Method and apparatus for power control with variable data rate

BACKGROUND OF THE INTENTION

I. FIELD OF THE INVENTION

0001The present invention relates generally to communication systems and particularly to power control in a code division multiple access communication system.

II. DESCRIPTION OF THE RELATED ART

0002The Federal Communications Commission (FCC) governs the use of the radio frequency (RF) spectrum, deciding which industry gets certain frequencies. Since the RF spectrum is limited, only a small portion of the spectrum can be assigned to each industry. The assigned spectrum, therefore, must be used efficiently in order to allow as many frequency users as possible to have access to the spectrum.

0003Multiple access modulation techniques are some of the most efficient techniques for utilizing the RF spectrum. Examples of such modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).

0004CDMA modulation employs a spread spectrum technique for the transmission of information. A spread spectrum system uses a modulation technique that spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal. The spread spectrum technique is accomplished by modulating each baseband data signal to be transmitted with a unique wide band spreading code. Using this technique, a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. Typical examples of spread spectrum techniques can be found in <nplcit id="ncit0001" npl-type="b"><text>Spread Spectrum Communications, Volume i, M. K. Simon, Chap. 5, pp. 262-358</text></nplcit>.

0005A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only 200-300 kHz of a signal is typically affected by a frequency selective fade, the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, will be affected less by the fade condition.

0006In a CDMA-type radiotelephone system, multiple signals are transmitted simultaneously at the same frequency. A particular receiver then determines which signal is intended for that receiver by the unique spreading code in the signal. The signals at that frequency without the particular spreading code intended for that particular receiver appear to be noise to that receiver and are ignored.

0007<figref idref="f0001">FIG. 1</figref> shows a typical prior art CDMA transmitter for use on the reverse channel of a radiotelephone system, the reverse channel being the link from the mobile to the base station. A digital baseband signal is first generated by a vocoder (voice encoder/decoder). The vocoder (100) digitizes an analog voice or data signal using an encoding process such as the Code Excited Linear Prediction (CELP) process that is well known in the art.

0008The digital baseband signal is input to a convolutional encoder (101) at a particular rate, such as 9600 bps. The encoder (101) convolutionally encodes the input data bits into data symbols at a fixed encoding rate. For example, the encoder (101) could encode the data bits at a fixed encoding rate of one data bit to three data symbols such that the encoder (101) outputs data symbols at a 28.8 ksym/s rate with a 9600 bps input rate.

0009The data symbols from the encoder are input to an interleaver (102). The interleaver (102) scrambles the symbols such that the lost symbols won't be contiguous. Therefore, if more than one symbol is lost in the communications channel, the error correcting code is able to recover the information. The data symbols are input into the interleaver (102) in a column by column matrix and output from the matrix row by row. The interleaving takes place at the same 28.8 ksym/s data symbol rate that the data symbols were input.

0010The interleaved data symbols are input to a modulator (104). The modulator (104) derives a sequence of fixed length Walsh codes from the interleaved data symbols. In 64-ary orthogonal code signaling, the interleaved data symbols are grouped into sets of six to select one out of the 64 orthogonal codes to represent the set of six data symbols. These 64 orthogonal codes correspond to Walsh codes from a 64 by 64 Hadamard matrix wherein a Walsh code is a single row or column of the matrix. The modulator outputs a sequence of Walsh codes, corresponding to the input data symbols at a fixed symbol rate, to one input of an XOR combiner (107).

0011A pseudo random noise (PN) generator (103) uses a long PN sequence to generate a user specific sequence of symbols. In a mobile radiotelephone having an electronic serial number (ESN), the ESN can be exclusive-ORed with the long PN sequence to generate the sequence, making the sequence specific to that radiotelephone user. The long PN generator (103) inputs and outputs data at the spreading rate of the system. The output of the PN generator (103) is coupled to the XOR combiner (107).

0012The Walsh code spread symbols from the combiner (107) are next spread in quadrature. The symbols are input to two XOR combiners (108 and 109) that generate a pair of short PN sequences. The first combiner (108) XORs the Walsh code spread symbols with the in-phase (I) sequence (105) while the second combiner (109) XORs the Walsh code spread symbols with the quadrature phase (Q) sequence (106).

