digital hardware and communications

Today, multi-gigabit per second (Gbit/s) signalling speeds are deployed even in the most basic consumer products. The associated data streams propagate across component interfaces using suitably designed transmission lines. The huge bandwidths necessary mean that the electrical transmission properties of these are critical to achieving reliable performance. These include PCB layout aspects: controlled impedance architecture, connector mismatches, discontinuities (through holes and interfaces) and differential delays. Electrical properties include: compliance voltages, common mode rejection and component Q factors. We know from the application of the Fourier transform to streams of digital data that, assuming no noise, perfect digital signals occupy an infinite bandwidth. That is not possible of course so we have to settle for the largest possible bandwidths that we can achieve. If we require to modulate a carrier and radiate this, freespace bandwidth is scarce, expensive and possibly not even available from the licensing authorities, so in practice it has to be limited. Noise in its many forms is always present. Additive white Gaussian noise (AWGN), also known as Johnson, Nyquist, thermal or simply kTB noise is the type which mostly influences the integrity of digital data. Some transmission errors must therefore be tolerated. Chris Angove, represented by FCL, has a good understanding of the related theories which apply to digital communications and how they may be applied, for example those originating from the works of Nyquist, Shannon, Hartley and Laplace. The frequency spectra of high speed data can extend well into the gigahertz region, generally considered part of the microwave spectrum. Accordingly, FCL's extensive experience in RF and microwave engineering has developed into a wider brief extending into many areas of digital hardware interfaces and communications equipment design. Some of this is summarised in the following sections.

Analog/Digital Signal Conversion (ADCs and DACs)

Although we live in a digital world, most natural physical phenomena are analog. For example, radio wave propagation and any form of modulation, even if it is described as a 'digital' is actually analog. Here, digital refers to the type of information being carried and not the way it is used for the process of modulation. For example, one form of digital modulation is quadrature amplitude modulation (QAM) which relies on changes in both amplitude and phase of the carrier. One type is 256QAM which is designed to distinguish amongst 256 different states of amplitude and phase, each with a unique 'symbol'. The modulated carrier may be propagated through free space by radio waves, such as a satellite communications channel, or perhaps carried on a cable. Even with the propagation path (wave or cable) being analog, we ultimately need to handle digital signals at each end of the communications channel. Therefore, there needs to be at least one digital to analog converter (DAC) at the transmit end and one analog to digital converter (ADC) at the receive end. A common architecture is a 'complex baseband' in which the amplitude and phase of the carrier is represented by quadrature components: the in-phase (I) channel and the in-quadrature (Q) channel. In this case there would be two identical ADCs or DACs, one for each quadrature component.

Early ADCs were designed to simply convert an analog input voltage into a digital form using a successive approximation register (SAR). The best conversion times were lengthy by today's standards. SAR based ADCs are still used today in accurate but slow responding devices such as digital multimeters and temperature sensors. More recent ADCs are sampling types, found in many different types of communication equipment. Sampling theory and the work of Nyquist and Shannon feature prominently. FCL has a good knowledge of many ADC architectures including: SAR, time interleaved, pipeline, sigma delta, flash/parallel and photonic sampling.

Typically, in today's electronics hardware, there appear to be more ADCs in use than DACs. Perhaps that is because more analog parameters are processed in digital form and there is a relatively infrequent need for an analog output. FCL also has a good level of knowledge and experience of DACs. With these we need to carefully select a suitable sampling frequency and make allowances for the shaping of the analog spectrum by an inverse sinc function. The quality of the output analog waveform is influenced by the 'order hold' that is used: what happens to the analog output between samples.

Over the years, as digital signal processing (DSP) overheads and speeds have increased, ADC signal input frequencies have moved from near DC, through audio and are now well into the radio frequency (RF) spectrum. In the typical receiver architecture, it has become possible to now perform many of the functions digitally that were previously done in analog. The configuration of digital hardware is usually programmable by software, often a form of hardware description language used to generate firmware: specific instructions for configuring the hardware. Such digital implementations have the major advantage that their configuration may be updated and modified in code, provided that this does not attempt to exceed the limitations of the hardware and of course the Nyquist sampling limitations. A common piece of digitally programmable harware may be used to operate both across a range of frequencies and to demodulate or modulate a variety of modulation types, both digital and analog.

Software defined radio (SDR) refers to a flexible technique which may be applied to re-configurable radio hardware in order to set up a specific radio architecture. The SDR configuration is defined by how the associated SDR software is programmed. The SDR software may define the frequency that the target system will operate at, the type of modulation used and various other radio-like parameters. It is ofter written in an intermediate language like C or C++ and compiled before programming the hardware. More recently, higher level applications like MATLAB have allowed the generation of SDR code. The dynamic range of a SDR radio is improving but is not yet equivalent to a good quality superheterodyne radio designed for a similar frequency. The superheterodyne architecture will not however have the re-programmable flexibility of the SDR version. Currently we are at an intermediate stage with SDR. ADC sampling speeds, allowing for the Nyquist (factor of two) sampling criterion, are still insufficient to allow direct (baseband) sampling for some of the higher operating frequencies, such as those used in digital cellular and satellite communications. So it is often necessary to use some analog circuits at the front end of a SDR receiver. Usually a low noise amplifier (LNA), some filtering and a mixer to change the incoming frequency band to a lower one which is more manageable for the current technology. Actual sampling may then be performed at baseband or zero-IF, where the converted frequency band starts at DC; across a frequency range starting near DC or across a band of frequencies (bandpass or undersampling).

