Reverse Link 

A passive RFID tag doesn’t have its own radio, and sends information back to the reader by changing the impedance state of its antenna, rather like using a tilted mirror to send signals back to someone holding a lamp. The reader

 
Class 0 tags backscatter during the on state of each symbol in the Forward Link, as shown in Figure 2. They use frequency-shift keying (FSK), with data ‘0’ and data ‘1’ symbols as tones at the relatively high frequencies of 2.2 and 3.3 MHz, respectively. This approach has the advantage of making the tag signal relatively easy to detect because it is displaced in frequency by several MHz from the much more powerful reader signals. However, the same large displacement means that other readers or devices operating 2-3 MHz from the wanted reader may interfere with it, and that, in some regulatory environments, the tags radiate out of the allowed bands, leading to compliance challenges. The tag data rate is the same as the reader data rate since one tag bit is sent for each reader bit.

The Class 1 Reverse Link has a symbol period half that of its Forward Link. The Data ‘0’ symbol has one transition, and Data ‘1’ has three transitions in this symbol period. This is a form of FSK that is sometimes called frequency/twice frequency (F2F), since Data ‘0’ corresponds to a tone of frequency 2/T0, and Data ‘1’ is a tone at twice that rate. The Class 1 tag data rate is therefore always twice the reader data rate, and so is typically around 140.35 kbps in US operation as shown in Table 1.

Gen 2 is quite a bit more complicated. There is a fundamental clock, known as the Backscatter Link Frequency (BLF), which specifies the pulse width of the shortest Reverse Link feature. There are then 4 permissible data encodings to map bits into symbols: FM0 (bi-phase space) baseband and 3 different Miller modulations of a BLF subcarrier. The simplest encoding, FM0, has transitions at the beginning of each data symbol. Data ‘0’ symbols have an additional mid-symbol transition. A long string of FM0 Data ‘0’ symbols produces a square wave at BLF; a string of Data ‘1’ generates a square wave at BLF/2. For FM0 the data rate is equal to the BLF, and they therefore share the same allowable range, from 40 kbps to 640 kbps. Compliant tags must support all these data rates, but a compliant reader need not implement the whole range, and in many cases will not.

Miller-modulated subcarrier (MMS) is a more elaborate encoding. MMS provides more transitions per bit and so is easier to decode in the presence of noise, but is slower for the same tag BLF. Three different MMS schemes are available, Miller-2, Miller-4 and Miller-8. The number specifies how many BLF periods define a data symbol. For example, using the slowest BLF of 40 kHz, the data rate for Miller-8 is the BLF/8 = 5 kbps. At such a slow rate, to transmit a 96-bit EPC and 16-bit error check will take 22.4 mS, corresponding to less than 45 tag reads per second (even fewer when all the overhead, such as the Forward Link commands, is included). As seen above, the other extreme is to use FM0 and the fastest BLF. The Reverse Link data rate is then 128 times faster (640 kbps), leading to a read time of 175 μS for the same EPC.

Figure 3: Gen 2 Reverse Link data encoding, MMS option, M=2; note M=4 and 8 are also possible.

MMS may provide superior performance when a large number of readers are operating simultaneously in the same facility, because it tends to put the Reverse Link spectrum into the frequency region between readers’ Forward Link channels. Because of this tradeoff between interference and data rate, an MMS option is likely to be provided on a reader that is certified for dense- or multiple-interrogator environments, and less likely to be available on readers that are only certified for single-interrogator use. The ability to vary both the backscatter link frequency and the data encoding (FM0 or an MMS) allows the user to empirically optimize the tradeoffs of tag data rate, read range, interference tolerance and multiple reader operation.

Table 3: Gen 2 Reverse Link Data Rates.