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Wireless Communications

Module by: Thomas Almholt, farrukh inam. E-mail the authorsEdited By: Gene Frantz

Summary: This module summarizes the various wireless communications products offered by Texas Instruments. It is specifically aimed at college seniors beginning their senior project.

Introduction

We first learned about the propagation of electromagnetic waves through the works of Maxwell and Hertz. Later, Tesla demonstrated the transmission of information using these waves, and in 1898, Marconi first demonstrated wireless communication from a boat to the Isle of Wight in the English Channel. In 1948, Claude Shannon’s work established the possibility of error-free communication under restriction for data rate (R) and signal-to-noise ratio (SNR) in a digital communication system. Thus began the era of active research in information theory and channel coding, with the goal to achieve data rates at channel capacity (C) in a digital communication system.

Digital vs. analog transmission

Digital signals are easy to regenerate, as they operate in binary state. This is not true for analog signals, which have infinite states. A pulse in a digital system is affected by distortion because of frequency characteristics and noise present in the channel. Before it can degrade to an ambiguous shape, amplifiers in the transmission path restore the pulse shape to its original form and retransmit. Digital circuits are also more reliable and less costly to design. Digital hardware is flexible and reconfigurable via software and can accommodate the operation of different communication techniques on the same hardware. Digital techniques lend themselves easily to signal-processing functions that protect against interference and jamming and allow for encryption and information security.

Requirements for wireless systems

In wireless systems, different applications have different requirements in terms of range, data rate and mobility energy consumption, for example. A few of these are listed below.

Data rate

Sensor networks that monitor temperature, humidity, speed and acceleration usually require data rates from a few bits per second to about 1 kbps. The central node in a sensor network might require data rates as high as 10 Mbps, since its tasks are coordination and data gathering. Speech communication requires between 5 kbps and 64 kbps, depending on the fidelity and amount of compression. Cellular networks with higher spectral efficiency operate at 10 kbps, while high-speed data services like WLAN and 3G/4G can go as high as 100 Mbps or more by utilizing space and time diversity techniques.

Coverage and number of subscribers

The task of a communication system is to convey information at a distance (d) with minimum probability of error (Pb) using a minimum amount of transmit power. In a mesh network, the coverage area of a system can be made independent of the range by adding multiple base stations. In sensor networks, nodes can be converted to routers that can communicate with the coordinator nodes over multiple hops.

Fixed or mobile installation

Wireless systems are designed to operate in mobile environments; this incurs costs in terms of system performance, as channel effects such as multipath (fading) and speed of mobility (Doppler shifts) can degrade bit-error-rate performance and reduce channel capacity.

Power consumption

Wireless devices are battery-operated and designed to consume a minimum amount of power. This allows longer battery life and fewer recharge cycles. In limited energy scenarios, power-efficient modulation schemes are used, along with control hardware, to regulate power consumption during the transmission and reception of packets. Control hardware puts radios in low-power modes during inactive periods.

Spectrum utilization

Frequency spectrum is a natural resource; its efficient use is stipulated by regulatory authorities. A radio service provider can buy or lease a portion of the spectrum, where it can have complete control over its operations. Alternatively, a portion of the spectrum can be allocated to a service, such as a cordless phone, where users can then set up hardware for that service without needing a license. The industrial, scientific and medical (ISM) bands are license-free bands where proprietary communication techniques can be used to implement wireless services.

In multiple user environments, multiple access schemes are employed. These can be implemented in the following ways. Frequency-division multiple access (FDMA) allocates bandwidth to users in a frequency spectrum, where they can operate in specific frequency bands. Time-division multiple access (TDMA) is based on time-slicing the occupied spectrum and creating a schedule of when each pair of radios is on the air. Code-division multiple access (CDMA) is a spread-spectrum technique where each user is provided with a unique uncorrelated pseudo-random noise (PN) sequence (code) that can be used to despread the received signal before demodulation. The main advantage of CDMA over either FDMA or TDMA is that the frequency reuse factor is 100 percent. This means that the entire allocated frequency band can be used for transmission. Frequency reuse in FDMA and TDMA systems depends on the isolation between areas of operation, depending on the path loss of the radio channel. CDMA can reuse the allocated spectrum for all areas.

