Wireless Local Loop Systems Wireline communications networks are expensive and time-consuming to con-figure.. If successful, the power supply network could be used in the future as a use
Trang 1Wireless Local Loop Systems
Wireline communications networks are expensive and time-consuming to con-figure Growing demand for telecommunications services and the imminent liberalization of the telecommunications market in Europe have greatly in-creased the interest in radio-based communication (Wireless Local Loops, WLL), also referred to as Radio in the Local Loop (RLL), see [3]
Future competitors of telecommunications monopoly service providers, such as power suppliers who already have their own wide-area networks, will usually lack access networks to the end users Message transfer over existing power supply lines is currently being attempted using CDMA transmission, and allows rates of several Mbit/s The CEN 50 065 standard, which enables the use of carrier frequencies of up to 140 kHz, is available for this purpose
In 1997 Northern Telecom conducted its first field tests in this area in Great Britain If successful, the power supply network could be used in the future
as a user access network for voice and data services, playing a major role in promoting competition and price reductions in telecommunications services Meeting the following requirements, wireless technology is very effective at bridging the “last mile” of the path between the fixed network access of private network operators (Point of Presence, POP) and the user:
• Fast and economic network configuration
• Economic network operations
• Flexible and expandable network structure
• Possible added-value through (restricted) mobility
For the user, a connection over a WLL system replaces the wireline fixed network connection and should guarantee the same quality of ser-vice characteristics—bit-error ratio, delay time and blocking probability—as the public analogue telephone network (Public Switched Telephone Network, PSTN) or even the Integrated Services Digital Network, ISDN
Figure 10.1 shows the different levels of mobility for a WLL user In ad-dition to connections without mobility in which the customer uses a normal telephone connected through a cable to an antenna in the wall of the house, systems with restricted mobility also exist, allowing a user to roam in a re-stricted area, e.g., in a radio cell defined by an internal base station Systems
in which users enjoy the same kind of mobility as in cellular mobile radio
ISBNs: 0-471-97595-8 (Hardback); 0-470-84193-1 (Electronic)
Trang 2584 10 Wireless Local Loop Systems
Figure 10.1: WLL without and with restricted mobility
!#"%$& '
4 57698
: % FG H:
:;=<?J@KLD EMN .03E CO:
:;=<?P@+ Q:
!]"^$&@_" $
`@a
bdcfe
$g_ h '^'
i e j
4 5_6*k
4 5_69l 45769m
&@r "
st`@`uo1v
4 5_69w
4 5_6Lx
Figure 10.2: ETSI reference model for WLL systems
networks (e.g GSM) are called Personal Communications Systems (PCS); see Section 3.15
Because of the great interest in WLL technology for the above reasons, the ETSI working party Radio Equipment and Systems 3 (ETSI RES 3), founded in January 1993, published a report [1] in November 1994 addressing the marketing aspects, different WLL technologies as well as scenarios and characteristics that are of interest from the standpoint of the dimensioning of WLL systems, such as radio range and capacity aspects The reference model presented in Figure 10.2 originates from this report
The reference model shows the interfaces of a WLL system and its elements
In addition to antennas, the base station (BS) has facilities for measuring and controlling radio connections with the radio termination of the customer terminal The controller connects the base station to the local exchange (LE) and controls the base station
10.1 Technologies for WLL Systems
The following technologies are suitable for WLL systems [2]:
• Analogue cellular mobile radio
Trang 3• GSM/DCS1800 derivations
• CDMA systems based on US-TIA standard IS-95
• Digital cordless radio networks:
– DECT
– Personal Handyphone System, PHS (Japan)
– US Personal Access Communication System (PACS)
• Digital point-to-multipoint (PMP) radio relay systems
Directional antennas with, e.g., 5–12 dB gain are normally provided for stationary residential lines to enable a low bit-error ratio on the radio channel
to be achieved even when base stations have a large coverage radius (typically 2–5 km) However, discussions are also taking place about systems in which the stationary customer line can get by with an indoor antenna, which then has to be small and unobtrusive The base station then of course has to operate with smaller supply radii
The suitability of the systems mentioned above depends heavily on the types of user—in other words on the services that are to be offered by the WLL system A differentiation is made between the following types of user:
• Residential users who require an analogue (PSTN) connection or an ISDN basic rate interface (BRI)
• Small-business customers who operate a small branch exchange for
for whom an ISDN primary rate interface (PRI) would be impractical and too expensive
• Large-business customers who operate large branch exchanges and re-quire a gateway to data networks such as X.21 and X.25 and frame relay along with normal telephone applications Their connection
n = 1, 2, )
10.1.1 Cellular Mobile Radio Networks
The Ericsson RAS 1000 System, which is based on the NMT standard (Nordic Mobile Telephony), was used by Deutsche Telekom AG (DTAG) in 1993 to connect around 13 000 customers in Potsdam, Germany However, analogue cellular systems are no longer competitive compared with the other systems presented here because of the high cost per connection and the disadvantages compared with digital networks
Experiments have also been made with digital mobile radio networks Stud-ies have been conducted with systems such as GSM900/1800 to determine the
Trang 4586 10 Wireless Local Loop Systems
243
5:9;
<4=
?A@CBEDFBHGI@JGLK MON
PAMQKNRGTSUGIMCSWVRMQK NRGQXY@CZOKM[X\DQ]@J^
PAMQKNRGTSUGIMCSW_a`C]GbK VcMQKNRGQXY@CZOKMdX\DO]@C^
efK gEDCZdhjiAkmlnkjMQNJNcDJoHGbKMQNa?AMCg
GLqrZOK X\DJoHGbKMQNR@Q]s@QNRGIDQNJNc@
Figure 10.3: Customer coverage by over the roofs supply
!
"#
%'&(*),+.-0/214351068797:)9;=<>&687@?26,(
?2A,B*)CB*<DA,<>&687
-0/2143'EGFH)HIA,J*B
K268&7L<MEN<D6,EPOL68&7L<8QRA,+H& 6SQT)8IA9J
K268&7L<MEN<D6,EPU@V,I<>&O:68&7L<8QRA,+H& 6.QT)HIA,J
Figure 10.4: Customer coverage by below the roofs supply
feasibility of the possibility shown in the lower part of Figure 10.1 This would allow customers to communicate cost-effectively indoors using their home base stations and to use the same terminal to access the corresponding cellular net-work However, mobile radio networks only offer narrowband channels and therefore cannot achieve the quality of service of a PSTN or ISDN/BRI access for wireline users Nevertheless, all users who just want to place a call have access to the wireless connection to the mobile radio networks
10.1.2 Digital Cordless Radio Networks
Some of the new network operators are pinning their hopes on WLL systems based on the DECT standard or on CT2 or PHS At around 300–400 US$, the cost per connection is considerably less than with GSM-based solutions DECT is able to support coverage over the roofs as well as below the roofs (see Figures 10.3 and 10.4) Over the roofs coverage would not be able to reach ditches, but with low-gain receiving antennas could penetrate through house walls Below the roofs systems operate with base stations (Radio Fixed Parts,
Trang 5Table 10.1: Frequency bands for WLL systems in Europe
Frequency band [GHz] Applications Channel grid [MHz] 24.549 26.061 PMP radio relay 3.5, 7, 14
17 Radio relay for Hiperlan/4
3.41 3.60 PMP radio relay 3.5
2.5 2.67 PMP radio relay
1.88 1.9 DECT-WLL 1.7
RFP) that are installed under the roof, thereby illuminating the ditches and enabling full mobility of the PCS system both indoors and outdoors
10.1.3 Digital PMP Systems
Point-to-multipoint radio relay systems offer attractive connection options particularly for business customers, because they can provide the customer with channels with high transmission rates The frequencies for PMP-WLL applications are listed in Table 10.1
WLL systems based on PMP technology are offered by a large number of suppliers, and represent an interesting connection option in urban and rural areas because of their ability to bridge up to 20 km in borderline cases Be-cause the transmission capacity needs of stationary customers connected over PMP systems vary, there exist PMP radio relay systems that can dynamically change their (switched) channels in terms of capacity and allocate them to the areas where they are most urgently needed (see Figure 10.5)
PMP systems are frequently also implemented as multihop systems, as shown in Figure 10.6 Several radio links (Line-of-Sight Radio, LOS) belong-ing to point-to-point or point-to-multipoint (PMP) systems are arranged se-quentially in order to bridge the path between the fixed network access (Point
of Presence, POP) and the customer’s connection Another interesting op-tion offered by a number of manufacturers is a combinaop-tion of PMP systems with DECT systems connected to a PMP end terminal for the connection of residential customers
10.2 Different WLL Scenarios
The competitive scenarios examined here are based on standard scenarios from ETSI RES-3 [1] The following scenarios were defined:
1 An existing operator, new area to be developed
2 Replacement of copper lines with WLL
3 Reaching the capacity limits of an existing fixed network
Trang 6588 10 Wireless Local Loop Systems
"!
