Mobile telephony is the telecom success story of the past decade and
its spectacular growth has several decades yet to run. This success,
however, has been based on “mobility,” not on “telephony.”
Subscribers have been willing to pay extra for the ability to communicate
on the fly, despite inconsistent service and poor voice quality. But in
the next five years, this will improve. In fact, you will be able to get
mobile phone service that sounds better than a landline phone call.
The significant difference between mobile and land-based telecom
services is the vibrant competitive environment in mobile telephony. While
residential landline customers have, in reality, only one viable provider
of local telephone service, wireless customers in U.S. markets have four
to six carriers to choose from -- Verizon, AT&T Wireless, Sprint,
Cingular, Nextel, VoiceStream, etc. Likewise, all major European markets
have multiple wireless service providers. Indeed, even third world
countries with PTT monopolies have introduced wireless telecom
competition. The result of this competition has been responsive service,
interesting new services, more innovative packaging of standard services
-- such as bundling of minutes and prepaid accounts -- and dramatic
subscriber growth.
IMPROVING COVERAGE
Of course, subscriber growth has stretched carrier facilities to the
limit, causing service problems in certain areas. Just getting enough
capacity to handle the growing subscriber base has been a problem. Radio
spectrum is limited and expensive and, until recently, the FCC restricted
how much radio spectrum any one carrier could purchase. But access to new
spectrum and ever improving radio technology is helping. And competitive
pressure drives a constant effort to add capacity -- both additional cell
sites and more efficient technology -- so as to guarantee service
availability.
In some European markets, mobile service has penetrated 80 percent of
the population and subscriber growth has begun to slow, giving carriers a
chance to catch up with traffic growth. But as coverage and availability
become a given, wireless carriers are looking for new competitive
frontiers. Among the choices are diverse new services -- e-mail, Internet
access, voice-activated dialing, voice portals, etc. – and improved
voice quality.
IMPROVING VOICE QUALITY
The first goal in improving mobile voice quality is to at least match the
quality of landline calls. Each successive generation of cellular
technology has improved voice quality. The initial wireless offering, AMPS
(Advanced Mobile Phone Service), was an analog system subject to
increasing noise the further you are from the base station.
Second-generation (2G) systems, such as GSM, CDMA (IS-95), and TDMA
(IS-136), are digital. These systems use aggressive digital speech coding
to limit the amount of information being sent over the air which, combined
with improved modulation, increases the number of voice channels per MHz
of available spectrum. These 2G systems eliminate earlier analog noise
problems, but their voice coders (vocoders) limit voice quality. For
example, the initial GSM vocoder uses only 13 Kbps (versus 64 Kbps for a
landline phone call). The analog noise is gone, but voice quality is still
much less than that of a landline telephone call.
As digital signal processors (DSPs) have improved, new vocoder
algorithms have emerged. The GSM community developed both a half-rate
vocoder that can pack twice as many calls into the same amount of spectrum
with little additional degradation, and an enhanced full-rate (EFR)
vocoder that uses additional DSP resources to deliver better voice quality
over the original 13 Kbps channel. Mobile service providers who initially
offered the better-sounding EFR service quickly gained a competitive
advantage. As a result, EFR has become standard in competitive GSM markets
and carriers are now aware that voice quality is a competitive issue.
IMPROVING THE USER EXPERIENCE
But whether it’s GSM, CDMA, or TDMA, none of the 2G vocoders match
landline quality. So while we wait for next-generation vocoders to squeeze
more quality into the available bandwidth, there are a number of other
interesting technologies available today to enhance the perceived voice
quality during a mobile call.
The first is improved echo cancellation. Because digital mobile
telephony systems involve speech coding, they introduce significant
latency into a call. Unlike landline systems, in which a local call can be
made with no echo canceller, 2G wireless systems require echo cancellers
on every call to a local landline subscriber. But these echo cancellers
differ in the amount of cancellation they provide, the rate at which they
adapt to changing conditions and the way in which they interact with other
echo cancelling devices used by long distance and VoIP service providers.
Less well-known but more significant is a series of wireless-specific
voice quality enhancements, which can be performed on the same platform as
the echo cancellation. The first of these enhancements is background noise
reduction. The amplitude and spectral content of the background noise is
monitored, especially during pauses in the conversation, and that
information is used to reduce the noise heard at the other end. Although
it would be possible to totally suppress background noise this is not done
-- a totally quiet line would make it seem that the mobile subscriber had
hung up.
At the same time, by knowing the level of background noise a caller is
experiencing, it’s possible to raise or lower the amplitude of the other
party’s speech to compensate. The more sophisticated systems only
provide gain for actual speech (not for noise) and dynamically adjust the
target level to match the changing background noise level, allowing the
subscriber to experience a comfortable listening level under all
conditions.
