US 20050250521 A1
The present invention provides a method and apparatus for maximizing throughput of a data call in a wireless communication system in which data is transmitted from a wireless station, such as a mobile station, on multiple assigned channels in accordance with a known transmission standard, such as IS-95B. The multiple assigned channels include a fundamental channel and at least one supplemental channel. Data is formatted into variable rate data frames and transmitted on the fundamental channel and the supplemental channel. A wireless receiver, such as a base station, receives the multiple assigned channels. The wireless receiver demodulates and decodes data frames associated with each of the multiple assigned channels. The wireless receiver determines a likely initial data rate for each demodulated and decode data frame. The wireless receiver correlates all of the likely data rates, by comparison to one another and to a relevant transmission protocol standard, to determine a maximum likelihood combination of data rates. The maximum likelihood combination of data rates includes a maximum likelihood data rate corresponding to each likely data rate. Decoded data frames are invalidated and erased when the likely data frame rates do not match corresponding maximum likelihood data rates.
1. A computer program product comprising computer usable media having computer readable program code means embodied in the media for causing application programs to execute on a computer processor in a wireless communication device to maximize throughput of a data call in a wireless communication system in which data is transmitted by a wireless station to the wireless communication device on multiple assigned channels, the wireless communication device including receiving means for receiving the multiple assigned channels, and demodulating and decoding means for demodulating and decoding each of the multiple assigned channels, the computer readable program code means comprising:
a first computer readable program code means for causing the processor to determine a likely data rate of each of the multiple assigned channels; and
a second computer readable program code means for causing the processor to correlate all of the likely data rates to determine a maximum likelihood combination of data rates.
2. The computer program product of
3. The computer program product of
wherein data can be transmitted at a first data rate on the fundamental channel, and
wherein data can be transmitted at a second data rate on the supplemental channel only when data is being transmitted at the first data rate on the fundamental channel, and
wherein it is more likely than not that data is being transmitted at the first data rate on the fundamental channel when a plurality of supplemental channels have likely data rates equal to the second data rate,
the computer program product further comprising a third computer readable program code means for causing the processor to invalidate and erase demodulated and decoded data associated with the fundamental channel when
a) the fundamental channel does not have a likely data rate equal to the first data rate, and
b) the plurality of supplemental channels have likely data rates equal to the second data rate.
4. The computer program product of
wherein the first data rate corresponds to a fundamental channel full rate and the second rate corresponds to a supplemental channel full rate,
the computer program product further comprising a fourth computer readable program code means for causing the processor to invalidate and erase demodulated and decoded data associated with each of the plurality of supplemental channels when
a) the fundamental channel does not have a likely data rate equal to the fundamental channel full rate, and
b) only one of the plurality of supplemental channels has a likely data rate equal to the supplemental channel full rate.
This application is a Divisional of U.S. application Ser. No. 09/790,358 filed Feb. 21, 2001.
1. Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly, to such a system for maximizing the useful data transmission throughput in a data call in which data is transmitted between wireless stations on multiple assigned channels.
2. Related Art
A wireless communication system can be used to transmit synchronous and asynchronous packet data between a wireless transmitter and a wireless receiver. For example, the wireless communication system can operate in accordance with a High Speed Packet Data (HSPD) feature of the “TLA/EIA/IS-95B Mobile Station-Base Station Compatibility Standard for Dual-mode Wideband Spread Spectrum Cellular Systems” (hereinafter referred to as IS-95B) to achieve a packet data transmission bandwidth of up to 115 kilobits-per-second (kbps). Under IS-95B, a mobile station can transmit data to a base station receiver on an IS-95B reverse-link traffic channel including a fundamental code channel (FCCH) and up to seven additional Supplemental Code Channels (SCCHs). The FCCH is a variable rate channel capable of operating at data transmission rates including a full rate, a half rate, a quarter rate, and an eighth rate. On the other hand, the SCCH operates only at a full rate when data is to be transmitted, and at a zero rate when no data is available.