0013The resulting I and Q channel code spread sequences are used to bi-phase modulate a quadrature pair of sinusoids by driving the power level of the pair of sinusoids. The sinusoidal output signals are then summed, bandpass filtered, translated to an RF frequency, amplified, filtered, and radiated by an antenna.

0014The typical prior art CDMA transmitter used on the forward channel of a radiotelephone system, the link from the base station to the mobile, is similar to the reverse channel. This transmitter is illustrated in <figref idref="f0001">FIG. 4</figref>. The difference between the forward and reverse channel transmitters is the addition of a Walsh code generator (401) and power control bit multiplexer (420) between the PN generator combiner (107) and the quadrature spreading combiners (108 and 109) for the forward channel transmitter.

0015The power control bit multiplexer (420) multiplexes a power control bit in place of another bit in the frame. The mobile knows the location of this bit and looks for this power control bit at that location. As an example, a "0" bit instructs the mobile to increase its mean output power level a predetermined amount and a "1" bit instructs the mobile to decrease its mean output level a predetermined amount.

0016The code division channel selection generator (401) is coupled to a combiner (402) and provides a particular Walsh code to the combiner (402). The generator (401) provides one of 64 orthogonal codes corresponding to 64 Walsh codes from a 64 by 64 Hadamard matrix wherein a Walsh code is a single row or column of the matrix. The combiner (402) uses the particular Walsh code input by the code division channel generator (401) to spread the input scrambled data symbols into Walsh code spread data symbols. The Walsh code spread data symbols are output from the XOR combiner (402) and into the quadrature spreading combiners at a fixed chip rate of 1.2288 Mchp/s.

0017The mobile can aid the base station in the control of the power on the forward channel by transmitting a power control message to the base station on the reverse link. The mobile gathers statistics on its error performance and informs the base station via the power control message. The base station may then adjust its power to the specific user accordingly.

0018The problem with the type of power control described above is that, for the forward link control, the power control message replaces voice or data bits, thereby reducing the quality of the voice or the data throughput. This fundamentally limits the rate at which mobile stations can send power control messages to the base station and in turn the rate at which the base station can adjust the output power to this specific mobile. A high update rate transmit power adjustment would allow the base station to tune the transmit power to each individual mobile station to a minimum level necessary to maintain a link of a specified quality. By minimizing each individual transmit power, the total interference generated is also minimized, thus improving the capacity of the system. There is a resulting need for updating the power output of a transmitter at a higher rate without substantially degrading the quality of the data in the transmission.

0019Attention is drawn to <patcit id="pcit0001" dnum="US5103459A"><text>US-A-5 103 459</text></patcit> which describes a system and method for communicating information signals using spread spectrum communication techniques. PN sequences are constructed that provide orthogonality between the users so that mutual interference will be reduced, allowing higher capacity and better link performance. With orthogonal PN codes, the cross-correlation is zero over a predetermined time interval, resulting in no interference between the orthogonal codes, provided only that the code time frames are time aligned with each other. In an example, signals are communicated between a cell-site and mobile units using direct sequence spread spectrum communication signals. In the cell-to-mobile link, pilot, sync, paging and voice channels are defined. Information communicated on the cell-to-mobile link channels are, in general, encoded, interleaved, bi-phase shift key (BPSK) modulated with orthogonal covering of each BPSK symbol along with quadrature phase shift key (QPSK) spreading of the covered symbols. In the mobile-to-cell link, access and voice channels are defined. Information communicated on the mobile-to-cell link channels are, in general, encoded, interleaved, orthogonal signalling along with QPSK spreading.

0020Further attention is drawn to <patcit id="pcit0002" dnum="EP0428099A2"><text>EP-A2-0 428 099</text></patcit> which relates to a digital radio link system and a method of adjusting the transmission power in a digital radio link system. The system comprises at a receiving end a first means for monitoring the error rate estimate and for producing a first control signal if the error rate estimate exceeds a predetermined threshold value. At a transmitting end the system comprises means for adjusting the transmission power, said means responding to the occurrence of the first control signal by increasing the transmission power. In the system of the invention there is further provided at the receiving end a second means for monitoring the rate of change of the received signal level and for producing a second control signal if the rate of change exceeds a predetermined threshold value.; The means for adjusting the transmission power respond to the occurrence of the first or the second control signal by increasing the transmission power temporarily close to the maximum transmission power.

