Introduction

In Chapters 2 and 9, I discussed studies that illustrate a considerable weakness of the GSM half-rate codec in the face of noise. I also brought to light study results showing that an effective NR algorithm may be able to remedy some of the HR weakness and lift the voice-quality performance back to par for signals with a relatively poor SNR (see Figure 9.13).

As new wireless codecs are introduced at the market place, many service providers wonder whether VQ systems are effective in enhancing quality, as higher compression ratios are used in an attempt to reduce spectrum requirements and augment air capacity.

In this chapter, I introduce a procedure for testing the hypothesis that a new codec with a higher compression ratio offers an inferior (or equivalent) voice-quality performance in comparison to an existing codec, specifically under noisy conditions.

If the hypothesis proves to be true, then there is a need to test the hypothesis that VQS can lift the performance level of the new codec to a close proximity (or beyond) of the existing codec (under noisy conditions and without VQS).

Procedure

The procedure outlined here involves two phases.

Phase 1 (see Figure 15.1)

1. (1) Send pre-recorded speech signals from point B without noise. Capture and record speech signals (pre-recorded) at point A.

2. (2) Send pre-recorded speech signals from point E with noise. Capture and record speech signals (pre-recorded) at point D.

3. (3) Send pre-recorded speech signals from point B without noise. Capture and record speech signals (pre-recorded) at point A while sending speech signals from point A (double talk).

4. […]

In survey after survey potential and actual users of wireless communications indicated that voice quality topped their reasons for selecting a specific service provider. While providers have been well aware of this key component powering their offering, they have not always been certain as to the specific methodology, resolution elements, equipment type, architecture, trade-offs, and rate of return on their particular investment that elevate the perceived voice-quality performance in their network.

It is only natural that voice quality in wireless networks has become a key differentiator among the competing service vendors. Network operators, network infrastructure planners, sales representatives of equipment vendors, their technical and sales support staff, and students of telecommunications seek information and knowledge continually that may help them understand the components of high-fidelity communicated sound.

Throughout the 1990s applications involving voice-quality enhancements, and specifically echo cancelation, have induced fresh inventions, new technology, and startling innovations in the area of enhanced voice performance. The initial echo canceler (EC) product implementations existed for about a decade before a diverse array of voice-quality enhancement realizations emerged to meet the evolving needs of digital wireless communications applications.

Early EC implementations were limited to very long distance (e.g., international) circuit-switched voice and fax applications where echo was perceived (in voice conversations) due to delays associated with signal propagation. The EC application soon expanded beyond strictly very-long-distance applications as further signal processing and dynamic routing along the communications path added delay to end-to-end voice transport.

Part I reviews the major voice codecs, their history, and their relative perceived quality. Voice-coding architectures are the building blocks of transmitted voice. They are the core that shapes the characteristics and quality of transmitted speech. Nevertheless, they are treated in this book only as background to the main subject, which deals with impairments due to transmission architecture and environment, and their corresponding remedies that immunize and repair any potential or actual spoil. Since the effectiveness of the various remedies depends on that underlying coding, it is essential that these designs be understood so that remedies can be fine tuned and customized to suit the particular characteristics of the underlying voice architecture.

Chapter 5 is devoted to the subject of noise reduction. Noise reduction is the most complicated feature among the voice-quality-assurance class of applications. It also requires a higher-level understanding of mathematics. This discussion, however, substitutes numerous mathematical expressions for intuition, ordinary analogies, and logical reasoning, supplemented by graphical and audio illustrations.

The analysis gets underway with the definition of noise, a definition consistent with the principles and characterization employed by a typical noise-reduction algorithm. It then introduces and explains the mathematical concept of time and frequency domains and the transformation process between the two. Once the reader is armed with the understanding of time- and frequency-domain representations, the analysis proceeds to a discussion of the noise-estimation process. The presentation then moves ahead to examine the suppression algorithm, which employs the noise-estimation results in its frequency-band attenuation procedures. The next segment contains a presentation covering the final algorithmic steps, which involve scaling and inverse transformation from frequency to time domains.