Packet data transmitted on the FCCH and SCCHs is partitioned into 20 millisecond (ms) variable rate data frames. Although the data rate can change rapidly, for example, on a frame by frame basis, rate information is typically not included in each transmitted data frame for at least two reasons. First, including rate information in each data frame wastes data bandwidth, and second, corruption of such transmitted rate information would adversely affect the entire frame. Since rate information is not included in each transmitted data frame, the receiver must determine from each received data frame (without the aid of embedded rate information) the rate at which the frame was transmitted, to thereby enable the receiver to properly process the data in the data frame. Known methods of determining data frame rates exist for voice only traffic. However, such methods are insufficiently accurate and thus unsuitable for packet data traffic.
Therefore, there is a need in a variable rate communication system to accurately determine a transmitted data rate for packet data traffic at a receiver without embedding rate information into the transmitted data.
In the above described communication system, the mobile station sends signaling requests for SCCH assignment and de-assignment to the base station based on the amount of data the mobile station needs to transmit. In response, the base station dynamically allocates and de-allocates SCCHs via signaling messages. Assigning and de-assigning SCCHs via such signaling can be a relatively slow mechanism and thus wastes valuable data transmission bandwidth. For example, assigning or de-assigning an SCCH can take up to a half-second. To reduce assignments and de-assignments and associated delays, a mobile station can operate in a discontinuous transmission (DTX) mode while a SCCH is assigned to the mobile station. The DTX mode permits the mobile station to stop transmitting on the assigned SCCH while data is unavailable. This is referred to as the DTX “black-out” period. The DTX mode also permits the mobile station to resume transmitting as soon as data becomes available, thus avoiding delays associated with assigning and de-assigning the SCCH. Transmitted data frames typically do not include DTX “on/off” information for similar reasons as mentioned above with regard to rate information. Since the receiver of the assigned SCCH receives no explicit indicator regarding the black-out periods, the receiver continuously demodulates and decodes the SCCH as long as the SCCH is assigned, even during the black-out period when no data is being transmitted, that is, when the demodulated and decoded data is invalid.
Therefore, it is desirable at a receiver in a communication system to discriminate between data transmission periods and black-outs so as to reduce a likelihood that invalid data is declared to be valid at the receiver.
In accordance with IS-95B, each transmitted SCCH data frame includes a 12 bit Cyclic Redundancy Code (CRC) for checking the validity of the data in the data frame at the receiver. Additional observable metrics, such as a Yamamoto measure, a symbol error rate, a frame energy, and so on, can be used to further improve on the CRC check. There is a finite probability (2−12=2.4×10−4) that demodulated random data associated with the black-out period, or noise corrupting a received data frame, will cause an erroneous match of the 12 bit CRC. In the case of a black-out period, a non-existent SCCH data frame or “random frame” corresponding to the erroneous CRC match, erroneously labels the invalid random frame as a valid data frame.
As is known, the transmitter and receiver typically implement complementary or parallel, layered, communication protocol layers including a physical protocol layer and an overlaying Radio Link Protocol (RLP) layer. One known RLP layer useable in wireless data communication stations is the IS-707 Radio Link Protocol. The physical layer sends (and receives) supposedly valid data frames (for example, data frames passing the CRC check as mentioned above) to (and from) the RLP. The RLP at the receiver tracks RLP frame sequence numbers embedded in the data frames for purposes of errored frames re-transmission and control.
During black out-periods, it has been observed that passing random frames as valid data frames to the RLP causes the RLP to initiate error control processes. This can occur on either the FCCH or SCCHs. For example, the RLP will reset and re-synchronize itself if the received sequence number, supposedly embedded in the random frame, is outside of a predetermined sequence number window (for example, 255) away from an expected sequence number. Alternatively, the RLP will request a retransmission of all of the data frames between the received and expected sequence numbers. In either case, the RLP error control processes disadvantageously reduce useful data throughput on the channel since most of the available bandwidth is utilized to re-sync the RLP or retransmit numerous data frames.
Therefore, there is a need to more accurately validate data frames at a receiver in a communication system, to thereby reduce the occurrence of such RLP error control processes and correspondingly increase channel bandwidth efficiency over conventional techniques.
