Design considerations for global short-range wireless systems in unlicensed bands below 1 GHz
The term short-range wireless devices covers wireless transceivers that enable one-way or two-way communication without causing interference to other wireless devices. SRD is widely used and can provide many different services. The more popular applications are: remote control applications in home or building automation systems, wireless sensor systems, alarms, cars (including remote keyless car door locks and remote car startup), and voice and Wireless transmission of video When selecting a wireless communication frequency, designers of wireless systems for short-range wireless devices must be very careful. In most cases, assuming that specific specifications and conditions of use can be met, the frequency range that can be selected is limited to unlicensed bands. Table 1 lists the available frequency bands for short-range wireless devices worldwide.
When designing a universal system, designers usually choose the 2.4GHz band. In fact, 2.4GHz has become Bluetooth, WLAN and
ZigBee and other standards are the preferred operating frequency bands. In the wireless phone or 802.11a version of WLAN applications, the 5.8GHz band also occupies some status.
However, for systems that require longer transmission distances and lower power consumption, the frequency bands below 1GHz are of great concern because of fewer coexistence issues and longer transmission distances, because both aspects of performance affect power consumption, and Power consumption is an important consideration in battery-powered applications.
The increase in the transmission distance of the low-frequency transmitter can be expressed by the simplified Friis transmission equation, which gives the relationship between the power Pr on the receiving antenna and the power Pt delivered to the transmitting antenna.
It is assumed here that both antennas have unity gain. This equation shows that for a fixed transmit power Pt, the received power will decrease with the square of the distance d and the square of the frequency f (or increase with the square of the wavelength λ). If the received power is lower than the minimum power required to correctly demodulate the signal (called sensitivity), the link will be interrupted.
Worldwide allocation of frequencies below 1 GHz Table 2 provides a more detailed description of communication frequency standards for short-range wireless devices below 1 GHz. Not all standards are listed here. For more details, please check the relevant links.
The 433MHz frequency band is one of the frequencies that can be used worldwide. In Japan, the frequency needs to be adjusted a little. Most transceivers with flexible frequency tuning can easily handle this problem, as shown in Figure 1 (ADF7020, or ADF7021). However, the available bandwidth of the 433MHz band is less than 2MHz, and it is usually not used for the transmission of voice, video, audio and continuous data, so it is often used in applications such as keyless door lock systems and basic remote control.
However, before the advent of the latest EN300 220 specification, the United States and Europe adopted very different solutions. The US uses frequency hopping, while Europe limits the duty cycle of each sub-band, as described in the ERC REC-70 document. These two solutions are very effective in reducing interference, but for system manufacturers, if the design is to target products in two regions at the same time, then they must completely rewrite the media access layer (MAC) in the system communication protocol. .
Frequency-hopping system frequency-hopping spread-spectrum (FHSS) transmission technology divides the spectrum into multiple channels and uses pseudo-random sequences (or "frequency hopping codes") recognized by both receivers and transmitters between these channels Switching to expand energy in the time domain. To facilitate new nodes joining the network, the controller node periodically sends beacon signals to enable the newly joined nodes to synchronize with it. The required synchronization time depends on the beacon transmission interval and the number of frequency hopping channels. The number of frequency hopping channels stipulated by the US and European standards is relatively close, and the maximum dwell time (ie, the time to stay on a specific frequency during a single frequency hop) is 400 ms.
Table 3 shows the number of channels, effective radiated power (ERP) and duty cycle required for the extended frequency band (less than 870MHz) of the European standard when using FHSS technology. When meeting the listen-before-talk (LBT) or duty cycle limits, the bandwidth can be as high as 7MHz, while in the past only a bandwidth range of 2MHz was available.
Listen first and say that it is a "polite" communication protocol. It will scan the activity on the channel before starting transmission. It is also called clear channel assessment (CCA). limits.
In addition to FHSS, direct sequence spread spectrum (DSSS) is also adopted in the new European specification. In the DSSS system, the narrow-band signal is multiplied by a high-speed pseudo-random number (PRN) sequence to obtain a spread spectrum signal. Each PRN pulse is called a "chip", and the rate of the sequence is called a "chip rate". The extent to which the initial narrowband signal is spread is called the processing gain, and is equal to the ratio between the chip rate (Rc) and the narrowband data symbol rate. Figure 2 compares the spectrum of FHSS and DSSS.
Table 3 shows the number of channels, effective radiated power (ERP) and duty cycle required for the extended frequency band (less than 870MHz) of the European standard when using FHSS technology. When meeting the listen-before-talk (LBT) or duty cycle limits, the bandwidth can be as high as 7MHz, while in the past only a bandwidth range of 2MHz was available.
Listen first and say that it is a "polite" communication protocol. It will scan the activity on the channel before starting transmission. It is also called clear channel assessment (CCA). limits.
