Local Multipoint Distribution Service LMDS
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Local Multipoint Distribution Service (LMDS) is the broadband wireless point-to-multipoint communication system operating above 20 GHz that can be used to provide two-way voice, data, video services and Internet. Services using LMDS technology include high-speed Internet access, real-time multimedia file transfer, and remote access to corporate local area networks, interactive video, video-on-demand, video conferencing, and telephony among other potential applications. My paper gives a general idea about LMDS, its architecture and applications.
The architecture part mainly deals with the equipments used in base station and customer premise. In the United States, 1.3 MHz of Bandwidth have been allocated for LMDS to deliver Broadband services in a point-to-point or point-to-multipoint configuration to residential and commercial customers. The LMDS has wide range of applications in wireless LAN, Broadband Wireless Local Loop and mainly transmission of video, voice and Internet. As a transport system LMDS can be engineered to provide 99.999 percent availability.
Local Multipoint Distribution Service (LMDS), or Local Multipoint Communication Systems (LMCS), as the technology is known in Canada, is a broadband wireless point-to-multipoint communication system operating above 20 GHz that can be used to provide digital two-way voice, data, Internet, and video services. The term Local indicates that the signals range limit. Multipoint indicates a broadcast signal from the subscribers; the term distribution defines the wide range of data that can be transmitted, data ranging anywhere from voice, or video to Internet and video traffic. It provides high capacity point to multipoint data access that is less investment intensive. Services using LMDS technology include high-speed Internet access, real-time multimedia file transfer, remote access to corporate local area networks, interactive video, video-on-demand, video conferencing, and telephony among other potential applications. In the United States LMDS uses 1.3 GHz of RF spectrum to transmit voice, video and fast data to and from homes and businesses. With current LMDS technology, this roughly translates to a 1 Gbps digital data pipeline. Canada already has 3 GHz of spectrum set aside for LMDS and is actively setting up systems around the country. Many other developing countries see this technology as a way to bypass the expensive implementation of cable or fiber optics into the twenty-first century.
1.1 IMPORTANCE OF LMDS
Point-to-point fixed wireless network has been commonly deployed to offer high-speed dedicated links between high-density nodes in a network. More recent advances in a point-to-multipoint technology offer service providers a method of providing high capacity local access that is less capital intensive than wireline solution, faster to deploy than wireline, and able to offer a combination of applications. Moreover, as large part of a wireless networkâ„¢s cost is not incurred until the Customer Premise Equipment (CPE) is installed, the network service operator can time capital expenditures to coincide with the signing of new customers. LMDS provides an effective last-mile solution for the incumbent service provider and can be used by competitive service providers to deliver services directly to end-users.
1.2 BENEFITS OF LMDS
The main benefits of LMDS are listed below:
Lower entry and deployment costs
Ease and speed of deployment (systems can be deployed rapidly with minimal disruption to the community and environment)
Fast realization of revenue (as a result of rapid deployment)
Demand based build out (scalable architecture employing open industry standards ensuring services and coverage areas can be easily expanded as customer demand warrants)
Cost, shift from fixed to variable components. (For wireline systems most of the capital investment is in the infrastructure, while with LMDS a greater percentage of investment is shifted to CPE)
No stranded capital when customers churn.
Cost-effective network maintenance, management, and operating costs.
1.3 TECHNICAL BASICS
Past communication technologies focused their attention lower in the RF spectrum because low frequency signals with enough power could be sent long distances and penetrate buildings. Such is the case with television and radio. LMDS, however, uses low powered, high frequency (25 -31 GHz) signals over a short distance. LMDS systems are cellular because they send these very high frequency signals over short line-of-sight distances. These cells are typically spaced 4-5 kilometers (2.5 - 3.1 miles) apart. LMDS cell layout determines the cost of building transmitters and the number of households covered. Direct line-of-sight between the transmitter and receiver is a necessity. Reflectors and/or repeaters can spray a strong signal into shadow areas to allow for more coverage. Various isolation techniques can be used to prevent interference between signals.