The present invention provides a method and apparatus for maximizing throughput of a data call in a wireless communication system in which data is transmitted from a wireless station, such as a mobile station, on multiple assigned channels in accordance with a known transmission standard, such as IS-95B. In one embodiment, the multiple assigned channels include a fundamental channel and at least one supplemental channel. Data is formatted into variable rate data frames and transmitted on the fundamental and supplemental channels. A wireless receiver, such as a base station, receives the multiple assigned channels. The wireless receiver demodulates and decodes data frames associated with each of the multiple assigned channels. The wireless receiver determines a likely initial data rate for each demodulated and decode data frame. The wireless receiver correlates all of the likely data rates, by comparison to one another and to a relevant transmission protocol standard, to determine a maximum likelihood combination of data rates. The maximum likelihood combination of data rates includes a maximum likelihood data rate corresponding to each likely data rate. Decoded data frames are invalidated and erased when the likely data frame rates do not match corresponding maximum likelihood data rates.
Features and Advantages
The present invention overcomes the above mentioned problems and represents an improvement over known rate determination and data validation techniques in a wireless data communication receiver.
The present invention accurately determines a variable transmitted data rate for packet data traffic at a wireless receiver without embedding rate information into the transmitted data.
The present invention advantageously reduces a likelihood that invalid data will be declared valid at the wireless receiver during both periods of data transmission and black-outs. More specifically, the present invention enhances the accuracies of rate determination and data validation at the receiver, and results in an increase in a traffic channel bandwidth efficiency over conventional techniques.
In a communication system including fundamental and supplemental channels operating in accordance with IS-95B, the present invention improves the accuracies of rate determination and data validation on the fundamental channel using supplemental channel signal quality measurements.
The foregoing and other features and advantages of the invention will be apparent from the following, more particular description of the exemplary embodiments of the invention, as illustrated in the accompanying drawings.
Exemplary transmitter 110 includes a controller 130 for controlling the operation of transmitter 110 and for exchanging communication signaling information with receiver 120 to assign and de-assign communication channels during call-setup and tear-down, for example. Transmitter 110 includes a transmit channel processor 132 for performing transmit channel processing for one or more communication channels assigned to transmitter 110.
A data source 134 provides data 136 at variable data rates to transmitter 110. Data 136 can be synchronous or asynchronous packet data, as is known in the art. In turn, transmitter 110 formats data 136 into consecutive, variable rate data frames, each having an exemplary duration of 20 milliseconds. In the exemplary embodiment, an RLP processing component (not shown) at transmitter 110 and operating in accordance with TIA/EIA/IS-707 (referred to as “IS-707”), embeds consecutive frame sequence numbers in consecutive data frames for purposes of error correction and control. Then, transmit channel processor 132 further processes the data frames to prepare the data frames for wireless transmission to receiver 120, as will be further described below.
Transmitter 110 transmits the data frames to receiver 120 on a traffic channel 140 assigned to transmitter 110. In the exemplary embodiment, traffic channel 140 is a reverse link IS-95B traffic channel operating in a accordance with the HSPD Feature of IS-95B. The IS-95B reverse link traffic channel 140 includes a fundamental code channel (FCCH) F, and can include up to seven additional supplemental code channels (SCCHs) S0, S1, S2, S3, S4, S5, S6. The FCCH is a variable rate channel capable of operating at data frame rates (also referred to herein as “rates”) including an FCCH full rate, a half rate, a quarter rate, and an eighth rate. The FCCH can carry data 136 from data source 134 and signaling information. Each of the assigned SCCHs S0-S6 can operate at only an SCCH full rate when data is to be transmitted and at a zero rate during DTX periods when no data is available to be transmitted. Under IS-95B, SCCHs S0-S7 can only transmit (at the SCCH full rate) when the FCCH is concurrently transmitting at the FCCH full rate. The present invention takes advantage of this IS-95B traffic channel restriction to improve the accuracies of determining FCCH frame rates and validating received data frames, as will be further described below.
In accordance with IS-95B, the above mentioned rates fall into two categories, namely, a first rate set RS1, and a second rate set RS2. RS1 includes the following rates:
On the other hand, RS2 includes the following rates:
Still with reference to
The above mentioned channel transmission requirements of the IS-95B HSPD feature are illustratively depicted in
Referring to timing diagram (a), transmitter 110 transmits on the FCCH at the full, quarter, half, eighth, and full rates during consecutive portions 172, 174, 176, 178 and 180 of the timing diagram. A time interval 182 represents the duration of a single transmitted data frame, such as 20 ms.
