The research work of RFID Air Interface Based on CDMA in foreign countries still stays in the research stage of active tag and sending but not receiving. The direct reason is usually attributed to the passive tag’s failure to realize the chip’s low-power design, so it can be seen that the chip’s low-power design is very necessary. Therefore, only by clarifying the power supply mechanism of passive tag and analyzing the application environment of UHF RFID Air interface, can we find a complete solution. This paper introduces the special power supply mechanism of UHF RFID passive tag chip.
2 power supply characteristics of UHF RFID passive tag
Wireless power transmission uses wireless electromagnetic radiation method to transmit electric energy from one place to another, and its working principle is shown in Figure 1. The working process is to convert the electric energy into the radio frequency energy through the radio frequency oscillation, the radio frequency energy into the radio electromagnetic field energy through the transmitting antenna, the radio electromagnetic field energy can be transmitted to the receiving antenna through space, and then converted back to the radio frequency energy from the receiving antenna, and the detection becomes the direct current electric energy.
In 1896, Guglielmo Marchese Marconi, an Italian, invented radio and realized the radio signal transmission across space. In 1899, Nikola Tesla, an American, put forward the idea of using wireless power transmission, and established a 60m high antenna with bottom sensing and top capacity in Colorado. With the frequency of 150kHz, 300kW input power was transmitted over a distance of 42km, and 10kW wireless receiving power was obtained at the receiving end.
UHF RFID passive tag power supply follows this idea, which is powered by the reader through radio frequency. However, there are great differences between UHF RFID passive tag power supply and Tesla test: the frequency is nearly ten thousand times higher, and the antenna size is thousands of times shorter. Because the wireless transmission loss is directly proportional to the square of frequency and the square of distance, obviously, the transmission loss growth is huge. The simplest wireless propagation mode is free space propagation. The propagation loss is inversely proportional to the square of the propagation wavelength and directly proportional to the square of the distance. The free space propagation loss is LS = 20lg (4 π D / λ). If the distance D is in M and the frequency f is in MHz, LS = -27.56 + 20lgd + 20lgf.
The UHF RFID system is based on the wireless power transmission mechanism. There is no self-contained power supply for the passive tag. It needs to use the RF energy sent by the receiving reader to establish the DC power supply through the double voltage rectification, that is, the Dickson charge pump.
The communication distance applicable to UHF RFID Air interface mainly depends on the transmitting power of the reader and the basic space transmission loss. The transmitting power of UHF RFID reader is usually limited to 33dbm. From the basic propagation loss formula, ignoring any other possible loss, the RF power transmitted to the tag through wireless power can be calculated. The relationship between the communication distance of UHF RFID Air interface and the basic transmission loss, and the RF power of the arrival tag are shown in Table 1:
Table 1 Relationship between communication distance, propagation loss and RF power of arrival tag
Note: it is assumed that the transmitting power of the reader is 33dbm.
It can be seen from table 1 that the UHF RFID wireless power transmission has the characteristics of high transmission loss. Because RFID complies with the national short distance communication rules and the reader’s transmission power is limited, the tag’s power supply is low. With the increase of communication distance, the power supply capacity of passive tag decreases rapidly.
2.2 power supply by on-chip energy storage capacitor charging and discharging
(1) Charging and discharging characteristics of capacitor
Passive tag uses wireless power transmission to obtain energy, which is converted into DC voltage, and then charges and stores energy to the on-chip capacitor, and then supplies power to the load through discharge. Therefore, the power supply process of passive label is the process of capacitor charging and discharging. The process of capacitor charging and discharging is shown in Figure 2. The establishment process is a pure charging process. The power supply process is a discharge and supplementary charging process. The supplementary charging must start before the discharge voltage reaches the minimum supply voltage of the chip.
(2) Charging and discharging parameters of capacitor
1) Charging parameters
Charging time: τ C = RC × C
Where RC is the charging resistance and C is the energy storage capacitance.
2) Discharge parameters
Discharge time length: τ d = Rd × C
Where RD is the discharge resistance and C is the energy storage capacitance.
The above shows that the power supply characteristics of passive tag are not constant voltage source or constant current source, but the charge and discharge of energy storage capacitor. When the on-chip energy storage capacitor is charged to the working voltage V0 of the chip circuit, it can supply power to the tag. When the energy storage capacitor starts to supply power, its power supply voltage begins to drop. When the power supply voltage drops below the working voltage V0 of the chip, the energy storage capacitor will lose its power supply ability and the chip will not continue to work. Therefore, the air interface label should have enough ability to recharge the label.
