Biblio5

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16.9 CORDLESS TELEPHONE TECHNOLOGY

16.9.1 Background

Cordless telephones began to become widely used in North America around 1981. Today their popularity is worldwide, with hundreds of millions of units in use. As the technology develops, they will begin to compete with cellular radio systems, where the cordless telephone will operate in microcells.

16.9.2 North American Cordless Telephones

The North American cordless telephone operates in the 50-MHz frequency band with 25 frequency pairs using frequency modulation. Their ERP is on the order of 20 mW. Ref. 19 suggests that this analog technology will continue for some time into the future because of the telephone’s low cost. These may be replaced by some form of the wireless local loop (WLL), operating in the 30-GHz or 40-GHz band.

16.9.3 European Cordless Telephones

The first-generation European cordless telephone provided for eight channel pairs near 1.7 MHz (base unit transmit) and 47.5 MHz (handset transmit). Most of these units could only access one or two channel pairs. Some called this “standard” CT0.

This was followed by another analog cordless telephone based on a standard known as CEPT/ CT1. CT1 has 40 25-kHz duplex channel pairs operating in the bands 914 –915 MHz and 959–960 MHz. There is also a CT1+ in the bands 885–887 MHz and 930 –932 MHz, which do not overlap the GSM allocation. CT1 is called a coexistence standard (not a compatible standard), such that cordless telephones from different manufacturers do not interoperate. The present embedded base is about 12.5 million units with some 5 million units expected to be sold in 1998.

Two digital standards have evolved in Europe: the CT2 Common Air Interface and DECT (digital European cordless telephone). In both standards, speech coding uses ADPCM (adaptive differential PCM). The ADPCM speech and control data are modulated onto a carrier at a rate of 72 kbps using Gaussian-filtered FSK (GFSK) and are transmitted in 2-ms frames. One base-to-handset burst and one handset-to-base burst are included in each frame.

The frequency allocation for CT2 consists of 40 FDMA channels with 100-kHz spacing in the band 864 –868 MHz. The maximum transmit power is 10 mW, and a two-level power control supports prevention of desensitization of base station receivers. As a byproduct, it contributes to frequency reuse. CT2 has a call reestablishment procedure on another frequency after three seconds of unsuccessful attempts on the initial frequency. This gives a certain robustness to the system when in an interference environment. CT2 supports up to 2400 bps of data transmission and higher rates when accessing the 32 kbps underlying bearer channels.

CT2 also is used for wireless pay telephones. When in this service it is called Telepoint. CT2 seems to have more penetration in Asia than in Europe.

Canada has its own version of CT2, called CT2+. It is more oriented toward the mobile environment, providing several of the missing mobility functions in CT2. For example, with CT2+, 5 of the 40 carriers are reserved for signaling, where each carrier provides 12 common channel signaling channels (CSCs) using TDMA. These channels

support location registration, updating, and paging, and enable Telepoint subscribers to receive calls. The CT2+ band is 944 –948 MHz.

DECT takes on more of the cellular flavor than CT2. It uses a picocell concept and TDMA with handover, location registration, and paging. It can be used for Telepoint, radio local loop (RLL), and cordless PABX besides conventional cordless telephony. Its speech coding is similar to CT2, namely, ADPCM. For its initial implementation, 10 carriers have been assigned in the band 1880 –1900 MHz.

There are many areas where DECT will suffer interference in the assigned band, particularly from “foreign” mobiles. To help alleviate this problem, DECT uses two strategies: interference avoidance and interference confinement. The avoidance technique avoids time/ frequency slots with a significant level of interference by handover to another slot at the same or another base station. This is very attractive for the uncoordinated operation of base stations because in many interference situations there is no other way around a situation but to change in both the time and frequency domains. The “confinement” concept involves the concentration of interference to a small time–frequency element even at the expense of some system robustness.

Base stations must be synchronized in the DECT system. A control channel carries information about access rights, base station capabilities, and paging messages. The DECT transmission rate is 1152 kbps. As a result of this and a relatively wide bandwidth, either equalization or antenna diversity is typically needed for using DECT in the more dispersive microcells.

