Part 2
Wireless Broadband Networks Handbook: 3G, LMDS & Wireless Internet
Chapter 7: Local Multipoint Distribution Service (LMDS) Design Technology
November 19, 2001
Brought to you by:
Telephony
As
explained previously, local multipoint distribution service (LMDS) is a
broadband wireless distribution technology at the millimeter-wave frequency
band. The U.S. FCC has approved over 1000 MHz of spectrum for LMDS near 28 GHz.
Approximately 850 MHz of the spectrum is assigned for downstream communications
(from network to customers), and about 150 MHz of the spectrum is dedicated for
upstream communications (from customers to network). With the large available
bandwidth, LMDS is capable of providing two-way broadband network access to the
home. It can support integrated applications such as telephony, high-speed
data, and video services.
The deregulation of the telecommunications industry, through the
1996 Telecommunications Act, has opened up opportunities for competitive local
exchange carriers (CLECs) to enter the local telephony service market. In the
highly competitive telecommunications world, time to market and cost of
deployment are critical factors when it comes to choosing the appropriate
infrastructure technology for any service. LMDS is an attractive communications
technology that can be used to deploy telephony service to the home. Because of
its wireless nature, LMDS can offer fast and low-cost deployment by avoiding
the need and the cost of installing underground cables or fibers and their
associated right-of-way problems. Another advantage of LMDS is that it is a
broadband infrastructure. Therefore, even with an initial deployment of only
telephony service over LMDS, any service provider can expand to provide other
broadband services with minimal additional infrastructure cost. Much of the
LMDS equipment at both the network and customer sides can be reused.
In this part of the chapter the support of telephony over LMDS using
existing technology for personal communications services (PCS) is presented.
The rest of this part of the chapter is organized to follow a design approach
and system architecture for a telephony solution. In particular, the overall
system does not have a stringent frequency stability requirement and therefore
can allow low-cost implementation of the LMDS millimeter-wave equipment.
Design Approach And System Architecture
Figure 7-13 shows the architecture
of a telephony system over LMDS.23 LMDS uses a cellular architecture
with a typical cell size of about 2 km in radius. Due to antenna and other
system design tradeoffs, each cell can be divided into different sectors. At
each headend location, an antenna is used to transmit/receive signals to/from
the customers within its sector. Correspondingly, each customer has an antenna
to transmit/receive signals to/from the headend. Figure 7-13 shows that each
cell is divided into four sectors. At the headend, equipment can be connected
to the wide-area communications infrastructure via fiber links. The downstream
path (headend to customers) is a broadcast channel in which the headend
broadcasts information to the customers within the cell. This feature
distinguishes LMDS from the traditional point-to-point microwave technology. In
the upstream direction (customer to headend), each customer communicates with
the headend using a point-to-point link.
A wireless local loop (WLL) architecture is used to deliver voice
services to customers. Compared with other approaches, the advantage of using
LMDS to build WLLs is that other services, such as video and high-speed data,
can be integrated easily into the same broadband LMDS infrastructure. In this
WLL architecture, fixed radio access units (FRAUs) are placed at headend
locations, and customer equipment (CE) is placed at each customer location. Figure 7-14 further illustrates the WLL architecture.24 The FRAU has an interface to the existing wire-line telecommunications infrastructure. It also
executes the protocol to coordinate customer access to the wire-line network.
In the downstream path, it delivers a properly conditioned IF signal (after
modulation and frequency conversion) to the LMDS millimeter-wave/rf radio
equipment. The LMDS radio equipment then converts the IF signal to the LMDS
band for transmission. At the customer end, the LMDS equipment receives and
downconverts the received signal to a lower IF signal before delivering it to
the CE. The CE then performs the demodulation and later produces the original
transmitted baseband signal. The upstream process is similar. The CE delivers a
modulated IF signal to the LMDS millimeter-wave/rf equipment at the customer
side. The LMDS equipment then upconverts the signal to the LMDS band for
transmission. After receiving the signal at the headend, the headend LMDS
equipment downconverts it to a lower IF before delivering it to the FRAU for
demodulation.