Cell size is also influenced by the amount of local rainfall. Because LMDS signals are microwaves, they are attenuated by water and lose strength. To correct this, LMDS operators can either increase the power of their transmissions when it rains in an attempt to ensure a strong signal reaches its destination, or they can reduce their cell size. Leaves, trees and branches can also cause signal loss, but overlapping cells and roof-mounted antennas generally overcome the problem.
2. ARCHITECTURAL ISSUES
Various network architectures are possible within LMDS system design. The majority of system operators will be using point-to-multipoint wireless access designs, although point-to-point systems and TV distribution systems can be provided within the LMDS system. It is expected that the LMDS services will be a combination of voice, video, and data. Therefore, both asynchronous transfer mode (ATM) and Internet protocol (IP) transport methodologies are practical when viewed within the larger telecommunications infrastructure system of a nation.
Basically, four parts in the LMDS architecture are
1. Network operations center (NOC)
2. Fiber based infrastructure
3. Base station
4. Customer Premise Equipment
The NOC contains the network management system (NMS) equipment that manages large regions of the customer network. LMDS network management is designed to meet a network operator's business objectives by providing highly reliable network management services. Multiple NOCs can be interconnected. The fiber based infrastructure basically consists of SONET OC-12 OC-3 and DS-3 links, the ATM and IP switching systems, Interconnections with the Internet and public switched telephone networks (PSTN), the central office equipment.
The conversion from fibered infrastructure to a wireless infrastructure happens at the base stations. Interface for fiber termination, modulation and demodulation functions, microwave transmission and reception equipment are a part of the base station equipment. Local switching can also be present in the base station. If local switching is present then customers communicating in the same base station can communicate with each other without entering the fiber infrastructure.
The customer premise equipment varies widely from vendor to vendor. All configurations include in door digital equipment include modulation and out door mounted microwave equipment. The customer premise equipment may attach to network using TDMA, FDMA or CDMA. Different customer premise equipment requires different configurations. And the customer premise locations can range anywhere from malls to residential locations. At the customer-premise site a Network Interface Unit (NIU) provides the gateway between the RF component and in-building appliances. NIUs are manageable by the network management systems provided in the network control center, which will be discussed latter.
2.1 Architectural Options
LMDS system operators offer different services and have different legacy systems, financial partners, and business strategies. As a result, the system architecture used will differ between all system operators. The most common architectural type uses co-sited, base-station equipment. The indoor digital equipment connects to the network infrastructure, and the outdoor microwave equipment mounted on the rooftop is housed at the same location (see Figure 2). Typically, the radio frequency (RF) planning for these networks uses multiple sector microwave systems, in which transmit- and receive-sector antennas provide service over a 90-, 45-, 30-, 22.5-, or 15-degree beamwidth. The idealized circular coverage area around the cell site is divided into 4, 8, 12, 16, or 24 sectors.
Alternative architectures include connecting base station indoor unit to multiple remote microwave transmission and reception systems with analog fiber interconnection between indoor data unit and out door data unit.
There are manufacturers such as Wav Trace, Ensemble communications and End Gate who have come up with different approaches. One idea from Angel technologies is to have an aircraft transmitting signals from overhead. They called it HALO (high altitude long operating). This Idea has various problems ranging from air traffic control to cost for medium sized cities.
While coming up with architecture a standard issue that is considered is Point to Multipoint communication (PMP). The question that arises is if PMP is actually required. PMP allows multiple microwave paths allowing spectrum and capacity to be shared as needed. So high bandwidth is required, then PTP (point to point) connection may be the best but otherwise, if bandwidth on demand is the case, then PMP is well suited. A new model that is ramping up quickly is IFU or the invisible fiber unit. Two IFU's that are set up in a line of sight link and placed back to back with other links. Thus in IFU transmit and receive create a link between source and destination.
2.2. Wireless Links and Access Options
Wireless system designs are built around three primary access methodologies: TDMA, FDMA, and CDMA. These access methods apply to the connection from the customer-premises site to the base station, referred to as the upstream direction. Currently, most system operators and standards activities address the TDMA and FDMA approaches.
