Figure 1. Two Layer LMDS
Abstract- Broadband wireless access to television, multimedia, Internet, and data services has not yet become widespread. This has been partially due to technical and economical circumstances, partially to a lack of standardization. These technological and price boundaries have now been broken. The IEEE 802.16 standardization effort has begun, and will be the last boundary to widespread deployment. In this paper we look at previous research into the area of two-layer LMDS for broadband wireless access. The current work and research into IEEE 802.16 will then be examined.
Broadband wireless access has been approached with many different methods. Third generation mobile phone systems promise up to a 2 Mbps data rate. Point to point links operating in the millimeter-wave band offer very high data rates, but are expensive and inflexible. Satellite delivery systems have been successful for broadband delivery, but uplink channels are slow with interactive delay being a problem. The large amount of bandwidth available in the 28 GHz and 40 GHz bands make these ideal for wireless local loop (WLL) implementations. However an affordable, feasible, multiple access system is needed. The Local Multipoint Distribution System (LMDS) architecture fits the bill for wireless local loop access.
CABSINET (Cellular Access to Broadband Services and Interactive Television) was a European Union ACTS
demonstration project conducted between 1997 and 1999. The goal of this was to realize the implementation
of LMDS at 40 GHz. This was completed with an active demonstration and field trials conducted in 1999. Extensive
simulations have been carried out on this architecture with an emphasis on IP over LMDS [1]. The
architecture developed in this project differs from the typical, using a two-layer cellular approach
illustrated in Figure 1.
The two-layer cellular approach allowed a more flexible and capable system than had been seen before. Communication between base stations and micro-cells is carried out at 40 GHz with Line-Of-Sight (LOS) propagation. Repeaters in each micro-cell use 5.8 GHz equipment to communicate with end users, eliminating LOS operation and making end user equipment much cheaper and smaller. The two-layer block diagram is seen in Figure 2. The greatest benefits of this architecture are system flexibility and implementability. System operators have much freedom in micro-cell design with the capability of supporting both private users and businesses (i.e. an ISP) requiring high data rates. Implementability is realizable since 5.8 GHz technology is cheaper for end user equipment, and smaller without the need of a LOS dish antenna. This also provides for limited mobility within a micro-cell.
Figure 2 shows the block diagram of this system. DVB-T standards [4] were followed for the downlink. 188 byte MPEG framing is used, with a multiplexing system. This allows unlimited services to be sent over LMDS. Video programs encoded with MPEG2 are multiplexed directly, along with any audio streams. IP packets are encapsulated in MPEG frames and sent over the downlink using a router in the base station. Any other data packets can be sent. At 40 GHz COFDM modulation is used in the macro-cell, which is down-converted to 5.8 GHz by the micro-cell repeater. The Set-top-box of the user then de-multiplexes all MPEG streams, playing Video, audio, while at the same time providing full Internet services to end user devices.
The uplink of CABSINET is originally based on the DAVIC [5] standard which specifies ATM sized framing and TDMA access. We evolved from this since it was not optimal for wireless links. Strong Reed-Solomon FEC and a selective ARQ link control was added with extra framing for the wireless channel. On-demand TDMA access allows flexible allocation for users and bandwidth. DSSS at 5.8 GHz is used in the micro-cell which is up-converted to 40 GHz and modulated using QPSK in the macro-cell. A DAVIC compatible MAC is used to provide user management and dynamic allocation of resources. This makes a very workable system capable of providing full services with high performance Internet over LMDS.
Internet has been the main emphasis in our research activities. Extensive simulation models and experiments have been carried out for
TCP/IP performance [1]. These findings were then compared to TCP/IP tests on the actual CABSINET demonstrator system.
It was seen that high performance TCP/IP over LMDS was possible and performs well. DVB-T compatibility, and a DAVIC MAC make
the transition to the IEEE 802.16 standard feasible.
"To develop standards and recommended practices to support the development and deployment of fixed broadband wireless access systems."Using a variety of working groups, the 802.16 effort is working towards an LMDS type architecture for broadband wireless access. This technology will be based in the 10-60 GHz bands, although currently the main frequency allocations are at 28 and 40 GHz. A special working group has recently begun research into possibilities under 10 GHz for this standardization, possibly to implement a 5.8 GHz two-layer type architecture as above. The technical requirements of the effort are quite ambitious. Support for up to 155 Mbps over the air interface is required, along with support for many protocols such as IP, ATM, Frame Relay, Voice, and Video. Quality Of Service (QoS) support for ATM and IP will be supported by the Medium Access Controller (MAC). The design of dynamic capacity assignment is important, and will be integrated for efficiency of bandwidth, user capacity, and QoS support. Overall the IEEE 802.16 standardization will produce a high performance broadband wireless access system.