Blocks of a wireless communication system

To simplify the drawings, let's split a typical transceiver into its two major components, the receiver and transmitter, and describe them individually.

Block diagram of a receiver

Figure 1 is a system block diagram of a direct conversion receiver and transmitter. An antenna is followed by a bandpass filter, used as a band select filter. This eliminates out-of-band noise and presents the signal to the low-noise amplifier (LNA). The LNA then amplifies the desired signal, adding a minimum amount of inherent noise. The signal processed by the LNA is then down-converted to the desired IF frequency by a set of mixers operating in quadrature. These mixers are often image-reject mixers and have some gain as well. After down-conversion, the low-frequency IF signal is lowpass-filtered to remove aliasing components and converted to digital samples by the analog-to-digital converter. In the digital domain, more filtering is applied for channel selection, and plenty of signal processing is performed to remove any channel effect before the detection stage.

Figure 1
Figure 1 (Picture 8.png)

On the transmitter side, the digital I and Q data – which are already processed by a digital-to-analog converter, filtered and amplified – are up-converted by quadrature mixers to the carrier frequency of interest. After combining, the signal is again filtered to contain the spectral content of the signal in the required bandwidth stipulated by the emission mask. After that, it is applied to the power amplifier and transmitted over the air with an antenna (Figure 2).

Figure 2
Figure 2 (Picture 7.png)

Now let's get into the details for each of the blocks in the transmitter and receiver.

Antennas

Antennas are coupling circuits to space that radiate or receive information-bearing electromagnetic waves. In a receiving antenna, the EM wave impinging on the surface produces currents, which in a 50-ohm system are applied to an LNA for amplification and subsequent processing. On the transmitter side, the surface current density on the antenna produces a magnetic field around the antenna. If the current density is time-varying, an accompanying electric field is also produced; propagation takes place in a direction perpendicular to both the electric and magnetic fields. The total radiated power is given by a surface integral of the Poynting vector over any surrounding surface. The value of resistance that would dissipate this amount of power is called the radiation resistance, which is caused by the power radiated. The total resistance of the antenna comprises radiation resistance and resistance due to power loss. For high efficiency, the value of radiation resistance should be large.

Filters

Filters remove the effect of broadband noise and thereby increase the SNR of a desired signal. They are also used to select channels in multiple transmission environments and to remove image frequencies in broadband services and other out-of-band interference. In the transmitter, digital pulse-shaping filters are used for efficient utilization of the RF spectrum and externally to suppress RF splatter in adjacent channels.

Amplifiers

The RF signal at a receiver’s antenna is very small in magnitude. The IEEE 802.15.4 standard defines a minimum signal of -85 dBm = 3.16 pW, whose voltage in a 50-ohm system is 12.6 µV. At the detector, the typical signal requirement is at 1 mVp-p for detection and decoding of digital waveforms. To achieve this, low-noise amplifiers are used in the front end to amplify the signal up to the detection stage. The gain required in the receiver is usually between 60-90 dB – very high. Therefore, to avoid oscillations, this gain is distributed over different stages of the radio-frequency integrated circuit.

On the transmitter side, power amplifiers (PAs) are used to transmit the EM wave. PAs come in various classes and can be linear and nonlinear. They usually employ matching circuits between the output and the load. In practice, the output impedance of the active device is complex and varies with load; thus nonlinear complex impedance must be matched to a linear load. More often, the antenna impedance may be complex and vary with both the position of the transceiver and surrounding objects. This makes PA input and output matching a nontrivial task. In practice, a technique called load pull is applied to a matching circuit design. In this test, the output power is measured and plotted as a function of the complex impedance load seen by the transistor output stage. A tuner can accurately vary the output impedance while a power meter measures the power, keeping it constant. The impedance gives a contour on the Smith chart. As the output impedance varies, this changes the input impedance of the transistor, thus requiring the use of a second tuner such that the impedance seen by the generator remains constant.

