Link Budgets
Let's summarize the message of the last couple of sections. To transmit to a tag, a reader uses amplitude modulation to send a series of digital symbols. The symbols are coded to ensure that sufficient power is always being transmitted regardless of the data contained within in. The received signal can be demodulated using a very simple power detection scheme to produce a baseband voltage, which is then decoded by the tag logic. The whole scheme is depicted in Figure 19.

Figure 20 shows the corresponding tag-to-reader arrangements. The tag codes the data it wishes to send and then induces changes in the impedance state of the antenna. The reader CW signal bounces off the tag antenna (competing with other reflections) and is demodulated by the reader receiver and then decoded back into the transmitted data.

While we have alluded several times to the fact that the reader must power the tag, so far we have avoided coming to grips with the crucial associated question of just how much power the tag needs to get and just how far we can go from the reader and still get it. The amount of power that one needs to deliver to a receiver across a wireless link in order that the transmitted data be successfully received is known as the link budget. Since readers and tags both talk, for an RFID system there are two separate link budgets, one associated with the reader-to-tag communication (the forward link budget) and one with the tag reply to the reader (the reverse link budget)².

In order to find the forward link budget, we need to know the following:

• How much power can the reader transmit?
• How much power does the tag receive as a function of distance from the reader?
• How much power does the tag need to turn on?
• How much power does the tag need to decode the reader signal?

Let's examine each question in turn.

Reader Transmit Power
The reader transmit power is set by a combination of practicality and regulation. Most RFID equipment operates in spectrum set aside for unlicensed use by the governmental body that regulates radio operation in a given jurisdiction. For example, in the United States, the FCC allows operation in the band 902-928 MHz without requiring that the person operating the equipment have a license to do so. However, the equipment itself must obey certain operating limitations in order to allow unlicensed use. Relevant for us at the moment is the maximum transmit power, which cannot exceed 1 W. While not all readers will deliver a watt, and in some applications, we may intentionally reduce transmitted power, in many cases a UHF reader will be operated at the legal limit. So let's assume we transmit 1 W of total power.

Path Loss The difference between the power delivered to the transmitting antenna and that obtained from the receiving antenna is known as the path loss. In general, finding the path loss requires knowing something about the details of the antenna operation, and we shall discuss the relevant measurements and terminology shortly. However, to get started, we will use the simplest possible (not very accurate) approach: let us assume that the transmitting antenna radiates in all directions with the same power density, that is the transmitter is isotropic. We can picture the radiated power as being uniformly distributed over a spherical surface at any given distance r from the reader antenna (Figure 21). Some of this power can be collected by a tag antenna. It is reasonable to guess that the amount of power collected should be proportional to the density of power impinging on the tag and dimensionally necessary that the constant of proportionality be an area, often known as the effective aperture A_e of the tag antenna.

Since in the isotropic case the power density at a distance r is the ratio of the transmitted power P_TX to the sphere area, we can find the power received by the tag P_RX: