TESTING DIGITAL COMMUNICATIONS TRANSMITTERS AND RECEIVERS
There is a fundamental tradeoff in communication systems. Simple hardware can be used in transmitters and receivers to communicate information. However, this method uses a lot of spectrum, which limits the number of users. Alternatively, more complex transmitters and receivers can be used to transmit the same information over less bandwidth. The transition to more spectrally efficient transmission techniques requires more complex hardware. Complex hardware is difficult to design, test and build. This tradeoff exists whether communication is over air or wire, analog or digital.
In the last few years, a major transition has occurred from simple analog amplitude modulation (AM) and frequency/phase modulation (FM/PM) to new digital modulation techniques. Examples of digital modulation include quadrature phase shift keying (QPSK), frequency shift keying (FSK), minimum shift keying (MSK) and quadrature amplitude modulation (QAM).
There are only three characteristics of a signal that can be changed over time: amplitude, phase or frequency. However, phase and frequency are just different ways to view or measure the same signal change.
Fig. 1 A typical polar diagram showing magnitude and phase.
Amplitude and phase can be modulated simultaneously and separately, but this is difficult to generate, and especially difficult to detect. Instead, in practical systems the signal is separated into another set of independent components: in-phase (I) and quadrature (Q). These components are orthogonal and do not interfere with each other.
POLAR DISPLAY - MAGNITUDE AND PHASE REPRESENTED TOGETHER
A simple way to view amplitude and phase is with a polar diagram shown in Figure 1. The carrier becomes a frequency and phase reference and the signal is interpreted relative to the carrier. The signal can be expressed in polar form as a magnitude and a phase. The phase is relative to a reference signal (the carrier in most communication systems). The magnitude is either an absolute or relative value. Both are used in digital communication systems. Polar diagrams are the basis of many displays used in digital communications, although it is common to describe the signal vector by its rectangular I and Q coordinates.
Fig. 2 I/Q formats.
In digital communications, modulation is often expressed in terms of I and Q (a rectangular representation of the polar diagram). On a polar diagram, the I axis lies on the phase reference, and the Q axis is rotated by 908. The signal vector's projection onto the I axis is its I component and the projection onto the Q axis is its Q component. Figure 2 shows the I/Q representation.
I AND Q IN A RADIO TRANSMITTER
Fig. 3 I and Q signals in a radio transmitter.
I/Q diagrams are particularly useful because they mirror the way most digital communications signals are created using an I/Q modulator. In the transmitter, I and Q signals are mixed with the same local oscillator (LO), as shown in Figure 3. A 908 phase shifter is placed orthogonal to each other or in quadrature. Signals that are in quadrature do not interfere with each other. They are two independent components of the signal. When recombined, they are summed to a composite output signal. There are two independent signals in I and Q that can be sent and received with simple circuits. This simplifies the design of digital radios. The main advantage of I/Q modulation is the symmetric ease of combining independent signal components into a single composite signal and later splitting such a composite signal into its independent parts.
I AND Q IN A RADIO RECEIVER
The composite signal with magnitude and phase (or I and Q) information arrives at the receiver input. The input signal is mixed with the LO signal at the carrier frequency in two forms. One is at an arbitrary zero phase, while the other has a 908 phase shift. The composite input signal (in terms of magnitude and phase) is broken into I and Q components. These two components of the signal are independent and orthogonal. One can be changed without affecting the other.
Normally, information cannot be plotted in a polar format and reinterpreted as rectangular values without doing a polar-to-rectangular conversion. This conversion is exactly what is done by the in-phase and quadrature mixing processes in a digital radio. An LO, phase shifter and two mixers can perform the conversion accurately and efficiently. Figure 4 shows the receiver demodulating the input signal into I and Q components.
Fig. 4 I and Q signals in a practical radio receiver.
WHY USE I AND Q?
Digital modulation is easy to accomplish with I/Q modulators. Most digital modulation maps the data to a number of discrete points on the I/Q plane. These points are known as constellation points. As the signal moves from one point to another, simultaneous amplitude and phase modulation usually results. Accomplishing this modulation with an amplitude modulator and a phase modulator is difficult and complex. It is also impossible with a conventional phase modulator. The signal may, in principal, circle the origin in one direction forever, necessitating infinite phase shifting capability. Simultaneous AM and PM is easy with an I/Q modulator. The I and Q control signals are bounded, but infinite phase wrap is possible by properly phasing the I and Q signals.
