Advanced Tutorial on Wireless Communication and Electronic Tracking: Appendix A
- 2.0Communications Systems Performance
- 3.0Electronic Tracking Systems Performance
- 4.0CT System Survivability, Reliability, And Availability
- 5.0CT System Safety
A.1 Definitions of Equipment Parameters
The equipment parameters that are typically required for coverage and electromagnetic compatibility (EMC) analysis of communications and tracking systems, and often specified in technical literature, are described in the following sections.
The equipment parameters that are generally applicable to a radio frequency (RF) emitting system are listed in Table A-1.
|Frequency band, lower limit||MHz|
|Frequency band, upper limit||MHz|
|RF channelization start||MHz|
|RF channelization increment||MHz|
|Duplex frequency offset||MHz|
The frequency band consists of the lower (minimum) and upper (maximum) frequencies over which a system may be tuned. This frequency band is also called the fundamental frequency band. Within its tuning band, the system is usually tuned to specific frequencies (i.e., channel frequencies), but tuning to any frequency (i.e., continuous tuning) is generally possible.
For systems having a specific set of channels and uniform channel spacing, the RF channelization specifies the start frequency and the frequency increment. For example, a UHF radio may have a tuning band of 450 to 470 MHz with 25 kHz channel increments starting at 450 MHz. A specific channel frequency might be 464.025 MHz. If the channel spacing is not uniform, a list of the specific channels should be provided.
If the system is capable of duplex operation, the duplex frequency offset specifies the constant difference in frequencies between a transmit channel and a receive channel. For example, if the transmit channel is 464.25 MHz and the receive channel is 464.85 MHz, the frequency offset is 0.6 MHz.
The modulation type refers to how information is coded or blended onto a carrier frequency. Example modulation types include:
- Frequency modulation (FM) - analog
- Single sideband (SSB) amplitude modulation (AM) - analog
- Binary phase shift key (BPSK) - digital
- Quadrature phase shift key (QPSK) - digital
The data rate is the number of bits per second (BPS) in a digital waveform. The units of kilobits per second (kbps) are also commonly used. This parameter is applicable only for digital modulation types and is normally the same for both the transmitter and the receiver.
Note: the characteristics listed in Table A-1 are assumed to be the same for both the transmitter and the receiver. If, for some reason, this is not the case, then a list of the characteristics unique to the transmitter and the receiver should be provided.
The equipment parameters that are generally applicable to the transmitter are listed in Tables A-2 through A-4.
The peak power is also known as the peak envelope power. The unit dBm is power in decibels with respect to one milliwatt; see Appendix Section B.1.1 for power conversions.
Harmonic frequencies occur at multiples of the fundamental tuned frequency. For instance, for a UHF radio having a fundamental tuning band of 450 to 470 MHz, the second harmonic frequency band occurs at 900 to 940 MHz, the third harmonic frequency band occurs at 1,350 to 1,410 MHz, etc. Note that the harmonic bands are wider than the fundamental band. Harmonic attenuation levels are generally provided for the second, third, and higher-order harmonics. The harmonic attenuation is relative to the in-band peak transmitter power. A typical harmonic level is -60 dB.
|Pulse repetition frequency||pps or Hz|
Some systems have a waveform that is turned on and then turned off. Such a waveform is called a pulsed waveform, and is defined by its pulse width (PW) and pulse repetition frequency (PRF). The PW and PRF are only relevant for systems with a pulsed modulation type. Typical units are microseconds (μs) for the PW, and pulses per second (pps) or Hz for the PRF. The duty cycle (DC), in dB, is computed as 10 times the log of the product of the PW in seconds and the PRF in Hz. In calculating DC, care should be taken to match the PW with the correct corresponding PRF. The mean power (i.e., average power) in dBm is computed by adding the DC in dB to the peak power in dBm. The signal level of the carrier is dBc. The unit dBc/Hz refers to the noise density with respect to the signal level of the carrier.
|At attenuation level||dB|
|Broadband transmitter noise (BBTN)||dBc/Hz|
The transmitter emission spectrum provides an indication of a transmitter’s frequency domain characteristics. An emission spectrum, which generally depends on the modulation type, is defined by the bandwidth (BW) of the spectrum at each of several attenuation levels. In general, these levels are the -3, -20, -40, and -60 dB points, although the -20, -40, and -60 dB points are sometimes not available. The BW and attenuation data points define an envelope for the spectrum. The minimum data required for an electromagnetic interference (EMI) analysis are the -3 dB BW and the rolloff in dB/decade.
