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White Paper on Spread Spectrum Wireless Technology

Abstract: Section 1 provides a brief introduction to the use of spread spectrum technology in  consumer cordless phone devices. Section 2 provides a general overview of spread spectrum technology including direct sequence schemes and frequency hopping schemes. Important terms such as processing gain are  introduced. Those already familiar with spread spectrum technology can skip Section 2 and proceed to Section 3. Section 3 looks at the Part 15 ISM bands where commercial spread spectrum devices can be used without FCC  spectrum licensing. Rules followed by these devices are introduced. Section 4 discusses various advantages of spread spectrum technology and how they can be applied to address current cordless phone user problems.  Section 5 describes the cordless phone market and its trends. Section 6 provides an overview of InfoWave, developed by InnoMedia. Finally, Section 7 provides some concluding remarks and Section 8 cites references.

Table of Contents

    1 Introduction
    2 Spread Spectrum Fundamentals
    2.1 Direct Sequence Spread Spectrum Systems
    2.2 Frequency Hopping Spread Spectrum Systems
    3 Use of Spread Spectrum Technology in the ISM Bands
    4  Advantages of Spread Spectrum Wireless Systems
    5 Consumer Cordless Phone Market
    6 InfoWave and Its Applications
    7 Concluding Remarks
    8 References

1. Introduction

Ever since cordless phones  were introduced, the popularity of their use has grown steadily. Worldwide the number of cordless phone sales has increased from just over 20 million units in the late 80's to over 40 million units in 1996. Today, sales  in the US alone have grown to more than 20 million units per year.

As usage of cordless phones increases, its problems have also become more evident. Some examples are: poor voice quality, high cross talk interference  (especially in densely-populated urban areas), lack of privacy, and short operating distances.

Spread spectrum technology, first developed by the military, offers some distinct advantages which can alleviate the  problems of conventional cordless telephones. However, because the technology was initially developed for military applications, it could not be readily applied for commercial use due to its high cost and large size. As  this technology and the components continue to develop, integrated circuit (IC) technology has also undergone drastic advancement. These two factors have made commercial use of spread spectrum technology a realistic  proposition. Some recent commercial applications of the technology include digital cellular telephony, digital cordless telephony, and the global positioning system.

In conjunction with increased cordless phone  usage, data applications are also increasingly finding their ways into homes and small office environments. This is due in part to the maturity of multimedia technologies and applications, and to the arrival of the  information era where global information through powerful networking vehicles (such as the Internet) is penetrating into many home and offices.

These developments have created demands for wireless data for homes and  small offices where wiring can often be either too costly or too inconvenient. ISM band spread spectrum devices can also be designed to address the wireless data needs of home and small office users. Section 6 looks at  how InfoFone, a high-performance low-cost device developed by InnoMedia, provides wireless voice and data solutions for home and small office users.

2. Spread Spectrum Fundamentals

 Since its early development in the mid 1950's for military applications, spread-spectrum communications have, over the years, become increasingly popular largely due to its interference tolerance and coexistence  capabilities. Today, the commercial use of spread spectrum technology ranges from digital cellular phones and wireless PCS (for wide-area wireless voice applications) to wireless LANs (for local-area wireless data  transmission). A general definition for spread spectrum technology is [1]:

Spread spectrum is a means of transmission in which the signal occupies a bandwidth in excess of the minimum necessary to send the  information; the band spread is accomplished by means of a code which is independent of the data, and a synchronized reception with the code at the receiver is used for despreading and subsequent data recovery.

Two of  the most popular techniques that provide spectrum spreading in a spread spectrum system are direct sequence and frequency hopping.

2.1 Direct Sequence Spread Spectrum Systems

In  a direct sequence system (as seen in Figure 1), the narrow band information carrier in the transmitter is modulated by a pseudo-random number (PN) pulse sequence. The PN sequence has a pulse frequency (chip rate) which  is much higher than that of the information rate (bit rate). The result of this modulation is spectrum spreading.

The amount of bandwidth spreading is dependent upon the ratio of the chip rate and information bit  rate. This ratio, called processing gain, has significant implications for system performance as will become evident (see below).

In the receiver, the information is recovered by demodulating the received signal by a  locally generated replica of the same PN pulse sequence used by the transmitter. This demodulation process despreads the wide band signal back to its original form (i.e. narrow-band signal), and at the same time,  spreads any narrow-band interferers that may have fallen within the receive band.

