Patent Application
20080061943
As shown in
When the tag is in a sniff mode 221, since no wake-up signal is available, the tag returns to a sleep mode. When the tag turns to a sniff mode again, a wake-up signal is coming from the reader 210. In this case, one of the time markers, such as marker K, will be captured. Subsequently, a calculator (not shown) of the tag will determine when the reader will start sending valid data, namely, a time length between the time for capturing the mark k and the time for starting to transmit valid data. For example, if every marker is stamped at the same interval along the entire duration of a wake-up signal, the reader 210 will begin to send valid data after a time length T lapses, where T=(N+1−K)×ΔT, wherein ΔT is equal to the width of a time marker. After obtaining the time length T, the tag 220 stores the time length T and will get into a sleep mode immediately for the time length T. A microprocessor or a controller (not shown) of the system can control the tag to be woken up at the end of the time length T, namely, at the time when the reader 210 starts to transmit valid data. Since a tag needs to reserve a guard time to ensure the function on the tag is stable before it gets into a receiving mode to receive the valid data or exchange data with the reader, the duration of the tag in the sleep mode shall be Tg=((N+1−K)×ΔT−Guard Time).
Thus, compared with the scheme of ISO18000-7, the wake-up scheme according to one embodiment of the present invention can significantly save energy for a tag, particularly for those frequently used tags which will be woken up hundreds of times a day. For example, if a tag is woken up 100 times a day and each wake-up signal lasts one second, a tag will waste about 0.5×100 seconds working period averagely before receiving valid data or exchanging data. The wake-up scheme according to one embodiment of the present invention can alleviate this problem.
In addition to power-saving, the modulated wake-up signal used in one embodiment of the invention also leads to an attractive anti-interference feature. In reality, there are tremendous RF interferences in the space and some of them are very strong. Using a modulated wake-up signal can prevent tags from being woken up by a spurious wake-up signal. Only when a spurious wake-up signal happens to be the same as a modulated wake-up signals as provided in one embodiment of the invention, which is almost impossible, can tags be woken. Thus, using a modulated wake-up signal enhances the reliability of an active RFID system and extends the effective working distance thereof if the energy of a tag is kept in a state of satiations.
Optionally, the reader 210 comprises a switcher (not shown) which can switch the operation of the reader from one protocol to another protocol according to applications. For example, the switcher can switch alternatively the reader to operate under two different protocols; or the reader can work for most of time in the protocol according to one embodiment of the present invention, and the switcher can switch the reader periodically to a conventional protocol mode to check if a conventional tag also exists and then switch the reader back to the protocol according to one embodiment of the present invention.
Now an illustration of the operation of the tag of the RFID system according to one embodiment of the present invention is exemplified by referring to
As shown in
The tag will return to the sleep mode after informing its own microprocessor (not shown) to store the calculated time length T or Tg, and be woken up at the end of the stored time length T or Tg in step 306.
After woken up, the tag is in a receiving mode for communicating with the reader in step 307 and enters into its normal state, namely, staying alternatively in a sniff mode and a sleep mode once the communication is completed.
For most of active RFID systems, the communication distance between a tag and a reader can be more than 100 meters. However, a tag is not always away from a reader as far as 100 m. For example, the distance between a tag and a reader may be less than 20 meters. For such a short distance, lower output power from the tag would be pertinent for power-saving. Thus, another adaptive power control scheme is proposed in one embodiment of invention, which may be used in combination with other schemes or separately, depending on the actual applications. According to the proposed adaptive power control scheme, when a tag is close to a reader, its output power will be automatically reduced for the purpose of power saving. On the contrary, when a tag is far from a reader but within a predetermined range, its output power will be increased to ensure the communication between the tag and the corresponding reader.
Specifically, the scheme is carried out with the following steps. It is understood, the output power of a tag will be preset so strong that the communication at the longest communication distance could be successful.
When the tag captures a wake-up signal at a first distance which is shorter than the longest distance, it detects the received signal strength (RSS), and determines whether the captured RSS is the same as the preset strongest one. If the RSS is less than the preset one, the captured RSS will be preset as a threshold, which shows that, as long as the distance between the tag and the reader equals to or is less than the first distance, the power of output of the tag should be reduced from the strongest to the same as the threshold.
