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3.6 Synopsis of some implemented adaptive array systems
This section provides a brief description/study of some adaptive antenna systems and the advantages and disadvantages of their implementation trends.
Reference [49] focuses on the stability problem that exists in adaptive antenna arrays when the weighting functions operate at a nonzero frequency. This paper shows how stability can be achieved by a simple time-compensation procedure. As a result. the weight processor can be implemented digitally. Nonzero frequency weighting in adaptive antenna arrays was introduced as a means to overcome some drawbacks encountered during implementation of conventional arrays whose weights operate around dc. Some of these drawbacks are dc offsets, feed through component imbalance. etc, and are usually associated with analog devices. The paper also cites an implementation of a four-element adaptive array with weights at an IF of 41 OMHz.
Perfonnance improvement in interference rejection over the conventional baseband array is also demonstrated. Also cited, is an array whose weight processor is implemented entirely with analog components. As a result, precise adjustments are difficult to achieve due to drifts and nonlinearity effects. Furthennore, analog integrators in adaptive processors suffer from imprecision, have a voltage limitation, and face an uncertain storage of infonnation. This study was initially prompted by the desire to design the nonzero frequency weight processor by digital means in order to make this storage digital, and hence improve upon the analog integration.
Reference [50] discusses a phase-locked HF (30MHz) receiving array using separate RF amplifiers for each element (6 elements in total). The signals from each element are combined at an intennediate frequency of 3. 7MHz. The type of array considered corrects essentially all phase errors between the distant transmitter and the point where the received signals are combined. This includes errors due to the propagation path, array element motion, near-field obstructions, and instabilities in electronic equipment and RF cables. The array also corrects phase shifts due to changes in angle
of arrival, thus giving it the highly desirable property of automatically tracking a desired signal.
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An adaptive antenna for a TDMA mobile telephony system is described in [51]. The adaptive array operates at a centre frequency of 1721MHz and consists of 10 elements arranged in a circular configuration. The weighting and summing of the received signals are performed on the received RF signal allowing the use of an ordinary base station as a receiver. The weighting is performed on the received RF signal using a phase shifter and an attenuator. The phase shifter accuracy is 1 degree and the attenuator is adjustable in I dB steps down to -50dB. The RF signal is split and down-converted to the baseband and separated into I and Q channels where sampling at bit rate is done (270kHz). The ADC used has an 8 bit resolution with a dynamic range from -32dBm (O.63J~W) to -80dBm (JOpW). Digital signal processing is thereafter employed.
Reference [52] discusses an adaptive antenna for CDMA cellular systems. The performance of the adaptive antenna in various scenarios has been studied. There are four antenna elements in the array and the modulation scheme is QPSK. A processing gain of 128 was used. The spread spectrum signal received by the antenna elements is first processed by a weighting summing circuit. The output is then fed into two branches, one for symbol detection using a rake receiver and the other for comparing with the reference signal. The advantage of this scheme is that only one Rake receiver is needed and the path searcher operates in an environment of high signal to interference ratio (SIR) once an appropriate beam is formed. In a real system. the pilot signal can be used as part of the reference signal, but in the work reported in this paper, the antenna array is operated in a decision directed mode which means that the reference signal is obtained by respreading the detected symbols with phase and amplitude adjustment.
In [53], a new type of adaptive antenna is presented, which is intended for use in mobile receivers. A nonlinear, adaptive, analog feedback controller has been developed to control the phase relationship between two receiving elements. Both the controller andRF prototype are described in this paper. The prototype system described in this paper would be noteworthy for several reasons: First, the antenna diversity does not come at the price of additional expensive RF parts. This is in contrast to the multiple antenna systems that do the combining in baseband
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processing, which requires that each antenna have its own LNA and mixer. Tt is common in modern spatial diversity systems to perform a down-conversion on each antenna signal before processing. The signals are digitized, then the computational power of a DSP is brought to bear on the problem of inteJJjgent combining. This approach to building spatial diversity systems can be expensive; each new diversity branch requires its own LNA, mixer and ADC. However, the prototype system in [53] is much more cost effective. Second, the adaptive antenna would be completely modular, essentially a stand-alone "smart" antenna. The command level voltage could either be derived from the IFlbaseband block or be simple, fixed reference.
Third, the command level essentially sets the level of the automatic gain control at the receiving antenna, which relaxes the linearity requirement of the LNA. Finally, the non linear, analog controller proposed is a novel design and for this application, more cost efficient than traditional microprocessor-driven approaches.
From the above discussion, it is evident that antenna diversity comes at the expense of additional expensive RF parts (LNA, mixer and ADC). These systems are also very complex and expensive due to the large number of receivers required. They also require accurate calibration procedures for the phase and amplitude mismatches between channels. In [54] a multichannel measurement system using a single receiver is described. The system is based on a complex wideband radio channel sounder and a fast RF switch. The carrier frequency of the sounder is 2.154GHz and the bandwidth is lOOMHz. The system provides real-time recording of signals from multiple input channels and this enables real dynamic array measurements of the radio channel of a moving mobile. The continuously recorded path lengths can be over one hundred metres. This method simulates a real adaptive array and therefore gives a realistic picture of the operation of practical adaptive array radio systems. In order to measure multiple antenna elements with a single receiver, a fast RF switching unit is required behind the antenna. The switching unit is based on a TTL-controlled GaAs switch with a switching time of 3ns. The system used in this paper has 8 channels.
The worldwide cellular and PCS infrastructure build-out has provided market incentives to develop better radio receivers. Better means smaller, lower power dissipation, high sensitivity, less factory tweaking, high manufacturing yields, field programmability, and operational flexible. Reference [55] discusses a new chipset
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that has been developed that can be used to design high sensitivity, lower power digital receivers for a variety of cellular and PCS standards, including AMPS, IS-136, PDC, GSM and CDMA. One of them, the AD6600 is a dual channel, gain ranging receIver ADC that can sample IF signals from 70MHz to 250MHz at 20 mega samples per second (MSPS). It has on chip peak detectors, and received signal strength indicator (RSSI). It also has two analog input pins that can be used to sample separate diversity antennas. At the rated iF frequencies, 90dB of dynamic range is achieved: 30dB from the gain ranging and 60dB from the 11 bit ADC. The other chip, the AD6620 is a dual channel digital decimation programmable filter designed to interface directly with the AD6600 and subsequent DSPs. This chip can accept 16 bit input words at up to 65MSPS; the decimated data can be accessed by serial or parallel words. The combination of high IF over-sampling with programmable digital filtering allows designers to replace many analogIRF functions with digital functions.
Due to the difference in application (centre frequency, bandwidth, technology, etc.), a performance comparison of the above systems cannot be made.
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