Referring to timing diagram (b), transmitter 110 concurrently transmits on SCCH Si at the SCCH full rate during portions 190 and 192 respectively coinciding with portions 172 and 180 of timing diagram (a), in accordance with IS-95B. Conversely, transmitter 110 transmits on SCCH Si at a zero rate (that is, transmitter 110 does not transmit) during portion 194 of timing diagram (b) coinciding with portions 174-178 of timing diagram (a). Portion 194 of timing diagram (b) corresponds to a black-out or DTX period on SCCH Si. Also, it is to be understood the FCCH frames can be transmitted at the FCCH full rate while the SCCH Si is at the zero rate.
Interleaver 212 provides a reordered data frame to a modulator 214 for modulating the data frame for transmission. In the exemplary embodiment, modulator 214 is a CDMA modulator. Modulator 214 provides a modulated data frame to a transmitter module 216. Transmitter module 216 up-converts and amplifies the up-converted signal for transmission via an antenna 218. Transmitter module 216 transmits data frames to receiver 120 on traffic channel 140.
Receiver 120 receives traffic channel 140 via an antenna 220. Antenna 220 provides the received traffic channel to a plurality of parallel receive channel processors 152 1-152 n. Each of receive channel processors 152 1-152 n is assigned by receiver controller 150 to perform receive channel processing on a corresponding one of the received traffic channels F and S0-Sn (also referred to herein as “F-Sn”). For example, receive channel processor 152 1 can be assigned to the FCCH, while the next receive channel processor 152 2 can be assigned to SCCH S0, and so on. In this manner, receive processing for any one of the received channels F-Sn can be performed independently of the receive processing for any of the other received traffic channels.
Receive channel processor 152 1 performs receive channel processing as is now described. Receive antenna 220 provides received traffic channel 140 to a receiver module 222. Receiver module 222 down converts and amplifies the received traffic channel and provides a down converted and amplified received traffic channel to a demodulator 224, which demodulates the received channel. In the exemplary embodiment, demodulator 224 is a CDMA demodulator. In another embodiment, each of receive channel processors 152 1-152 n can share a single demodulator. Demodulator 224 provides a demodulated signal, namely, demodulated data frames, to de-interleaver 228. De-interleaver 228 re-orders demodulated data frame symbols in accordance with a predetermined format, as is known in the art.
De-interleaver 228 provides a re-ordered data frame to a decoder 230 for decoding the data frame. In the case where receive channel processor 152 1 is assigned to the FCCH, decoder 230 is preferably a multi-rate Viterbi decoder capable of decoding FCCH full rate, half rate, quarter rate and eighth rate received data frames associated with the FCCH, as is known in the art. In the case where receive channel processor 152 1 is assigned to an SCCH, Si, decoder 230 need only decode full rate data frames since SCCH Si can operate at only the SCCH full rate or the zero rate. As mentioned above, although the transmitted date frame rate can change on a frame by frame basis, rate information is typically not included in each transmitted data frame. Therefore, receiver 120 determines the transmitted rate for each received data frame to accurately decode and validate the data frame.
The decoding and CRC checking processes for a received FCCH frame are now described. In the exemplary embodiment, decoder 230 decodes symbols in the received FCCH frame for each of the four possible transmitted rates (that is, the FCCH full, half, quarter, and eighth rates) so as to provide four separately decoded frames, each of which is provided to a CRC check detector 232. Using conventional techniques, CRC check detector 232 determines whether the CRC bits for each of the four decoded frames are correct. CRC check detector 232 performs a CRC check for the CRC bits in each of the four decoded frames to determine at which of the full, half, quarter, or eighth rates the currently received frame was transmitted. As a result, in one embodiment, CRC check detector 232 provides four check bits, C1, C2, C4, C8, where the subscripts “1”, “2”, “4”, and “8” respectively corresponding to the full rate, half rate, quarter rate, and eighth rate, and where a binary value of “1” for a given CRC check bit can indicate that the CRC check bits passed, while a binary value of “0” can indicate that the CRC bits failed.
In addition, decoder 230 provides decoded frame data to a Symbol Error Rate (SER) detector 234. Specifically, SER detector 234 receives decoded frame bits and an estimate of the received symbol data from decoder 230. As is known, SER detector 234 re-encodes and re-decodes the decoded bits, and compares them to the estimate of the received symbol data from decoder 230. The SER is a count of the number of discrepancies between the re-encoded symbol data and the received symbol data. Therefore, SER detector 234 generates four SER values: SER1, SER2, SER3, and SER4.
Furthermore, decoder 230 provides information to a Yamamoto check detector 236 for providing a confidence metric based on the difference between the selected path through a trellis and the next closest path through the trellis. The Yamamoto quality metric is well known in the art, and is further described, for example, in U.S. Pat. Nos. 5,710,784 and 5,872,775. While the CRC check is dependent on the bits in each of the four decoded frames, the Yamamoto check is dependent on the decoding process of receiver 120. Yamamoto detector 136, similar to detectors 232 and 234, provides four Yamamoto values for each of the four possible rates: Y1, Y2, Y4, and Y8. Although detectors 232, 234, 236 are shown as separate elements, the detectors can be incorporated within the hardware and/or software processes of decoder 230.
Receive channel processor 152 1 collectively provides the CRC check bits, SER values, and Yamamoto values from respective detectors 232, 234 and 236 to controller or control processor 150 as a data frame quality metric signal 240 1. Data frame quality metric signal 240 1 is indicative of the quality (and thus validity) of decoded data corresponding to the data frame. Using data frame quality metric signal 240 1, control processor 150 determines at which of the four rates the currently received FCCH data frame was transmitted. For example, in the exemplary embodiment, the control processor selects a rate corresponding to a passed CRC and a favorable SER value.
Receive channel processor 152 1 also provides a decoded frame signal 242 1 to the control processor. Decoded frame signal 242 1 includes each of the separately decoded frames corresponding to the four different frame rates. Decoded frame signal 242 1 can be provided to a decoded data memory buffer so as to be accessible to the control processor.
The decoding and CRC checking processes performed on a received SCCH frame are similar to those processes described above for a received FCCH frame, as is now described. In the case where a receive channel processor (such as receive channel processor 152 2) is assigned to SCCH Si, the associated decoder 230 decodes each received data frame at only the SCCH full rate. In this case, the assigned receive channel processor provides a single decoded SCCH data frame to control processor 150. Also, the receive channel processor provides the associated data frame quality metrics (for instance, the CRC, SER and Yamamoto values) associated with the decoded SCCH frame to control processor 150. Thus, in the case where multiple receive channel processors 152 1-152 n respectively process multiple receive channels F, S0-Sn, the receive channel processors respectively provide data frame quality metrics signals 240 1-240 n and decoded frame signals 242 1-242 n to the control processor.
Receiver controller 150 uses the above described signal quality metrics signals 240 1-240 n to initially determine current FCCH and SCCH frame rates and to initially validate the associated, decoded FCCH and SCCH frames. The present invention then refines and thus improves the accuracy of such initial determinations, as is further described below.
Method 300 begins at a step 305 when transmitter 110 transmits data frames on multiple assigned traffic channels F-Sn. At a next step 310, receiver 120 receives traffic channels F-Sn. At a next step 315, receiver 120 demodulates, de-interleaves and decodes each of the received channels F-Sn as described in connection with
At a next step 320, a rate for each of the received channels F-Sn is initially determined independent of the other received channels. Each determined or detected rate can be considered a “likely” rate because it may be incorrect if, for example, errors have corrupted the corresponding transmitted frame. In the exemplary embodiment, the likely rate for each SCCH is determined to be the SCCH full rate when the CRC check bits pass and the SER values are favorable for the decoded SCCH frame. When the likely rate is equal to the SCCH full rate, the associated SCCH decoded frame is assumed valid. On the other hand, when the likely rate is determined to be the zero rate, the associated SCCH data frame is assumed invalid.
In the exemplary embodiment, the likely rate for the FCCH is determined based on CRC check bits C1, C2, C4, C8, and SER values SER1, SER2, SER3, and SER4, provided that CRC check bits are available. Specifically, the likely FCCH rate is determined to be the one of the four possible rates corresponding to the one of the four decoded frames having a passing CRC and a favorable SER value. The decoded frame associated with the selected likely rate is initially assumed valid.
