHY test has changed because the frequency offset of the 2Msps modulation method is different. These limits of the LE 2M PHY have doubled. For the LE coding specification (S=8), the measurement patterns are different. The first code is generated by assigning all 1s. After encoding and mapping, the pattern becomes 00111100. If the input pattern of the encoder and mapper are all 0, a second pattern 00110011 is generated. The standard also states that this measurement starts with the 33rd symbol.

Stable modulation characteristics: This is a new indicator that was not available in previous Bluetooth test specifications. The LE device is equipped with a transmitter with a stable modulation index, which can be used to inform the receiving LE device through a function matching mechanism. The modulation index of these transmitters is between 0.495 and 0.505. If applicable to all LE transmitter physical layers it supports, the device shall only indicate that the transmitter has a stable modulation index. If the transmitter does not have a stable modulation index, but is still within the 1% margin of the ideal modulation index of 0.5, then we call it a standard modulation index.

Frequency Offset and Drift: The frequency offset can be calculated by finding the average of the frequency deviations in a specified interval alternating between the 1 and 0 patterns. The interval between the previous low energy standards was 10 or 10 μs. This frequency offset is calculated in the preamble and payload. These frequency offsets are then calculated for drift in the 50 [mu]s interval (5 intervals apart). For the LE 2M PHY, the interval is still 10μs, but consists of 20 bits instead of 10 (because it is 2Msps). The drift measurement is still performed in 5 groups or 5 intervals apart. For the LE-encoded physical layer standard, a 16-bit interval is selected instead of 10, and then the drift is calculated by three interval durations (48 μs) because the pattern is 00110011.

20dB bandwidth: Measure the bandwidth until the spectrum falls to a point 20dB below the peak power.

Output Power: Calculates the power of the entire packet.

In-depth Bluetooth analysis: In addition to the above measurements, some Bluetooth analysis software provides additional information about the test signal. These analyses can help you debug and optimize the performance of your target application, including:

1) decoded packet information, that is, all headers and packet information that have been decoded;

2) Summary or screenshot of all measurements and package information after decoding;

3) Multiple display screens, display frequency deviation changes with time, used when debugging or interpreting modulation map and drift measurement;

4) The drift table shows the frequency offset calculated in the 10-bit interval and the drift in 50 μs (the distance between the five intervals);

5) Constellation diagram, eye diagram and symbol table display.

achieve

It is also useful to use a real-time spectrum analyzer in Bluetooth applications, which can show problems hidden under broadband noise that are not visible with other instruments. Figure 6 (right) shows what the swept spectrum analyzer sees in a 40MHz scan and what the real-time spectrum analyzer (left) sees.

Figure 6: Real-time spectrum analyzer can show hidden problems that traditional swept spectrum analyzers miss

Bluetooth 5.0 has been fully improved over Bluetooth 4.2 LE. By paying close attention to test and measurement strategies, your design will be able to take advantage of every single advantage offered by the new standard.