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LMDS has proven to be a cost-effective technology that has no hassles of physical connections and can do two-way wireless broadband microwave transmission of mixed video, audio, and data. LMDS, the 28-GHz band in the United States (Europe uses the 40 GHz for LMDS), is the one that is being used for the wireless broadband LANs. Basically, it is a wireless broadband 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 a wide range of one- 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 an appropriate protocol, access method, and speed, LMDS is given the potential to transform society. When implemented with a multiservice protocol such as Asynchronous Transfer Mode (ATM), LMDS can transport among others, voice, data, and even video. As a transport system, LMDS can be engineered to provide 99.999 percent availability. A few of the various advantages of LMDS for local loops and LANs are as follows:

1. Overall, it is very cost-effective.

2. A major percentage of investment is shifted to customer premises equipment (CPE), which means that the operator spends money on equipment only if a customer signs up. Equipment cost can be shifted entirely to the customer in this model.

3. It has a very scalable architecture, and it uses open industrial standards—ensuring services and expendability.

4. Network management and maintenance are very cost-effective.⁶

The advent of the LMDS channel was driven initially by digital TV applications. Standardizing for the digital TV was first initiated in Europe with the establishment of the Digital Video Broadcasting (DVB) project by the European Broadcasting Union. The technical specifications given by the DVB project were passed over to the European Telecommunications Standard Institute (ETSI) for the publication of standards. Focus on microwave transmission was then begun. The DVB created the standard for short-range millimeter-wave radio systems. Initially, it was called the multipoint video distribution system by the DVB.

Another international body called the Digital Audio Video Council (DAVIC) has come into existence. This body groups major network operators, service providers, and consumer electronics, telecommunications, and computer industries. Though DAVIC is not a part of any official standard making body, it is very powerful.

DVB Specifications

In order for LMDS to benefit from the mass market of broadcasting satellites, specifications for LMDS downlink channels are the same as those of direct-to-home satellite services. Both use quarternary phase-shift key (QPSK) modulation and a concatenated forward error correction (FEC) coding scheme with a convolutional inner code and a Reed Solman (RS) outer code. The transmission frame is based on MPEG2 transport data stream.

The outer code carries 188 information bytes. It has a block length of 204 bytes and can correct up to 8 byte errors per each block. This code is obtained by shortening the RS (255,239) code. A convolution interleaver (see Figure 7-7) with interleaving depth of I = 12 is inserted between inner and outer encoders.¹² This is done in order to uniformly distribute errors that occur by bursts at the VD output in the receiver. The interleaved and deinterleaved block diagram is sketched in Figure 7-7.

DAVIC Specifications

The DAVIC specification for LMDS basically is the same as the DVB specification except for an option of alpha values for channel filtering and either QPSK or 16 QAM for modulation. Basically, there is a lot of similarity between DAVIC and DVB specifications. DAVIC also seems to define future extensions. Along with the MPEG2 scheme use for detail video broadcasting (as discussed in the preceding section), a mapping function to the ATM data in the downstream channel is also made. Two 187-byte packets are formed when 3 control bytes are appended to 7 consecutive 53-byte ATM channels. A description of this is provided in Figure 7-8.¹³

The MAC protocol is used to allocate resources to various user terminals. Both the downstream and upstream frames are encapsulated as one ATM cell. Each frame on the downstream frame includes two slots. There is a frame start slot, followed by a random access slot. The upstream frame has three slots, namely, the polling response slots, the contention slots, and the reserved time slots. The polling response slots are obviously used to respond to a poll message. The contention slots are shared and used by more than one terminal and may result in a collision. The contention when a collision occurs can be resolved in numerous ways—one by waiting for a random amount of time before retransmitting. Reserved time slots are reserved for use by the terminal. The terminal transmits on these slots whenever it has data, and when it does not have any data, it transmits an empty cell. The MAC protocol also has an option for a combination of circuit mode reservation for constant-bit-rate services, and it also has a dynamic reservation for variable-bit-rate and unspecified-bit-rate services. Polls are repeated periodically at intervals of less than or equal to 2 seconds. If a new user comes in, it listens to the downstream channel to find a message sent to it. If it does not find the message for 2 seconds, then it switches to the next downstream channel and listens. This goes on until the terminal finds the message transmitted to it.