At a next step 325, all of the likely rates determined at step 320 are correlated to produce a Maximum Likelihood (ML) combination of rates for the received traffic channels. The ML combination of rates includes an ML rate corresponding to each likely rate. Each such ML rate can be a probabilistically more accurate estimate of the transmitted rate than is the corresponding likely rate. This is because each likely rate is determined independent of the other traffic channels, whereas the ML rate is determined by correlating all of the independent likely rates. Correlating the independent likely rates adds relevant cross-channel rate information, such as traffic channel interdependencies, to each of the ML rate determinations, to thereby produce a probabilistically better rate estimate.
The correlation includes a comparison of each likely rate to each of the other likely rates. In addition, the correlation can include a comparison of the likely rates to a relevant set of rules, such as the traffic channel transmission requirements for the particular standard (for example, IS-95B) under which the traffic channels were transmitted. Such a comparison adds further relevant information to the process of generating the ML rates. A correlation in accordance with the exemplary embodiment is further described below in connection with
At a next step 330, one or more of the likely rates determined at step 320 are compared or matched against corresponding ML rates in the ML combination of rates to determine whether to invalidate any of the decoded frames (such as the decoded FCCH frame) initially assumed valid in previous step 320.
Then, all of the decoded frames confirmed as valid in step 330 are provided to the next level of processing, such as the RLP processing layer and/or data sync 160. On the other hand, data frames invalidated at step 325 (and previous step 320) are “erased,” that is, such invalidated frames are not provided to the next level of processing. For example, the FCCH frame and one or more SCCH frames may be invalidated at step 330, based on the results from step 325.
Next, at a decision step 410, it is determined whether the likely FCCH rate is at the FCCH full rate. If the likely FCCH rate is at the FCCH full rate, the decoded FCCH frame is assumed valid for use at the next processing stage, and flow proceeds to a step 415.
At step 415, decoded frames associated with received SCCHs are validated based on the respective likely rates of the decoded frames, as follows. First, the likely rates for the SCCHs (that is, the likely rate of each SCCH frame transmitted concurrently with the FCCH frame) are determined as described above. Then, decoded SCCH frames associated with full rates and zero rates are respectively assumed valid and invalid. Invalid SCCH frames are erased.
On the other hand, if at step 410 it is determined that the FCCH rate is at other than full rate, then flow proceeds to a next decision step 420. At decision step 420 it is determined whether at least two of the SCCHs have likely rates equal to the SCCH full rate. If at least two of the SCCHs have likely rates equal to the SCCH full rate, then flow proceeds to a step 425 where the decoded FCCH data frame (determined to be at other than the FCCH full rate at step 410) is assumed invalid and erased. The SCCHs are validated in accordance with their respective likely rates as described in connection with step 415.
On the other hand, if at step 420 it is determined that less than two of the SCCHs are at the SCCH full rate, then flow proceeds to a step 430 where all of the concurrently received SCCH decoded frames are erased.
The decisional logic embodied in method 400 is supported by a combination of the IS-95B requirements described above and by a probability analysis now described. The probability analysis considers two relevant probabilities. A first relevant probability Pe arises when an FCCH frame is transmitted at the FCCH full rate. In this case there is a finite probability, Pe, that the likely FCCH rate initially determined at step 410 will be erroneous, that is, the likely rate may be determined to be a rate other than the full rate (such as a half, quarter or eighth rate). This finite probability Pe is referred to as the “rate determination error for a full rate frame”. The probability of detecting a full rate frame as other than a full rate frame, that is, the “probability of a rate determination error for a full rate frame” can be determined from Table 1 below. Table 1 is an excerpt from the TIA/EIA-IS-98B “Recommended Minimum Performance Standards for Dual Mode Wideband Spread Spectrum Cellular Mobile Stations” (referred to herein as “IS-98B”). Table 1 tabulates for the FCCH the minimum probabilities of rate determination error for IS-95B Rate Set 1 (RS1) and Rate Set 2 (RS2) full rate frames. RS1 full rate frames are assumed for the present discussion.
