Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-07-01
2004-04-06
Appiah, Charles (Department: 2686)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S456100, C455S457000, C455S063300, C370S310000, C370S320000, C370S342000
Reexamination Certificate
active
06718170
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to communications telephone systems. In particular, the invention relates to allocation of microprocessor resources in a wireless communication system.
BACKGROUND OF THE INVENTION
A wireless communication system may comprise multiple remote units and multiple base stations.
FIG. 1
exemplifies an embodiment of a terrestrial wireless communication system with three remote units
10
A,
10
B and
10
C and two base stations
12
. In
FIG. 1
, the three remote units are shown as a mobile telephone unit installed in a car
10
A, a portable computer remote
10
B, and a fixed location unit
10
C such as might be found in a wireless local loop or meter reading system. Remote units may be any type of communication unit such as, for example, handheld personal communication system units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment.
FIG. 1
shows a forward link
14
from the base station
12
to the remote units
10
and a reverse link
16
from the remote units
10
to the base stations
12
.
Communication between remote units and base stations, over the wireless channel, can be accomplished using one of a variety of multiple access techniques which facilitate a large number of users in a limited frequency spectrum. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). An industry standard for CDMA is set forth in the TIA/EIA Interim Standard entitled “Mobile Station—Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, TIA/EIA/IS-95, and its progeny (collectively referred to here as IS-95), the contents of which are incorporated herein by reference. Additional information concerning a CDMA communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” (the '307 patent) assigned to the assignee of the present invention and incorporated in its entirety herein by reference.
In the '307 patent, a multiple access technique is disclosed where a large number of mobile telephone system users, each having a transceiver, communicate through base stations using CDMA spread spectrum communication signals. The CDMA modulation techniques disclosed in the '307 patent offer many advantages over other modulation techniques used in wireless communication systems such as TDMA and FDMA. For example, CDMA permits the frequency spectrum to be reused multiple times, thereby permitting an increase in system user capacity. Additionally, use of CDMA techniques permits the special problems of the terrestrial channel to be overcome by mitigation of the adverse effects of multipath, e.g. fading, while also exploiting the advantages thereof.
In a wireless communication system, a signal may travel several distinct propagation paths as it is transmitted between base stations and remote units. The signal generated by the multipath characteristics of the wireless channel presents a challenge to the communication system. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. For example, if an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation factor. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than other received pulses. Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than other received pulses.
In the wireless channel, the multipath is created by reflection of the signal from obstacles in the environment such as, for example, buildings, trees, cars, and people. Accordingly, the wireless channel is generally a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over the time varying multipath channel, the received stream of pulses changes in time delay, attenuation, and phase as a function of the time that the ideal impulse is transmitted.
The multipath characteristics of a channel can affect the signal received by the remote unit and result in, among other things, fading of the signal. Fading is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors add destructively, yielding a received signal that is smaller in amplitude than either individual vector. For example if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB, a time delay of &dgr; with a phase shift of ⊖ radians, and the second path has an attenuation factor of X dB, a time delay of &dgr; with a phase shift of ⊖+&pgr; radians, no signal is received at the output of the channel because the two signals, being equal amplitude and opposite phase, cancel each other. Thus, fading may have a severe negative effect on the performance of a wireless communication system.
A CDMA communications system is optimized for operation in a multipath environment. For example, the forward link and reverse link signals are modulated with a high frequency pseudonoise (PN) sequence. The PN modulation allows the many different multipath instances of the same signal to be separately received through the use of a “rake” receiver design. In a rake receiver, each element within a set of demodulation elements can be assigned to an individual multipath instance of a signal. The demodulated outputs of the demodulation elements are then combined to generate a combined signal. Thus, all of the multipath signal instances must fade together before the combined signal experiences a deep fade.
In the remote unit, a microprocessor is used to assign demodulation elements to the available multipath signal instances. A search engine is used to provide data to the microprocessor concerning the multipath components of the received signal. The search engine measures the arrival time and amplitude of the multipath components of a pilot signal transmitted by the base stations. The effect of the multipath environment is the same on the pilot signal and the data signal. Determining the multipath environment's effect on the pilot signal allows the microprocessor to assign demodulation elements to the data channel multipath signal instances.
The search engine determines the multipath components of the pilot signal by “searching” through a sequence of potential path offsets and measuring the energy of the pilot signal received at each of the potential path offsets. The microprocessor evaluates the energy associated with a potential offset, and, if it exceeds a certain threshold, assigns a signal demodulation element to that offset. A method and apparatus of demodulation element assignment based on searcher energy levels is disclosed in U.S. Pat. No. 5,490,165 entitled “DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS”, assigned to the assignee of the present invention.
FIG. 2
shows an exemplary set of multipath signal instances of a single pilot signal from a base station arriving at a remote unit. The vertical axis represents the power received in decibels (dB). The horizontal axis represents the delay in the arrival time of a signal instance due to multipath delays. The axis (not shown) going into the page represents a segment of time. Each signal spike in the common plane of the page has arrived at the remote unit at a common time but has been transmitted by the b
Appiah Charles
Brown Charles D.
Iqbal Khawar
Kordich Donald C.
Wadsworth Philip
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