SUMMARY

0021In accordance with the present invention a method for providing a variable data rate communication forward link, as set forth in claim 1, and an apparatus for providing a variable data rate communication forward link, as set forth in claim 5, are provided. Preferred embodiments of the invention are claimed in the dependent claims.

0022The process of the present invention enables a transmitter to update the power output to each mobile station to which it is communicating on a frame by frame basis. The process is accomplished through a feedback mechanism from the mobile station to the base station. Through the feedback mechanism, the mobile station informs the base station on whether it is receiving frames correctly or incorrectly by including such information on every frame of data transmitted to the base station.

0023The process first determines whether the power output of the transmitter, with which communication is established, is to be increased or decreased. The process then informs that transmitter to change its power accordingly by including power control bits in each frame of data transmitted.

0024Another embodiment of the process of the present invention enables a communication link to have a higher data rate input signal while maintaining a constant data rate output signal. The method first convolutionally encodes the input data signal to produce a plurality of convolutionally encoded signals. Each of the convolutionally encoded signals are comprised of a plurality of data symbols. Each data symbol is repeated a predetermined number of times to produce a code repetition data sequence at a predetermined and fixed rate. The data sequence is then punctured such that symbols in predetermined locations of the data sequence are deleted thus generating a data sequence at a predetermined and fixed rate which is lower than that of the original data sequence. The encoded signals with the repeated data symbols are multiplexed to produce a data sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

0025<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> shows a typical prior art CDMA, reverse link transmitter for use in a radiotelephone system.</li><li><figref idref="f0002">FIG. 2</figref> shows the forward communication link process of the present invention as used in a CDMA radiotelephone system.</li><li><figref idref="f0002">FIG. 3</figref> shows the mobile radio process which is not claimed but useful for the understanding of the present invention as used in a CDMA radiotelephone system.</li><li><figref idref="f0001">FIG. 4</figref> shows a typical prior art CDMA, forward link transmitter for use in a radiotelephone system.</li><li><figref idref="f0003">FIG. 5</figref> shows the forward link power control process of the present invention.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

0026The variable data rate communication link process of the present invention enables the data rate of a signal input to a convolutional encoder to be variable without changing the data rate of the encoded signal. This enables a higher quality voice channel or a faster facsimile or data channel to be used without increasing the fixed output rate of 19.2 kbps. The variable data rate is obtained by puncturing a rate ½ convolutional code to obtain a rate ¾ convolutional code. For example, a fixed input data rate of 9600 bps encoded by a rate ½ convolutional code produces a fixed output data rate of 9600 · 2 = 19.2 kbps. Equivalently, a fixed input data rate of 14400 bps encoded by a rate 3/4 convolutional code produces a fixed output data rate of 14400 · ⁴/₃ = 19.2 kbps.

0027The forward communication link process of the present invention is illustrated in <figref idref="f0002">FIG. 2</figref>. The process begins with a data signal, I(D), being input to the convolutional encoder (201). The process enables the data rate of this signal to be variable and as high as 14.4 kbps. The convolutional encoder (201), in the preferred embodiment, is a rate ½ encoder.

