Pulse or digital communications – Spread spectrum
Reexamination Certificate
1998-10-27
2003-09-23
Liu, Shuwang (Department: 2734)
Pulse or digital communications
Spread spectrum
C375S136000, C375S147000, C370S208000, C370S342000
Reexamination Certificate
active
06625197
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for demodulating code division multiple access (CDMA) signals.
II. Description of the Related Art
In a wireless radiotelephone communication system, many users communicate over a wireless channel to connect to wireline telephone systems. Communication over the wireless channel can be one of a variety of multiple access techniques that allow 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).
The CDMA technique has many advantages. An exemplary CDMA system is described in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of the present invention and incorporated by reference herein.
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 satellite repeaters or terrestrial base stations using CDMA spread spectrum communication signals. The base station-to-mobile station signal transmission path is referred to as the forward link and the mobile station-to-base station signal transmission path is referred to as the reverse link.
In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. Each base station provides coverage to a limited geographic area and links the mobile stations in its coverage area through a cellular system switch to the public switched telephone network (PSTN). When a mobile station moves to the coverage area of a new base station, the routing of that user's call is transferred to the new base station.
The CDMA modulation techniques discussed in the '307 patent and in U.S. Pat. No. 5,102,459 entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein, mitigate the special problems of the terrestrial channel, such as multipath and fading. Instead of being an impediment to system performance, as it is with narrowband systems, separable multipaths can be diversity combined in a mobile rake receiver for enhanced modem performance. The use of a RAKE receiver for improved reception of CDMA signals is disclosed in U.S. Pat. No. 5,109,390, entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein. In the mobile radio channel, multipath is created by reflection of the signal from obstacles in the environment, such as buildings, trees, cars, and people. In general the mobile radio channel is 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 would change in time location, attenuation, and phase as a function of the time that the ideal impulse was transmitted.
The multipath properties of the terrestrial channel produce, at the receiver, signals having traveled several distinct propagation paths. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. As described in the '390 patent, the spread spectrum pseudonoise (PN) modulation used in a CDMA system allows different propagation paths of the same signal to be distinguished and combined, provided the difference in path delays exceeds the PN chip duration. If a PN chip rate of approximately 1 MHz is used in a CDMA system, the full spread spectrum processing gain, equal to the ratio of the spread bandwidth to the system data rate, can be employed against paths having delays that differ by more than one microsecond. A one microsecond path delay differential corresponds to a differential path distance of approximately 300 meters.
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. If, for example, 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. This can result in signal fading.
A fade occurs when multipath vectors are added destructively, yielding a received signal that is smaller 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 d with a phase shift of Q radians, and the second path has an attenuation factor of X dB, a time delay of d with a phase shift of Q+&pgr; radians, no signal would be received at the output of the channel.
As described above, in current CDMA demodulator structures, the PN chip interval defines the minimum separation two paths must have in order to be combined. Before the distinct paths can be demodulated, the relative arrival times (or offsets) of the paths in the received signal must first be determined. The demodulator performs this function by “searching” through a sequence of offsets and measuring the energy received at each offset. If the energy associated with a potential offset exceeds a certain threshold, a demodulation element, or “finger” may be assigned to that offset. The signal present at that path offset can then be summed with the contributions of other fingers at their respective offsets.
A method and apparatus of finger assignment based on searcher and finger energy levels is disclosed in U.S. Pat. No. 5,490,165, entitled “FINGER ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS”, assigned to the assignee of the present invention and incorporated by reference herein. In the exemplary embodiment, the CDMA signals are transmitted in accordance with the Telecommunications Industry Association TIA/EIA/IS-95-A, entitled “MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEM”. An exemplary embodiment of the circuitry capable of demodulating IS-95 forward link signals is described in detail in U.S. Pat. No. 5,764,592, entitled “MOBILE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein. An exemplary embodiment of the circuitry capable of demodulating IS-95 reverse link signals is described in detail in U.S. Pat. No. 5,654,979, entitled “CELL SITE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM,” assigned to the assignee of the present invention and incorporated by reference herein.
FIG. 1
shows an exemplary set of signals from a base station arriving at the mobile station. It will be understood by one skilled in the art that
FIG. 1
is equally applicable to the signals from a mobile station arriving at the base station. The vertical axis represents the power received on a decibel (dB) scale. The horizontal axis represents the delay in the arrival time of a signal due to multipath delays. The axis (not shown) going into the page represents a segment of time. The signals in the common plane traveled along different paths arriving at the receiver at the same time, but having been transmitted at different times.
In a common plane, peaks to the right were transmitted at an earlier time by the base station than peaks to the left. Fo
Lundby Stein
Razoumov Leonid
Wheatley III Charles E.
Baker Kent
English Sean
Liu Shuwang
Qualcomm Incorporated
Wadsworth Philip
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