digital hardware and communications

Today, multi-gigabit per second signalling speeds exist even in the most basic consumer products such as personal computers. In these many properties, previously insignificant, are now critical. For example, PCB layout, controlled impedance lines, connector mismatches, transmission line discontinuities, differential delays, compliance voltages, common mode rejection and passive component Q factors. We know from the Fourier transform that, theoretically with no noise, perfect digital signals occupy an infinite bandwidth. Bandwidth is however scarce and expensive so in practice it has to be limited. Noise in its many forms is always present, especially additive white Gaussian noise (AWGN).  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, for example Nyquist, Shannon, Hartley, Laplace and Nyquist. 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 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 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. A common 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 by a radio wave, 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 and the best conversion times were lengthy by today's standards. Although this was the best technology at the time, the conversion rate was relatively slow and could not accommodate rapid changes of the input voltage. ADC resolution and accuracy in equipment like digital voltmeters and temperature monitors were more important than parameters like analog bandwidth and maximum sampling rate. However, as technologies developed sampling optimised ADCs were developed, digital signal processing (DSP) overheads and speeds increased, ADC signal input frequencies moved from DC through audio and eventually well into the radio frequency (RF) spectra. These ADCs are specified for sampling applications and are the ADC type most commonly used today.

Software defined radio (SDR) uses software, or more precisely firmware, to manipulate the modulated waveform. Suitable resources directed at writing software in a language like C or C++ can develop executable programs to allow the DSP to demodulate almost any form of modulation, analog or digital provided the frequency spectrum was within the Nyquist sampling capability of the radio. The noise performance of a SDR radio and therefore dynamic range is however usually less than could be achieved with an equivalent superheterodyne receiver. With the present technology an SDR receiver usually requires either a low noise amplifier at the front end perhaps followed by a block downconverter, dependent on the frequency range of interest. 

FCL is experienced in specifying many different of ADCs: successive approximation, sigma delta, flash, pipelined and time interleaved. The sampling types have been implemented at baseband and at higher order Nyquist zones using over-sampling and under-sampling or baseband sampling. Parameters addressed include integrated non-linearity (INL), differential non-linearity (DNL), 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 the second, third and fourth generation digital cellular networks (2G, 3G and 4G respectively). 4G used to be known as long term evolution (LTE) but in 2011 LTE-advanced (LTE-A) was released which specified several improvements including multiple antenna technologies and greater data rates, especially for mobile and semi-mobile stations. This is achieved using carrier aggregation which enables the effective channel capacity to be increased by combining contiguous or discontinuous 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 cellular services into tunnels, buildings or areas which would otherwise receive poor coverage. The ROF work was focused on the development of a new low cost remote unit, the electrical and optical aspects of it. FCL has also supported work on the class AB power amplifiers using parallel combined bipolar devices. The parameters covered included power ramp measurements, non-linear distortion measurements and feed forward linearisation techniques.

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Direct Conversion and Software Defined Radio

FCL has been working with traditional superheterodyne receiver architectures for some 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 these types compared to the superheterodyne.

The issues of software defined radio (SDR) compared with the more traditional physical hardware architectures is another area that is engaged. 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 equivalent frequency spectra. It is a relatively straightforward transformation then to apply baseband filtering, such as the raised cosine or Gaussian laws, to minimise the inter-symbol interference (ISI) and optimise the occupied bandwidth. 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 submarine optical fiber cables for connecting high capacity telecommunications infrastructures. The modulation source was applied at baseband using the synchronous digital hierarchy (SDH) standard at a speed of 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 amplifiers (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 digital cellular 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 uses a relatively wide bandwidth 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 such as 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 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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