In addition to FHSS, direct sequence spread spectrum (DSSS) is also adopted in the new European specification. In the DSSS system, the narrow-band signal is multiplied by a high-speed pseudo-random number (PRN) sequence to obtain a spread spectrum signal. Each PRN pulse is called a "chip", and the rate of the sequence is called a "chip rate". The extent to which the initial narrowband signal is spread is called the processing gain, and is equal to the ratio between the chip rate (Rc) and the narrowband data symbol rate. Figure 2 compares the spectrum of FHSS and DSSS.
At the receiver, the received spread spectrum signal is multiplied by the same PRN code to despread the signal, thereby extracting the initial narrowband signal. At the same time, any narrow-band interference signal at the receiver will be expanded and appear on the demodulator in the form of broadband noise. Each user in the system is assigned a different PRN code, thereby achieving user isolation in the same frequency band. This method is called Code Division Multiple Access (CDMA).
1. Anti-interference—The basic principle of the anti-interference ability of DSSS is to multiply the useful signal and the PRN code twice (spreading and despreading), while any interference signal is multiplied only once (spreading);
2. Low power spectral density-minimum interference introduced in existing narrow-band systems;
3. Security—Because spread spectrum / de-spread spectrum is implemented, it has strong resistance to interference;
4. Alleviate multipath effects.
Wideband modulation methods other than DSSS or FHSS In addition to FHSS and DSSS wideband spread spectrum modulation methods, European specifications also support FSK / GFSK (Gaussian Frequency Shift Keying) modulation technology with a bandwidth greater than 200 kHz. Table 4 gives the main specifications applicable to European broadband modulation schemes (including DSSS).
2. Low power spectral density-minimum interference introduced in existing narrow-band systems;
3. Security—Because spread spectrum / de-spread spectrum is implemented, it has strong resistance to interference;
4. Alleviate multipath effects.
Wideband modulation methods other than DSSS or FHSS In addition to FHSS and DSSS wideband spread spectrum modulation methods, European specifications also support FSK / GFSK (Gaussian Frequency Shift Keying) modulation technology with a bandwidth greater than 200 kHz. Table 4 gives the main specifications applicable to European broadband modulation schemes (including DSSS).
The ADF7025 ISM band transceiver is a device that supports the FSK modulation broadband standard. In order to work in the 865 ~ 870MHz sub-band, the device design must meet the limits of maximum occupied bandwidth (99%) and maximum power density, and the limit of the maximum power of the channel (or frequency band) edge is -36dBm. If the ADF7025 is set according to the parameters in Table 4, the device can meet these three constraints. Figure 3 is the result of ADF7025's wideband modulation test, which occupies a bandwidth of 1.7569MHz and a peak spectral density of -1.41dBm / 100kHz.
1. The minimum 6dB bandwidth is at least 500kHz;
2. For digital modulation systems, within any 3 kHz bandwidth during any continuous transmission interval, the power spectral density conducted from the transmitting end to the antenna is not greater than 8 dBm.
Anyone wishing to use a non-FHSS system must usually limit the radiated field strength to 50mV / m (-1.5dBm
ERP), but in the case of "digital modulation", as long as the maximum power spectral density limit can be met, the maximum output power is 1W. Therefore, when using the ADF7025, the width of its FSK frequency deviation is sufficient to ensure that the 6dB bandwidth is greater than 500kHz, so 1W ERP is allowed. In addition, due to the wide signal bandwidth, the system can also achieve higher data rates (up to 384kbps for the ADF7025).
ADF7025's co-channel suppression varies in the same channel from -2dB (worst) to + 24dB, depending on the bandwidth of the interference source. This is comparable to a commercial 802.15.4 DSSS transceiver with co-channel rejection of -4dB, which uses an artificial interference signal that is an IEEE 802.15.4 modulated signal.
With these methods, similar broadband modulation systems can be used in the US and Europe, simplifying product development for the global market. The ADF7025 transceiver architecture can work not only in the "digital modulation" mode defined by the US standard, but also in the "broadband modulation" mode defined by the European standard.
2. For digital modulation systems, within any 3 kHz bandwidth during any continuous transmission interval, the power spectral density conducted from the transmitting end to the antenna is not greater than 8 dBm.
Anyone wishing to use a non-FHSS system must usually limit the radiated field strength to 50mV / m (-1.5dBm
ERP), but in the case of "digital modulation", as long as the maximum power spectral density limit can be met, the maximum output power is 1W. Therefore, when using the ADF7025, the width of its FSK frequency deviation is sufficient to ensure that the 6dB bandwidth is greater than 500kHz, so 1W ERP is allowed. In addition, due to the wide signal bandwidth, the system can also achieve higher data rates (up to 384kbps for the ADF7025).
ADF7025's co-channel suppression varies in the same channel from -2dB (worst) to + 24dB, depending on the bandwidth of the interference source. This is comparable to a commercial 802.15.4 DSSS transceiver with co-channel rejection of -4dB, which uses an artificial interference signal that is an IEEE 802.15.4 modulated signal.