0028The convolutional code has the generating polynomials G₁=753 and G₂=561. In polynomial notation, the generating polynomials appear as: <maths id="math0001"><math display="block"><mtable columnalign="left"><mtr><mtd><msub><mi mathvariant="normal">G</mi><mn mathvariant="normal">1</mn></msub><mfenced><mi mathvariant="normal">D</mi></mfenced><mo mathvariant="normal">=</mo><mn mathvariant="normal">1</mn><mo mathvariant="normal">+</mo><mi mathvariant="normal">D</mi><mo mathvariant="normal">+</mo><msup><mi mathvariant="normal">D</mi><mn mathvariant="normal">2</mn></msup><mo mathvariant="normal">+</mo><msup><mi mathvariant="normal">D</mi><mn mathvariant="normal">3</mn></msup><mo mathvariant="normal">+</mo><msup><mi mathvariant="normal">D</mi><mn mathvariant="normal">5</mn></msup><mo mathvariant="normal">+</mo><msup><mi mathvariant="normal">D</mi><mn mathvariant="normal">7</mn></msup><mo mathvariant="normal">+</mo><msup><mi mathvariant="normal">D</mi><mn mathvariant="normal">8</mn></msup></mtd></mtr><mtr><mtd><msub><mi mathvariant="normal">G</mi><mn mathvariant="normal">2</mn></msub><mfenced><mi mathvariant="normal">D</mi></mfenced><mo mathvariant="normal">=</mo><mn mathvariant="normal">1</mn><mo mathvariant="normal">+</mo><msup><mi mathvariant="normal">D</mi><mn mathvariant="normal">2</mn></msup><mo mathvariant="normal">+</mo><msup><mi mathvariant="normal">D</mi><mn mathvariant="normal">3</mn></msup><mo mathvariant="normal">+</mo><msup><mi mathvariant="normal">D</mi><mn mathvariant="normal">4</mn></msup><mo mathvariant="normal">+</mo><msup><mi mathvariant="normal">D</mi><mn mathvariant="normal">8</mn></msup></mtd></mtr></mtable></math><img file="EP2017971B1_D0001.tif" /></maths>

0029Since this is a rate ½ encoder (201), for every one bit input to the encoder (201), two symbols will be output. By way of example, if the input signal is comprised of bits b₀, b₁, and b₂, the output symbol sequences are: C₁₁, C₁₂, C₁₃, C₁₄, C₁₅, C₁₆ ... for G₁ and C₂₁, C₂₂, C₂₃, C₂₄, C₂₅, C₂₆ ... for G₂. Therefore, without the process of the present invention, the input must be 9.6 kbps in order to maintain the standard 19.2 kbps output of the rate ½ encoder.

0030The next step of the process inserts a repeat (202 and 203) of each of the output symbols into the symbol sequence. The data rate is set by the speech encoder or by the data service controller so it knows how many symbol repeats need to be inserted to obtain the proper data rate. In the preferred embodiment, the symbols are repeated once so that the output symbol sequences are: <maths id="math0002"><math display="block"><mtable columnalign="left"><mtr><mtd><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">11</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">11</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">12</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">12</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">13</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">13</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">14</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">14</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">15</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">15</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">16</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">16</mn></msub><mo mathvariant="normal">…</mo><msub><mi>for G</mi><mn mathvariant="normal">1</mn></msub><mspace width="1em" /><mi>and</mi></mtd></mtr><mtr><mtd><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">21</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">21</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">22</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">22</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">23</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">23</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">24</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">24</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">25</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">25</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">26</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">26</mn></msub><mo mathvariant="normal">…</mo><msub><mi>for G</mi><mn mathvariant="normal">2</mn></msub><mn mathvariant="normal">.</mn></mtd></mtr></mtable></math><img file="EP2017971B1_D0002.tif" /></maths>

0031A parallel to serial conversion is performed on these symbol sequences by a multiplexer (204). The two symbol sequences are input to the multiplexer (204) at a 14.4 kbps rate and are output from the multiplexer as a single sequence having a data rate of 28.8 kbps. This multiplexing step generates the symbol sequence: <maths id="math0003"><math display="block"><mtable columnalign="left"><mtr><mtd><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">11</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">21</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">11</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">21</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">12</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">22</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">12</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">22</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">13</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">23</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">13</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">23</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">14</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">24</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">14</mn></msub><mo mathvariant="normal">,</mo></mtd></mtr><mtr><mtd><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">24</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">15</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">25</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">15</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">25</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">16</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">26</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">16</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">26</mn></msub><mo mathvariant="normal">…</mo></mtd></mtr></mtable></math><img file="EP2017971B1_D0003.tif" /></maths>