In the downstream direction, from base station to customer premises, most companies supply time division multiplexed (TDM) streams either to a specific user site (point-to-point connectivity) or multiple user sites (a point-to-multipoint system design). Figure 4 illustrates an FDMA scheme in which multiple customer sites share the downstream connection. Separate frequency allocations are used from each customer site to the base station
FDMA scheme allows a fixed bandwidth, or a bandwidth varying slowly over time. If the user requirement is a constant bandwidth (a dedicated one) and expecting continuous availability like a wireless DS3 or multiple structured DS1 connection, FDMA access links fit in well. FDMA links terminate in a dedicated FDMA demodulator, which as it should be, is in the base station. When the customer does not have a very heavy upstream traffic and just needs a 10 base T port, TDMA makes sense. So the choice is based on customer requirements and system design. CDMA or the code division multiple access supports significantly smaller number of users that a TDMA. There are two classes of CDMA that are available, one is an Orthogonal CDMA called as OCDMA and other is the Non orthogonal CDMA. Systems may often use a combination of the two. OCDMA is said to have identical capacity with TDMA. OCDMA allocates using a mutually orthogonal spreading sequence. The other class of CDMA, which is the Pseudo noise CDMA (non orthogonal), all users interfere with each other and the capacity depends on how much Interference one is prepared to tolerate. Both CDMA and TDMA have once again case based advantages and both can be advocated to be good in a particular type of situation. When using smart antennas, using TDMA is an advantage.
How does a system operator decide when to use TDMA and when to use FDMA First, it is necessary to estimate the peak and average expected traffic data rate from all of the potential or estimated offices. Second, it is important to determine which traffic may be multiplexed and traffic-shaped to smooth out the traffic burstiness. If the resulting burstiness is smooth enough, the upstream traffic requirements can be handled effectively using FDMA techniques. Alternately, if burstiness persists within the traffic stream, TDMA may be a better choice.
There are additional system issues relating to the choice of TDMA and FDMA such as the efficiency of the wireless medium access control (MAC), customer-premises multiplexer efficiency, channel structure efficiency, amount of forward error correction (FEC) used on the channel, maximum data rate during peak hours, sharing of the base-station equipment during commercial off-peak hours, blocking levels allocated to the wireless access links, asymmetrical and symmetrical traffic mixtures, and link distance which can be sustained for the various access methods. These issues are discussed in Table 1.
Issue TDMA FDMA
User burstiness efficiency TDMA allows for bursty response and does not request slots unless necessary. FDMA link is always on, regardless of whether or not the user sends data.
Wireless MAC MAC efficiency ranges from 65â€œ90 percent or higher depending on the burstiness characteristics of the users and the MAC design. Efficiency is estimated at 100 percent, no MAC.
Customer-premises mix Both the FDMA and TDMA systems allow higher-priority user traffic to be sent first. Both systems multiplex various streams through the same wireless pipe.
Channel efficiency Efficiency is estimated at 88 percent, based on preamble and ranging. Efficiency is 100 percent.
FEC percent 75 to 85 percent 91 percent
Maximum data rate TDMA allows bursting to the maximum rate of the channel, based on fairness algorithms for the wireless MAC and the customer-premises multiplexer. FDMA provides a constant pipe, with bursting occurring based on fairness algorithms within the customer-premises multiplexer.
Table 1. TDMA and FDMA System Issues
2.3 Data rate capacity
For discussing the data rate capacity in both the accesses, we use the Bits per second per hertz, measurement unit. For the various modulation schemes, the rate varies. Two areas where comparisons can be made would be data rate capacity and maximum number of customer premise sites.
Maximum data rate:
For the FDMA, the bandwidth spectrum efficiency is 1.5b/s/Hz for a 4-QAM modulation where b is bits and s is seconds. For the 16-QAM and 64-QAM the bandwidth spectrum efficiency is 3.5 b/s/Hz and 5b/s/Hz respectively. The TDMA band doesn't use the 64- QAM modulation. For the other modulations it has a reduced data rate.
Maximum number of customer premise sites:
For the FDMA assuming an "x" MHz spectrum with a reuse frequency of "r", the LMDS system provides x/r MHz usable spectrum per sector. If we assume the downlink spectrum to be "d" times the uplink spectrum, the downlink will have d*(x/r)/(d+1) spectrum and the uplink would have a (x/r)/(d+1) spectrum. If the channel bandwidth is assumed to be "b", then the maximum number of customer premise equipment would be (x/r)/((d+1)*b).