Recently, there has been strong interest in various WLL
applications. In particular, many communications techniques, ranging from
analog FM to digital mobile radio, have been considered for WLL deployment.
These technologies have their own characteristics. You need to choose an
appropriate technology that can achieve the following in your LMDS telephony
solution:
- Voice
quality. As mentioned in the introduction, it is envisioned that
LMDS can be used to deploy telephone service to the home. As a result, the
voice quality of the telephony solution must be comparable with that of
existing wired service.
- Time
to market/costs. We would like our telephony solution to be based
on proven technology to reduce development cost.
- Tolerance
to frequency drifts. LMDS operates at millimeter-wave frequency
band near 28 GHz.25
At such a high frequency, current millimeter-wave component
technology cannot provide a stable frequency source at low cost. In order to
avoid expensive millimeter-wave equipment, particularly for CE, the LMDS
telephony solution must be able to tolerate frequency drifts without incurring
severe impairment to voice quality.
Because of time-to-market/cost considerations, existing wireless
broadband mobile telephony technology has been found to be most suitable for
the WLL application in LMDS. Existing wireless broadband mobile telephony
technology can be classified into two categories: high-tier cellular systems
and low-tier personal communications services (PCS) systems. Unfortunately, all
existing high-tier cellular technologies, such as analog FM, IS-54, and global
system for mobile communications (GSM), employ narrow frequency channels
(channel bandwidth less than a couple hundred kilohertz). The narrow frequency
channels mean that these technologies require strict frequency stability, which
is hard to achieve at low cost in the LMDS frequency band. Furthermore, the
digital cellular technologies, such as IS-54, IS-95, and GSM, employ speech coding
to reduce their bandwidth requirements. Their voice quality is generally
inferior to that of wire-line voice because their speech rates are usually more
than four times slower than the wire-line 64 kbps. As a result, high-tier
cellular technologies are not applicable for integration to LMDS.
One approach is to base your LMDS WLL application on the low-tier
PCS technology that is available in the market today. Due to the PCS design
philosophy, all these systems support wire-line speech quality using 32 kbps
ADPCM speech coding. Currently, there are three different low-tier PCS systems:
digital enhanced cordless telecommunications (DECT), personal access
communications system (PACS), and Japanese personal handiphone system (PHS).
DECT is a European standard (Table 7-1), whereas PACS and PHS are U.S. and
Japanese standards, respectively.26
| Frequency (MHz) | 1880-1900 |
| Multiple access | TDMA/TDD (10 carriers) |
| Modulation | GMSK |
| Raw bit rate (Mbps) | 1.152 per carrier |
| Speech coding | ADPCM |
Integrating with the LMDS system can be accomplished by building
frequency converters to translate their respective operating frequency to the
LMDS IF frequency (see Figure 7-14). Other system components, such as network
interface and multiple access protocol, are already supported by the hardware
developed for these systems. While these PCS technologies have been proposed to
support WLL by themselves, the drawback of these systems comes from the fact
that they were designed originally for low-power operations with long battery
life. Consequently, they have only a limited practical coverage range of
several hundred meters. The main advantage of the LMDS/PCS WLL approach is that
coverage can be extended to about 2 km for each antenna tower, thus saving
the infrastructure cost to provide service for a certain area.
As mentioned already, the LMDS/PCS WLL approach is satisfactory in
terms of voice-quality and development-cost considerations. The remaining issue
is whether these PCS technologies can tolerate the frequency drifts expected in
LMDS radio equipment. This approach has been proven by developing a telephony
solution based on DECT. DECT was chosen because it has the highest channel
bandwidth (more than 1.5 MHz) among all the PCS candidates. As a result, it is
expected to have the best frequency tolerance. Table 7-1 shows the system
parameters for DECT.
Now let's look at the measurement results—showing the
bit error rate (BER) and frequency tolerance performance of a DECT-based LMDS
telephony system.
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REPORTS
ANALYTICS
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