Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-08-23
2002-12-10
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S252000, C370S500000
Reexamination Certificate
active
06493329
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to wireless communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for adaptively estimating the channel conditions of a wireless communication channel.
II. Description of the Related Art
In a wireless radiotelephone communication system, many users communicate over a wireless channel. 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”, issued Feb. 13, 1990, assigned to the assignee of the present invention, and incorporated herein by reference. An exemplary CDMA system is further described in U.S. Pat. No. 5,103,459, entitled “System And Method For Generating Signal Waveforms In A CDMA Cellular Telephone System”, issued Apr. 7, 1992, assigned to the assignee of the present invention, and incorporated herein by reference.
In each of the above patents, the use of a forward-link (base station to mobile station) pilot signal is disclosed. In a typical CDMA wireless communication system, such as that described in EIA/TIA IS-95, the pilot signal is a “beacon” transmitting a constant zero symbol and spread with the same pseudonoise (PN) sequences used by the traffic bearing signals. The pilot signal is typically covered with the all-zero Walsh sequence. During initial system acquisition, the mobile station searches through PN offsets to locate a base station's pilot signal. Once it has acquired the pilot signal, it can then derive a stable phase and magnitude reference for coherent demodulation, such as that described in U.S. Pat. No. 5,764,687 entitled “Mobile Demodulator Architecture For A Spread Spectrum Multiple Access Communication System,” issued Jun. 9, 1998, assigned to the assignee of the present invention, and incorporated herein by reference.
A functional block diagram of a typical prior art forward link data formatter as used by a CDMA base station is shown in FIG.
1
. Data source
102
may be, for example, a variable rate vocoder such as that described in U.S. Pat. No. 5,657,420, entitled “Variable Rate Vocoder”, issued Aug. 8, 1997, assigned to the assignee of the present invention and incorporated herein by reference. Data source
102
generates traffic channel information in the form of frames of digital data. CRC and tail bit generator
104
calculates and appends cyclic redundancy check (CRC) bits and tail bits to the frames generated by data source
102
. The frame is then provided to encoder
106
, which provides forward error correction coding, such as convolutional encoding, upon the frame as is known in the art. The encoded symbols are provided to repetition generator
120
, which repeats the reordered symbols to provide the appropriate modulation symbol rate. The repeated symbols are then provided to interleaver
108
, which re-orders the symbols in accordance with a predetermined interleaver format. The repeated, interleaved symbol stream is then covered with one of a set of orthogonal Walsh sequences in traffic Walsh coverer
122
, and gain adjusted in gain element
124
. It should be understood that other forward link data formatters are also known in the art. For example, it is well known that the repetition generator
120
may be placed after the interleaver
108
.
Pilot signal generator
128
generates a pilot signal, which may be a sequence of all ones. The pilot signal is then covered with the all-one Walsh sequence and combined with the output of gain element
124
in combiner
136
. The combined pilot channel and traffic channel data (which may be plus or minus ones) is then spread in PN spreader
138
using a complex PN code generated by PN generator
140
, and then transmitted by radio frequency transmitter
142
over antenna
144
. A similar forward link data formatter is disclosed in co-pending U.S. patent application Ser. No. 08/886,604, entitled “High Data Rate CDMA Wireless Communication System”, assigned to the assignee of the present invention and incorporated by reference herein.
Other data formatting techniques also exist. For example, in the cdma2000 reverse link, the pilot signal is time-multiplexed with power control commands. Additionally, in W-CDMA, the forward link uses dedicated pilot signals that are time-multiplexed with other information.
FIG. 2
illustrates a functional block diagram of a typical prior art data demodulator for use in a CDMA mobile station. Receiver
202
receives and downconverts the signals transmitted by transmitter
142
of FIG.
1
. The digital baseband output of receiver
202
is despread in PN despreader
204
using the complex PN code generated PN generator
206
, which is the same complex PN code as that generated by PN generator
140
of FIG.
1
.
The despread signal is then Walsh uncovered in traffic channel Walsh uncoverer
208
using the same Walsh sequence as that of the traffic channel Walsh coverer
122
of FIG.
1
. The Walsh-uncovered chips are then accumulated into Walsh symbols in Walsh chip summer
210
and provided as a traffic channel signal to dot product circuit
212
. In some applications, an additional delay element (not shown) is introduced between Walsh chip summer
210
and dot product circuit
212
to account for delays introduced by pilot filter
216
. However, if pilot filter
216
is a causal filter, such a delay element (not shown) is not necessary. The dot product circuit is also known as a “conjugate product” circuit. It performs the operation expressed mathematically by one of the following equivalent forms: <a,b>=a·b=ab*, where b* is the complex conjugate of b.
The despread signal is also provided to Walsh chip summer
214
where they are accumulated into Walsh symbols and provided to pilot filter
216
as pilot channel symbols. Note that since the pilot channel is covered with the all-one Walsh sequence in Walsh coverer
134
of
FIG. 1
, a vacuous operation, the corresponding uncoverer is also vacuous in operation. However, in the general case, the pilot signal may be uncovered using any same Walsh sequence as is used to cover it. The pilot filter
216
serves to reject the noise in the pilot symbols, providing a phase and scale reference for the dot product circuit
212
.
Once per symbol, the dot product circuit
212
computes the component of the traffic channel signal in phase with the pilot channel signal generated by the pilot filter
216
. As described in U.S. Pat. No. 5,506,865, entitled “Pilot Carrier Dot Product Circuit”, issued Apr. 9, 1996, assigned to the assignee of the present invention and incorporated herein by reference, the dot product adjusts both the received signal's phase and scale as needed for coherent demodulation.
The symbols output from dot product circuit
212
are de-interleaved in de-interleaver
218
, using the same format used by interleaver
108
of FIG.
1
. The de-interleaved symbols are then decoded in decoder
220
according to the error correcting codes employed by encoder
106
of FIG.
1
. The resulting decoded symbols are analyzed on a frame-by-frame basis by CRC Check
222
to ensure that the frame was properly decoded. If the frame was properly decoded, then that decoded frame is forwarded for further processing. CRC Check
222
typically would examine the CRC portion of the frame, but may also use other frame quality indications such as Yamamoto metrics.
A typical pilot filter
216
is implemented as an equal-weight finite impulse response (FIR) filter with all defining parameters (e.g., weighting, window width, window center) remaining constant regardless of the channel condit
Baker Kent D.
Kizou Hassan
Martin Roger
Qualcomm Incorporated
Tsegaye Saba
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