For the TDMA for a given (x/r)/(d+1) spectrum, if we assume about 16 DS0 connections possible with 1 MHz then the total number of simultaneous users would be
If the values of concentration over entire sector and cell are assumed to be in the ratio 1 then the total connections would be s*16*(x/r)/((d+1)*b). Which would be very high when compared to what is possible with FDMA.
3. EQUIPMENTS USED IN ARCHITECTURE
3.1 NETWORK-NODE EQUIPMENT
The network-node equipment (NNE) provides the basic network gateway for connecting wireline network traffic to the LMDS bandwidth (see Figure 7). The NNE is equivalent to the base-station digital equipment. The network-node products provide processing, multiplexing, demultiplexing, compression, error detection, encoding, decoding, routing, modulation, and demodulation. The NNE may also provide ATM switching.
The following functions may be performed at the network node:
3.1.1 Digital Signal Compression
The conversion of analog television signals to highly compressed digital signals for distribution by the microwave system.
3.1.2 Wireline/Wireless Protocol Interfaces
Depending on an operator's service offerings, NNE may be configured to extend video, IP, and voice services over LMDS bandwidth. (ATM is emerging as a likely standard for the delivery of voice, data, Internet, and video services over LMDS.)
3.1.3 Modulation and Demodulation
Signals from the voice, video, and data multiplexing system are modulated before wireless transmission occurs. Similarly, traffic from the microwave receiver is demodulated before wireline transmission.
3.2 RADIO FREQUENCY EQUIPMENT
3.2.1 Network Node
LMDS network node RF equipment includes transmitters and receivers as well as transceivers and the antennas they feed. If there is one carrier per transmitter, the system is said to be channelized. If there are multiple carriers per transmitter, the system is said to be broadband.
Individually modulated signals are combined and applied to the broadband transmitter. Within the transmitter, the very-high-frequency (VHF) signals are converted up to the desired carrier frequency, amplified, and applied to the antenna for transmission. Separate transmitters, receivers, and antennas can be used in each direction to minimize the near-end crosstalk effects between transmit and receive signals.
Separate broadband receiver receives the entire band at carrier frequency and converts the signals to the VHF band. The VHF signals are then applied to coaxial or fiber cable for distribution to the NNE.
Combined transmitter and receiver functions can be provided in a single broadband transceiver.
Antennas are chosen based on the desired coverage of potential subscribers, taking into consideration the terrain, interfering objects, antenna azimuth pattern, antenna elevation pattern, and antenna gain.
3.2.2 Customer-Premises Site
For two-way data network applications, a transceiver is used to provide a return path for LMDS services. The antenna may be an integral part of the transceiver. The transceiver may be broadband or channelized.
Customer Antenna Systems
Typical technology choices available include microstrip design, parabolic and grid-parabolic reflectors, and horn designs. The selection is an engineering decision based on the customer's location. As well, vendors will have various levels of integration with specific antenna technologies.
3.3 Network Interface Equipment (Customer Premises)
At the customer-premises site, a network interface unit (NIU) provides the gateway between the RF component and in-building appliances. NIUs are manageable by the network management system provided in the network control center (see Figure 4.2). These NIUs are available in scalable and nonscalable forms depending on customer requirements.
3.3.1 Fully Scalable/Configurable NIU
A scalable NIU is flexible, fully configurable, and chassis-based. It is located at the subscriber site and supports two-way digital wireless voice, data, and video communications for commercial and business uses. As part of the wireless broadband network, the NIU communicates with the base-station equipment through a two-way transceiver forming a part of the point-to-multipoint network solution. This solution allows network operators to deploy their services instantly without the need for building the subscriber wireline infrastructure, thereby, capturing the rapidly growing telecommunications market just in time.
The NIUs basic building blocks consist of the following components:
A radio-modem module supporting 4, 16, and 64â€œQAM and featuring either FDMA or TDMA access mode
A data-processor module (DPM) supporting various services such as T1/E1, 10BaseT, and ATM 25.6 services through an ATM SARing processor
A chassis-interface module (controller providing processing resources)
A power supply
Modular design of the NIU allows network operators to meet each subscriber's requirements efficiently. The network operator can configure multiple radio modems to support the total bandwidth required by the configured services. The NIU should work in conjunction with the broadband microwave system to maximize the use of available RFâ€œspectrum resources. The radio-modem and DPM ratios can be optimized with mix and match options from one-to-one mapping to multiplexing the data streams from several DPMs.