Further, the strength of a signal will be affected by the path loss and fading property in a given environment. The total attenuation is the sum of the path loss and the fading loss, if other factors affecting the strength of a signal are omitted. The path loss of a wireless uplink channel (from a tag to a reader) is the same as that in a downlink channel (from a reader to a tag). Thus, a tag can estimate the total attenuation after it gets the strength of a received signal from a reader, given that the transmitted power keeps constant, and thereby determine how to adjust its output power for the purpose of power saving.
As shown in
Assuming that the sensitivity of the reader is of −100 dBm, when a tag is at the location A, B, C, D, E and F, respectively, as long as the received signal strength is greater than −100 dBm, there is no need for a tag to output the predetermined power of 5.6 dBm. Instead, by using the proposed adaptive power control scheme, the output power of the tag at these six locations may be −40.4 dBm, −37.4 dBm, −35.4 dBm, −32.4 dBm, −28.4 dBm and −24.4 dBm, respectively, to satisfy the reliability of communication between the tag and the reader. Thus, the power received by the reader is still −100 dBm, but the energy of a tag could be saved significantly.
It is noted that, when the communication between a reader and a tag cannot be successfully performed, the tag should adaptively increase its output power until the communication between tags and the reader successes. That is to say, the output power of a tag is dynamically adjusted to ensure a good communication.
As for active RFID tags operating under ISO18000-7 protocol, especially for those tags that are not used frequently, most of the energy of battery will run out due to its alternatively and continually switching between a sniff mode to a sleep mode. Thus, adjustment of the period of the alternative period will also benefit to save power. From this point of view, the present inventors further propose the following scheme. It is understood that this scheme may be applied in combining with the above mentioned power-saving schemes or independently.
First, the tag records the frequency of the tag being woken in the memory. The microprocessor of the tag is configured to adjust the alternative period according to the frequency. The lower the stored frequency is, the longer the alternative period should be. Users could preset an upper threshold and a lower threshold for the frequency. If the frequency of a tag being woken is less than said preset lower threshold, the alternative period should be increased. Say, the tag will stay in a sniff mode after a longer period in a sleep mode. To the contrary, if the frequency is higher than the preset upper threshold, which means the tag is used often, the period during which the tag is in a sleep mode will be shorten in hopes of not missing any desirable wake-up signal.
According to one embodiment of the present invention, another embodiment shows a process of automatically switching between two protocols, which enable one RFID system to operate at two protocols.
In order to make an RFID system according to one embodiment of the present invention compatible to ISO18000-7 protocol, an active RFID tag preferably comprises a switcher which can automatically determine whether a wake-up signal captured by the tag is the one under the ISO18000-7 protocol or adaptable to one embodiment of the present invention (which may be taken as a new protocol of active RFID system in future, hereinafter, it will be referred to as “power-saving protocol”). The switcher will enable the tag to switch to an appropriate protocol according to the difference between wake-up signals under two protocols.
It is not difficult to distinguish the wake-up signal under the ISO18000-7 protocol from that under one embodiment of the present invention since these two kinds of wake-up signals are totally different in time domain and in frequency domain. In ISO18000-7 protocol, the wake-up signal is an f0±30 KHz sub-carrier which is near the operating frequency. In the proposed power saving protocol, the wake-up signal is a modulated RF signal which incorporates time markers.
When a wake-up signal is captured during the period of a tag staying at a sniff mode, the switcher on a tag will firstly determine which protocol the captured wake-up signal belongs to. Since the demodulated 30 KHz sub-carrier of the wake-up signal under ISO18000-7 protocol is a DC signal, it is easy for a tag to be understood. Thus, the tag will automatically switch to ISO18000-7 and begin to receive data under ISO18000-7 protocol. Otherwise, if the tag could sense a time marker from the captured wake-up signal, it will operate according to the power saving protocol.
While the forgoing embodiments of the invention have been described and shown, it is understood that alternatives and modifications, such as those suggested and others, may be made thereto which fall within the scope of the invention.
