Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-04-15
2002-08-27
Urban, Edward F. (Department: 2685)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S335000, C375S147000, C375S150000, C455S063300
Reexamination Certificate
active
06442154
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a direct sequence CDMA receiver and, more particularly, for demodulating using one sample per chip to reduce receiver complexity.
BACKGROUND OF THE INVENTION
Wireless communications has been expanding at a phenomenal rate, as more radio spectrum becomes available for commercial use and as cellular phones become commonplace. Technology is currently evolving from analog communications to digital communications. Speech is represented by a series of bits. The bits are modulated and transmitted between a base station and a mobile station. Each of the base station and mobile station has a transmitter and a receiver. The receiver demodulates the received wave form to recover the bits, which are then converted back into speech. There is also a growing demand for data services, such as e-mail and Internet access, which require digital communications.
There are numerous types of digital communication systems presently available. Frequency-division multiple access (FDMA) divides the spectrum into plurality of radio channels corresponding to different carrier frequencies. These carrier frequencies may be further divided into time slots, referred to as time-division multiple access (TDMA) as is known in the D-AMPS, PDC and GSM digital cellular systems. Alternatively, if the radio channel is wide enough, then multiple users can use the same channel using spread spectrum techniques and code-division multiple access (CDMA).
Direct sequence (DS) spread spectrum modulation is commonly used in CDMA systems. Each information symbol is represented by a number of “chips”. Representing one symbol by many chips gives rise to “spreading”, as the latter typically requires more bandwidth to transmit. The sequence of chips is referred to as the spreading code or signature sequence. The code has a chip rate that is higher than the bit rate of the information signal. At the receiver, the received signal is despread using a despreading code, which is typically the conjugate of the spreading code. IS-95 and J-STD-008 are examples of DS-CDMA standards.
Coherent rake reception is commonly used with coherent DS-CDMA systems. The received signal is despread by correlating to the chip sequence, and the spread value is weighted by the conjugate of a channel coefficient estimate, removing the phase rotation of the channel and weighting the amplitude to indicate a softer confidence value. When multi-path propagation is present, the amplitude can vary dramatically. Multi-path propagation can also lead to time dispersion, which causes multiple, resolvable echoes of the signal to be received. Correlators are aligned with the different echos. Once the despread values have been weighted, then they are summed. This weighting and summing operation is commonly referred to as rake combining.
Modulation schemes which allow non-coherent demodulation may be used in DS-CDMA systems. For example, differentially encoded phase may be used, such as DPSK, allowing differential detection at the receiver. Also, as in the IS-95 uplink, M'ary orthogonal modulation may be used, allowing non-coherent detection at the receiver. The advantage of non-coherent detection is that channel estimation is not needed. Channel estimation may be difficult due to how fast the channel changes or how noisy the signal is. With non-coherent detection, rake combining of detection values corresponding to different echoes can still be performed to gain path diversity. Combining signals from different antennas is also possible.
Referring to
FIG. 1
, a digital communications systems
10
is illustrated. Digital symbols are provided to a transmitter
12
. The transmitter
12
maps the symbols into a representation appropriate for the transmission medium or channel
16
, such as a radio channel, and couples the signals to the transmission medium via an antenna
14
. The transmitted signal passes through the channel
16
and is received at an antenna
18
. The received signal is passed to a receiver
20
. The receiver
20
uses a radio processor
22
, a baseband processor
24
, and a post-processing unit
26
.
The radio processor
22
tunes to the desired band and desired carrier frequency and amplifies, mixes and filters the received signal down to baseband. The signal is sampled and quantized, ultimately providing a sequence of baseband received samples. Since the original radio signal has in-phase (I) and quadrature (Q) components, the baseband samples typically have I and Q components, giving rise to complex baseband samples. The baseband processor
24
detects the digital signals that were transmitted. It may also produce soft information, which gives information regarding the likelihood of the detected symbol values. The post processing unit
26
performs functions that depend on the particular communications application. For example, it may use the soft detected values to perform forward error correction decoding or error detection decoding. It may convert digital symbols into speech using a speech decoder.
The performance of DS-CDMA systems is limited by interference from other users. The despreading operation provides some degree of interference suppression, allowing multiple users to overlap in time and frequency. However, the capacity is limited. To improve receiver performance, interference cancellation has been used. One approach is successive cancellation of interference in which users are detected and subtracted in signal strength order, starting with the strongest user. Ideally, subtraction is based on the user's symbol value and channel response information, referred to as coherent successive cancellation. In practice, the symbol value may be unreliable and the channel response may be unknown. A form of non-coherent cancellation can be used, as described in U.S. Pat. No. 5,151,919, assigned to the assignee of the present application, the specification of which is incorporated by reference herein. In this patent the despread value is used for signal subtraction rather than channel information.
With reference to
FIG. 2
, a prior art method of non-coherent successive cancellation is illustrated. The method begins at a block
32
. At a block
34
, user signals are ordered by signal strength. Signals may be ordered as described in U.S. Pat. No. 5,151,919. For each user, starting with the strongest, despreading is performed to obtain correlation value using normal spreading waveforms at a block
36
. These correlation values are used to detect the information symbols at block
38
. The correlation values are respread using the normal spreading waveform at block
40
, and the respread signal is subtracted from the composite received signal at a block
42
. A decision block
44
determines whether more users need to be demodulated. If so, then the processing returns to the block
36
. If not, the routine ends at a block
46
.
While non-coherent successive cancellation improves performance, there is a relatively high error floor when plotting performance versus signal-to-noise ratio. This factor is analyzed in P. Patel and J. Holtzman, “Analysis of A Simple Successive Interference Cancellation Scheme In A DS/CDMA System,”
IEEEJ SeL Areas Commun.,
vol. 12, pp. 796-807, June 1994. This shows that interference from previously canceled users is not entirely eliminated.
The performance of non-coherent successive cancellation can be improved by accounting for how each cancellation step affects the remaining signals. One approach is to use signal orthogonalization, which is described in U.S. Pat. No. 5,615,209, assigned to the assignee of the above application, which is incorporated by reference herein. This approach is initially described for signals which are synchronized in time. Orthogonalization is performed using the spreading sequences of the different users. In one embodiment, the method in U.S. Pat. No. 5,615,209 is similar to that shown in
FIG. 2
herein, except that respreading uses a modified sequence instead of the normal sequence. The modified sequence is obtained by applying the Gram-S
Ericsson Inc.
Gregory A. Stephens, P.C.
Urban Edward F.
Zewdu Meless N
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