0032This sequence is then punctured (205) using 110101 as the puncturing pattern, each 0 being the punctured bit. This pattern is implemented by deleting from the symbol sequence all bits that are in locations 6n+3 and 6n+5, where n is an integer in the range of 0 to ∞. Alternate embodiments can puncture the symbol sequence in different locations and at a different rate. The result of this operation is the following symbol sequence: <maths id="math0004"><math display="block"><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">11</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">21</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">21</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">22</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">12</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">22</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">23</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">23</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">14</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">24</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">24</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">25</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">15</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">25</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">25</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">26</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">26</mn></msub><mo mathvariant="normal">…</mo></math><img file="EP2017971B1_D0004.tif" /></maths>

0033The symbols are then input into a block interleaver (207). It will be appreciated by those skilled in the art that other types of interleaving can be used in alternate embodiments without departing from the scope of the present invention. The interleaved data symbols are output by the interleaver (207) at the same data symbol rate that they were input, 19.2 kbps. The interleaved symbol sequence is input to one input of the XOR combiner (226).

0034The interleaving is necessary to reduce the likelihood that a fade or interference will cause a large gap in the data sequence. In the case in which symbols are also repeated, losing a symbol will not necessarily cause a total loss of data, thus providing improved performance.

0035A long pseudo-noise (PN) generator (220) is coupled to the other input of the XOR combiner (226) to provide a spreading sequence to the XOR combiner (226). The long PN generator (220) uses a long PN sequence to generate a user specific sequence of symbols or unique user code at a fixed rate, 19.2 kbps in the preferred embodiment. In addition to providing an identification as to which user sent the traffic channel data bits over the communication channel, the unique user code enhances the privacy of the communication in the communication channel by scrambling the traffic channel data bits. The XOR combiner (226) uses the unique user code input by long PN generator (220) to spread the input Walsh coded data symbols into user code spread data symbols. This spreading by the XOR combiner (226) provides a factor increase in the overall spreading of the traffic channel data bits to data symbols. The user code spread symbols are output from the XOR combiner (226) at a fixed chip rate, 1.228 Mchp/s in the preferred embodiment.

0036The code spread symbols are input to a combiner (260) that is also coupled to a code division channel selection generator (250) that provides a particular length Walsh code to the combiner (260). The generator (250) provides one of 64 orthogonal codes corresponding to 64 Walsh codes from a 64 by 64 Hadamard matrix wherein a Walsh code is a single row or column of the matrix. The combiner (260) uses the particular Walsh code input by the code division channel generator (250) to spread the input scrambled data symbols into Walsh code cover data symbols. The Walsh code cover data symbols are output from the XOR combiner (260) and into the quadrature covering combiners (227 and 229) at a fixed chip rate of 1.2288 Mchp/s.

0037A pair of short PN sequences (i.e. short when compared to the long PN sequence used by the long PN generator (220)) are generated by an I-channel PN generator (225) and a Q-channel PN generator (228). These PN generators (225 and 228) may generate the same or different short PN sequences. The XOR combiners (227 and 229) further spread the input Walsh code spread data with the short PN sequences generated by the PN I-channel generator (225) and PN Q-channel generator (228), respectively. The resulting I-channel code spread sequence and Q-channel code spread sequence are used to bi-phase modulate a quadrature pair of sinusoids by driving the power level controls of the pair of sinusoids. The sinusoids are summed, bandpass filtered, translated to an RF frequency, amplified, filtered and radiated by an antenna to complete transmission of the symbol sequence on the forward communication link.

0038In a CDMA cellular radiotelephone system, a process is required in the mobile radio unit to interpret the symbol sequence transmitted on the forward communications link. This mobile unit process which is not claimed but useful for the understanding of the present invention is illustrated in <figref idref="f0002">FIG. 3</figref>.

0039The mobile unit process first demodulates the received symbol sequence (301). The demodulated signal is then input to a deinterleave process (302) to reverse the interleaving of the forward link process. The result of this operation is the original sequence of symbols, including the repeated symbols, as it was input to the interleaver of the forward link process.

