Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-08-31
2001-10-30
Marcelo, Melvin (Department: 2663)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S342000
Reexamination Certificate
active
06310869
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The invention relates to communication systems. More particularly, the invention relates to methods and apparatus for reducing amplitude and interference in wireless communication systems using inserted pilot symbols.
II. Description of the Related Art
Several multiple access communication techniques are known in the art, such as time division multiple access (TDMA) and frequency division multiple access (FDMA). However, the spread spectrum modulation techniques of code division multiple access (CDMA) provide significant advantages over other multiple access modulation techniques. CDMA techniques in a communication system are disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention.
CDMA modulation techniques provide capacity improvements over other techniques based in part on CDMA's use of orthogonal functions. The CDMA codes are generated by, e.g., Walsh functions that mathematically form an orthogonal set Thus, any two Walsh functions are orthogonal to each other, and signals encoded with two separate Walsh functions should cause no mutual interference if they are time aligned. An example of Walsh functions employed in a CDMA communication system is disclosed in U.S. Pat. No. 5,602,833, entitled “METHOD AND APPARATUS FOR USING WALSH SHIFT KEYING IN A SPREAD SPECTRUM COMMUNICATION SYSTEM,” assigned to the assignee of the present invention.
Since CDMA employs a wideband signal, it spreads the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. CDMA also provides space or path diversity through multiple signal paths that simultaneously link a mobile station or user with two or more cell-sites. Furthermore, CDMA can exploit the multipath environment by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled “METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,” and U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention.
Under one CDMA standard, described in the Telecommunications Industry Association's
TIA/EIA/IS
-95-
A Mobile Stations
-
Base Station Compatibility Standard for Dual
-
Mode Wideband Spread Spectrum Cellular System
, each base station transmits pilot, sync, paging and forward traffic channels to its users. The pilot channel is an unmodulated, direct-sequence spread spectrum signal transmitted continuously by each base station. The pilot channel allows each user to acquire the timing of the channels transmitted by the base station, and provides a phase reference for coherent demodulation. The pilot channel also provides a means for signal strength comparisons between base stations to determine when to hand off between base stations (such as when moving between cells).
CDMA modulation techniques require that all transmitters be under precise power control to manage interference in the system. If the transmission power of signals transmitted by a base station to a user (the forward link) are too high, it can create problems such as interfering with other users. As a result, most base stations have a fixed amount of power at which to transmit signals, and therefore can transmit to only a limited number of users. Alternatively, if the transmission power of signals transmitted by the base station is too low, then some users can receive multiple erroneous transmitted frames. Terrestrial channel fading and other known factors also affect the transmission power of signals transmitted by the base station. Thus, each base station needs to adjust the transmission power of the signals it transmits to its users. A method and apparatus for controlling transmission power is disclosed in U.S. Pat. No. 5,056,109, entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the present invention.
Recent CDMA modulation techniques have been proposed using dedicated time multiplexed (“DTMP”) pilot symbols. Under the DTMP approach, separate pilot symbols are time multiplexed on each user's traffic channel. Each user sequentially de-spreads the pilot symbols (and information symbols). Under an alternative common code multiplexed pilot (“CCMP”) approach, one co-channel is dedicated to broadcasting a pilot signal. No pilot symbols are multiplexed with dedicated channels, and all users de-spread both the pilot symbols and the modulated information signals in parallel.
Under the DTMP approach, the base station must employ a fraction of its total power for pilot symbols and pilot data for each user. The total amount of power required for the pilot symbols and pilot data is based on a sum of the power required for all pilot symbols and pilot data for all of the base station's users. The CCMP approach need only allocate a fraction of its total power to the common pilot based on a maximum pilot power required by the “worst-case” user. Additionally, the DTMP approach may suffer from further shortcomings.
SUMMARY OF THE INVENTION
The inventors have found that, with inserted pilot symbols (as opposed to a common, continuous pilot signal), the pilot symbols transmitted to different mobile stations or users in the DTMP approach add linearly and cause large peak to average amplitude variations. Such amplitude variations require either large power amplifiers and/or cause interference in the system. As explained more fully below, a sign or value is identical in one position of each orthogonal code assigned to concurrent users (the “common sign chip position”), which can linearly add to produce a large amplitude.
The inventors have identified at least four classes of solutions to this problem. Under a first solution, the base station multiplies the signals of each user by a plus or minus random variable or phase rotation between 0 and 360 degrees, such as 0 or 180 degrees. The orthogonality of the codes is still maintained between the orthogonal functions, but the value of the common sign chip position of some codes is varied. The user station can determine the value of the random variable by either observing the sign of the Walsh demodulation, or by receiving additional data sent from the base station.
Under a second class of solutions, the base station transmits empty symbols or chips in the common sign position of each orthogonal function. The user station then reinserts the missing chip after receiving the remainder of the orthogonal function sequence. The user station can reconstruct the first chip with the Walsh functions. For example, if all the transmitted Walsh functions would sum to 0 if fully sent, then the user station sums all of the received Walsh functions (without the first chips) over all the Walsh chips. The negative of this sum is the value that the received signal would have had if the Walsh functions were fully transmitted. If one of the Walsh functions did not sum to 0 (for example, all chips were equal to 1), then a first Walsh demodulation of all the received Walsh functions provides simultaneous equations for solving for the first Walsh chip amplitudes.
Under a third class of solutions, instead of providing each user station with its own pilot symbols, the base station first identifies symbol positions common to different users. For example, four users may expect to receive a pilot symbol in symbol position six. Instead of transmitting four separate pilot symbols, the base station only transmits one pilot symbol to be used by all four users. This is a hybrid DTMP and CCMP approach. Individual pilot symbols are effectively shared or c
Holtzman Jack
Razoumov Leonid
Terasawa Daisuke
Baker Kent D.
Kalousek Pavel
Marcelo Melvin
Qualcomm Incorporated
Wadsworth Philip
LandOfFree
Method and apparatus for reducing amplitude variations and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for reducing amplitude variations and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for reducing amplitude variations and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2596001