MUTUAL FLUCTUATIONS BETWEEN CARRIERS IN A BROADBAND

MUTUAL FLUCTUATIONS BETWEEN CARRIERS IN A BROADBAND
Abstract – Mutual fluctuations between two carriers in a broadband millimeter-wave communication system may be observed from the heterodyne signal spectrum of the carriers. Laboratory measurements made on equipment developed for local multipoint distribution system (LMDS) applications indicated the degradation in the heterodyne spectrum was mainly in the form of increased background noise with L(f) close to the beat-tone carrier essentially unaffected.  Synchronization and orthogonal relationship between the carriers are therefore well preserved by the millimeter-wave link. This property is a key requirement for supporting modulation schemes with high spectral efficiency.
 I. Introduction
To meet with the need for higher information throughput and to provide better flexibility in geographical coverage, multipoint distribution systems employing millimeter-wave carriers have been introduced in recent years [1, 2].  Such systems offer unprecedented instantaneous bandwidth and versatility in spectrum usage that have not been previously offered by a commercial wireless connectivity.  Because of the flexibility in system configuration and baseband formats, there are often several ways to accomplish  the same set of goals using rather diverse design approaches.  Nevertheless, it is not unusual to find a local multipoint distribution system (LMDS) built upon a broadcast or point-to-multipoint type of architecture, with a headend communicating with a multitude of subscribers within a region, often referred to as a cell.  While there may be similarities to a cellular phone system or a satellite communication system, LMDS finds itself rather unique in a variety of aspects.  Many of these are due to the frequency of operation being in the millimeter-wave bands and the magnitude of the bandwidth, leading to special device requirements and composite signal propagation characteristics.
As complex modulation methods are introduced to achieve higher spectral efficiency, the ability of the radio frequency (RF) channel in maintaining the synchronization or orthogonality of two carriers is of fundamental interest to the system designer, for this aspect has direct influence on the quality of transmission as well as on receiver design.  In this paper, an experiment to directly observe the mutual fluctuations between two carriers after being transmitted through a millimeter-wave communication system is described.  Various components in the system may exhibit nonlinear effects [3] and may contribute to the noise in the received broadband signal. Through the spectrum of the heterodyne signal of the two carriers, one is able to observe the mutual fluctuations between them.
II. System Description
Since the measurements involved the high-frequency portion of the millimeter-wave system, the description will only concentrate on this part of the equipment.  The system accepts an intermediate fre-quency (IF) input at the transmitter side at 3.5 – 4.5 GHz and block-converts it to 27.5 – 28.5 GHz. The upconverted signal is amplified by a traveling wave tube and a portion of the output is radiated by a horn antenna.  A signal path attenuation representing 3 km free-space loss was introduced in the laboratory.
The transmitted signal is received by a horn antenna and downconverted to IF for observation. In a typical measurement, two synchronized carriers at 3.96 GHz and 4.00 GHz are inserted to a multi-carrier IF and fed to the transmitter. These two tones are retrieved by bandpass filters at the receiver and mixed to generate a beat signal at 40 MHz nominally. The spectrum of this beat signal is monitored by a spectrum analyzer as shown in figure 1.
As in phase-noise measurements, fluctuations be-tween two oscillators can be revealed by mixing their outputs and displaying the beat signal on a spectrum analyzer.  The spectrum of the transmitter input IF around the two carriers are shown in figure 2, while their heterodyne signal spectrum (lower sideband) centered at 40 MHz, their difference frequency, is shown in figure 3.
After being upconverted to Ka-band, transmitted, re-ceived, and downconverted to IF at the receiver, the signal exhibits some degradation, as shown in figure 4. Notable from the spectrum are the appearance of intermodulation products and the increase in noise level.  To observe the mutual fluctuations between these received carriers, they are each extracted by 20 MHz bandpass filtering and the heterodyne signal spectrum is shown in figure 5.  The 20 MHz bandwidth was chosen to ensure that the filters do not have influence on the near carrier spectral contents of each of the carriers.  A comparison of the spectra in figure 3 and figure 5 shows that the millimeter-wave communication system has minimal effect on the mutual fluctuations between the two carriers.
III. Discussions
Millimeter-wave frequency bands offer several dis-tinct advantages as the vehicle for providing the capacity and flexibility in system deployment needed by emerging broadband communication system initia-tives.  Channel characteristics in this frequency range call for special attention being given to system design.  With the employment of modulation techniques that rely on the synchronization or orthogonal relation between carriers and subcarriers, RF channel induced mutual fluctuations between the carriers are of fundamental importance.  Laboratory measurements made on a broadband millimeter-wave communication system indicated that fluctuations caused by the equipment is only significant in raising the background noise of the heterodyne signal for a pair of carrier 40MHz  apart from each other at 28 GHz, for which a carrier-to-noise ratio of over 30 dB can be sustained.  This indicates that the current millimeter-wave technology is effective in providing support for broadband information access with significant throughput and spectral efficiency.
www.crimico.org/doc/Example_E.doc
MARIA GABRIELA MEDINA
C.I.16779553
CAF