Personal Communication Services PCS

PCS is difficult to strictly define. It is a radio system; it is small; it can provide multiple services; and it often can be considered an extension of cellular radio. Consider that paging, various types of remote control devices, cordless telephones, wireless PBXs, and wire loop replacement and wireless LANs may be considered PCSs. In our brief discussion below, we will consider only wireless telephones, which are operated in a cellular-like manner. Of course, in this case, cells are much smaller. We might call them microcells or picocells.

There are two standards that have developed in the recent past: Digital European Cordless Telephone (DECT) and Personal Access Communication System (PACS), a North American initiative (see Chapter 10). We briefly describe PACS hardware implementation in this subsection.

Some key features of PACS are:

• Small, inexpensive radio ports (RPs) and small coverage area per port

• Low complexity per-circuit signal processing

• Low transmit power and small batteries for subscriber units (SUs)

• Capability to provide PSTN access comparable to wire line

• Optimized to provide service to and in-building, and for pedestrian and city traffic operating environments

• Cost effective to service high traffic capacities

Figure 15.49 illustrates the PACS functional architecture. We make comparisons to cellular functions where appropriate.

The radio port (RP in Figure 15.49) can be likened to a cell site. This is a radio unit consisting of a digital transceiver. Its transmitter has 200-mW RF peak power output; its average power is 25 mW. RPs are powered by wire pair from a central power source. The radio interface uses tt/4 quadrature phase-shift keying (QPSK) modulation in a TDMA/TDM type format where the downlink is TDM. The radio frame is 2.5 ms in duration with 8 bursts/frame. A RP range is about 650 ft, so for full coverage, the maximum RP spacing is about 1300 ft.

The RPCU (radio port control unit) may roughly be likened to a combination of cell site control and MTSO control and carries out functions of both. An access manager (AM) (Figure 15.49) can support multiple RPCUs with network-related tasks such as querying remote databases for visiting users, assisting in network call setup and delivery, coordinating link transfer between RPCUs, and multiple RP management. Between the PACS AM and RPCU, nearly all of the MTSO responsibilities are covered.

The RPCU connects to the serving PSTN switch or PABX by ISDN BRI.* Data are transmitted using the X.25 protocol with link access by LAPB.

* BRI = basic rate interface. It consists of two 64-kbps clear channels and a 16-kbps signaling channel. For a good overall description of ISDN and BRI, consult Practical Data Communications (Ref. 20).

Personal Communication
Figure 15.49. PACS functional architecture. SS7 = CCITT Signaling System No. 7. (From Figure 1, ''PACS: Personal Access Communications System—A Tutorial,'' IEEE Personal Communications, June 1996; Ref. 21.)

PACS can operate as a FCC-licensed operation or in the PACS-UA or UB mode, in unlicensed operation. When operating as unlicensed, the frequency band authorized by the FCC is the 1920- 1930-MHz band. Otherwise, as licensed, it may operate in the 1850-1910-MHz and 1930-1990-MHz bands.

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