0040The output symbol sequence is then processed to fill in the symbols that were deleted in the forward link puncture process (303). Since the receiving mobile has the same puncturing pattern as the base, it knows which symbols were deleted and can therefore replace these deleted symbols with empty spaces, also known as erasures. The output of this operation is as follows, where E is the erasure: <maths id="math0005"><math display="block"><mtable columnalign="left"><mtr><mtd><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">11</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">21</mn></msub><mo mathvariant="normal">,</mo><mi mathvariant="normal">E</mi><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">22</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">12</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">22</mn></msub><mo mathvariant="normal">,</mo><mi mathvariant="normal">E</mi><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">23</mn></msub><mo mathvariant="normal">,</mo><mi mathvariant="normal">E</mi><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">23</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">14</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">24</mn></msub><mo mathvariant="normal">,</mo><mi mathvariant="normal">E</mi><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">24</mn></msub><mo mathvariant="normal">,</mo><mi mathvariant="normal">E</mi><mo mathvariant="normal">,</mo></mtd></mtr><mtr><mtd><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">25</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">15</mn></msub><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">25</mn></msub><mo mathvariant="normal">,</mo><mi mathvariant="normal">E</mi><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">26</mn></msub><mo mathvariant="normal">,</mo><mi mathvariant="normal">E</mi><mo mathvariant="normal">,</mo><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">26</mn></msub><mo mathvariant="normal">…</mo></mtd></mtr></mtable></math><img file="EP2017971B1_D0005.tif" /></maths>

0041This sequence is then input to a buffer (304) for temporary storage. The buffer allows the Viterbi decoder to process the sequence of symbols multiple times to determine the data rate.

0042The Viterbi decoder (305) also assigns a null metric to the erasure bits as is well known in the art. The output of the Viterbi decoder is digital data that is converted to an analog signal by a digital to analog converter (306). The analog signal can then be used to drive a speaker (307) in the mobile unit.

0043The symbols transmitted on the forward and reverse channels are formatted into frames, each frame having a 20 millisecond length. Copending patent application <patcit id="pcit0003" dnum="US07822164B"><text>U.S. Serial No. 07/822,164 to Padovani et al</text></patcit>. and assigned to the assignee of the present invention recites a more detailed explanation of these frames. The amount of data transmitted in each frame depends on the data rate. The frame composition for each data rate for the forward and reverse channels is illustrated in the following table: <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="6"><colspec colnum="1" colname="col1" colwidth="19mm" /><colspec colnum="2" colname="col2" colwidth="12mm" /><colspec colnum="3" colname="col3" colwidth="11mm" /><colspec colnum="4" colname="col4" colwidth="14mm" /><colspec colnum="5" colname="col5" colwidth="15mm" /><colspec colnum="6" colname="col6" colwidth="14mm" /><thead><row><entry align="center" valign="top">Raw # bits</entry><entry align="center" valign="top">CRC</entry><entry align="center" valign="top">Tail</entry><entry align="center" valign="top">Rsrvd</entry><entry align="center" valign="top">Info bit</entry><entry align="center" valign="top">Rate</entry></row></thead><tbody><row><entry>288</entry><entry align="center">12</entry><entry align="center">8</entry><entry align="center">3</entry><entry align="center">265</entry><entry align="center">13250</entry></row><row><entry>144</entry><entry align="center">10</entry><entry align="center">8</entry><entry align="center">2</entry><entry align="center">12A</entry><entry align="center">6200</entry></row><row><entry>72</entry><entry align="center">8</entry><entry align="center">8</entry><entry align="center">2</entry><entry align="center">54</entry><entry align="center">2700</entry></row><row><entry>36</entry><entry align="center">6</entry><entry align="center">8</entry><entry align="center">2</entry><entry align="center">20</entry><entry align="center">1000</entry></row></tbody></tgroup></table></tables>

0044The rate listed in the table is the information bit rate. The reserved bits for the forward and reverse channels, in the preferred embodiment, are for signaling, power control, and future use.

0045The transmit power of the forward channel transmitters can be controlled on the reverse channel by the power control process of the present invention, illustrated in <figref idref="f0003">FIG. 5</figref>. The power control process will be described as being used in the CDMA cellular radiotelephone system, but the process can be used in other communication systems.