With these methods, similar broadband modulation systems can be used in the US and Europe, simplifying product development for the global market. The ADF7025 transceiver architecture can work not only in the "digital modulation" mode defined by the US standard, but also in the "broadband modulation" mode defined by the European standard.
Transient power requirements Design engineers should be aware of a new technical specification requirement in the European regulations, namely the limitation of transient power. Transient power is defined as the power that falls into the adjacent spectrum when the transmitter is turned on and off during normal operation. The latest specification adds this restriction to prevent spectral scattering.
As the current delivered to the power amplifier increases or decreases, the load seen from the voltage controlled oscillator (VCO) terminal also changes, which causes the phase-locked loop (PLL) to lose lock in an instant, and When the loop tries to regain the locked state, it generates stray radiation or spectral scattering. In those systems that only transmit intermittently, spectral dispersion can cause a significant increase in power falling into adjacent channels.
Figure 4 shows the spectrum scattering problem. The green line represents the PA output spectrum when the PA of the ADF7020 transmitter is turned on and off every 100ms. Obviously, a large amount of power falls into the channels on both sides of the carrier. During the test, the spectrum analyzer maintains the maximum hold state. The red line represents the output of the PA output when it gradually rises and falls in 63 steps every 100 ms, so it can be seen that the power falling into the adjacent channel drops significantly. SpecificaTIon 8.5 in the EN 300 220 specification limits the power that falls into these adjacent channels.
To ensure that this specification is met, the easiest way is to gradually transition the PA from the off state to the on state, or from the on state to the off state. This can usually be achieved by using a microcontroller to turn on / off the PA in stages . By using the ADF7020 transceiver, the PA can be adjusted from shutdown to output + 14dBm in up to 63 steps. A faster and simpler method is to use a transceiver with automatic PA power ramp control. For example, the ADF7021 has a programmable power ramp control function. The user can set the number of steps and the duration of each step.
Communication protocol considerations
ADI's ADIsimLINK protocol software can be used with any ADF702x transceiver. The agreement for this plan for the world's sub-1GHz frequency band incorporates new European specifications and is based on the star network shown in Figure 5 (up to 255 endpoints).
The core of the protocol is a non-slotted, non-persistent, anti-collision carrier sense multiple access / collision avoidance (CSMA / CA) scheme, whose endpoint (EP) can monitor the channel before transmission (LBT) To avoid collision.
The gapless feature in the protocol means that endpoints can send data as soon as they have data, so the "listen before talk" operation must be performed first. This method also ensures that no synchronization is required. If the endpoint detects that the channel is busy, it will wait a random length of time before performing the next LBT. The number of waits is limited, so the protocol also has a non-persistent nature. In FHSS mode, the protocol uses this CSMA / CA system on each frequency hopping channel, so it can meet the LBT requirements of the new European standard.
The physical layer (PHY) and media access layer (MAC) parameters of the ADIismLINK protocol are highly configurable, which allows full evaluation of devices and systems. ADI also provides source code to simplify the system development process. The agreement is provided as part of the ADF702x development kit (ADF70xxMB2). Figure 6 is the overall system architecture of ADIismLINK.
ADI's ADIsimLINK protocol software can be used with any ADF702x transceiver. The agreement for this plan for the world's sub-1GHz frequency band incorporates new European specifications and is based on the star network shown in Figure 5 (up to 255 endpoints).
The core of the protocol is a non-slotted, non-persistent, anti-collision carrier sense multiple access / collision avoidance (CSMA / CA) scheme, whose endpoint (EP) can monitor the channel before transmission (LBT) To avoid collision.
The gapless feature in the protocol means that endpoints can send data as soon as they have data, so the "listen before talk" operation must be performed first. This method also ensures that no synchronization is required. If the endpoint detects that the channel is busy, it will wait a random length of time before performing the next LBT. The number of waits is limited, so the protocol also has a non-persistent nature. In FHSS mode, the protocol uses this CSMA / CA system on each frequency hopping channel, so it can meet the LBT requirements of the new European standard.
The physical layer (PHY) and media access layer (MAC) parameters of the ADIismLINK protocol are highly configurable, which allows full evaluation of devices and systems. ADI also provides source code to simplify the system development process. The agreement is provided as part of the ADF702x development kit (ADF70xxMB2). Figure 6 is the overall system architecture of ADIismLINK.
This article summarizes the new European specifications that set very specific requirements for wireless transmission protocols in the 863 ~ 870MHz band. Regardless of whether the system uses a single-channel protocol or FHSS or DSSS, it must comply with specific rules, which makes the protocol design more complicated. However, the advantage of these new specifications is that they are similar to the FCC Part 15 247 specifications in many respects, which can simplify the protocol design for multi-regional applications. ADI's development kit includes various protocol examples that can alleviate the design challenges of short-range wireless networks.
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