Table 1 includes a first column listing FCCH rates, a second column listing error probabilities for RS1 FCCH full rate frames, and a third column listing error probabilities for RS2 FCCH full rate frames. Table 1 includes three rows respectively corresponding to half, quarter and eighth rates. The first row indicates the probability of erroneously detecting a full rate frame as a half rate frame. Similarly, the second row indicates the probability of erroneously detecting a full rate frame as a quarter rate frame, and so on.
The total minimum probability of a rate determination error (Pe) for detecting an RS1 FCCH full rate frame as other than a full rate frame is the addition of the error probabilities, from Table 1, of detecting the frame rate as one of the other three frame rates. In other words, the probability of erroneously detecting an FCCH frame transmitted at the full rate as other than the full rate is given by:
A second probability, Pc, relates to erroneously detecting an invalid received SCCH frame as a valid frame, for example, during a DTX period. As mentioned above, a transmitted SCCH data frame includes a 12 bit CRC. When the CRC passes at receiver 120, the corresponding SCCH frame is assumed valid. It is to be understood that SER can also used for supplemental rate decisions, but that it is ignored here to simplify this probability analysis. Invalid frames can be received, for example, during a DTX period, when transmitted frames are corrupted with noise, or when transmitted frames are substantially attenuated during transmission. In such circumstances, there is the finite probability Pc of detecting a valid CRC at receiver 120 even though invalid data is being received and demodulated. The random probability Pc of a 12 bit CRC matching any random bit sequence at receiver 120 is 2.4×10−4. Further, assuming SCCH channels are statistically independent from each other for the purpose of calculating such a random probability, then the random probability Pcc of two SCCHs both passing CRCs is given by:
A comparison between Pcc and Pe reveals that Pcc<<Pe, by several orders of magnitude. Since SCCH data frames can only be transmitted (at the SCCH full rate) when FCCH data frames are transmitted at the FCCH full rate under IS-95B, the probabilistic comparison Pcc vs. Pc definitively suggests the following conclusion: when a FCCH frame is detected at a rate other than the full rate (for example, at the half, quarter or eighth rate) and at the same time or concurrently (that is, for the same 20 ms frame interval) at least two SCCH data frames associated with two SCCHs are detected at the full rate, it is much more likely than not that the FCCH non full rate determination is erroneous and that the FCCH data frame was actually transmitted at the full rate. In other words, the initial FCCH non full-rate determination is most likely wrong, and therefore should be overruled.
Under such circumstances, it is likely the FCCH data frame is corrupted (or a DTX period is in progress) and probabilities dictate that it is safer to invalidate and erase the FCCH frame than it is to provide such a corrupted frame to the RLP. Method 400 thus improves FCCH rate detection during HSPD calls by filtering-out invalid FCCH data frames in accordance with the result of the above described correlation between all of the received traffic channel rates, and the further comparison of the rates against the IS-95B transmission requirements.
The above described probabilistic comparison Pcc vs. Pc definitively suggests the FCCH non full-rate determination should be overruled when at least two SCCHs are at the full-rate. On the other hand, when only one SCCH is determined to be at the full rate, a relevant probabilistic comparison Pc(2.4×10−4) vs. Pc (3.31×10−4) is much less definitive since Pc and Pe are substantially the same, that is, within an order of magnitude of one another. Relative to the earlier probability comparison, this comparison suggests it is just as likely the FCCH data frame was transmitted at the FCCH full rate as it was not transmitted at the FCCH full rate when only one SCCH channel is detected at the SCCH full rate. Under such conditions, probability does notjustify overruling a determination that the FCCH is not full-rate based on a single SCCH channel being full rate.
Therefore, in the exemplary embodiment, when the FCCH rate is not full rate and only one SCCH is full rate, the SCCH frame is invalidated/erased while the FCCH data frame is assumed valid and provided to the next processing stage. This approach is taken because experience has shown erasure of a valid SCCH data frame is less harmful than providing an invalid SCCH frame to the RLP.
Method 400 is now illustrated with reference to
During interval 505, while the FCCH rate is at the half rate, SCCH2 is erroneously determined to be at the SCCH full rate (that is, the SCCH likely rate is equal to the SCCH full rate). Such a condition is not allowed under IS-95B. In this situation, method 400 invalidates and erases a decoded SCCH2 frame associated with interval 505 in favor of the FCCH half rate detected during the same time interval.