After the despreading operation, the interferers (although having the same total power) will have a much lower spectrum density per  Hertz (reduced by the processing ratio factor) because their bandwidths have been increased by the processing ratio. Thus, after the receive filtering process where noise outside of the narrow-band signal have all been  removed, the in-band interference noise power is reduced by the processing gain. This is shown in Figure 2.

2.2 Frequency Hopping Spread Spectrum Systems

In a frequency hopping  system, the carrier of the narrow band information in the transmitter hops according to the output bits of a PN sequence generator. The rate of the hops is typically higher than the information bit rate. Therefore, the  transmitted spectrum appears as having occupied a much wider frequency band (by a ratio of the processing gain) than its original signal spectrum.

In the receiver, the same PN sequence is used to demodulate the  carrier of the receive signal to retrieve the original information. Frequency hopping systems achieve interference tolerance differently from direct sequence systems. In a direct sequence system, the interference  tolerance is achieved by attenuating the interference power (described above), thereby obtaining a higher signal to noise ratio (SNR) to reduce bit error rate (BER). In a frequency hopping system, because each  information bit is transmitted at several (pseudo) random frequencies, the probability that all these frequencies are being interfered (or jammed) is low. Thus, by applying any majority rule scheme, the BER is reduced.

3. Use of Spread Spectrum Technology in the ISM Bands

The FCC has allocated three frequency bands for Part 15 devices to operate on a secondary basis:
· 902-928 (26 MHz)
· 2400-2483.5 (83.5 MHz)
· 5725-5850 (125 MHz)

To operate in these bands on a secondary basis, the devices must tolerate interference from other devices operating in the same band, and cannot cause interference to  the primary users which may emit at a higher signal power. The primary users include government systems as well as industrial, scientific, and medical (ISM) users.

As a secondary user, certain rules must be followed.  For devices with operating power above 1 mW, spread spectrum with direct sequence or frequency hopping must be used. In addition, the spread bandwidth of a direct sequence system must be greater than or equal to 500  KHz, with a minimum processing gain of 10 dB. For low power devices (whose transmitted power is no more than 1 mW), the above rules may be waived.

Once these rules are followed, the users of these devices can enjoy  the free-of-spectrum-licensing benefit, resulting in free air time charges for using the spectrum.

4. Advantages of Spread Spectrum Wireless Systems

Some of the advantages of ISM  spread spectrum wireless systems over conventional systems are:

1. No cross talk interference - Conventional cordless phones frequently suffer from cross talk interference especially when used in densely populated residential areas (such as apartment complexes). This problem  disappears in spread spectrum cordless phone systems because:

a) Cross talk interference is greatly attenuated due to the processing gain of the spread spectrum system as described earlier.

b) The effect of the  suppressed cross talk interference can be essentially removed with digital processing where noise below certain threshold results in negligible bit errors. These negligible bit errors will have little effect on voice  transmissions.

c) The 900 MHz ISM band is a much wider frequency bandwidth than the conventional 45-49 MHz band. This further reduces the possibility of cross talk interference among ISM spread spectrum cordless  phone users.

2. Better voice quality/data integrity and less static noise - Due to the processing gain and digital processing nature of spread spectrum technology, a spread spectrum-based system is more immune to  interference and noise. This greatly reduces consumer electronic device-induced static noise that is commonly experienced by conventional analog wireless system users.

3. Lowered susceptibility to multi-path  fading - Because of its inherent frequency diversity properties (thanks to wide spectrum spread), a spread spectrum system is much less susceptible to multi-path fading. This makes the reception of a spread  spectrum-based cordless phone much less sensitive to the location and pointing direction of the handset than that of a conventional analog wireless system.

4. Inherent security - In a spread spectrum system, a PN  sequence is used to either modulate the signal in the time domain (direct sequence systems) or select the carrier frequency (frequency hopping systems). Due to the pseudo-random nature of the PN sequence, the signal  in the air has been "randomized". Only the receiver having the exact same pseudo-random sequence and synchronous timing can despread and retrieve the original signal. Consequently, a spread spectrum system  provides signal security that is not available to conventional analog wireless systems.

5. Co-existence - A spread spectrum system is less susceptible to interference than other non-spread spectrum systems. In  addition, with the proper designing of pseudo-random sequences, multiple spread spectrum systems can co-exist without creating severe interference to other systems. This further increases the system capacity for  spread spectrum systems or devices.

6. Longer operating distances - A spread spectrum device operated in the ISM band is allowed to have higher transmit power due to its non-interferencing nature. Because of the  higher transmit power, the operating distance of such a device can be significantly longer than that of a traditional analog wireless communication device.