FCL is experienced in specifying many different types of ADCs and incorporating them into multiple designs. Parameters addressed include integrated non-linearity (INL), differential non-linearity (DNL), kTB and quantisation noise and effective number of bits (ENOB), missing codes, aperture jitter, dithering and multiple carry issues. These have naturally led onto several digital signal processing (DSP) phenomena such as spectral leakage, windowing and the implementation of forward and inverse fast Fourier transforms.

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Digital Cellular Communications

FCL has experience of base station and remote station hardware used for 2G, 3G and 4G digital cellular networks. 4G used to be known as long term evolution (LTE), but the performance of LTE, especially for mobile operation, was disappointing so, in 2011 LTE-advanced (LTE-A) was released. This specified several improvements over LTE including multiple antenna technologies and higher data rates for mobile and nomadic stations. This is achieved using carrier aggregation which enables the effective channel capacity to be increased by combining contiguous or discontiguous frequency bands, even if they are in different service bands. This resulted in LTE informally being relegated to 3.9G with LTE-A taking up 4G. FCL has supported the development of products incorporating RF over optical fiber (ROF) technology intended to extend all of these digital cellular services into tunnels, buildings or areas which would otherwise receive poor or no coverage. The ROF work was focused on the development of a new low cost remote unit, especially the electrical and optical aspects.

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Direct Conversion and Software Defined Radio

FCL has been working with traditional superheterodyne receiver architectures for several years now and has a good understanding of the common digital receiver front end configurations. These include zero-IF (homodyne or direct conversion), low-IF and direct IF (or bandpass) detection. FCL has a good appreciation of the properties, advantages and disadvantages of each of these types compared to the superheterodyne.

The issues of SDR compared with the more traditional physical hardware architectures is another working area. Many of the SDR algorithms are already established so most of FCL's support has been working with the DSP implementation in developing fully integrated, re-configureable and upgradeable receiver products.

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Baseband Waveforms and Digital (Quadrature) Modulation

FCL has implemented Fourier transforms to convert regular voltage-time pulse trains found in the baseband to their associated frequency spectra. It is a relatively straightforward transformation then to apply the baseband filtering law as required to shape the waveform to minimise the inter-symbol interference (ISI) and to optimise the occupied bandwidth after modulation. A further transformation (complex frequency shift) may then be applied to (digitally) modulate the baseband onto a carrier.

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Synchronous Telecommunications and RF Over Optical Fiber (ROF)

Whilst supporting a client's team responsible for designing terminal equipment for optical fiber submarine cables, FCL worked on techniques for dense wavelength division multiplexing (DWDM) using Mach Zehnder (MZ) optical intensity modulators. These were used to impart high speed digital data streams onto optical fiber cables for carrying high capacity telecommunications information. The modulation source was applied at baseband using the synchronous digital hierarchy (SDH) standard at a speed known as STM-64, equivalent to a synchronous optical network (SONET) speed of OC-192, or approximately 10 Gbit/s for each optical wavelength, around 1550 nm. FCL's input was in designing a device for controlling the phase of the clock source using I-Q modulators. The clock source itself was at approximately 10 GHz, and the bandwidth of the whole baseband after filtering ranged from approximately 50 kHz to nearly 20 GHz.

One of FCL's assignments with another team was to predict the performance of a proposed 'RF over optical fiber' (ROF) system end to end through the electrical to optical and then back again to the electrical domain. This system was proposed to carry various communications channels via a remote location as a contingency. FCL was responsible for specifying the optical and electrical components for procurment, followed by building and demonstrating the prototype to the customer hands-on, and all to tight timescales. The optical components used included MZ intensity modulators, DWDM filters, circulators, attenuators and an erbium doped fiber amplifier (EFDA). The demonstration was highly successful, giving the client new expertise and enhancing their range of products.

Another ROF project was supporting the development of a remote unit designed to extend the coverage of digital cellular networks into tunnels or areas of previously inadequate coverage. It was also used for effectively increasing the capacity inside large buildings such as airports and sports stadia. Essentially the same hardware, except for a few frequency dependent components, was common across all of the European and American digital cellular services: 2G, 3G and 4G/LTE/LTE-A. The stand-alone unit was suitable for supporting single (one uplink and one downlink) band or dual (two uplink and two downlink) bands simultaneously.

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Some of FCL's more recent work has been concerned with the latest generation of carrier access types for wireless networks, orthogonal frequency division multiplex (OFDM) which supports relatively wide bandwidths containing closely spaced sub-carriers. OFDM access methods are used in many areas including 4G/LTE/LTE-A digital cellular communications, WiMAX, WiFi and digital terrestrial broadcast (DAB, DVB-T and DVB-T2). Actually, OFDM access (OFDMA) is not a particularly new technology. It was suggested many years ago, soon after the discovery of the 'fast' algorithm for the Fourier transform by Cooley and Tukey. However, it has only been in the last few years that small, portable, battery operated devices such as radios and mobile handsets have included sufficient processing power to make it a realistic solution in mobile services. FCL's knowledge of the principles of OFDM, extend to its practical used in a heavy multipath environment, together with link calculations and special properties such as Doppler shift, cyclic prefix, convolutional coding, Reed Solomon coding, scalable bandwidths and modulation methods.

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Sampling Theory

Sampling theory is of course central to the operation and understanding of many types of ADCs, DACs and digital signal processing. With the help of well known and reliable references, FCL has dedicated time to researching and understanding this important subject. This has helped immeasurably to provide a better understanding of many of the issues that clients' digital hardware presents.

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Laplace and Fourier Transforms

With todays math CAD applications such as Matlab® and MathCad®, both of which FCL has invested in, performing important transforms for signal processing, such as those of Laplace and Fourier is quite routine. FCL has dedicated resources to understanding the theories involved and, importantly, how they are used and how the results may be interpreted in the real world. 

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