Japan has developed the personal handyphone system (PHS). Its frequency allocation is 77 channels, 300 kHz in width, in the band 1895–1918.1 MHz. The upper-half of the band, 1906.1–1918.1 MHz (40 frequencies), is used for public systems. The lower-half of the band, 1895–1906.1 MHz, is reserved for home/ office operations. An operational channel is autonomously selected by measuring the field strength and selecting a channel on which it meets certain level requirements. In other words, fully dynamic channel assignment is used. The modulation is p/ 4 DQPSK; average transmit power at the handset is 10 mW (80-mW peak power) and no greater than 500 mW (4-W peak power) for the cell site. The PHS frame duration is 5 ms. Its voice coding technique is 32-kbps ADPCM (Ref. 9).

In the United States, digital PCS was based on the wireless access communication system (WACS), which has been modified to an industry standard called PACS (personal access communications services). It is intended for the licensed portion of the new 2-GHz spectrum. Its modulation is p/ 4 QPSK with coherent detection. Base stations are envisioned as shoebox-size enclosures mounted on telephone poles, separated by some 600 m. WACS/ PACS has an air interface similar to other digital cordless interfaces, except it uses frequency division duplex (FDD) rather than time division duplex (TDD) and more effort has gone into optimizing frequency reuse and the link budget. It has two-branch polarization diversity at both the handset and base station with feedback. This gives it an advantage approaching four-branch receiver diversity. The PACS version has eight time slots and a corresponding reduction in channel bit rate and a slight increase in frame duration over its predecessor, WACS.

Table 16.2 summarizes the characteristics of these several types of digital cordless telephones.

16.10 WIRELESS LANs

Wireless LANs (WLANs), much as their wired counterparts, operate in excess of 1 Mbps. Signal coverage runs from 50 ft to less than 1000 ft. The transmission medium

aGeneral allocation to PCS; licensees may use PACS.

Source: Ref. 19, Table 2.

can be radiated light (around 800 nm to 900 nm) or radio frequency, unlicensed. Several of these latter systems use spread spectrum with transmitter outputs of 1 W or less.

WLANs using radiated light do not require FCC licensing, a distinct advantage. They are immune to RF interference but are limited in range by office open spaces because their light signals cannot penetrate walls. Shadowing can also be a problem.

One type of radiated light WLAN uses a directed light beam. These are best suited for fixed terminal installations because the transmitter beams and receivers must be carefully aligned. The advantages for directed beam systems is improved S/ N and fewer problems with multipath. One such system is fully compliant with IEEE 802.5 token ring operation offering 4- and 16-Mbps transmission rates.

Spread spectrum WLANs use the 900-MHz, 2-GHz, and 5-GHz industrial, scientific, and medical (ISM) bands. Both direct sequence and frequency hop operation can be used. Directional antennas at the higher frequencies provide considerably longer range than radiated light systems, up to several miles or more. No FCC license is required. A principal user of these higher-frequency bands is microwave ovens with their interference potential. CSMA and CSMA/ CD (IEEE 802.3) protocols are often employed.

There is also a standard microwave WLAN (nonspread spectrum) that operates in the band 18–19 GHz. FCC licensing is required.

Building wall penetration loss is high. The basic application is for office open spaces.

16.11 MOBILE SATELLITE COMMUNICATIONS

16.11.1 Background and Scope

This section contains a brief review of satellite PCS/ cellular services. It is hard to discern whether these services are PCS or cellular. Most of the active and proposed low earth orbit (LEO) satellite systems discussed here utilize frequency reuse and are based on a cellular concept. By the year 2000 or just after we expect at least two “broadband” satellite systems designed to deliver such services as Internet and various other forms of “high-speed” data. By broadband, we mean bandwidths well in excess of those bands available between 1.5 GHz and 2.6 GHz. Among this group are Teledesic and Celestri.

508 CELLULAR AND PCS RADIO SYSTEMS

2. Why is transmission loss so much greater on a cellular path compared to a LOS microwave path on the same frequency covering the same distance?