 

5. Consumer Cordless Phone Market

As stated earlier, the cordless phone market has been growing steadily since its debut. The number of units sold in the US alone is now over 20  million units a year. Even though these units are still predominantly conventional analog cordless telephones, digital cordless phones are expected to increase rapidly in sales volume and approach the volume of analog  cordless phones before the end of this century. The numbers and their trend are represented in Figure 4.

6. InfoWave and Its Applications

InfoWave, a high-performance low-cost 900  MHz ISM band spread device developed by InnoMedia, has all the advantages associated with spread spectrum systems. In addition, InfoWave has the following unique strengths which further distinguish it from other  competing products:

1. Digital transmission over the air - The signal modulated by the PN sequence in InfoWave is still in its digital form. The conversion of the digital signal into analog form takes place only after the PN  modulation process. Thus, the digital signal recovery scheme is used in the receiver after its despreading process. The digital signal recovery scheme, if operated above a signal to noise threshold, has a noise  masking effect which allows high data bit rate, low error rate reception possible. Consequently, an InfoWave system can carry data at a rate up to 85 or170 Kilo bits per second occupying 2 or 4 MHz of spread  bandwidth. This high data rate capability enables users to have a high speed wireless data exchange. This is in contrast with some spread spectrum systems where analog voice signals are modulated by the PN sequence.  In those systems, the analog signal must be retrieved after the despreading process, and any residual noise that is in the analog signal can no longer be removed. This severely limits the analog signal quality as  well as the potential data carrier capacity of those systems. For those systems, not only is simultaneous voice and data transmission impossible, but also, the ultimate data rate is limited to the voice band modem  data rate. In addition, data communication can take only the form of analog voice signals where a voice band modem is needed to convert the digital data into an analog signal in the voice band.

2. Frequency  agility for reliable low interference transmission - InfoFone has built-in intelligence to conduct auto-channel scanning to select the clearest channel for a low error-rate communication. It can also intelligently  switch channels to avoid new interference after the communication link has been established.

With its high performance and a diverse feature set, InfoWave can be used in these various environments:

1. Single  pair base unit (DTE) remote unit (DCE) configuration for remote access of peripheral devices - Figure 5 shows a configuration which is especially useful for a home user who does not want his PC to be stringed close  to its peripheral devices, especially modem devices which is close to the wall jack. As shown in Figure 5, sometimes it is very inconvenient or impractical to wire between the PC and those peripheral devices. Using  an InfoWave in this case allows the remote PC to access the printer or the modem/ISDN TA without having to wire between them.

2. Peer-to-peer multiple point-to-point data/voice communications- In a small office  environment, some PCs may need to frequently transfer files among them, while others may require access to an external WAN access device (modem) or a printer. Figure 6 shows such a setup. Once again, with InfoWave  these convenient connections can be achieved easily without wiring. Notice that in this arrangement the PC pairs work in a peer-to-peer fashion (as opposed to the client-server model) where the InfoWave remote units  can talk between each other. Even though InfoWave allows only one pair of devices to talk to each other, the InfoWave units are intelligent enough to allow multiple point-to-point communications to occur at the same  time without interferencing with one other.

3. Multiple point-to-point data communications in a single server setup - In some small office environments, it may be more desirable to allocate one PC to act as the  print server, the file server, or even the fax server. Thus, all the remote accesses to the printer, hard disk, and fax modem can go through the server PC via wireless links. This is also especially useful for a  home user who has multiple PCs but only one set of peripheral devices. InfoWave allows this by configuring the InfoWave base unit to be in a DCE mode (as shown in Figure 7). Under this setup, the client PCs can  still communicate between them in pairs in a peer-to-peer mode and communicate with the Server PC as clients. This feature will be available in a upgrade version of the InfoWave offering.

 7. Concluding Remarks

A few key elements will determine the success of the commercial use of spread spectrum technology, namely: maturity of spread spectrum technology, advancement of baseband as well as RF  IC technologies, and system integration where all these pieces are integrated to offer the best value to end users.

As these elements continue to advance, spread spectrum technology will find more and more  commercial applications ranging from cordless telephony, wireless LAN and wireless data, digital cellular telephony, and even personal communication services (PCS). The end users will be the ultimate beneficiary as  the quality of these products improves while the cost to the users continues to decline.

8. References

Pickholtz, D. Schilling, and L. Milstein, "Theory of Spread-Spectrum  Communications - A Tutorial" IEEE Trans. Comm., vol. COM 30, pp. 855-884, May 1982