Fixed Wireless Internet

Over MMDS

Proposed System Overview

The architecture of the WIA Wireless proposed system should utilize standard DOCSIS cable equipment as the network elements.  Complementing this standard DOCSIS setup should be RF hardware designed to translate specific sub-bands of the DOCSIS cable frequency plan into the frequency allocations of each specific customer.

Key Characteristics of the system should be:

• 27 Mbps time division multiplexed downstream using 64QAM in a 6MHz channel, or 36 Mbps in an 8MHz channel.

• 256 Kbps to 10 Mbps time division multiple access burst upstream using QPSK or 16QAM in 200 kHz to 3.2 MHz bandwidths.

• Near line of sight performance.

• System should operate in the … GHz range.

• Audible installation alignment beeper to facilitate customer self-install and avoid a truck roll.

• Economies of scale by reusing existing DOCSIS products which are now being deployed in high volume worldwide.

DOCISIS Overview

DOCSIS is an open industry standard developed and coordinated through the efforts of cable MSOs and technology manufacturers such as Cisco and Motorola.  DOCSIS technology is mature and in operation in many countries around the world as the standard for Internet access on hybrid coax cable systems.  A fundamental advantage of using DOCSIS is the wide variety of manufacturers producing cable modems and the volumes currently being deployed which translate into very cost effective subscriber equipment.  Over 15 million DOCSIS cable modems are currently deployed worldwide with over 200,000 upstream DOCSIS ports.  DOCSIS cable modem pricing is now less than $55 USD in quantity.

The technology behind DOCSIS also lends itself very well to use as a fixed broadband wireless access mechanism.  A well-developed physical layer, sophisticated media access control, and a complex network reference model provide for a commercial grade wireless system.

Physical Layer Characteristics

Key parameters of the physical layer include:

Downstream

• 30 Mbps in a 6 MHz RF channel (standard DOCSIS)

• 64QAM modulation

• Reed-Solomon forward error correction (~ 10% overhead)

• Time division multiplexing

• Continuous transmission

Upstream

• Variable channel bandwidths (to be defined by bandwidth licenses)

• QPSK or 16QAM modulation

• Reed-Solomon forward error correction (~ 10% overhead)

• Burst mode transmission

• Adaptive power control over 50 dB range

Media Access Control

The DOCSIS MAC utilizes a request/grant mechanism under central management of the CMTS.  The time division multiple access (TDMA) reservation-based protocol requires each CPE to request a time to transmit data.  The CMTS examines all of the incoming requests and grants a time to transmit based on a multitude of parameters including CPE data rate limitations and service priorities.  A DOCSIS compliant system with full QoS is required.

Security

Baseline Privacy Plus should be included as part of the DOCSIS protocol, providing advanced security and privacy features for both the service provider and the customer.  All over-the-air communications should operated with a minimum 56-bit DES encryption ( or 40 bit if reulations require).  Additionally, encrypted key management between the CMTS and modems requires subscriber authorization and registration to ensure reliable and secure billing for multiple server classes which prevents unauthorized access to the system.  Various cable modems support end to end 3DES encryption.

RF Systems

Point to Multipoint Configuration

Figure 1 shows the overall system diagram for point to multipoint communication.  Detailed network capacity and RF planning is necessary to determine the optimum solution.

Base Station

Figure 2 shows a detailed diagram of a 2.5 GHz (another frequency range is possible, dependent on Licenses) base station configured with an omni- directional downstream and 4 sector upstream; the transport/network connection ties directly into the hub CMTS.  WIA Wireless’ proposed capability at 2.5 GHz requires three components to complement the CMTS at the base station (hub) site.  These additional components serve to convert the standard DOCSIS cable frequency plan to the required 2.5 GHz frequency plan and include the following:

            1 x Agile Upconverter (Indoor Unit)

            2 x Power Amplifer (Indoor Unit)

            2 x (Headend Downconverter (Outdoor Unit)

Downstream subscriber traffic is time-division multiplexed and modulated by the DOCSIS CMTS into a 6 MHz (27 Mbps) downstream RF channel.  This modulated signal at 44 MHz is fed into the upconverter for translation to any desired RF channel between the band edges.  Following upconversion to the MMDS/MCS band, amplification through the Power Amplifier brings the total output power to a maximum of +37 dBm.  A channel combiner should be used to filter the output spectrum for harmonic content and allow for future combining of the outputs of multiple downstream channels for higher capacity.  Low loss 50-ohm feeder cable runs up the tower to the omni-directional or sectorized hum transmit antenna.

Subscribers within the cell are split into four 90 degree sectors to minimize the effect of multipath interference and increase upstream capacity.  Each of these four sectors should have a separate flat panel antenna and low-loss feeder cable that feeds into the Headend Down Converter.  The proposed Headend Down Converter down-converts the upstream band into the 14.375 to 20.375 MHz (or what is required by license) return band required by the CMTS.  The CMTS demodulates each upstream sector and routes to the appropriate network segment accordingly.

Subscriber

Figure 3 details the proposed configuration for a standard subscriber installation.  There are two fundamental elements, each with multiple options depending on performance and feature set;  the outdoor RF transceiver and the indoor DOCSIS modem.

The subscriber transceiver serves to frequency translate and amplify the upstream and downstream signals to the appropriate cable frequencies for use by the indoor DOCSIS modem.  A single low cost 75-ohm cable is used to connect between the transceiver IF port and indoor AC/DC power inserter.  A short jumper cable is used to connect to the DOCSIS modem.  Recommended modem depends on the application.

Element and Network Management

All network and RF elements with the exception of the subscriber transceiver are fully visible on the network.  A network server should display cable modem and network statistics as well as maintain a historical log.

Secondly a network/subscriber management systems should be provided, by offering DHCP, TFTP, customer database, individual control of cable modems, and latitude/longitude mapping.