3. What is the function of the MTSO or MSC in a cellular network?

4. What is the channel spacing, in kHz, of the AMPS system? What type of modulation does it employ?

5. Why are cell site antennas limited to just sufficient height to cover cell boundaries?

6. Why do we do cell splitting? What is the approximate minimum practical cell diameter (this limits splitting)?

7. When is handover necessary?

8. Cellular transmission loss varies with what four factors besides distance and frequency?

9. What are some of the fade ranges (dB) we might expect on a cellular link?

10. If the delay spread on a cellular link is about 10 msec, up to about what bit rate will there be little deleterious effects due to multipath fading?

11. Space diversity reception is common at cells sites. Antenna separation varies with

.

12. For effective space diversity operation, there is a law of diminishing returns when we lower the correlation coefficient below what value?

13. What is the gain of a standard dipole over a reference isotropic antenna? Differentiate ERP and EIRP.

15. What would the maximum transmission loss to the 39-dBm contour be if the cell site EIRP is +52 dBm?

16. A cell site antenna has a gain of +14 dBd. What is the equivalent gain in dBi?

17. What are the three generic access techniques that might be considered for digital cellular operation?

18. If we have 10 cellular users on a TDMA frame and the frame duration is 20 ms, what is the maximum burst duration without guard time considerations?

19. What are the two basic elements in digital cellular transmission with which we may improve users per unit bandwidth?

20. What power amplifier advantage do we have in a TDMA system that we do not have in an FDMA system, assuming a common power amplifier for all RF channels?

21. Cellular radio, particularly in urban areas, is gated by heavy interference conditions, especially cochannel from frequency reuse. In light of this, describe how we achieve an interference advantage when using CDMA.

22. A CDMA system has an information bit stream of 4800 bps, which is spread 10 MHz. What is the processing gain?

23. For effective frequency reuse, the value of D/ R must be kept large enough. Define D and R. What value of D/ R is large enough?

24. In congested urban areas, where cell diameters are small, what measure can we take to reduce C/ I?

25. What is the effect of the Fresnel ellipse?

26. What range of transmitter output power can we expect from cordless telephone PCS?

27. Why would it be attractive to use CDMA with a RAKE receiver for PCS systems?

28. Speech coding is less stringent with PCS scenarios, typically 32 kbps. Why?

29. What are the two different transmission media used with WLANs?

30. Give two decided advantages of LEO satellite systems over their GEO counterparts.

REFERENCES

1. T. S. Rappaport, Wireless Communications: Principles and Practice, IEEE Press, New York, 1996.

2. Telecommunications Transmission Engineering, 3rd ed., Bellcore, Piscataway, NJ, 1989.

3. R. Steele, ed., Mobile Radio Communications, IEEE Press, New York, and Pentech Press, London, 1992.

4. Y. Okumura, et al., “Field Strength and Its Variability in VHF and UHF Land Mobile Service,” Rec. Electr. Commun. Lab., 16, Tokyo, 1968.

5. M. Hata, “Empirical Formula for Propagation Loss in Land-Mobile Radio Services,” IEEE Trans. Vehicular Technology, VT-20, 1980.

6. F. C. Owen and C. D. Pudney, “In-Building Propagation at 900 and 1650 MHz for Digital Cordless Telephones,” 6th International Conference on Antennas and Propagation, ICCAP, Pt. 2, Propagation Conf., Pub. No. 301, 1989.

7. IEEE Standard Dictionary of Electrical and Electronics Terms, 6th ed., IEEE Std. 100-1996, IEEE, New York, 1996.

8. W. C. Y. Lee, Mobile Communications Design Fundamentals, 2nd ed., Wiley, New York, 1993.

9. Recommended Minimum Standards for 800-MHz Cellular Subscriber Units, EIA Interim Standard EIA/ IS-19B, EIA, Washington, DC, 1988.

10. Cellular Radio Systems, seminar given at the University of Wisconsin—Madison, by A. H. Lamothe, consultant, Leesbury, VA, 1993.