0046The selector of the land network determines the rate at which a frame is sent to a mobile (501) and sends the frame to all base stations communicating with that particular mobile. The selector is part of the base station and is responsible for the call processing requirements of the base station.

0047During soft hand-off, more than one base station is communicating with a mobile. The base stations transmit the frame to the mobile (505). After combining the data from possible multiple base stations, the mobile determines whether the last frame (510) has been received and decoded correctly. If the mobile correctly decoded the last frame, the mobile sets the power control bit in the next frame (520) that is transmitted to the base stations.

0048Since the selector knows the rate at which it transmitted the last frame to the mobile and it now has feedback from the mobile on whether that frame was correctly decoded, the selector compiles a table of statistics (525) on the error rates that the mobile station is incurring at each rate. The "received correctly" entries in the table are incremented only if the reverse link frame from the mobile, containing the feedback bit, was received and decoded correctly (515). <tables id="tabl0002" num="0002"><table frame="all"><tgroup cols="5"><colspec colnum="1" colname="col1" colwidth="23mm" /><colspec colnum="2" colname="col2" colwidth="22mm" /><colspec colnum="3" colname="col3" colwidth="21mm" /><colspec colnum="4" colname="col4" colwidth="23mm" /><colspec colnum="5" colname="col5" colwidth="25mm" /><thead><row><entry align="center" valign="top" /><entry align="center" valign="top">TX at full rate</entry><entry align="center" valign="top">TX at ½ rate</entry><entry align="center" valign="top">TX at ¼ rate</entry><entry align="center" valign="top">TX at ¹/₈ ratee</entry></row></thead><tbody><row><entry align="center">RX correctly</entry><entry align="center">I1</entry><entry align="center">J1</entry><entry align="center">K1</entry><entry align="center">L1</entry></row><row><entry align="center">Erased</entry><entry align="center">I2</entry><entry align="center">J2</entry><entry align="center">K2</entry><entry align="center">L2</entry></row><row><entry align="center">Total</entry><entry align="center">I=I1+I2</entry><entry align="center">J=J1+J2</entry><entry align="center">K=K1+K2</entry><entry align="center">L=L1+L2</entry></row><row><entry align="center">Error rate</entry><entry align="center">^I2/_I</entry><entry align="center">^J2/_J</entry><entry align="center">^K2/_K</entry><entry align="center">^L2/_L</entry></row></tbody></tgroup></table></tables>

0049The selector also maintains a table of predetermined target error rates T1, T2, T3, and T4, one for each rate. If the present invention is used in a cellular radiotelephone system, these error rates can be set by the cellular service carrier in order to provide a specific grade of service.

