HK1068745B - Method and apparatus for supervising a potentially gated signal in a wireless communication system - Google Patents

Description

Method and apparatus for monitoring a potential gating signal in a wireless communication system

This application is a divisional application of a patent application filed on 22/6/2000 under the name of 00809366.0 entitled method and apparatus for monitoring a potential gating signal in a wireless communication system.

Technical Field

The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for monitoring the performance of a potentially gated channel.

Background

The telecommunications industry association proposed a standard for Code Division Multiple Access (CDMA) communication systems in the interim standard IS-95A (hereinafter IS-95) entitled "mobile station-base station compatibility standard for dual mode broadband spectrum cellular systems". In IS-95 systems, a mobile station controls its transmit energy by a combination of open-loop and closed-loop power control methods. In open loop power control, the mobile station measures the received forward link signal energy from the serving base station and adjusts the reverse link transmit energy based on the measurement. In closed loop power control, a serving base station measures the transmit energy of a mobile station and transmits a series of up/down commands to the base station based on the measurements, and the mobile station adjusts its transmission in response. A power control system that benefits from both closed loop and open loop power control is described in detail in U.S. Pat. No.5,056,109, entitled "Method and Apparatus for control 11 transmission power in a CDMA cellular mobile telephone system," which is assigned to the assignee of the present invention and is incorporated herein by reference.

In IS-95, the mobile station IS required to monitor the performance of the forward traffic channel during the call. When the mobile station receives 12 (N) consecutive signals_2m) In bad frames, the mobile station is required to stop its transmitter so as not to block the reverse link. Subsequently, if the mobile station receives two (N) in succession_3m) When a frame is good, its transmitter should be reactivated. The mobile station also maintains a fade timer. When a mobile station activates its transmitter at the beginning of a call, a fade timer is first activated and whenever two consecutive tones are received on the forward traffic channel(N_3m) Reset for 5 seconds (N) with good frame_5m). If the decay timer expires, the mobile station disables its transmitter and declares that the forward traffic channel is lost and terminates the call.

The international telecommunications union has recently requested that a discussion be presented of methods for improving high speed and high quality voice services over wireless communication channels. The telecommunications industry association issued the first of these recommendations, named "cdma 2000 ITU-R RTT candidate proposal" (hereinafter cdma 2000). In cdma2000, equivalent to the forward traffic channel in IS-95 are the forward fundamental channel (F-FCH) and the forward dedicated control channel (F-DCCH). Data frames transmitted on these channels may have a20 ms or 5 ms duration. For F-FCH, a frame (20 or 5 ms) is transmitted every 20 ms interval, based on the CDMA system time start. For F-DCCH, it may be transmitted on and off, so data frames are not necessarily transmitted every 20 milliseconds based on CDMA system time.

The use of Code Division Multiple Access (CDMA) modulation techniques is one of several techniques that facilitate communication for a large number of system users. Other multiple access communication system techniques such as Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) are also known in the art. The spread spectrum modulation technique of CDMA has significant advantages over the modulation techniques of these multiple access communication systems. The use of CDMA techniques in multiple access communication systems is disclosed in U.S. patent No.4,901,307, entitled "Spread spectrum multiple access communication system using satellite or terrestrial repeaters," which is assigned to the assignee of the present invention and is incorporated herein by reference. The use of CDMA techniques in multiple access communication systems is further disclosed in U.S. patent No.5,103,459, entitled "System and method for generating signal communications in a CDMA cellular telephone System," which is assigned to the assignee of the present invention and is incorporated herein by reference.

CDMA, by its inherent nature as a wideband signal, provides a form of frequency diversity by spreading the signal energy over a wide band. The selective attenuation of frequency affects only a small portion of the CDMA signal bandwidth. Space or path diversity may be obtained by providing multiple signal paths on the simultaneous link from a mobile user to two or more cell sites. Path diversity is also obtained by exploiting the multipath environment that is processed by the spread spectrum, and for this purpose allows a signal arriving with different propagation delays to be separately received and processed. Examples of path Diversity are shown in U.S. Pat. No.5,101,501 entitled "Method and system for providing a handoff in communication with a CDMA ce11 cellular telephone system" and U.S. Pat. No.5,109,390 entitled "Diversity Receiver in a CDMA ce11 cellular telephone system," both of which are assigned to the assignee of the present invention and are incorporated herein by reference.

