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Wireless Broadband Modems
PDF of this tutorial4. Frequency Bands and Limitations
The frequency band determines the type of receiving antenna and the coverage, or range, of the transmitter. MMDS, instructional television fixed service (ITFS) and multipoint distribution service (MDS) analog television transmitters were the first used for Internet access. The bands were underutilized for television and one, two, or even three digital 2-MHz subchannels could be made to work with adjustments to the analog-transmitter diplexers and filters. Digital transmitters have recently been made available from several vendors.Wireless communications service (WCS) is a newly available band with wireless transmitters and downconverters under development. Ultra-high frequency (UHF) and very-high frequency (VHF) low-power transmitters are in service for Internet downstream access using experimental licenses. Most bandwidths are 6-MHz downstream, whereas WCS is 5 MHz.
| Name | Basic Usage |
| MMDS | Most of these T.V. transmitters are analog and require upgrade to digital unless sub-channelization is used (see Note 1). |
| MDS | analog T.V. or not used |
| WCS | new |
| ITFS | educational service, includes Internet access |
| LMDS | local multipoint distribution service; new |
| ISM | instructional, scientific and medical; unlicensed bands used for LANs and for the return path of two-way modem systems |
| Note 1: Sub-channelization is a way to increase the power of the adaptive equalizer in the cable modem by applying it to a narrower bandwidth. A standard equalizer can equalize three times the amplitude tilt or triple the delay if it operates over a 2-MHx channel instead of a 6-MHz channel. The narrower bandwidth may allow operation without upgrading the diplexer or removing sound subcarrier stop fileters of an existing analog transmitter. In the latter case, only one or two subchannels may operate instead of three. | |
Table 1. Frequency Band Designations
| Name | Frequency | Notes |
| MMDS | 2500-2686 MHz | thirty-one individual 6-MHz television channels (transmitters) including ITFS; some operators have only four channels; range to 35 miles, requires LOS and is affected by multipath (see Note 2). |
| MDS1 | 2150-2156 | single channel 6 MHz; see MMDS. |
| MDS2 | 2156-2162 | single channel 6 MHz; see MMDS. |
| MDS2A | 2156-2160 | MDS2 truncated on one side to 4 MHz |
| WCS | 2305-2320 | 5- or 10-MHz blocks; new in May 1997 |
| WCS | 2345-2360 | 5- or 10-MHz blocks; new in May 1997 |
| ITFS | 2500-2690 | 6-MHz channels shared with MMDS |
| low-power television service | 54-72 78-88 174-216 470-806 | low-power broadcast; 6-MHz channels, experimental licenses. Low power can be 50-Kwatts effective radiated power (ERP) (This includes the antenna gain). LOS operation is advised. |
| LMDS | 27500-28350 31000-31300 | short range, 3 miles, 20-MHz channels, new |
| ISM | 902-928 2,400-2,483.5 | short range 0.5 miles spread; spectrum omnidrectional. Short range, similar to 900 MHz, but can also be engineered beyond 15 miles point to point as the return path for a cable-modem system. |
| Note 2: Multipath is the reception of two or more signals over different paths. The direct signal may combine with a reflection off a roof, wall, or other surface, refraction off trees or an atmospheric inversion layer. The receved signal is the vector sum of the two signals creating both an amplitude and phase change. The distortion may move rapidly across the frequency band. | ||
Table 2. Frequency Bands