The next section in Chapter 5 reflects on key potential side effects associated with noise-reduction algorithms including treatment of non-voice signals. It points to key trade-offs and adverse-feature interactions that may occur in various GSM and CDMA networks – a subject that is covered much more thoroughly in Part V – Managing the network. The final section offers an examination of the network topology and placement of the noise-reduction application within it.

Noise in wireless networks

Background acoustic noise is a major voice-quality irritant that is, unfortunately, abundant in wireless communications.

Introduction

Chapter 7 reviews the optional placements of the VQS functions relative to the mobile-switching center and the base-station controller, since placement impacts voice performance, applications, deployment cost, and data-detection algorithms. The first section of this chapter covers wireless-network architectures that provide comprehensive signal processing coverage for mobile-to-mobile call applications. The topic of economics and architectural trade-off associated with voice-enhancement systems is also addressed. The second part of the chapter presents an analysis of the techniques employed by a voice-quality system when coping with data communications without interfering or blocking its error-free transmission. The analysis includes descriptions of data-detection algorithms based on bit-pattern recognitions. The scope encompasses circuit-switched and high-speed circuit-switched data (CSD and HSCSD respectively) services. Finally, the third section describes tandem-free operation (TFO), its potential impact on speech transmission and data communication, and potential features and architectures.

The chapter characterizes two major themes and their joint interplay: (1) mobile-to-mobile network architectures with voice-quality enhancements, and (2) mobile data communications. It elaborates on various applications, technical challenges, and potential solutions. It is also intended to impart a sharper awareness of where technology is heading, and what constitutes winning features in the race to provide products that deliver superior voice quality in the wireless-communications arena.

The surge in data communications has spilled its fervor into wireless applications. Demand for higher-speed data access and the remarkable growth of internet applications have fueled the growth of this industry.

Introduction

This chapter portrays the 2G, and 3G network topologies, and their impact on VQA feasibility and architecture. It provides an evolutionary examination of the process leading from the 2G to 3G wireless architecture, and it presents a parallel progression of placement and applicability of the VQS that supports the evolving infrastructure.

3G promotions and promises

Anyone following the latest developments in wireless telecommunications has certainly noticed the hype concerning the 3G wireless era. “It sets in motion high-speed web-cruising via your cellphone,” the exuberant technologists exclaim. “It lets you watch a movie on your cellphone,” the ecstatic entrepreneurs predict, waiting for you to part your lips in awe. “It represents an amazing political transformation,” the sociological gurus gasp. “Imagine – a single standard, with the GSM and TDMA (IS-54) advocates surrendering to a competing religion – CDMA,” the 3G zealots chant in unison. Although these are exciting and enticing statements, they represent rumour and, at best, are actually only half-truths.

Considering the complexities, it may take quite some time beyond launching the early pieces of 3G networks for wireless-data rates to match existing wireline digital subscriber line (DSL) and cable-modem speeds. Presently, the 3G theoretical speed limits are severely constrained by implementation intricacies.

The 3G IMT-2000, UMTS, and cdma2000 capabilities are founded on the basis of foresight and planned service capabilities that require wide-band virtual channels, which enable full-motion video transmission and very high-speed data-transfer options.

Introduction

It must have happened to most of us. At some point through our lives we came across someone whom we deemed an “audio nut.” (Some of us may have even impersonated that one special character.) That singular person would not listen to music unless it was played back on an exceedingly pricey hi-fi system. He or she actually did hear a titanic disparity in sound quality between what we would be taking pleasure in on a regular basis and what he or she regarded as a minimum acceptable threshold.

In all probability, we might have adopted the same mind-set had we been accustomed to the same high-quality sound system. It is a familiar human condition – once a person lives through luxury, it grows to be incredibly hard getting used to less. How quickly have we forgotten the pleasure we took in watching black-and-white TV, listening to the Beatles on vinyl records, Frank Sinatra on AM radio, etc. But hey, we have experienced better sound quality and, thus, we refuse to look back.