In a communication system that provides data using a quadrature phase shift keying modulation format, very useful information can be obtained by cross-multiplying the I and Q components of a QPSK signal. Knowing the relative phase of the two components, the velocity of the mobile station relative to the base station can be determined. A circuit for determining the cross product of I and Q components in a QPSK modulated communication system is disclosed in U.S. patent No.5,506,865, entitled "Pilot carrier dot product circuit," which is assigned to the assignee of the present invention and is incorporated herein by reference.

There is an increasing demand for wireless communication systems capable of transmitting digital information at high rates. One method of transmitting high rate digital data from a remote station to a central base station is to allow the remote station to transmit the data using the spread spectrum technique of CDMA. One proposed method for allowing a remote station to transmit information using a small set of orthogonal channels is described in detail in co-pending U.S. patent No.08/886,604 entitled High data rate CDMA wireless communication system, which is assigned to the assignee of the present invention and is incorporated herein by reference.

When the F-DCCH is in Discontinuous Transmission (DTX) mode, a new method of monitoring the F-DCCH is required since the mobile station must currently determine whether a received frame is a good frame, a bad frame, or an empty frame (i.e., no transmission).

Disclosure of Invention

The present invention is a new and improved method and apparatus for monitoring a potentially gated channel in a wireless communication system.

The first approach IS to simply ignore the null frame but augment the approach used in IS-95 with a potentially different threshold. The mobile station maintains a counter CNT1 for consecutive bad frames and a counter CNT2 for consecutive good frames. COUNT1 and COUNT2 are set to zero at the start of the call. For each frame received, the mobile station determines whether it is a good frame, a bad frame, or a null frame. If the received frame is a good frame, COUNT1 is reset to zero and COUNT2 is incremented by 1. If the received frame is a bad frame, COUNT1 is incremented by 1 and COUNT2 is reset to zero. If the received frame is a null frame, COUNT1 and COUNT2 remain unchanged. If COUNT1 reaches threshold TH1, the mobile station disables its transmitter. Subsequently, if COUNT2 reaches threshold TH2, the mobile station reactivates the transmitter. Whenever COUNT2 is greater than or equal to TH3, the mobile station resets its fade timer to X seconds.

In a second exemplary embodiment, if no data frames are transmitted on the F-DCCH at this time, the base station periodically transmits a "supervisory frame" (e.g., at the beginning of every N-second interval relative to the beginning of the CDMA system time). The supervision frames are sent at the lowest data rate that has been agreed upon for the base station and the mobile station. The mobile station then performs F-DCCH monitoring on frames transmitted at a preset timing with potentially different values of various thresholds in a manner similar to that defined in IS-95. In addition to these periodic frames, the mobile station may include other non-null frames received for monitoring purposes.

In the third exemplary embodiment, the base station transmits a "supervisory frame" whenever the number of consecutive null frames exceeds a threshold, or when the number of (consecutive or non-consecutive) null frames within a given interval exceeds a certain threshold. This ensures that the mobile station has some non-null frames to monitor from time to time.

In a fourth exemplary embodiment, the mobile station sends a message requesting a response (e.g., the response may simply be an acknowledgement) from the base station when the number of consecutive null frames detected exceeds a threshold. This ensures that the mobile station will receive non-null frames for which monitoring is done.

In a fifth exemplary embodiment, when the number of null frames (continuous or discontinuous) detected within a given interval exceeds a threshold, the mobile station sends a message requesting a response (e.g., the response may simply be an acknowledgement) from the base station. This ensures that the mobile station will acquire non-null frames to monitor from time to time.

