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07 November 2009
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Wireless and DSP Backgrounder
Today's wireless industry is driven by the explosive
interest in mobile communications, particularly voice
telephony, but also data and even video traffic. It is
estimated that one in five individuals in the United States now
use portable cellular telephones and the growth continues
unabated. Users of wireless services have come to depend upon
their devices for staying in touch. Gone are the days when a
cell phone was a perk; today it is a tool. Wireless data modems
will become as commonplace as wired data modems. Users are
demanding connectivity, anytime, anywhere. That includes when
they are in their car, outside with their portable, or inside
public and private structures. The wireless industry is the
focus of numerous vendors of systems, semiconductors, and
software designers who are driving improvements in cost, power
consumption, and spectrum efficiency.
The existence of a single analog cellular standard enabled
rapid deployment in the U.S. In Europe, multiple analog
standards in individual countries prevented cellular from
gaining wide acceptance. But with digital cellular, the tables
have been reversed.
In 1985, European countries made a clean break from analog
wireless and agreed to support a single digital standard, and
the first GSM network began operation in 1991. Japan has its
own standard, Personal Digital Cellular (PDC). But in the US,
the market is fragmented. The first standard (IS-54 TDMA) was
released in 1990, promising 3x the spectrum efficiency compared
to analog systems, and a second standard (IS-95 CDMA) followed
three years later, promising a 10x improvement over analog
systems. Many wireless network operators are planning to move
from analog technology to digital. Digital cellular offers
subscribers the benefits of enhanced services such as fax and
data. But its main advantage is efficient spectrum management,
allowing operators to combat congestion problems resulting from
the success of analog cellular. However, the multiplicity of
standards has created confusion in the market.
Nevertheless, mobile telephone technology is clearly moving
away from analog and toward digital technology. PCS cellular
phones will use only digital technology, and non-PCS cellular
phones are moving toward digital as well. While 95% of the
North American mobile phone users just two years ago were using
analog systems, by 1999, according to insider sources at Lucent
Technologies, digital-based users will represent 1/3 of the
total. Many of the handsets being introduced today are dual
mode sets; they have both analog and digital capabilities,
easing the market transition.
It is easy to become lost in the wireless alphabet soup. The
terminology is confusing because it reflects various approaches
taken to provide wireless services over the past decade. This
backgrounder will review the cellular industry, the different
wireless technologies, and the semiconductor technologies and
applications driving the wireless industry.
How Did Digital Cellular Begin?
Before cellular technology was implemented in the 1980s,
mobile phone service was provided by placing a single
transceiver (transmitter/receiver) at a high elevation point
within a serving area. The analog signals from the antenna
could travel about 50 miles. Because of the limited spectrum
allocation made by the Federal Communications Commission (FCC),
only 44 calls could be supported by a single system. AT&T,
which spun off its equipment arm into Lucent Technologies, did
research to circumvent this problem and in 1970 combined a
number of technologies to create cellular telephony with the
Advanced Mobile Phone Service (AMPS). Cellular communications
systems, as depicted in Figure 1, are based on three
components: the mobile phone itself, the base station, and the
cellular system switching office.
Figure 1: Simple schematic of cellular system
What Does Cellular Mean?
A cellular phone company divides its service area into
sections ranging in size from several miles to just a few city
blocks. These sections are called cells. Each cell has its own
transceiver. Because the cells are small and the transmitters
are low powered, another cell in another part of the city can
reuse the same frequencies without interference to communicate
with phones in its area.
What is a DSP Core?
A DSP core is a DSP engine (or DSP design) that can be part
of an Application Specific Integrated Circuit (ASIC). Cores are
not stand-alone products. The core approach allows a customer
to obtain a very integrated, low-cost, low power consumption
single chip solution. For example, the ASIC might contain the
DSP engine, Random Access Memory (RAM), Read Only Memory (ROM),
a serial Input/Output (I/O) interface, or Peripheral Component
Interconnect (PCI) bus interface, special peripheral logic,
timers, etc. All the building blocks are connected to the DSP
core on a single chip ASIC and can be custom designed for a
specific application. In other words, the systems designer has
a chance to "author" his or her own silicon by placing on-chip
exactly the peripheral devices he needs in just the right
configuration or his or her intended application. In some
cases, users may even opt to have either multiple DSP cores or
combinations of DSP cores and Reduced Instruction Set Computer
(RISC) cores on the same silicon.