Another voice quality issue is acoustic echo. Wireless standards
require manufacturers to minimize the amount of coupling from the earpiece
to the microphone in a mobile telephone. Unfortunately, many wireless
phones do not meet standards, so acoustic echo contributes to speech
quality problems. What’s more, traditional echo cancellers can make this
problem worse. Traditional echo cancellers are designed to compensate for
electrical echoes that are mostly stable for the duration of a call.
Acoustic echoes are not stable, however, as sound bounces off of multiple
nearby surfaces, and the movement of people or objects during a phone
conversation creates different, rapidly changing, acoustic echo effects.
In addition, the echo canceller sees this acoustic echo through the
non-linearity of a speech compression algorithm. While many echo canceller
equipment vendors claim to do acoustic echo control, the effectiveness of
acoustic echo solutions varies widely from vendor to vendor.
A typical sequence of voice enhancement features includes Noise
Reduction (NR), Acoustic Echo Control (AEC), Automatic Gain Control (AGC),
and Intelligent Level Control (ILC) which integrates AGC with Noise
Compensation (NC) (i.e. ILC = NC+AGC). The combination of these seemingly
minor enhancements allows wireless carriers to dramatically improve the
perceived quality of a mobile call, opening up a new area of competition.
Already some wireless carriers are branding their sound quality and using
it as a marketing tool to attract and retain subscribers. And as mobile
subscriber densities increase, voice quality becomes an issue for
mobile-to-mobile calls as well as mobile-to-landline calls. This means the
voice quality enhancement technology must move to the radio side of the
mobile switch to provide voice quality enhancements on every call.
REDUCING DELAY
Another factor degrading the quality of mobile calls is delay. Delay is
introduced by digital modulation schemes and each time speech is coded. In
2G networks, the coded speech from the handset is converted to 64 Kbps
G.711 so it can be switched with traditional switching equipment, even if
the call is destined for another mobile subscriber. Beyond that, many
service providers send speech through an inter-exchange network where, in
order to save money, the speech is recompressed in a different format.
Every time the speech is re-coded, additional delay is introduced, plus
each sequential coding reduces quality.
To solve this, the industry has developed specifications for
Tandem-Free Operation (TFO), at least for mobile-to-mobile calls. With TFO,
intermediate transcoding is turned off on mobile-to-mobile calls between
compatible handsets (GSM-to-GSM or CDMA-to-CDMA). The result is greatly
reduced delay and less voice quality degradation. TFO-capable equipment is
becoming available and beginning to be deployed. However, TFO only
provides an advantage on mobile-to-mobile calls. And, with today’s
equipment, you lose the voice quality enhancements (NR, AEC, AGC and ILC)
when TFO is turned on as the voice quality equipment works on 64 kbps
signals and, under TFO, speech is never converted back to G.711 -- it’s
kept in the vocoded domain. Luckily, there’s technology in the lab that
will allow the voice quality enhancements to be applied to vocoded speech,
on a frame-by-frame basis. By the time TFO is widely needed, this
technology should be commercially available.
GOING BEYOND LANDLINE QUALITY
Everything discussed so far makes mobile calls sound better, but still not
as good as a landline call. And the voice quality of a landline call is
not all that great -- it’s based on 120 year old technology. For
example, the frequency response of a landline telephone call is limited to
just over 3 kHz, even though the human voice has frequencies in it that go
up easily to 7 kHz. That why it’s so difficult for people to understand
you when you are spelling words that contain “s” or “f.”
The breakthrough will come with 3G mobile technology. The 3GPP (Third
Generation Partnership Project), has specified vocoders for use in the 3G
world. While they have defined an adaptive vocoder that is a superset of
existing coders (AMR-NB: Adaptive Multi-Rate Codec for narrowband speech),
they have also defined a new vocoder called AMR-WB (Adaptive Multi-Rate
Codec for wideband speech). Here the speech coder is able to encode
frequencies up to 7 kHz with the result that an AMR-WB telephone call
actually sounds better than a landline telephone. While 7 kHz is still not
enough for music, it’s enough for speech and will make a wireless voice
call sound as though you were speaking face-to-face.
Actually getting AMR-WB service will have to wait for widespread
deployment of 3G wireless and the availability of new wideband handsets.
This will take a few years, but it will happen. The technology works in
the lab and wireless service providers recognize the importance of voice
quality. So in an extremely competitive market place, we can expect to be
offered mobile service that sounds better than traditional telephony in
the not too distant future.
Brough Turner is senior vice president of
technology at NMS
Communications, a leading provider of hardware and
software technologies for developers of high-value
telecommunications solutions. For more information,
call NMS Communications at (508) 271-1000. E-mail to
the author (addressed to brough_turner@nmss.com)
is also welcome.
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