In a sixth exemplary embodiment, the mobile station uses the received pilot strength (Ec/Io) of the pilot signal in the active set to complete F-DCCH monitoring. If the accumulated active set pilot Ec/Io is above a preset threshold, the mobile station considers the data (if transmitted within the frame) to be received correctly and therefore a good frame. Otherwise, the mobile station considers the frame as a bad frame. Similar monitoring rules as defined in IS-95 (along with the definition of good and bad frames described above) may then be used at the same threshold or a modified threshold.

Drawings

The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like parts are designated with like reference numerals throughout and wherein:

FIG. 1 is a schematic diagram illustrating elements of a wireless communication system;

FIG. 2 is a block diagram of a base station of the present invention; and

fig. 3 is a block diagram of a mobile station of the present invention.

Detailed Description

In fig. 1, base station 2 transmits forward link signal 6 to mobile station 4. The mobile station 4 transmits a reverse link signal 8 to the base station 2. In The illustrative embodiment, forward link signal 6 and reverse link signal 8 are Code Division Multiple Access (CDMA) communication signals expected in a Candidate proposal filed by The telecommunication industry Association with The International Telecommunication Union (ITU), entitled "The CDMA2000 ITU-R RTT conference sub" and further elaborated in The draft provisional Standard entitled "deployed balloon Text for CDMA2000 Physical Layer".

Referring to fig. 2, the elements required to transmit the F-DCCH on the forward link signal 6 and receive the reverse link signal 8 are shown in greater detail. The message sent on the F-DCCH is generated by an F-DCHH message generator (DCCH MSG GEN) 100. These messages may include rate scheduling messages, switch direction messages, and response messages (described further below). As previously described, the F-DCCH is a DTX channel, which is transmitted when there is a message to be transmitted and is not transmitted when there is no message to be transmitted thereon.

The message is provided to F-DCCH processing unit 102. F-DCCH processing unit 102 performs the necessary F-DCCH message (when present) preprocessing and coding and channelizes the message transmitted on the F-DCCH of forward link signal 6. The F-DCCH message is provided to CRC and tail bit generator 104. CRC and tail bit generator 104 is responsive to generate a set of Cyclic Redundancy Check (CRC) bits from bits in the F-DCCH message and to append the CRC bits to the F-DCCH message. The CRC and tail bit generator 104 then appends a series of tail bits to clear the decoder's memory at the receiver and provides the final packet to the encoder 106.

In the illustrative embodiment, encoder 106 is a convolutional encoder. The design and implementation of convolutional encoders is well known in the art. Those skilled in the art will appreciate that the present invention is equally applicable to other encoders such as block encoders and high efficiency (Turbo) encoders. The encoded symbols are provided to an interleaver 108. To provide time diversity in the transmission of F-DCCH messages, interleaver 108 reorders the symbols in a predetermined manner. Errors in wireless communication systems typically occur in bursts. Decoders have significant processing advantages when dealing with errors that do not occur in bursts. Interleaving helps to distribute the erroneous burst errors into the packets to improve the performance of the decoder at the receiver.

The interleaved symbols are supplied to a power control insertion (boosting) unit 109. Insertion unit 109 receives reverse link power control bits and inserts the power control bits into the interleaved symbol stream. Power control bits are sent to mobile station 4 and used to adjust the transmit energy of reverse link signal 8.

The symbols from the insertion unit 109 are provided to a demultiplexer 110 which alternately outputs the symbols to two different processing paths. The first output of the demultiplexer 110 is provided to an expansion unit 112A and the next output of the demultiplexer 110 is provided to an expander 112B, and so on. The spreader 112 spreads the signal according to an orthogonal spreading function W_DCCHThe de-multiplexed symbols are spread. Orthogonal spreading is well known in the art and a preferred embodiment of spreader 112 is disclosed in the aforementioned U.S. Pat. No.5,103,459. The spread signal is provided to a complex PN spreader 116.

In addition to the dedicated control channel, in the illustrative embodiment, base station 2 transmits a pilot channel to enable mobile station 4, which may also be referred to as a remote station, to coherently demodulate the received F-DCCH. The pilot symbols, which are typically all 1 sequences, are provided to spreading unit 114. Pilot symbol correlation and spreading sequence W_DCCHOrthogonal spreading sequence W_pilotAnd (5) performing expansion.