Wireless telecommunications is entering its third generation (3G). Infancy started with analog communications. It developed through childhood in the form of GSM and CDMA, and has crossed the threshold to puberty with cdma2000 and W-CDMA – its third generation. Voice quality in wireless telecommunications is still young and looking up to adolescence, but technology has advanced appreciably, and most of us have been content with its voice performance.

Introduction

My sound is a concept that provides a preferred-user segment (business-class segment) with control over customized sound atmosphere, flavor, clarity, and quality.

Speech and sound quality are an essential part of wireless communications. Unadvertised step-ups and upgrades in network voice-quality capabilities go, for the most part, unnoticed by subscribers, particularly when these capabilities address insufficiencies rather than impairments. For example, echo is a clear impairment while inadequate speech level in the presence of listener noise would be viewed as insufficiency. Intrusive background noise and poor signal-to-noise ratio (SNR), more often than not, would be considered as an insufficiency, while punctured radio coverage and even echo would seize top awareness on the impairment list.

Voice-quality insufficiencies can cross the threshold of a subscriber's awareness (or lack of it) if control over their intensity is given to users throughout or prior to a communication session. The exercise of control may attract users' attention and have them notice and appreciate the enhancement, while simultaneously provide a specific subscriber segment with the pleasure of being in command of their sound output.

One way of letting a subscription-based class of preferred subscribers assume control over certain parameters affecting speech levels and noise-reduction aggressiveness may be implemented via a DTMF sequence, which these users may apply throughout a phone conversation. When striking a DTMF sequence during a call, an authorized user may assume control over the VQS channel he or she is seizing.

Introduction

Chapter 10 presents a basic template that may be used by service providers as part of their request for information from vendors. The chapter elaborates on the various elements beyond voice performance that make the VQS easy to manage and easy to integrate within the operation of the network. The information is rather dry, but highly useful as a reference. Readers of the book who are not interested in the system engineering and operational requirements may skip this chapter in their pursuit for understanding of the magic that make voice-quality systems enhance speech communications.

Management-systems overview

General requirements

Owing to business requirements, a centralized network-management system for VQS is necessary. A VQS management system is an integral part of the operations-support system, which provides the management backbone for the entire service offered by the mobile carrier. As a rule, a management-system architecture operates in a hierarchical structure, as shown in Figure 10.1. The lowest level in the hierarchy is the element-management system (EMS). It allows network operators to monitor the alarm status of individual systems from a centralized (remote) location; it reviews current and historical alarm records; it controls individual systems; it examines equipment-inventory information; it provides equipment parameters; it operates equipment functions; and it analyzes alarm information.

The second level in the hierarchy is the network-management system. This system collects information from all EMS in the network to provide a network view.

The accompanying website (www.cambridge.org/9780521855952) contains audio illustrations of various aspects discussed in the book. These examples are presented in the wav format; they may be listened to on a personal computer with Microsoft Windows Media Player, RealPlayer, or other audio programs that play the wav format. Specialized programs such as Cooledit are best suited for the demonstrations. They offer an audiovisual experience and the ability to analyze and manipulate the signal.

The audio illustrations are divided into the following groups:

• Codec illustrations, where the same speech segment is played through a variety of particular codecs.

• Hybrid echo, where the same speech segment produces echo as a function of delay and ERL combinations.

• Various recordings of noise types, including different crowd noises, wind, seashore, busy street, and white noise.

• Acoustic-echo control under noisy conditions with different comfort-noise matching.

• Recordings that demonstrate the “before” and “after” VQS processing of a conversation taking place under noisy conditions.

• Examples related to cases discussed in Chapter 12.

• DTMF and ring-back tones.

The intent of the audio illustrations is to highlight certain elements that can be better depicted by having them listened to. They can also serve as raw material for those who wish to use the demos in their own presentations and test platforms.

Codec illustrations

The codec illustrations on the website are all produced without background noise or transmission errors.

