Donald L. Schilling

The capacity of broadband CDMA overlaying a GSM cellular system

The author proposes that one can overlay an existing GSM cellular network with a broadband CDMA cellular system utilizing direct-sequence spread spectrum alone with agile notch filters at the base station. Notch filter depths of 35 dB on receive and 20 dB in transmit were found to be adequate. Capacity calculations show that the B-CDMA overlay can approximately quadruple the nominal capacity of GSM, which is 16 channels per sector, by providing an additional 51 voice channels at 16 kbps each. Also, the author has calculated that the B-CDMA mobile transmits on average with 16 dB less power than the GSM mobile, while the CDMA base transmit power per channel would be about 5.5 dB less than the GSM base due to the lower CIR required by the B-CDMA overlay system.<<ETX>>

Wireless communications going into the 21st century

The evolution of telecommunications, from the wired phone to personal communications services, is resulting in the availability of wireless products not previously considered practical. The user of a cellular or personal communication system wants to use one phone for all of his intended needs. Thus, a single portable phone should be able to operate in a residence, as a cordless phone; in a vehicle using a cellular system, in an office using a WPBX; and outside with wireless local access. Broad-band code-division multiple access (B-CDMA) is a technique which allows PCS operation in the cellular frequency band in conjunction with existing cellular service, as well as in the PCS band (1850-1990 MHz in the U.S.). Using B-CDMA, high-quality voice with no dropped calls as well as data-rate-on-demand can be achieved, which will permit ISDN and multimedia communications at power levels which are much less than that required for other technologies. This paper describes the present cellular and PCS environments, as well as the evolution of these environments into the 21st century, and explains how broad-band CDMA can provide the one-phone service required by business people as well as people at home. >

Broadband spread spectrum multiple access for personal and cellular communications

Bandwidth is the most important parameter of a code division multiple access (CDMA) system. It is governed by the system chip rate and affects capacity, fade margin, data rate, voice quality, performance in indoor and outdoor environments, transition plan, frequency management, overlay capability, and adaptive power control. The author discusses how each of these parameters is affected by the choice of bandwidth. A comparison is made between the capacity of a B-CDMA system in the cellular and PCS bands.

On the statistics of multipath fading using a direct sequence CDMA signal at 2 GHz, in microcellular and indoor environment

This paper describes the results of field tests consisting of received signal variations and multipath returns seen by a mobile receiver using direct-sequence (DS) code-division multiple-access (CDMA) waveforms. The experimental data for outdoor (microcellular environment) reveals that the instantaneous received power lost due to fading (sometimes called the fade margin) increases dramatically as the signaling bandwidth decreases below 11 MHz. It also shows that for most outdoor locations the significant multipath was confined primarily to 1Μs of excess delay with average delay of less than 200 ns. A broader-band CDMA system, having a bandwidth exceeding 10 MHz, could resolve the multipath components and could use a RAKE receiver to further reduce the transmitted power. However, a narrower-band CDMA system could not resolve these multipath components and thereby would suffer signal fading which would have to be compensated by 15 dB or more signal amplification. As a result, a narrower-band CDMA system would interfere with its other users and reduce its user capacity if no other forms of diversity are used. Our results also reveal that at some locations, returns with delays of more than 1Μs can be seen by the receiver with enough energy that could be collected using a RAKE receiver. However, these locations were found to be confined to a very low probability of existence within a prescribed cell (3-mile radius).

Multipath fading statistics of a direct sequence CDMA signal at 2 GHz, in microcellular and indoor environment

This paper describes the results of field tests consisting of received signal variations and multipath returns seen by a mobile receiver using direct sequence (DS) code division multiple access (CDMA) waveforms. The results reveal that the instantaneous received power lost due to fading (sometimes called the fade margin) increases dramatically as the signalling bandwidth decreases. It also shows that for most of the outdoor locations the significant multipath was confined primarily to 1/spl mu/s of excess delay with average delay of less than 200 ns. A broader band CDMA system, having a bandwidth exceeding 10-MHz could resolve the multipath components and could use a RAKE receiver to further reduce the transmitted power. However, a narrower band CDMA system could not resolve these multipath components and thereby would suffer signal fading which would have to be compensated by 15 dB or more signal amplification. As a result, a narrower band CDMA system would interfere with its other users and reduce its user capacity if no other forms of diversity are used.<<ETX>>

Broadband-CDMA overlay

A Broadband Code Division Multiple Access (B-CDMA) technique is presented that will overlay the existing cellular telephone spectrum (825 to 894MHz). The overlay will provide additional capacity to the network while allowing high quality voice and high speed data services to be coexistent with the existing cellular services (AMPS and TDMA). The advantages of using B-CDMA in a fading environment are analyzed. The low level of mutual interference between the existing cellular telephone system and the B-CDMA overlay system is shown.

Impact on capacity to AMPS jamming CDMA/CDMA jamming AMPS in adjacent cells

The authors address the mutual interference caused by the coexistence of narrowband code division multiple access (N-CDMA) and narrowband cellular telephone technologies, i.e., advanced mobile phone system and time division multiple access (AMPS and TDMA). It is shown that when adjacent service areas are using AMPS or TDMA, a dead zone of up to two cells is required before N-CDMA can be deployed. The resulting capacity of the N-CDMA service area is lessened to an extent dependent upon the service area geometry. The dead zone can be extensive enough where the N-CDMA would yield only marginal capacity improvement over AMPS and TDMA.

Capacity limitations due to adjacent cell dissimilar technologies

A direct sequence spread spectrum code division multiple access (CDMA) communication system for cellular or PCN applications should be designed to use the entire bandwidth allocated. In this way the peak power spectral density is minimized. However, some CDMA systems have been designed to have a bandwidth which is much less than the allocated bandwidth. Such narrowband CDMA (N-CDMA) systems lack the LPI characteristics of spread spectrum. The authors address the mutual interference caused by the coexistence of N-CDMA and existing narrowband cellular telephone technologies (AMPS FDMA or TDMA). It is shown that when adjacent service areas are using AMPS or TDMA a dead zone of up to two cells is required before N-CDMA can be deployed. The resulting capacity of the N-CDMA service area is lessened to an extent dependent upon the service area geometry. The dead zone can be extensive enough where the N-CDMA would yield only marginal capacity improvement over AMPS and TDMA. Furthermore, significantly higher N-CDMA mobile transmit power would be required than would be true without adjacent cell interference.<<ETX>>

Spread spectrum technology for the command post on the move

Discusses the operating conditions for broadband CDMA cellular systems as they may be used for mobile command posts. The authors conclude that the mobile command post will permit communication of voice and data with no performance degradation as compared to a fixed command post.<<ETX>>

Standardization in a wireless environment

The mobile communications industry was built upon a fundamental technological stability in the radio transmission link: analog FM. For nearly forty years this stability supported a comfortable environment in which uniform standards were feasible. Single standards supported the manufacturers goal of lowering production costs. Consumers benefitted as competition forced these lowered costs to be passed through to the public. This comfortable equilibrium was shattered in the 1980s with the introduction of digital radio into the commercial marketplace. Digital radio will end, forever, the era of a single standard and a complacent marketplace. This paper presents the authors opinion of the role of standardization in a wireless environment.<<ETX>>