The spread signals from spreading units 112 and 114 are provided to a complex PN spreader 116. The complex PN spreader 116 is based on two Pseudo Noise (PN) sequences PN_IAnd PN_QThe signals from the spreaders 112 and 114 are spread. Complex PN spreading IS well known in the art and IS described in detail in the cdma2000 candidate proposal, the draft IS 2000 procedures, and the aforementioned co-pending U.S. patent application No.08/856,428. The complex PN spread signal is provided to a transmitter (TMTR) 118. Transmitter 118 upconverts, amplifies, and filters the spread signal, which is transmitted via antenna 120 as forward link signal 6. In the illustrative embodiment, transmitter 118 modulates the trellis in accordance with QPSKThe equation modulates the signal.

Referring to fig. 3, a forward link signal 6 is received at an antenna 200 and provided to a receiver (RCVR)204 via a demultiplexer 202. Receiver 204 downconverts, amplifies, and filters the forward link signal 6. According to an exemplary embodiment, receiver 204 demodulates forward link signal 6 according to a QPSK demodulation format and outputs in-phase and quadrature-phase signals to complex PN despreader 206. The complex PN despreader 206 spreads the signal (PN) based on the signal_IAnd PN_Q) The two pseudo-noise sequences of (a) despread the received signal.

The complex PN despread signal is provided to the pilot filter 208. The pilot filter 208 is based on the orthogonal spreading sequence W_pilotThe signal is despread. The despread pilot symbols are provided to the Ec/Io calculator 214 and the dot product circuit 216.

The complex PN despread signal is also provided to demodulator 210. Demodulator 210 based on orthogonal spreading codes W_DCCHThe PN despread signal is demodulated. The despread signal is then provided to the dot product circuit 216. Dot product circuit 216 calculates the dot product of the F-DCCH and the pilot channel. Since the pilot channel and the dedicated control channel travel the same propagation path, they will experience the same phase shift. By computing the dot product of the pilot and DCCH channels, a set of scalar values results, wherein channel-induced phase uncertainty is removed. A preferred implementation of the dot-product circuit 216 is described in detail in the aforementioned U.S. Pat. No.5,506,865.

The final demodulated symbols from the dot product circuit 216 are provided to a deinterleaver/decoder 218 and a null frame detector 220. Deinterleaver/decoder 218 deinterleaves and decodes the F-DCCH message and provides a message or signal estimate to DCCH control processor 222 indicating the bad frame declaration. There are various methods of detecting bad frames. The first is to determine whether the cyclic redundancy bits generated locally by the remote station 4 coincide with the decoded CRC bits. The second is to calculate the symbol error rate of the received symbols by comparing the received encoded symbols with a set of re-encoded symbols generated locally from the decoded bits.

The demodulated symbols from the dot product circuit 216 are also provided to a null frame detector 220. In the illustrative embodiment, null frame detector 220 calculates the signal-to-noise ratio of the demodulated symbols and compares the measured signal-to-noise ratio to a threshold. If the signal-to-noise ratio is below a threshold, an empty frame is declared. It is noted that there are other ways to determine the null frame, which fall within the scope of the present invention. A method and apparatus for detecting null frames is disclosed in co-pending U.S. patent application No.09/150,493 entitled "energy based communication detection system and method" filed 9/9 of 1998, which is assigned to the assignee of the present invention and is incorporated herein by reference.

The non-null data frames are provided to DCCH control processor 222 which extracts the inserted power control commands and responsively transmits a signal to transmitter 232 which adjusts the transmit energy of reverse link signal 8. The loss of this stream of power control commands results in an inability to control the power of the reverse link signal 8 and may cause the reverse link to become blocked.