What are the Important Attributes of a Wireless Telephone?
OEMs designing cellular handsets must be concerned with four
major design constraints:
- Performance
Communications in general, and wireless in particular, is a
computationally demanding application. The quality of a call
depends on implementation of the voice coding algorithm,
noise filtering and echo cancellation circuits, and the
signal path between the mobile phone and the base station.
Besides the main performance criteria of size and weight,
handset performance is also a function of the features
designed into the electronics, such as paging, email and
voice mail capabilities; handset display devices; memory
size; and the availability of special function keys.
- Power Consumption
Any untethered, battery operated device designed to be
carried in the field must be designed for minimum power
consumption. Reduced power consumption translates to longer
battery life, a significant contributor to mobile phone
consumer satisfaction.
- Cost
Because the device is primarily a consumer appliance, and
because the target is high volume markets, overall selling
price and hence design costs are of paramount
importance.
- Time-to-Market
With ever decreasing product life cycles and smaller windows
of market opportunity, time to market is a key driver to
profitability. OEMs must look for semiconductor solutions
that come with the appropriate design tools and technical
support. Being six months late to market can totally
eliminate a product's "profitability window."
What is the Role of a DSP in a Wireless Telephone?
DSPs play two crucial roles in wireless telephone handsets.
They perform a function known as "speech coding" in which the
analog output from the microphone is digitized and then
compressed or "coded" according to one of several available
algorithms. The goal of all speech coders is to reduce the
amount of data while maintaining the maximum speech quality.
Speech coding is a computationally intensive task and must be
done in "real-time" to maintain an interactive, natural-feeling
call. DSPs also do what is known as "channel coding." This is a
function analogous to that of ordinary modems; the data of
interest is connected to a communications channel, in this case
a wireless radio signal. In channel coding, noise levels are
equalized to obtain the best possible quality; encryption is
performed, if available, and the resulting digital stream of
data, which uses about 25 kHz of bandwidth, is "modulated" onto
a carrier with the appropriate frequency, typically in the 1900
kHz range for a PCS system. Coding, modulation, encryption,
noise filtering, and equalization are functions for which a DSP
chip is especially well suited.
Figure 2: Overview of cellular handset
What is the Role of the Wireless Handset?
The base station is the center of every wireless cell. The
base station contains filters, amplifiers, and demodulators to
capture a usable voice signal from the radio waves and pass it
on to a telephone line. Control logic manages the two-way
transmission links between the base station and the mobile
units and sends relevant information to the mobile telephone
company's switching office. Base stations can transmit and
receive on multiple frequencies simultaneously to provide
several individual voice channels at the same time. Hence, the
base station must be able to multiplex, detect, sort, and
enable user features and it must simultaneously service
multiple subscribers.
How are DSPs Used in Wireless Base Stations?
Base stations face a daunting task. Multiple senders may be
moving about within the cell, and signal strengths can ebb and
flow. Some signals may reflect off buildings and other
structures, so that the base station receives both the original
signal and a slightly delayed "shadow" signal. Other signal
generators or even adverse weather in the cell may create
intermittent noise. The challenge is to receive the user's
signal, recover the data, unravel the voice data from the
received digital stream, and pass the voice along to the
system's switching office. Such signal processing tasks, known
an equalization, filtering, channel decoding, encryption, and
forward error correction, are optimally performed by DSP
engines.
What Does the Switching Office Do?
