Radio Frequency Essentials

Radio Basics

The term radio refers to electromagnetic transmission through free space at millimeter wave frequencies and below. It encompasses a wide range of applications, from AM and FM car radios to cellular phone radios and terrestrial digital microwave radios. Some, like AM and FM radios, are one-way or broadcast radios. The electromagnetic transmission occurs in only one direction, and it is often in a one-to-many type configuration. Alternatively, two-way radios allow transmission and reception by all parties; they can either be a point-to-point configuration, which is common in telecommunications backhaul applications, or a point-tomultipoint configuration, such as WLAN and cellular networks.

An important architectural distinction with two-way radio communication is the difference between frequency division duplex (FDD) and time division duplex (TDD). In FDD, you use a different frequency to carry information in each direction, and the two frequencies are separated enough in frequency that they don't interfere. Provided adequate separation is maintained, FDD can significantly simplify the radio and system designer's task, and it allows full-duplex type operation where simultaneous transmission and reception can occur. However, it also suffers the problem that you must allocate spectrum in two bands, and it can be spectrally inefficient because it locks up bandwidth in each direction. The alternative, TDD, does all radio communication on the same channel but with alternating periods of transmitting and listening. Although TDD does create a half-duplex situation at the physical layer and does require the radio to be able to very quickly switch from transmitting to receiving, it can be more spectrally efficient. It allows for easier channelization, and it can provide for time-varying allocation of bandwidth in each direction.

One of the most important characteristics of a radio is power. Specifically, the output power of the radio is presented to the transmission line, cable, or antenna and is usually measured in watts or milliwatts (mW). Comparisons of power values use a logarithmic scale to express the ratio in decibels (dB). The radio manufacturer provides the output power in dBm, which is decibels per 1 mW, or in dBW, which is decibels per 1 W. Table 7-1 provides some sample conversions between powers in watts and decibels.

Antenna Basics

What is an antenna? The IEEE defines an antenna as "that part of a transmitting or receiving system that is designed to radiate or receive electromagnetic waves" (IEEE Standard 145-1993). In other words, it is the antenna that takes the radio frequency (RF) signal that the radio generates and radiates it into the air or that receives or captures electromagnetic waves for the radio. Usually, the transmit and receive properties are reciprocal, meaning that the parameters such as gain or radiation pattern or frequency are the same.

The next question you might ask is, "How does an antenna work?" According to the textbook that most antenna designers learned with—Stutzman and Thiele's Antenna Theory and Design—the key is radiation, a "disturbance in the electromagnetic field that propagates away from the source of the disturbance…disturbance is created by a time-varying current source." So the radio creates a time-varying voltage source at a particular frequency, which induces a time-varying current on the antenna that creates the aforementioned electromagnetic field.

Considerations of antenna performance make a distinction between the near field, close to the antenna, and the far field. In the far field, the distance from the antenna is much larger than the wavelength at which you are operating or much larger than the dimension of the antenna, in contrast to the near field. It is under far-field assumptions that antenna vendors specify the characteristics of the antenna. Keep this point in mind if you ever find yourself operating in the near field of antennas because the characteristics are different.

An important concept to understand is that of an isotropic radiator or antenna. It is a mathematical construct for an idealized lossless antenna that radiates equally in all directions. If you define a sphere with an isotropic radiator in its center, the electromagnetic field will be equal at all points on the surface of the sphere. The isotropic antenna is a useful reference point when we consider different antennas.


Receiver Performance Basics

Radio receivers are characterized by their receiver sensitivity, which is the minimum signal level for the receiver to be able to acceptably decode the information. The acceptability threshold is governed by a particular bit error rate (BER), packet error rate (PER), or frame error ratio (FER). For example, the 802.11a standard specifies that the minimum compliant receiver performance at a 54 Mbps data rate is –65 dBm at a 10 percent PER. Note that the receiver sensitivity is also at a specific data rate because each modulation scheme has its own signal-to-noise ratio (SNR) requirement. In general, the higher the data rate, the higher the SNR required and hence the higher the receiver sensitivity level. The receiver sensitivity of the radio is also governed by the receiver noise figure. All receivers have some base underlying noise level, either from the precision of the digital processing or from the performance of the analog components. This noise level is the noise floor. As the noise floor rises, so too does the receiver sensitivity because the minimum signal level over the noise, SNR, is fixed for the modulation scheme. This concept is depicted in Figure 7-5. To evaluate the performance of a radio, receiver sensitivity is one of the important inputs to your linkbudget calculation that ultimately determines the achievable data rates and ranges. In general, you want the lowest receiver sensitivity that is economically feasible.


802.11b Minimum Radio Performance

To ensure satisfactory system performance between equipment manufactured by different vendors, the 802.11b PHY standard defines minimum radio performance levels that all equipment must satisfy for compliance. At an FER less than .08 with a Physical Layer Convergence Procedure service data unit (PSDU) length of 1024 octets at an 11 Mbps data rate, the minimum receiver sensitivity is –76 dBm at the antenna connector. Under the same conditions, the receiver maximum input level is –10 dBm at the antenna connector, and the adjacent channel rejection of another compliant 802.11b transmitter is 35 dB at the antenna connector. For adjacent channel rejection, the receiver must be able to adequately filter or operate in the presence of an adjacent signal to maintain the 0.08 FER. To test adjacent channel rejection, the desired signal is placed 6 dB over the minimum receiver sensitivity level, and the adjacent channel signal is placed 41 dB over that same level. In the discussion of the 802.11b spectral mask, you will see what the likely resultant channel signal contribution is from the interferer.


802.11a Minimum Radio Performance

Similar to 802.11b, 802.11a also defines minimum radio performance parameters. Table 7-2 provides the minimum receiver sensitivity, adjacent channel rejection, and alternate adjacent channel rejection at the antenna connector for the 802.11a data rates at a PER less than 10 percent with a 1000-byte PSDU length. In the rejection performance, the desired signal is placed 3 dB over the minimum sensitivity level and the interferer at the level given by the ratio indicated.


The receiver maximum input level under the same conditions is –30 dBm. 802.11a also specifies the clear channel assessment (CCA) sensitivity, which states that a compliant radio must indicate busy with 90 percent probability within 4 microseconds if the received level is greater than or equal to –82 dBm during the preamble. If the preamble is missed, then the level is –62 dBm.