Broadband Wireless Access over two-layer LMDS

with an IEEE 802.16 future

Petri Mähönen (, Zach Shelby (
VTT Research, Wireless Internet Laboratory

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 and LMDS

Figure 1. Two Layer LMDS
Figure 1. Two Layer LMDS

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.

Local Multipoint Distribution System

LMDS is a cellular access technology designed for high bit rate and low mobility. A base station in each cell communicates with all subscriber nodes in that cell (Multipoint Distribution). Wireless communication is carried out in the millimeter-wave bands, typically at 28 or 40 GHz. The large amounts of bandwidth available gives LMDS systems the capability of very high data rates. Aggregate rates of up to 1 Gbps are possible over the air channel. Downlink channels are normally divided among users to give high throughput, while the uplink channel uses on-demand multiple access since uplink data rates are usually lower. This architecture allows digital video, audio, and telephone services to be delivered to the user while also supporting high performance TCP/IP over the interface.

Research in the European Union: CABSINET

Cabsinet Project 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.

CABSINET Overall Block Diagram
Figure 2. CABSINET block diagram

New IEEE 802.16 Activity

Previous research into LMDS such as above and by companies in North America have made this technology more feasible. However a cost boundary has existed for millimeter-wave technology along with a lack of standardization. The technology has currently become affordable, now the last boundry for widespread deployment into the marketplace is standardization.

IEEE 802.16 Standardization Effort

The IEEE has undertaken standardization of BWA beginning in March, 1999 [6]. Their goal is stated as the following:
"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.

New Research Initiative

As of January 2000 IEEE 802.16 is at the point of accepting proposals and will soon resolve on a set of technology for standardization. There is varied research going on around the world to help with the standardization process. This includes putting together proposals for working groups and doing research on particular issues which are pertinent to this technology. We are continuing LMDS research in the interesting area of computer based simulation. The use of simulation both in helping with the design of the IEEE 802.16 standard, and with the deployment of products and systems afterwards is tremendous. Currently our efforts are in designing simulation models using OPNET which are flexible enough to model various proposals of 802.16, and to finalize a model when standardization is completed. These models will be used for large scale performance tests, optimization of dynamic bandwidth allocation, and in the future with the deployment of commercial systems.


  1. Zach Shelby, Petri Mähönen, "Wireless Internet over LMDS" Moscow Internet 99', 1999.
  2. E. Russo, Petri Mähönen, "40 GHz LMDS system architecture development" International Conference on Telecommunications, 1998.
  3. T. Sukuvaara, P. Mähönen, Z. Shelby, "Wireless Internet and Multimedia Services by Two-Layer LMDS System" International Conference on Multimedia & Telecommunications Management (ICMTM), 1998.
  4. ETSI, "Digital broadcasting system for television, sound, and data services; Framing structure, channel coding and modulation for digital terrestrial television" ETS 300 744, 1996.
  5. DAVIC, "Digital Audio Video Council 1.2 and 1.3 Specifications" DAVIC, 1996.
  6. IEEE 802.16 Working Group, "Broadband Wireless Access Standardization Home Page" IEEE, 2000.
  7. H. Balakrishnan, "A comparison of mechanisms for improving TCP performance over wireless links" ACM SIGCOMM, 1996.