In a first embodiment of the present invention DCCH control processor 222 receives an indication from decoder 218 or detector 220 that the frame is good, bad or empty. Two counters (CNT1)224 and (CNT2)226 are initialized to zero at the beginning of the call. If the received frame is a good frame, counter 224 is reset to zero and counter 226 is incremented by 1. If the received frame is declared to be a bad frame, counter 224 is incremented and counter 226 is reset to zero. If the frame is declared empty, the values of counters 224 and 226 remain unchanged. If the value of counter 224 reaches threshold TH1, DCCH control processor 222 signals transmitter 232 to deactivate it (i.e., turn off the output power). Subsequently, if the value of counter 226 reaches threshold TH2, DCCH control processor 222 sends a signal to transmitter 222 to reactivate the transmitter.

In the second exemplary embodiment, if no data frame is transmitted on the F-DCCH at this time, the base station 2 transmits one frame, referred to herein as a supervisory frame, every N-second interval. In the preferred embodiment, the supervisory frame contains predefined bits known to the mobile station and is sent at the lowest data rate that the base station 2 and mobile station 4 have agreed upon.

Referring to fig. 2, timer 134 tracks the N-second interval and sends a signal to control processor 132 when the interval expires. The control processor 132 determines if there is a message to send and if there is no message to send, provides a signal to the message generator 100 to generate a supervisory frame. The supervisory frames are transmitted on the F-DCCH channel as the other DCCH messages described above. The mobile station then performs F-DCCH monitoring on frames transmitted at a preset timing with potentially different values of various thresholds in a manner similar to that defined in IS-95. In addition to these periodic frames, the mobile station may include other non-null frames received for monitoring purposes. Note that the supervisory frame is periodically generated according to the count value of the counter 130 in fig. 2.

In the third exemplary embodiment, the base station transmits a frame referred to herein as a supervisory frame whenever the number of consecutive null frames exceeds a threshold. In the preferred embodiment, the supervisory frame contains predefined bits known to the mobile station and is sent at the lowest data rate that the base station 2 and mobile station 4 have agreed upon.

Referring to fig. 2, control processor 132 tracks the number of consecutive null frames based on the signal from message generator 100. When the number of consecutive empty frames exceeds the threshold, the control processor sends a signal to issue a Supervisory frame to the message Generator 100 to generate a Supervisory frame. The supervisory frames are transmitted on the F-DCCH channel as the other F-DCCH messages described above. Mobile station 4 then performs F-DCCH monitoring on all non-null frames with potentially different values of various thresholds in a manner similar to that defined in IS-95.

In the fourth exemplary embodiment, the mobile station 4 sends a message requesting a response (e.g., the response may simply be an acknowledgement) from the base station 2 when the number of consecutive null frames detected exceeds a threshold. Referring to fig. 3, control processor 222 receives an indication of whether a frame is empty from empty frame detector 220. In this embodiment, counter 224 tracks the number of consecutive empty frames and resets when a bad or good frame is detected. When the count of consecutive empty frames exceeds the threshold, the control processor 222 sends a signal to the message generator (MSG GEN)228, which in response generates a request message. The request message is encoded in encoder 228, modulated in modulator 230, upconverted, amplified, and filtered into a predetermined channel of reverse link signal 8. The request message may be any existing message that has been defined in the standard and that does not cause any base station action other than sending an acknowledgement. Such as power measurement report messages. The request message may also be a special message that triggers the base station 2 to send a supervision frame on the F-DCCH.

Referring to fig. 2, a request message is received at antenna 8 and provided to a receiver 124, which receiver 124 downconverts, amplifies, and filters the reverse link signal 8 and provides a received signal to a demodulator 126. Demodulator 126 demodulates signals and decoder 128 decodes the demodulated symbols that provided the request message to control processor 132. Control processor 132 is responsive to determining whether the message is in an F-DCCH transmit queue and not transmitting a signal requesting message generator 100 to generate a message for transmission on the F-DCCH. In the exemplary embodiment, the message generated by generator 100 is simply an acknowledgement of receipt of the request message from mobile station 4.