All the cells in a system report information to a switching
office, known as a Mobile Telephone Switching Office, or MTSO,
which is a computer system connected to each base station cell
via wires and to the local telephone network. The switch
controller is responsible for decision making and routing calls
between a cell and the local wired phone system or between two
cells if both parties are on mobile phones. When a user turns
on a mobile phone, a signal is transmitted and received by one
or more base station receivers. The switching office decides
which cell is receiving the strongest signal and will serve the
phone user while the user is within that cell area. If a mobile
user leaves the range of its current cell while a call is in
progress, the cell's transceiver starts to receive a weak
signal and notifies the switch office, which uses information
from that cell and neighboring cells to "hand off" the call or
switch control from the cell being left to the one being
entered. The MTSO monitors all cellular calls and keeps track
of billing information.
What are the Standards for Wireless Telephony?
There are now many wireless systems in operation worldwide
using both analog and digital technologies. In the United
States alone there are many standards for the analog cellular
bands and three major proposed standards in the digital PCS
band: CDMA, GSM, and TDMA (Figure 3).
Figure 3: Technologies and Standards for Analog and
Digital Wireless Telephony
What is PCS?
PCS is a term for wireless communications using a higher
frequency band than traditional cellular technology. The
definition is somewhat loose, but generally refers to systems
operating in the 1.8 to 1.9 GHz range. Traditional cellular
operates in the 800-900 MHz range. Because the higher frequency
PCS signals can travel only over shorter distances, the size of
each PCS cell must smaller than traditional cells. Hence, more
PCS cells are needed than is the case with older technology.
However, PCS signals can carry much more information than their
lower-frequency counterparts because they have a higher
bandwidth. Also, since PCS cells are closer together, the
mobile phone transceiver requires less battery power. As a
result, PCS handsets can be smaller and lighter due to smaller
battery size.
What is GSM?
Global System for Mobile communications is a digital
European cellular phone system that allows European travelers
to use a single cellular phone in more than 35 countries and
have every call billed to one account. The internal access
method is TDMA. GSM digitizes speech at 13 kbps and can carry
eight users per 200 kHz frequency band.
What is TDMA?
Time Division Multiple Access allows multiple users to share
a single frequency band; each user's speech is stored,
compressed, and transmitted as a quick packet within a very
tightly controlled time slot. In GSM, eight users share a given
channel; in the U.S. IS-54 protocol, channels are narrower, and
the number of users is limited to three.
What are the Claims Being Made to Support CDMA?
Code Division Multiple Access, CDMA, uses what is called
spread spectrum technology to expand capacity from 6 to 18
times over that of Advanced Mobile Phone Systems (AMPS), the
present analog system. CDMA eliminates cross talk and
interference problems and can use high quality voice coders,
which gives cellular quality the equivalent of a wireline call.
CDMA sends a weak broadband signal spread over multiple
frequency ranges in a technique known as frequency hopping.
Each signal is combined with a code which the receiver uses to
separate the signal from the noise. The typical power
consumption of a CDMA telephone terminal is only 2 milliwatts
(mw), which is significantly smaller than the 125 mw consumed
by a typical GSM handset. Hence batteries last much longer.
Finally, proponents of CDMA claim that spread spectrum
technology can cover a larger area than is possible with GSM,
TDMA, or AMPS. CDMA also supports simultaneous voice, data and
mobile fax transmissions as well as paging, caller ID, and
voice mail services. CDMA has technical advantages but is
complex; TDMA is field-proven.
| Category |
Analog |
TDMA Digital |
CDMA Digital |
| Speech quality |
Fair (4 kHz) |
Fair (13 Kbps) |
Fair (13 Kbps) |
| Terminal Cost/Complexity |
Low |
Medium |
Medium |
| Base Station/Cost |
Low |
High |
High |
| Modularity (channels/base station) |
Low |
Medium |
High |
| Robustness |
Good |
Fair |
Good |
| Subscriber Density Supported |
Medium |
Medium |
High |
| Maximum Range |
40 km |
30 km |
25 km |
| Example Product |
AMPS |
GSM |
IS-95 |
Table 1: Comparison of analog cellular, TDMA
digital, and CDMA digital systems
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