3.3.2 Nonscalable NIU
A nonscalable NIU is a stand-alone, cost-effective piece of CPE that provides a fixed combination of interfaces. The combination is designed to meet the requirements of small- to medium-sized business market segments. Using this interface unit, subscribers can deploy various one-way or two-way voice, video, Internet and/or computer multimedia applications in a chassis using a single carrier frequency spectrum. This nonscalable NIU communicates with the base station through a two-way transceiver consisting of the following components:
A variable-bandwidth radio modem (supporting 4, 16, and 64â€œQAM, TDMA or FDMA depending on the type of services provided by the NIU)
An ATM segmentation-and-reassembly (SAR) processing unit.
A subscriber equipment interface
4. DESIGN ISSUES
Network planning for LMDS includes cell design where in a design of a LMDS cell is discussed. Then the issue of planning the frequency comes in. After planning the use of frequency, a very major issue, which could make a very big difference when it comes to data transmission speeds, is the cell reuse and reuse optimisation. Each of the following issues has been discussed in brief below.
4.1 Cell design issues
The attributes that require attention while designing the LMDS cell are:
1. Cell size selection- Based on the desired reliability level the cell size has to be decided.
2. Cell overlap- an issue that has to be taken into consideration while designing the cells.
3. Subscriber penetration- The number of subscribers having required signal level to achieve quality of service.
4. Number of cells-The number of cells in a sector is dependent on the cell size decided.
5. Traffic capacity- Based on the traffic capacity of the area, the cell size and properties are fixed.
6. Quality of service- Cell overlaps that exceed the allowed normal can effect the quality of service.
7. Link budget- Estimation of the maximum distance that a user can be located from the cell while the cell while still achieving acceptable service reliability.
8. Capital cost per cell-Used to estimate the network capital requirement.
Bellcore the engineering consultant firm formerly owned by AT&T, publish a study of LMDS prior to LMDS auction, concluded that only 25 cells covering only 2% of the land area should be built to yield an economical business. This may sound very attractive to the CLEC's (competitive local exchange carriers).
4.2 Frequency planning
The channel spacing that is usable by the operators in Europe is 112, 56, 28, 14, 7, 3.5. (All in MHz). These are obtained by successive division of 112 by 2. The capacity in upstream and down stream usually differs because, even if the bandwidth allocated is same, physical layer function of both the channels are different. So even if the bandwidth is equally distributed among the upstream and downstream channels, it is not possible to get same capacity. So physical layer issues such as channel coding and filtering have to be taken into consideration when planning channels if, equal capacity for down and up links is desired.
4.3 Reuse schemes
A very important issue that can substantially change the speed of transmission and utilization of bandwidth is frequency reuse. In a given geographical area how effectively can the frequencies be reused. First possibility is to use a hexagonal cellular pattern (same old mobile cells). As illustrated in the figure below, this frequency allocation scheme requires three times the bandwidth allocated to one cell.
Another possibility will be to use rectangular cells. Each quadrant of the cell in this figure is labelled with a digit, which indicates the frequency or group frequencies used in that sector. The frequency reuse pattern reduces the bandwidth requirements by 2 by using two orthogonal polarizations. This is shown in the figure below. This is the initial state, after optimisation the distribution is made only with two colours.
Antenna sectoring with in a cell has advantage of reducing the maintenance costs.
Few techniques to optimise frequency reuse are
1. Maximization of Isolation between adjacent sectors by use of polarization.
2. Maximization of the directivity of the cell antennas by sectoring the distribution system. Minimization of cross-polarization and multipathing.
4.4 Modulation schemes
Modulation schemes can tune the data rate to some extent. Low-density modulation allows greater distance at a given power, but sacrifices data throughput rates. LMDS however utilizes QPSK therefore realize about 1.8 Gbps of raw capacity even thought they had five times the MMDS bandwidth (MMDS can give 1 Gbps using 64QAM for its downstream links). Recently broadband developers have been taking more risk at using advanced coding methods to achieve efficient use of bandwidth. Thoughts of using coding techniques like OFDM (orthogonal frequency division multiplexing) for LMDS have been put forth.