For the analysis of a transmitter’s frequency domain characteristics, the midpoint of the -3 dB BW (and all other BWs) is usually assumed to be identical to the channel frequency. With this assumption, a modeled spectrum is symmetric with respect to the channel frequency. The channel BW is also usually assumed to be the same as the -3 dB BW. The frequency difference, Δf, to any arbitrary frequency (e.g., a receiver tuned frequency) is then the difference between that frequency and the channel frequency.
The spectrum rolloff defines the rate of attenuation of a spectrum’s envelope skirt outside of the -60 dB points and is used in cases of a large frequency offset. This rate of attenuation is usually given in dB per decade of frequency offset. The rolloff may be computed from the emission spectrum data. In the event of missing emission spectrum data or an extraordinarily large computed rolloff, a default rolloff of -40 dB per decade may be assumed.
The spectrum of a transmitter at very large frequency offsets is dominated by broadband transmitter noise (BBTN). BBTN is generated within the transmitter components and radiated by the transmit antenna. A typical level for BBTN from solid-state components, -160 dBc/Hz, may be used as a default value.
The equipment parameters that are generally applicable to the receiver are listed in Tables A-5 and A-6.
|Sensitivity criterion type||-|
The sensitivity is the power level in dBm required for some particular standard response for the receiver. The sensitivity criterion is usually either a required signal-to-noise power ratio (SNR) in dB for an analog system or a bit error rate (BER) for a digital system. Typical sensitivity criteria are 12 dB SNR for an analog system or a 10-4 BER for a digital system. Noise figure (NF) is a measure of degradation of the SNR caused by components in the RF signal chain. The noise figure is the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard noise temperature T0 (usually 290°K). The noise figure is thus the ratio of actual output noise to that which would remain if the device itself did not introduce noise. It is a number by which the performance of a radio receiver can be specified. A typical value of 6 dB may be used for the receiver noise figure (NF).
|At attenuation level||dB|
The receiver selectivity provides an indication of the frequency domain characteristics for the receiver. The selectivity is defined by the BW of the intermediate frequency (IF) stage at each of several attenuation levels. In general, these levels are the -3, -20, and -60 dB points. The BW and attenuation points define an envelope for the selectivity. The final IF stage is usually selected because it generally provides the most rejection to out-of-band (OOB) signals. In addition, the receiver noise level is usually computed using the -3 dB BW of the final IF stage. The minimum data required for an EMI analysis are the -3 dB BW and the rolloff.
For the analysis of a receiver’s frequency domain characteristics, the midpoint of the -3 dB BW is assumed to be identical to the channel frequency. The selectivity is then symmetric with respect to the channel frequency. The frequency difference, Δf, is the difference between any frequency and the channel frequency.
The rolloff defines the rate of attenuation (in dB per decade of frequency offset) of a selectivity’s envelope skirt outside of the -60 dB points. The rolloff may be computed from available selectivity data. In the event of missing data or a large computed value of rolloff, a default rolloff of -80 dB per decade may be assumed.
The equipment parameters that are generally applicable to the antenna and the transmission line are listed in Tables A-7 and A-8, respectively.
The maximum gain is sometimes called the mainbeam gain, although only directional antennas can be said to have a "beam".
The pattern type is a text data item and typical values are:
Other pattern types are also possible.
An antenna beamwidth is obtained from a pattern for the antenna, and is the angular difference between the half-power (-3 dB) points on the pattern.
The line type is a text data item and typical values are:
- Twin lead
- Coaxial cable
Other line types are possible.
The length of the transmission line is either from the transmitter to the antenna or from the receiver to the antenna. All transmission lines have some losses, and the line attenuation is a rating for the line. The loss along a particular section of line is then simply the line attenuation in dB per meter multiplied by the length in meters. Some line ratings are given in terms of dB per 100 feet, so to get dB per meter simply multiply the value in dB per 100 feet by 0.0328. Line ratings given in other units (e.g., dB per foot) may be converted to dB per meter by an appropriate scaling.
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