Mixers

Mixers are fundamental building blocks that translate frequencies from one band to another for further processing without changing the information content. On the transmitter side, they up-convert a baseband signal for efficient transmission over a channel. At the receiver, they down-convert to a suitable intermediate frequency for the extraction of information. The frequency translation occurs with the help of an oscillator and RF signal applied to a strong nonlinearity and then filtering of the desired frequency band.

Oscillator

Oscillators produce sinusoidal signals that up-convert or down-convert an RF signal to the required frequency, where subsequent processing might begin. They are designed to operate at a specified frequency. Generally, there is an amplifier and feedback circuit that returns a portion of the amplified signal back to the input. When feedback is aligned in phase, sustained oscillators occur. In practice, they are not perfect, and drift in frequency from time to time. They are also susceptible to phase noise. Due to this, many transceivers operate them in a phase-locked loop (PLL) that can provide frequency stability and lower phase noise. Oscillators use an external crystal to provide a reference signal to PLL-based signal sources. The accuracy of the crystal is specified in ppm. Crystal accuracy is important because the transmit or receive bandwidth may have to be changed according to the drift in the crystal frequency.

Analog-to-digital converter

Analog-to-digital converters are required to convert analog signals to digital signals for baseband processing. After digitizing, signal channel selection can occur in the digital domain, as can equalization.

Transceiver system parameters

This section identifies concepts essential to understanding and evaluating an RF system. The information on these parameters is available in the product’s data sheet. Understanding the data sheet is key to integrating efficient communication systems.

RF communication range

Receiver sensitivity, transmitter output power, signal frequency and propagation environment determine how far apart the receiver must be from the transmitter for error-free communication. A complete expression encompassing these paramaters is given by the Friis equation, Equation 1:

graphics1.png

With isotropic antennas in free space, the signal power at distance d can be calculated with a path loss equation (Equation 2):

graphics2.png

Where:

Pt = the signal power in dBm at distance d

L = overall system loss

λ = wavelength

Pr = the signal power at the antenna

f = the signal frequency in MHz

d = the distance in meters from the antenna

n = the path loss exponent whose value is determined experimentally and varies under different propagation environments

For example, the required sensitivity of a ZigBee device operating at 2.4 GHz is -85 dBm. Assuming isotropic antennas with unity gains and ideal line-of-sight conditions and a transmit power of 1 mW (0 dBm), the maximum separation between the radios can be approximately 175 m. Increase the power by 6 dB, however, and the range approximately doubles, to 350 m.

Receiver sensitivity

Noise is a phenomenon that degrades signal quality and impairs the receiver’s ability to make correct decisions about symbol detection. Thermal noise is a zero-mean Gaussian random process and processes the same power spectral density for all frequencies of interest. It is superimposed on a signal as it travels through the communication chain, and therefore has an additive effect. It is thus commonly referred to as additive white Gaussian noise (AWGN). The power spectral density of double-sided white noise is N0/2. At room temperature, single-sided noise power spectral density in the 1-Hz bandwidth is calculated with Equation 3:

graphics3.png

The signal at the receiver is very small in magnitude and must be amplified before any meaningful information can be extracted from it. This is done using high gain amplifiers at the front end. The amplifiers also amplify noise and interference already present in the signal, and might even add their own noise to the processed signal. Such broadband noise is another reason why bandpass filters are used in the front end of a receiver. They attenuate out-of-band noise while keeping the signal of interest unchanged, thereby increasing the SNR.