4.5 Receiver design.
The customer premise equipment has one outdoor unit with transmitter and receiver antenna of an indoor unit, which in-turn communicates with subscriberâ„¢s equipment such as telephones and PC's. The indoor unit accepts the signal from the outdoor unit, demodulates and de multiplexes it and then interfaces with the connected subscriber equipment. The downstream intermediate frequency in LMDS is the satellite intermediate frequency (950-2050 MHz) in LMDS system. A major design issue of a receiver could be to achieve a large frequency acquisition range in the carrier recovery loop.
5. APPLICATIONS OF LMDS
LMDS the 28GHz band in America (Europe uses the 40GHz for LMDS) is the one that is being used for wireless LAN. Basically it is a wireless service that transmits fixed broadband microwave signals in the 28 GHz band of the spectrum within small cells roughly 2 to 3 miles in diameter.
It offers wide range of one way and two-way voice, video and data service transmission capabilities with a very large capacity, better than what many current services offer. With millpond radio technology combined with appropriate protocol, access method and speed, which give LMDS the potential to transform the society.
When implemented with a multi service protocol such as Asynchronous Transfer Mode (ATM) can transport among others, voice, data and even video.
The Broadband Wireless Local Loop (B-WLL) can benefit from advances in Local Multipoint Distribution Services (LMDS) system with large usable bandwidth to support full-duplex, high data rate applications such as high-speed Internet, interactive video, and simultaneously including hundreds of traditional broadcast and digital television channels.
Its capability of handling thousands of voice channels with the existing bandwidth makes it a good contestant in the voice industry. With current industry trends, that are tending to merge the telecommunication and the networking industries, LMDS seems to be a solution that suits all their needs.
LMDS promises a wireless alternative to fiber and coaxial cables, It has the potential to replace the existing wired networks, it may prove to be the easiest way to deliver high speed data and two way video service. Its capability of handling thousands of voice channels with the existing bandwidth makes it a good contestant in the voice industry. Gallium Arsenide (GaAS) integrated circuits, digital signal processors, video compression techniques and advanced modulation systems have all made significant improvements in cost and performance. A major rewrite of telecommunication regulations has removed many of the barriers. With current industry trends, that are tending to merge the telecommunication and the networking industries, LMDS seems to be a solution that suits all their needs. For the recent digital TV world, LMDS is a very good choice considering the fact that LMDS was designed with Digital TV broadcast in mind.
Wide band Wireless Digital Communication by Andreash Molish
1. INTRODUCTION 1
1.1 IMPORTANCE OF LMDS 1
1.2 BENEFITS OF LMDS 2
1.3 TECHNICAL BASICS 2
2. ARCHITECTURAL ISSUES 3
2.1 ARCHITECTURAL OPTIONS 5
2.2 WIRELESS LINK AND ACCESS OPTIONS 6
2.3 DATA RATE CAPACITY 10
3. EQUIPMENTS USED IN ARCHITECTURE 11
3.1 NETWORK-NODE EQUIPMENT 11
3.1.1 DIGITAL SIGNAL COMPRESSION 11
3.1.2 WIRELINE/ WIRELESS PROTOCOL INTRFACE 12
3.1.3 MODULATION AND DEMODULATION 12
3.2 RADIO FREQUENCY EQUIPMENT 12
3.2.1 NETWORK NODE 12
3.2.2 CUSTOMER PREMISE SITE 13
3.3 NETWORK INTEFACE EQUIPMENT 14
3.3.1 FULLY SCALABLE/ CONFIGUARBLE NIU 14
3.3.2 NON SCALABLE NIU 15
4. DESIGN ISSUES 16
4.1 CELL DESIGN ISSUES 16
4.2 FREQUENCY PLANNING 17
4.3 REUSE SCHEMES 17
4.4 MODULATION SCHEMES 19
4.5 RECEIVER DESIGN 20
5. APPLICATION OF LMDS 21
6. CONCLUSION 22
7. REFERENCES 23
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