2.3 supply and demand balance
Floating charge power supply is another way of power supply. The floating charge power supply capacity is compatible with the discharge capacity. But they all have a common problem, that is, the power supply of UHF RFID passive tag needs to balance supply and demand.
(1) Supply and demand balanced power supply for burst communication
The current standard ISO / IEC18000-6 of UHF RFID passive tag belongs to the burst communication system. For the passive tag, the signal is not transmitted in the receiving period, while the carrier is received in the response period, but it is equivalent to acquiring the oscillation source, so it can be considered as a simplex working mode. For this kind of application, if the receiving period is taken as the charging period for the energy storage capacitor, and the response period as the discharging period for the energy storage capacitor, the equal charge amount of the charge and discharge and the balance between supply and demand become the necessary conditions to maintain the normal operation of the system. According to the above power supply mechanism of UHF RFID passive tag, the power supply of UHF RFID passive tag is neither constant current source nor constant voltage source. When the label energy storage capacitor is charged to higher than the normal working voltage of the circuit, the power supply starts; when the label energy storage capacitor is discharged to lower than the normal working voltage of the circuit, the power supply stops.
For burst communication, such as passive tag UHF RFID Air interface, it can be charged enough before the response burst is sent, enough to ensure that enough voltage can be maintained before the response is completed. In addition to receiving strong RF radiation, the chip is required to have large on-chip capacitance and long charging time. Response power consumption and response time must also be compatible. Because the distance between the tag and the reader is different, the response time is different, and the area of the energy storage capacitor is limited, it may be difficult to use the time division supply and demand balance.
(2) Floating charge power supply for continuous communication
For continuous communication, in order to maintain the uninterrupted power supply of the energy storage capacitor, it is necessary to achieve the same charging speed as the discharging speed, that is, to maintain the power supply capacity before the end of communication.
The passive tag code is divided into RFID and UHF RFID passive tag current standard ISO / IEC18000-6, which has the common characteristics. The tag receiving state needs to be demodulated and decoded, and the response state needs to be modulated and sent. Therefore, the tag chip power supply system should be designed according to the continuous communication. In order to make the charging speed close to the discharging speed, most of the energy received by the label must be used for charging.
3 sharing RF resources
3.1 RF front end of passive tag
In addition to being the power supply of label postcard, the more important thing is to realize the instruction signal transmission of the reader to the label and the response signal transmission of the label to the reader through wireless data transmission. The application of passive tag to RF energy from reader is shown in Figure 3:
It can be seen from Figure 3 that the RF energy received by the tag is divided into three parts, which are used for power supply, demodulation signal (including instruction signal and synchronous clock) and response carrier.
The current standard UHF RFID working mode has the following characteristics: the downlink channel uses the broadcast working mode, and the uplink channel uses the single channel sorting response mode shared by multiple tags. Therefore, in terms of information transmission, it belongs to the simplex working mode. However, since the tag itself cannot provide the transmission carrier, the tag response needs to provide the carrier with the help of the reader. Therefore, when the tag response is in the transmission state, the two ends of the communication are in the duplex working mode.
In different working states, different circuit units are put into operation, and the power required by different circuit units is not the same. All the power comes from the RF energy received by the tag. Therefore, it is necessary to control the RF energy distribution when it is appropriate.
3.2 application of RF energy in different working periods
When the tag enters the RF field of the reader and starts to establish the power supply, no matter what signal the reader sends at this time, the tag will provide all the received RF energy to the voltage multiplying rectifier circuit, and charge the on-chip energy storage capacitor to establish the chip power supply.
When the reader sends an instruction signal, the sending signal of the reader is an amplitude modulated signal encoded by instruction data and spread spectrum sequence. In the received signal, there are carrier components and sideband components representing instruction data and spread spectrum sequence. The total energy, carrier energy and sideband component size of the received signal are related to modulation. At this time, the modulation component is used to transmit the synchronous information of command and spread spectrum sequence, the total energy is used to charge the on-chip energy storage capacitor, which starts to supply power to the on-chip synchronous extraction circuit and the command signal demodulation circuit unit at the same time. Therefore, in the period of reader sending instruction, the received RF energy of tag is used for tag charging, synchronous signal extraction, instruction signal demodulation and identification. The energy storage capacitor is in the state of floating charge power supply.