11. R. C. V. Macario, ed., Personal and Mobile Radio Systems, IEE/ Peter Peregrinus, London, 1991.

12. W. F. Fuhrmann and V. Brass, “Performance Aspects of the GSM System,” Proc. IEEE, 89(9), 1984.

13. Digital Cellular Public Land Mobile Telecommunication Systems (DCPLMTS ), CCIR Rep. 1156, Vol. VIII.1, XVIIth Plenary Assembly, Dusseldorf, 1990.

14. M. Engelson and J. Hebert, “Effective Characterization of CDMA Signals,” Wireless Rep., London, Jan. 1995.

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15. R. C. Dixon, Spread Spectrum Systems with Commercial Applications, 3rd ed., Wiley, New York, 1994.

16. C. E. Cook and H. S. Marsh, “An Introduction to Spread Spectrum,” IEEE Communications Magazine, March 1983.

17. D. C. Cox, “Wireless Personal Communications. What Is It?” IEEE Personal Communications, 2(2), 1995.

18. H. L. Bertoni et al., “UHF Propagation Prediction for Wireless Personal Communication,”

Proc. IEEE, 89(9), 1994.

19. J. C. Padgett, C. G. Gunter, and T. Hattori, “Overview of Wireless Personal Communications,” IEEE Communications Magazine, Jan. 1995.

512 ADVANCED BROADBAND DIGITAL TRANSPORT FORMATS

17.2 SONET

17.2.1 Introduction and Background

The SONET standard was developed by the ANSI T1X1 committee with first publication in 1988. The standard defines the features and functionality of a transport system based on the principles of synchronous multiplexing. In essence this means that individual tributary signals may be multiplexed directly into a higher rate SONET signal without intermediate stages of multiplexing.

DS1 and E1 digital hierarchies had rather limited overhead capabilities for network management, control, and monitoring. SONET (and SDH) provide a rich built-in capacity for advanced network management and maintenance capabilities. Nearly 5% of the SONET signal structure is allocated to supporting such management and maintenance procedures and practices.

SONET is capable of transporting all the tributary signals that have been defined for the digital networks in existence today. This means that SONET can be deployed as an overlay to the existing network and, where appropriate, provide enhanced network flexibility by transporting existing signal types. In addition, SONET has the flexibility to readily accommodate the new types of customer service signals such as SMDS (switched multimegabit data service) and ATM (asynchronous transfer mode). Actually, it can carry any octet-based binary format such as TCP/ IP, SNA, OSI regimes, X.25, frame relay, and various LAN formats, which have been packaged for long-distance transmission.

17.2.2 Synchronous Signal Structure

SONET is based on a synchronous signal comprised of eight-bit octets, which are organized into a frame structure. The frame can be represented by a two-dimensional map comprising N rows and M columns, where each box so derived contains one octet (or byte). The upper left-hand corner of the rectangular map representing a frame contains an identifiable marker to tell the receiver it is the start of frame.

SONET consists of a basic, first-level, structure called STS-1, which is discussed in the following. The definition of the first level also defines the entire hierarchy of SONET signals because higher-level SONET signals are obtained by synchronously multiplexing the lower-level modules. When lower-level modules are multiplexed together, the result is denoted as STS-N (STS stands for synchronous transport signal), where N is an integer. The resulting format then can be converted to an OC-N (OC stands for optical carrier) or STS-N electrical signal. There is an integer multiple relationship between the rate of the basic module STS-1 and the OC-N electrical signals (i.e., the rate of an OC-N is equal to N times the rate of an STS-1). Only OC-1, -3, -12, -24, -48, and -192 are supported by today’s SONET.

17.2.2.1 Basic Building Block Structure. The STS-1 frame is shown in Figure 17.1. STS-1 is the basic module and building block of SONET. It is a specific sequence of 810 octets (6480 bits), which includes various overhead octets and an envelope capacity for transporting payloads.3 STS-1 is depicted as a 90-column, 9-row structure. With a frame period of 125 ms (i.e., 8000 frames per second). STS-1 has a bit rate of 51.840

3The reference publications use the term byte, meaning, in this context, and 8-bit sequence. We prefer to use the term octet. The reason is that some argue that byte is ambiguous, having conflicting definitions.