A DIGITAL COMMUNICATIONS TRANSMITTER
A digital communications transmitter begins and ends with an analog signal. The first step is to convert a continuous analog signal to a discrete digital bit stream. This step is called digitization.
The next step is to add voice coding for data compression. Then some channel coding is added. Channel coding encodes the data in such a way as to minimize the effects of noise and interference in the communications channel. It also adds extra bits to the input data stream and removes redundant ones. Those extra bits are used for error correction or sometimes to send training sequences for identification or equalization, making synchronization (or finding the symbol clock) easier for the receiver. The symbol clock represents the frequency and exact timing of the transmission of the individual symbols. At the symbol clock transition, the transmitted carrier is at the correct I/Q (or magnitude/phase) value to represent a specific symbol (a specific point in the constellation). Then the values (I/Q or magnitude/phase) of the transmitted carrier are changed to represent another symbol. The interval between these two times is the symbol clock period. The reciprocal of this clock period is the symbol clock frequency. The symbol clock phase is correct when the symbol clock is aligned with the optimum instants to detect the symbols.
Fig. 5 A typical digital communications transmitter.
Fig. 6 A typical digital communications receiver.
The next step in the transmitter, as shown in the simplified block diagram of Figure 5, is filtering. Filtering is essential for good bandwidth efficiency. Without filtering, signals would have very fast transitions between states and therefore very wide frequency spectra - much wider than is needed for the purpose of sending information. A single filter is shown for simplicity, but in reality there are two filters; one each for the I and Q channels. This filtering creates a compact and spectrally efficient signal that can be placed on a carrier.
The output from the channel coder is then fed into the modulator. Since there are independent I and Q components in the radio, half of the information can be sent on I and the other half on Q. This method is one reason digital radios work well with this type of digital signal. The I and Q components are separate.
The remainder of the transmitter looks similar to a typical RF transmitter or microwave transmitter/receiver pair. The signal is converted up to a higher intermediate frequency (IF) and then further upconverted to a higher radio frequency (RF). Any undesirable signals that were produced by the upconversion are then filtered out.
A DIGITAL COMMUNICATIONS RECEIVER
The receiver shown in Figure 6 is similar to the transmitter but in reverse. It is more complex to design. The incoming RF signal is first downconverted to IF and demodulated. The ability to demodulate the signal is hampered by factors including atmospheric noise, competing signals and multipath or fading.
Generally, demodulation involves carrier frequency recovery (carrier lock), symbol clock recovery (symbol lock), signal decomposition to I and Q components, determining I and Q values for each symbol (slicing), decoding and de-interleaving, expansion to the original bit stream and digital-to-analog conversion, if required.
However, in more and more systems, the signal starts out digital and stays digital. It is never analog in the sense of a continuous analog signal such as audio. The main difference between the transmitter and receiver is the issue of carrier and clock (or symbol) recovery.
Both the symbol-clock frequency and phase (or timing) must be correct in the receiver in order to demodulate the bits successfully and recover the transmitted information. A symbol clock could be at the right frequency but at the wrong phase. If the symbol clock was aligned with the transitions between symbols rather than the symbols themselves, demodulation would be unsuccessful.
Symbol clocks are usually fixed in frequency and this frequency is accurately known by both the transmitter and receiver. The difficulty is to get them both aligned in phase or timing. There are a variety of techniques and most systems employ two or more. If the signal amplitude varies during modulation, a receiver can measure the variations. The transmitter can send a specific synchronization signal or a pre-determined bit sequence such as 10101010101010 to "train" the receiver's clock. In systems with a pulsed carrier, the symbol clock can be aligned with the power turn-on of the carrier.
In the transmitter, it is known where the RF carrier and digital data clock are because they are being generated inside the transmitter itself. In the receiver, this luxury does not exist. The receiver can approximate where the carrier is but has no phase or timing symbol clock information. A difficult task in receiver design is to create carrier and symbol-clock recovery algorithms. That task can be made easier by the channel coding performed in the transmitter.