When the tag responds to the reader, the transmitted signal of the reader is amplitude modulated by the spread spectrum chip rate division clock. In the received signal, there are carrier components and sideband components representing the spread spectrum chip rate division clock. At this time, the modulation component is used to transmit the chip rate division rate clock information of the spread spectrum sequence. The total energy is used to charge the on-chip energy storage capacitor and modulate the received response data and send the response to the reader. The on-chip energy storage capacitor starts to supply the on-chip synchronous extraction circuit and the response signal modulation circuit unit at the same time. Therefore, in the reader receiving response period, the tag receiving RF energy is used to continue charging the tag, and the chip synchronous signal is extracted and the answered data is modulated and the response is sent. The energy storage capacitor is in the state of floating charge power supply.
In short, in addition to the tag entering the reader RF field and starting to establish the power supply period, the tag is to provide a voltage doubling rectifier circuit for all received RF energy, and charge the on-chip energy storage capacitor, so as to establish the chip power supply. Then, the tag extracts the synchronization from the received RF signal, demodulates the instruction, or modulates and transmits the response data, all of which use the received RF energy.
3.3 RF energy requirements for different applications
(1) RF energy requirements for wireless power transmission
Wireless power transmission establishes power supply for tag, so it requires not only enough voltage to drive chip circuit, but also enough power and continuous power supply.
The power supply of wireless power transmission is to establish the power supply by receiving the RF field energy of the reader and voltage doubling rectifier when the tag has no power supply. Therefore, its receiving sensitivity is limited by the voltage drop of the front-end detector diode. For CMOS chips, the receiving sensitivity of voltage doubling rectifier is between – 11 and – 0.7dbm, which is the bottleneck of passive tag.
(2) RF energy requirements for receiving signal detection
When the power supply of the chip is established by the voltage doubling rectifier, the radio frequency received by the tag should be divided into two parts to provide signal detection circuit, including instruction signal detection and synchronous clock detection. Since the signal detection is carried out under the condition that the power supply of the tag has been established, the demodulation sensitivity is not limited by the voltage drop of the front-end detector diode, so the receiving sensitivity is much higher than the receiving sensitivity of wireless power transmission, and belongs to the signal amplitude detection, without the requirement of power intensity.
(3) RF energy demand of tag response
When the tag is sent in response, in addition to detecting the synchronous clock, the received carrier (including the clock modulation envelope) needs to be pseudo PSK modulated and reverse transmitted. At this time, a certain power level is required, whose value depends on the distance of the reader to the tag and the sensitivity of the reader to receive. Because the reader working environment allows a more complex design, the receiver can achieve a low-noise front-end design, code division radio frequency identification using extended spectrum modulation, as well as the extended spectrum gain and PSK system gain, the reader sensitivity may be designed to be high enough, so that the requirements for the return signal of the tag are reduced to low enough.
To sum up, it is possible and reasonable to allocate the RF power received by the tag as the rectifier energy for wireless power transmission, then allocate the appropriate tag signal detection level and the appropriate return modulation energy to realize the reasonable energy distribution and ensure the continuous charging of the energy storage capacitor.
It can be seen that the RF energy received by the passive tag has multiple application requirements, so it needs to have RF power distribution design; the application requirements of RF energy are different in different working periods, so it needs to have RF power distribution design according to the requirements of different working periods; different applications have different requirements for the size of RF energy, among which the wireless power transmission requires the largest power, so RF power distribution should focus on the demand of wireless power transmission.
UHF RFID passive tag uses wireless power transmission to build tag power supply. Therefore, the power supply efficiency is very low and the power supply capacity is very weak. The tag chip must be designed with low power consumption. With the help of charging and discharging of on-chip energy storage capacitor, the chip circuit is powered. Therefore, in order to ensure the label to work continuously, it is necessary to charge the energy storage capacitor continuously. There are three different applications of the RF energy received by the tag: voltage doubled rectifier power supply, command signal receiving and demodulation, response signal modulation and transmission. Among them, the receiving sensitivity of voltage doubled rectifier is restricted by the voltage drop of rectifier diode, which becomes the bottleneck of air interface. Therefore, the basic functions of RFID system are to receive, demodulate and modulate the response signal. The stronger the power supply capacity of the voltage doubled rectifier tag, the more competitive the product will be. Therefore, the principle of reasonable distribution of received RF energy in the design of tag system is to increase the RF energy supply of voltage doubling rectifier as much as possible on the premise of ensuring the demodulation of received signal and the transmission of response signal.
Editor in charge: CT