Receiver sensitivity is the weakest RF signal that can be processed to develop a minimum SNR for achieving the required bit-error-rate (BER) performance. In AWGN, the sensitivity of a receiver can be derived from its noise figure. The noise factor of a receiver from the antenna port to the output of a detector is expressed in Equation 4 as the ratio of the SNR at the input to the SNR at the output.

graphics4.png

Noise figure is a measure of how much a system adds noise to a signal as it passes through it. It is given by 10 * log(F)(dB). Assuming that minimum SNR is required for obtaining the defined error rate, the corresponding sensitivity level is calculated with Equation 5:

graphics5.png

Where:

10 log(kTo) = -174 dBm/Hz is from (2.8)

Receiver noise bandwidth B is in Hz

NF = the overall noise figure of the receiver in dB

Data sheets specify this quantity for a specific bandwidth and BER performance. Therefore, it is often advantageous to use the smallest possible data rate in order to be able to use the smallest possible receiver bandwidth. Consider a Wi-Fi device with -83 dBm input sensitivity, a ZigBee at -97 dBm and a narrowband radio (CC1120) at -123 dBm.

From the Friss equation, you learned that for about every 6 dB in output power or input sensitivity, the range of the wireless system doubles. Therefore, a 40-dB improvement will equate to approximately 100 times the range.

Adjacent channel selectivity

Many different users must be able to broadcast at the same time. This necessitates separating the desired transmission from all the others at the receiver. One standard method is to allocate different frequency bands to various users; signals from different users can be separated using bandpass filters. Practical receive filters do not completely attenuate the frequency content of out-of-band signals, nor do they pass in-band signals completely distortion-free. Therefore, there is a requirement on the transmitters to spill the least amount of power in the adjacent bands.

This value is typically measured by applying a signal 3 dB above the sensitivity level of the system, adding a blocking signal in the adjacent channel, and increasing the power of the blocker until the receiver BER becomes the same as measured with 3 dB less signal and no blocker.

Maximum power is limited by nonlinearities

The building blocks of a receiver generate harmonics (tones at an integer multiple of the fundamental frequency) due to nonlinearities. This causes the translation of out-of-band frequencies onto in-band channels, thereby degrading SNR.

Harmonic distortion is the ratio of the amplitude of a particular tone to the fundamental. It is usually not a problem in a receiver and can be filtered out after the LNA. Cross-modulation occurs when a strong interferer and a weak desired signal present themselves at the front end. Amplitude modulation on the strong interferer is transferred to the desired signal through interaction with a receiver’s nonlinearity. A figure of merit of a receiver is the 1-dB compression point, P1dB, where the gain of the system is reduced by 1 dB as input power increases. Receiver sensitivity and the input 1-dB compression point set the dynamic range of a receiver.

Multitone distortions

Intermodulation distortions arise when more than one tone is present at the input. The third-order intercept point (IP3), which is measured by a two-tone test, shows to what extent the receiver can handle an environment with strong undesired signals. Second- and third-order intermodulation products are problematic in a receiver, as they fall very close to its operating frequency. The amplitude of the fundamental signal increases in proportion to the input signal, whereas the third-order intermodulation product increases as a cube of the fundmental. The IP3 is defined as the intersection of two lines in Figure 3 below.

Picture 10.png

Figure 3

Wireless systems operate in an environment where they encounter strong interfering signals from other transmitters. Since a large signal tends to reduce the average gain of the circuit, the weak signal may experience a diminishingly small gain. This is called desensitization, and for a large-enough interferer, the gain for the weak desired signal goes to zero and the signal is blocked.

An interferer that desensitizes a circuit even if the gain doesn’t go to zero is called a blocker. Receivers must be able to withstand a blocking signal 60 to 70 dB greater than the desired signal. This information is specified in data sheets as adjacent and alternate channel-rejection performance of radio-frequency integrated circuits.

Conclusion

TI’s wireless technology boasts a range of products, from low-power applications to cellular baseband processors. In low-power applications, TI has a broad range of devices that can suit any application, from consumer/personal networking to industrial monitoring and asset tracking. TI can provide complete solutions with reference designs, hardware development kits and software to jumpstart any wireless project, ranging from sub-1 GHz to ZigBee low-power networking and the Internet of things.

A few of TI's products for low-power wireless applications are listed in Table 1.

Table 1. TI Products for low power wireless applications.

graphics6.jpg

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