MEASUREMENTS ON DIGITAL RF COMMUNICATIONS SYSTEMS
Complex tradeoffs in frequency, phase, timing and modulation are made for interference-free, multiple-user communications systems. It is necessary to accurately measure parameters in digital RF communications systems to make the right tradeoffs. Measurements include analyzing the modulator and demodulator, characterizing the transmitted signal quality, locating causes of high bit-error-rate and investigating new modulation types. Measurements on digital RF communications systems generally fall into four categories: power, frequency, timing and modulation accuracy.
Fig. 7 Power measurement.
Fig. 8 Power and timing measurements.
Fig. 9 Frequency measurements.
Power measurements include carrier power and associated measurements of gain of amplifiers and insertion loss of filters and attenuators. Signals used in digital modulation are noise-like. Band-power measurements (power integrated over a certain band of frequencies) or power spectral density (PSD) measurements are often made. PSD measurements normalize power to a certain bandwidth, usually 1 Hz. Figure 7 shows an example of a power measurement in an I and Q format.
ADJACENT CHANNEL POWER
Adjacent channel power is a measure of interference created by one user that effects other users in nearby channels. This test quantifies the energy of a digitally-modulated RF signal that spills from the intended communication channel into an adjacent channel. The measurement result is the ratio (in decibels) of the power measured in the adjacent channel to the total transmitted power, as shown in Figure 8. A similar measurement is alternate channel power, which looks at the same ratio two channels away from the intended communication channel.
For pulsed systems such as time division multiple access (TDMA), power measurements have a time component and may have a frequency component, as well. Burst power profile (power versus time) or turn-on and turn-off times may be measured. Another measurement is average power when the carrier is on or averaged over many on/off cycles.
Frequency measurements, as shown in Figure 9, are often more complex in digital systems since factors other than pure tones must be considered. Occupied bandwidth is an important measurement. It ensures that operators are staying within the bandwidth that they have been allocated. Adjacent channel power is also used to detect the effects one user has on other users in nearby channels.
Occupied bandwidth (BW) is a measure of how much frequency spectrum is covered by the signal in question. The units are in hertz, and measurement of occupied BW generally implies a power percentage or ratio. Typically, a portion of the total power in a signal to be measured is specified. A common percentage used is 99 percent. A measurement of power versus frequency (such as integrated band power) is used to add up the power to reach the specified percentage. For example, it can be stated "99 percent of the power in this signal is contained in a bandwidth of 30 kHz" or "The occupied bandwidth of this signal is 30 kHz" if the desired power ratio of 99 percent was known.
Typical occupied BW numbers vary widely, depending on symbol rate and filtering. The BW is approximately 30 kHz for the North American Dual-mode Cellular p/4 differential quadrature phase shift keying signal and approximately 350 kHz for a GSM 0.3 Gaussian minimum shift keying signal. For digital video signals the occupied BW is typically 6 or 8 MHz.
Simple frequency-counter-measurement techniques are often not accurate or sufficient to measure center frequency. A carrier "centroid" can be calculated which is the center of the distribution of frequency versus PSD for a modulated signal.
Timing measurements are made most often in pulsed or burst systems. Measurements include pulse repetition intervals, on-time, off-time, duty cycle and time between bit errors. Turn-on and turn-off times also involve power measurements.
Modulation accuracy measurements involve measuring how close either the constellation states or the signal trajectory is relative to a reference (ideal) signal trajectory. The received signal is demodulated and compared with a reference signal. The main signal is subtracted and what remains is the difference or residual. Modulation accuracy is a residual measurement.
Modulation accuracy measurements usually involve precision demodulation of a signal and comparison of this demodulated signal with a (mathematically-generated) ideal or reference signal. The difference between the two is the modulation error, and can be expressed in a variety of ways including error vector magnitude (EVM), magnitude error, phase error, I-error and Q-error. The reference signal is subtracted from the demodulated signal, leaving a residual error signal. Residual measurements such as this are very powerful for troubleshooting. Once the reference signal has been subtracted, it is easier to see small errors that may have been swamped or obscured by the modulation itself. The error signal itself can be examined in many ways; in the time domain or (since it is a vector quantity) in terms of its I/Q or magnitude/phase components. A frequency transformation can also be performed and the spectral composition of the error signal alone can be viewed.