0050The selector next calculates the following differences. <maths id="math0006"><math display="block"><mi mathvariant="normal">E</mi><mo>⁢</mo><mn mathvariant="normal">1</mn><mo mathvariant="normal">=</mo><mmultiscripts><msub><mo mathvariant="normal">/</mo><mi mathvariant="normal">I</mi></msub><mprescripts /><none /><mrow><mi mathvariant="normal">I</mi><mo>⁢</mo><mn mathvariant="normal">2</mn></mrow></mmultiscripts><mo mathvariant="normal">-</mo><mi mathvariant="normal">T</mi><mo>⁢</mo><mn mathvariant="normal">1</mn></math><img file="EP2017971B1_D0006.tif" /></maths><maths id="math0007"><math display="block"><mi mathvariant="normal">E</mi><mo>⁢</mo><mn mathvariant="normal">2</mn><mo mathvariant="normal">=</mo><mmultiscripts><msub><mo mathvariant="normal">/</mo><mi mathvariant="normal">J</mi></msub><mprescripts /><none /><mrow><mi mathvariant="normal">J</mi><mo>⁢</mo><mn mathvariant="normal">2</mn></mrow></mmultiscripts><mo mathvariant="normal">-</mo><mi mathvariant="normal">T</mi><mo>⁢</mo><mn mathvariant="normal">2</mn></math><img file="EP2017971B1_D0007.tif" /></maths><maths id="math0008"><math display="block"><mi mathvariant="normal">E</mi><mo>⁢</mo><mn>3</mn><mo mathvariant="normal">=</mo><mmultiscripts><msub><mo mathvariant="normal">/</mo><mi mathvariant="normal">K</mi></msub><mprescripts /><none /><mrow><mi mathvariant="normal">K</mi><mo>⁢</mo><mn mathvariant="normal">2</mn></mrow></mmultiscripts><mo mathvariant="normal">-</mo><mi mathvariant="normal">T</mi><mo>⁢</mo><mn mathvariant="normal">3</mn></math><img file="EP2017971B1_D0008.tif" /></maths><maths id="math0009"><math display="block"><mi mathvariant="normal">E</mi><mo>⁢</mo><mn mathvariant="normal">4</mn><mo mathvariant="normal">=</mo><mmultiscripts><msub><mo mathvariant="normal">/</mo><mi mathvariant="normal">L</mi></msub><mprescripts /><none /><mrow><mi mathvariant="normal">L</mi><mo>⁢</mo><mn mathvariant="normal">2</mn></mrow></mmultiscripts><mo mathvariant="normal">-</mo><mi mathvariant="normal">T</mi><mo>⁢</mo><mn mathvariant="normal">4.</mn></math><img file="EP2017971B1_D0009.tif" /></maths> The selector determines the power level at which the next frame is to be transmitted by comparing to zero the respective difference just calculated. For example, if the frame is to be transmitted at a full rate and E1>0 (530), the power level will be P_nominal + P (535), where P is a function of the value of E1 and P_nominal is the power level set by the carrier for that geographical area. If E1 = 0 (540), the power level will be P_nominal (545). If E1 < 0, the power level is P_nominal - P (550). The other data rates follow the same procedure. The selector forwards the next frame to be transmitted to the mobile to the base stations that are communicating with the mobile. An indication of the power level at which the frame is to be transmitted is included with this frame.

0051Alternate embodiments of the present invention insert more than one repeat of each symbol into the symbol sequence, depending on the data rate input to the encoder. For example, if a 2.4 kbps data rate is input to the encoder, the symbols should be repeated three more times, for a total of 4 of the same symbols, in the output sequence to maintain a 19.2 kbps output data rate. By adding more or less repeats, the input data rate can be varied while maintaining the output at 19.2 kbps as required by the CDMA interim specification from the Electronic Industries Association/ Telephone Industries Association, IS-95.

0052Alternate embodiments may puncture first and repeat after the puncturing process. However, the preferred embodiment doesn't destroy the symbol as would be done if the symbols were punctured before the repeat process. By repeating first, the repetition of the symbol still exists after the puncture and, therefore, this information can still be transmitted.

0053Alternate embodiments may also require an output rate different from the 19.2 kbps required by the CDMA specification for the base station to mobile station link. An example of such an embodiment is the mobile station to base station link in which the specification calls for a 28800 bps rate. In this case, a 14400 bps information rate coupled with a rate ½ convolutional code achieves the desired rate of 14400 · 2 = 28800 bps.

0054By puncturing a rate ½ code to obtain a rate ¾ code, the process of the present invention enables a higher data rate to be supported by an encoder while the output remains constant. The puncturing process and the code symbol repetition process also enables also enables the encoder to support variable data rates, such as 14.4, 7.2, 3.6, and 1.8 kbps, while keeping the output of the encoder stable at 19.2 kbps by increasing the number of repetitions of the symbols. By using the puncturing process in a radiotelephone having the capability of operating in the CDMA radiotelephone system, higher voice quality and faster data and facsimile transmissions are achieved.

0055The fast forward power control process of the present invention enables a mobile to instruct a base station to change its power output at a faster rate. This process enables the mobile to send a power change command every frame of data without degrading the voice or data quality.

0056The performance degradation associated with the puncturing process of a rate ½ code in the base station to mobile station link is more than compensated for by the fast forward power control process of the present invention. The fast forward power control process of the present invention enables a mobile to instruct the base stations to adjust their power output at a 50 Hz rate (every frame) in comparison to rates of 0.2 Hz that can be achieved through other signaling methods that replace complete frames with power control information. This process enables the mobile to send a power change request every frame of data by using a single information bit per frame and, therefore, without degrading the voice quality or considerably reducing the data throughput.