Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1996-09-25
2001-03-20
Pham, Chi H. (Department: 2731)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C704S220000, C455S212000, C455S296000, C375S264000
Reexamination Certificate
active
06205130
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The invention generally relates to mobile telephone systems employing Code Division Multiple Access (CDMA) and in particular to techniques for detecting bad data packets.
II. Description of the Related Art
FIG. 1
is an illustrative block diagram of a variable rate CDMA transmission system
10
described in the Telecommunications Industry Association's Interim Standard TIA/EIA/IS-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System. This transmission system may be provided, for example, within a base station of a cellular transmission system for use in transmitting signals to mobile telephones within a cell surrounding the base station.
A microphone
11
detects a speech signal which is then sampled and digitized by an analog to digital converter (not shown). A variable rate data source
12
receives the digitized samples of the speech signal and encodes the signal to provide packets of encoded speech of equal frame lengths. Variable rate data source
12
may, for example, convert the digitized samples of the input speech to digitized speech parameters representative of the input voice signal using Linear Predictive Coding (LPC) techniques. In the exemplary embodiment, the variable rate data source is a variable rate vocoder as described in detail in U.S. Pat. No. 5,414,796 which is assigned to the assignee of the present invention and is incorporated by reference herein. Variable rate data source
12
provides variable rate packets of data at four possible frame rates 9600 bps, 4800 bps, 2400 bps and 1200 bps, referred to herein as full, half, quarter, and eighth rates. Packets encoded at full rate contain
172
information bits, samples encoded at half rate contain
80
information bits, samples encoded at quarter rate contain
40
information bits and samples encoded at eighth rate contain
16
information bits. Packet formats are shown in
FIGS. 2A-2D
. The packets regardless of size all are one frame length in duration, i.e. 20 ms. Herein, the terms “frame” and “packet” may be used interchangeably.
The packets are encoded and transmitted at different rates to compress the data contained therein based, in part, on the complexity or amount of information represented by the frame. For example, if the input voice signal includes little or no variation, perhaps because the speaker is not speaking, the information bits of the corresponding packet may be compressed and encoded at eighth rate. This compression results in a loss of resolution of the corresponding portion of the voice signal but, given that the corresponding portion of the voice signal contains little or no information, the reduction in signal resolution is not typically noticeable. Alternatively, if the corresponding input voice signal of the packet includes much information, perhaps because the speaker is actively vocalizing, the packet is encoded at full rate and the compression of the input speech is reduced to achieve better voice quality.
This compression and encoding technique is employed to limit, on the average, the amount of signals being transmitted at any one time to thereby allow the overall bandwidth of the transmission system to be utilized more effectively to allow, for example, a greater number of telephone calls to be processed at any one time.
The variable rate packets generated by data source
12
are provided to packetizer
13
which selectively appends cyclic redundancy check (CRC) bits and tail bits. As shown in
FIG. 2A
, when a frame is encoded by the variable rate data source
12
at full rate, packetizer
13
generates and appends twelve CRC bits and eight tail bits. Similarly, as shown in
FIG. 2B
, when a frame is encoded by the variable rate data source
12
at half rate, packetizer
13
generates and appends eight CRC bits and eight tail bits. As shown in
FIG. 2C
, when a frame is encoded by the variable rate data source
12
at quarter rate, packetizer
13
generates and appends eight tail bits. As shown in
FIG. 2D
, when a frame is encoded by the variable rate data source
12
at eighth rate, packetizer
13
generates and appends eight tail bits.
The variable rate packets from packetizer
13
are then provided to encoder
14
which encodes the bits of the variable rate packets for error detection and correction purposes. In the exemplary embodiment, encoder
14
is a rate ⅓ convolutional encoder. The convolutionally encoded symbols are then provided to a CDMA spreader
16
, an implementation of which is described in detail in U.S. Pat. Nos. 5,103,459 and 4,901,307. CDMA spreader
16
maps eight encoded symbols to a 64 bit Walsh symbol and then spreads the Walsh symbols in accordance with a pseudorandom noise (PN) code.
Repetition generator
17
receives the spread packets. For packets of less than full rate, repetition generator
17
generates duplicates of the symbols in the packets to provide packets of a constant data rate. When the variable rate packet is half rate, then repetition generator
17
introduces a factor of two redundancy, i.e. each spread symbol is repeated twice within the output packet. When the variable rate packet is quarter rate, then repetition generator
17
introduces a factor of four redundancy. When the variable rate packet is eighth rate then repetition generator
17
introduces a factor of eight redundancy.
Repetition generator
17
provides the aforementioned redundancy by dividing the spread data packet into smaller sub-packets referred to as “power control groups”. In the exemplary embodiment, each power control group consists of 6 PN spread Walsh Symbols. The constant rate frame is generated by consecutively repeating each power control group the requisite number of times to fill the frame as described above.
The spread packets are then provided to a data burst randomizer
18
which removes the redundancy from the spread packets in accordance with a pseudorandom process as described in copending U.S. patent application Ser. No. 08/291,231, filed Aug. 16, 1994 assigned to the assignee of the present invention. Data burst randomizer
18
selects one of the spread power control groups for transmission in accordance with a pseudorandom selection process and gates the other redundant copies of that power control group.
The packets are provided by data burst randomizer
18
to finite impulse response (FIR) filter
20
, an example of which is described in U.S. patent application Ser. No. 08/194,823, and assigned to the assignee of the present invention. The filtered signal is then provided to digital to analog converter
22
and converted to an analog signal. The analog signal is then provided to transmitter
24
which upconverts and amplifies the signal for transmission through antenna
26
.
FIG. 3
illustrates pertinent components of a mobile telephone
28
or other mobile station receiving the transmitted signal. The signal is received by antenna
30
, downconverted and amplified, if necessary, by receiver
32
. The signal is then provided to frame rate detection unit
33
which subdivides the signal into packets and determines the corresponding frame rate for each packet. The frame rate may be determined, depending upon the implementation, by detecting the duration of individual bits of the frame. The packet and a signal identifying the detected frame rate for the packet are then forwarded to CRC unit
34
for performing cyclic redundancy checks or related error detection checks in an attempt to verify that no transmission errors or frame rate detection errors occurred. A frame rate detection error results in the packet being sampled at an incorrect rate resulting in a sequence of bits that are effectively random. A transmission error typically results in only one or two bits being in error. Usually, if a transmission error or frame rate detection error occurs, the CRC unit detects the error. “Bad” frames failing the CRC are erased or otherwise discarded by frame erasure unit
36
. “Good” frames which pass the CRC are routed to
English Sean
Nguyen Steven
Pham Chi H.
Qualcomm Incorporated
Rouse Thomas R.
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