Since the early 1990s and for the 13 years that followed I was in charge of specifying and managing the design and development of voice-quality enhancement algorithms and systems in Lucent Technologies, and later in NMS Communications. When taking on the task, I was astonished by the secrecy surrounding the make-up of the minute algorithm details that separated the exceptional from the second-rate performers; the ones that elevate the performance of the voice-quality algorithms to significant heights versus their counterparts, which adhere to standard textbook and public-domain prescriptions.

And although I found out that there was no lack of technical material addressing the subject of voice quality, I learned that the many books, articles, and papers devoted to the subject focused on the math, while steering clear of the practical heuristics that are the backbone of any successful implementation. Their analysis was mostly academic – deep, technically precise, but nonetheless narrow. It addressed a single aspect of voice quality, whether it was electrical or hybrid-echo cancelation, non-linear acoustic-echo control and suppression, adaptive and automatic gain control, or noise reduction, rather than an interactive blend. It was intended for the few subject experts and algorithm designers rather than the user, the product manager, the troubleshooter, the marketing and sales personnel, and – most importantly – those responsible for making decisions affecting quality of service and investment in voice-quality products.

Introduction

This chapter describes test and evaluation procedures of performance associated with the various voice-quality applications. The telecommunications equipment marketplace is filled with a variety of echo canceler (EC) and voice-quality systems (VQS) promoted by different vendors. Noticeably, the characteristics and performance of these products are not identical. In addition, the non-uniformity and arbitrary judgment that is often introduced into the EC and VQS product-selection process makes the network operator's final decision both risky and error prone. This chapter describes the criteria and standards that are available to facilitate methods for objectively analyzing the benefits of EC and VQA technology when confronted with multiple EC and VQS selection choices. The scope includes procedures for evaluating the performance of electrical (hybrid), acoustic-echo control, noise reduction, and level optimization via objective, subjective, laboratory, and field-testing methods.

This chapter brings to light a list of tools and standards designed to facilitate the voice-quality assessment process. It is intended to provide a methodology for objectively analyzing the benefits of voice-quality assurance technology in situations where network operators are confronted with multiple VQA system choices.

Voice-quality application features required by network operators typically include the following capabilities:

• network hybrid (electrical)-echo cancelation,

• acoustic-echo control,

• automatic volume control,

• adaptive level control (noise compensation) as a function of surrounding noise,

• noise reduction,

• non-voice applications.

Voice-quality application and performance assessment criteria encompass the following areas:

• application features and performance,

• maintenance features,

• environmental considerations,

• reliability thresholds,

• density and heat-dissipation characteristics,

• level of integration with key infrastructure elements.

Judging audio quality and performance of hybrid-echo cancelation

The literature and descriptive material distributed by many VQS vendors tends to concentrate on specific strengths, while glossing over the technical details that are vital in deriving a complete and accurate assessment of the VQS capabilities.

Introduction

This chapter examines the sources and the reasons for the existence of acoustic echo in wireless networks. It explains how acoustic echo is different from hybrid or electrical echo, and how it can be diagnosed away from its hybrid relative. The chapter follows the description of the impairment by examining the present methods for properly controlling acoustic echo in wireless communications. It also gives details of how background noise makes it more difficult to control acoustic echo properly. It describes those particular impairments that may be set off by some acoustic-echo control algorithms, specifically those built into mobile handsets, and it describes how they can be remedied by proper treatment brought about by means of voice-quality systems (VQS) in the network.

Acoustic echo and its derivatives are common problems facing customers of wireless communications services. Unlike electrical echo, these problems are rarely anticipated during initial deployments of wireless infrastructures because they are supposed to be addressed by handset manufacturers and controlled inside the mobile phone.

Unfortunately, many handset solutions do not control acoustic echo properly. In fact, a considerable number of mobile handsets introduce solutions that exacerbate the impairment by spawning severe side effects in the form of noise clipping and ambiance discontinuity. These side effects are, in most cases, more damaging than the impairment affected by the acoustic echo on its own.