During interval 510, while the FCCH is erroneously determined to be at the FCCH half rate, at least two concurrent SCCH full rate frames are detected, namely, full rate frames for SCCH, and SCCH2. Such a condition is not allowed under IS-95B. In this situation, method 400 invalidates and erases the decoded FCCH frame in favor of the two SCCH full rate frames.
Table 2 below provides an exemplary illustration of the operation of method 400. Table 2 tabulates SCCH and FCCH frame erasure decisions in accordance with method 400 when up to four SCCH are assigned and received at receiver 120. The legend or key for interpreting Table 2 is as follows:
!F=not full rate (that is, Quarter, Half or Eighth Rate);
Receiver 120 can perform specific features of the present invention using receiver controllers, which in effect comprise a computer system. Although communication-specific hardware can be used to implement the present invention, the following description of a general purpose computer system is provided for completeness. The present invention is preferably implemented in software. Alternatively, the invention may be implemented using hardware or a combination of hardware and software. Consequently, the invention may be implemented in a computer system or other processing system.
An example of such a computer system 600 is shown in
Computer system 600 also includes a main memory 608, preferably random access memory (RAM), and may also include a secondary memory 610. The secondary memory 610 may include, for example, a hard disk drive 612 and/or a removable storage drive 614, representing a floppy disk drive, a magnetic tape drive, an optical disk drive, etc. The removable storage drive 614 reads from and/or writes to a removable storage unit 618 in a well known manner. Removable storage unit 618, represents a floppy disk, magnetic tape, optical disk, etc. which is read by and written to by removable storage drive 614. As will be appreciated, the removable storage unit 618 includes a computer usable storage medium having stored therein computer software and/or data.
In alternative implementations, secondary memory 610 may include other similar means for allowing computer programs or other instructions to be loaded into computer system 600. Such means may include, for example, a removable storage unit 622 and an interface 620. Examples of such means may include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an EPROM, or PROM) and associated socket, and other removable storage units 622 and interfaces 620 which allow software and data to be transferred from the removable storage unit 622 to computer system 600.
Computer system 600 may also include a communications interface 624. Communications interface 624 allows software and data to be transferred between computer system 600 and external devices. Examples of communications interface 624 may include a modem, a network interface (such as an Ethernet card), a communications port, a PCMCIA slot and card, etc. Software and data transferred via communications interface 624 are in the form of signals 628 which may be electronic, electromagnetic, optical or other signals capable of being received by communications interface 624. These signals 628 are provided to communications interface 624 via a communications path 626. Communications path 626 carries signals 628 and may be implemented using wire or cable, fiber optics, a phone line, a cellular phone link, an RF link and other communications channels.
In this document, the terms “computer program medium” and “computer usable medium” are used to generally refer to media such as removable storage drive 614, a hard disk installed in hard disk drive 612, and signals 628. These computer program products are means for providing software to computer system 600.
Computer programs (also called computer control logic) are stored in main memory 608 and/or secondary memory 610. Computer programs may also be received via communications interface 624. Such computer programs, when executed, enable the computer system 600 to implement the present invention as discussed herein. In particular, the computer programs, when executed, enable the processor 604 to implement the process of the present invention. Accordingly, such computer programs represent controllers of the computer system 600. By way of example, in a preferred embodiment of the invention, the processes performed by receiver controller 150 can be performed by computer control logic. Where the invention is implemented using software, the software may be stored in a computer program product and loaded into computer system 600 using removable storage drive 614, hard drive 612 or communications interface 624.
In another embodiment, features of the invention are implemented primarily in hardware using, for example, hardware components such as application specific integrated circuits (ASICs). Implementation of the hardware state machine so as to perform the functions described herein will be apparent to persons skilled in the relevant art(s).
While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention.
The present invention has been described above with the aid of functional building blocks illustrating the performance of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Any such alternate boundaries are thus within the scope and spirit of the claimed invention. One skilled in the art will recognize that these functional building blocks can be implemented by discrete components, application specific integrated circuits, processors executing appropriate software and the like or any combination thereof. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.