# $ %&(')* +,
.0/ 1 2
1 4
6578 9;: <>=6? @0ABDC E FHG
? I
EHKE F
LNM>O P QRO PTSVUXW YZ\[
QROP0SVUXWY Z>]
QROP0SVUXWY Z>^
^_OW`aPTY Z
bdcVbdefQROP0SVUXWY Z
gHhikjVl>m U`6noM
LNM>OP
LNMpO P
Figure 10.5: PMP radio system with a fixed or dynamic channel allocation con-trolled by the central terminal
Figure 10.6: Multihop connection-oriented PMP system
4 New operator in competition with existing operator
ETSI scenario 4 is the one being considered as a model for capacity studies (see Figure 10.7) The assumption is that a provider of telecommunications services is in competition with the national telephone company, and perhaps with other operators Of the three areas defined in the model scenario, only areas A and B will be considered (see the assumptions in Table 10.2) It
Trang 7Figure 10.7: ETSI scenario 4
Table 10.2: Definition of areas of ETSI scenario
Big city Small city Number of connections 500 000 50 000
Density of inner area 2000/km2 1000/km2
Density of outer area 500/km2 500/km2
Radius of inner area 4.5 km 2 km
Radius of outer area 16 km 5 km
Average traffic 70 mErl./user 70 mErl./user
Penetration 1 % increase of users p a during the first 10 years
is assumed that it would not be viable to supply coverage to the sparsely populated area C
On the assumption of a certain penetration of the respective area, e.g., con-nection of x % of the users of the WLL system, and with the knowledge of the
in the model show that it is possible to implement voice telephony with any
of the WLL technologies mentioned above However, the initial investment required in developing a connection area is high, and operation of the service especially during the early phases is not economical because of the time in-volved in gradually enticing customers away from the fixed network operator
In the long run, all WLL technologies would appear to be cost-effective and promising If the legal conditions allow a competitor direct access to individ-ual subscribers via the wired local loop network by the so-called unbundled access, and if the costs for this use are low (as is the case in Germany at 2.3 Pfg./min), then this will have a considerable effect on the competitiveness
of radio-based WLL systems, because they might appear to be too costly The most suitable systems for introduction are those that can manage with-out much infrastructure—in other words, those that can operate in frequency ranges with high diffraction (preferably below 1 GHz) and are therefore able
Trang 8590 10 Wireless Local Loop Systems
to illuminate a coverage area with small technical effort A prerequisite is suf-ficiently high transmitter power of 2–8 W, because otherwise too many base stations would be required The more users there are, the more base stations are needed, and the transmitter power decreases—as experienced with cellular systems
10.3 Direct User Connection in Access Network
In deregulated markets the onus is typically on the previous monopoly owner
of the wireline local loop network (incumbent) to provide competitors with unbundled user access over their fixed networks In so doing, the incumbent must allow the competitor, against an appropriate reimbursement of costs,
to have direct access to the two-wire lines in the local loop Non-unbundled access, on the other hand, means that competitors must use (and pay for) the incumbent’s multiplex systems every time they want to reach one of their subscribers
Owing to the changeover from analogue to digital technology (ISDN), old-established telecom companies (incumbents) actually have sufficient space in their main distribution frames to allow them to provide direct access within the corresponding rooms It is not currently clear to what extent new network operators will be taking advantage of direct user access
References
[1] ETSI (RES) Radio in the Local Loop., Nov 1994
DECT-Anwendungen im Frequenzband 1880–1900 MHz, Aug 1995 Studie im
[3] W Webb Introduction to Wireless Local Loop Artech House, Boston, 1998
... 24.549 26.061 PMP radio relay 3.5, 7, 1417 Radio relay for Hiperlan/4
3.41 3.60 PMP radio relay 3.5
2.5 2.67 PMP radio relay
1.88 1.9 DECT-WLL 1.7
RFP)... point-to-multipoint (PMP) radio relay systems
Directional antennas with, e.g., 5–12 dB gain are normally provided for stationary residential lines to enable a low bit-error ratio on the radio channel... because of the high cost per connection and the disadvantages compared with digital networks
Experiments have also been made with digital mobile radio networks Stud-ies have been conducted