In the fifth exemplary embodiment, the mobile station 4 transmits a message requesting a response from the base station 2 when the number of null frames detected within a predetermined number of reception frames (whether the null frames are continuous or discontinuous) exceeds a threshold value. Referring to fig. 3, control processor 222 receives an indication of whether a frame is empty from empty frame detector 220. Counter 224 tracks the number of empty frames in a moving accumulator manner. When the number of empty frames in a predetermined number of received frames exceeds a threshold, the control processor sends a signal to a message generator (MSG GEN)228, which generates a request message in response. The request message is encoded in encoder 229, modulated in modulator 230, and upconverted, amplified, and filtered onto a predetermined channel of reverse link signal 8.

Referring to fig. 2, a request message is received at antenna 122 and provided to receiver 124, which receiver 124 downconverts, amplifies, and filters reverse link signal 8 and provides a received signal to demodulator 126. Demodulator 126 demodulates signals and decoder 128 decodes the demodulated symbols that provided the request message to control processor 132. Control processor 132 is responsive to determining whether a message is in the F-DCCH transmit queue and if not, transmitting a signal requesting message generator 100 to generate a message for transmission on the F-DCCH. In the illustrative embodiment, the message generated by the generator 100 is simply an acknowledgement of receipt of the request message.

In the sixth exemplary embodiment, mobile station 4 completes F-DCCH monitoring using the pilot strength of the pilot signal in the active set (Ec/Io). If the accumulated active set pilot Ec/Io is above the preset threshold, then the mobile station 4 considers the data (if transmitted within the frame) to be correctly received and therefore a good frame. Otherwise, the mobile station 4 considers the frame as a bad frame. Similar monitoring rules as defined in IS-95 (along with the definition of good and bad frames described above) may then be used at the same threshold or a modified threshold.

Referring to fig. 3, the signal-to-noise ratio (Ec/Io) of the received pilot symbols is calculated in an Ec/Io calculator 214. The Ec/Io value of the pilot signal of forward link signal 6 is combined with the Ec/Io values of the pilot signals from other base stations in the active set of mobile station 4 to provide an accumulated Ec/Io. The active set of base stations is the set of base stations currently communicating with mobile station 4. The accumulated pilot signal Ec/Io is provided to the control processor 222, which compares the accumulated Ec/Io to a threshold. If the accumulated Ec/Io exceeds a threshold, a good frame is declared, and if the accumulated Ec/Io is less than the threshold, a bad frame is declared. This enables the mobile station 4 to infer whether a received non-null frame is a good frame or a bad frame without decoding the frame. Based on these counts, the mobile station 4 will either activate or deactivate the transmitter 232 as described above.

While the apparatus and methods have been described in various forms, the true spirit and scope of the invention is not limited thereto but only by the following claims and their equivalents. The invention is claimed in the claims and their equivalents.

Claims (9)

1. A base station, comprising:

a timer that transmits a timing signal;

a control processor coupled to the timer, the control processor determining if there is a message to send when it receives the timing signal, and if not, sending a signal indicating the condition;

a message generator, coupled to the control processor, that generates a supervisory frame when the message generator receives the signal; and

a transmitter coupled to the message generator, the transmitter transmitting the supervisory frame.

2. A base station, comprising:

a control processor that determines when the number of consecutive empty frames on the channel exceeds a threshold and transmits a signal indicative thereof;

a message generator, coupled to the control processor, that generates a supervisory frame when the message generator receives the signal; and

a transmitter coupled to the message generator, the transmitter transmitting the supervisory frame.

3. A method of monitoring a channel, comprising the steps of:

detecting the number of continuous empty frames on a channel; and

and when the number of the continuous empty frames exceeds a threshold value, sending a message, wherein the message requires the base station to respond.

4. The method of claim 3, wherein the reply is an acknowledgement.

5. The method of claim 3, wherein the transmitted message causes the base station to transmit the supervisory frame over the channel.

6. A remote station, comprising:

a null frame detector which detects when a null frame is present and transmits a signal indicating the same;

a counter for counting the number of consecutive empty frames;

a control processor coupled to the null frame detector and the counter, the control processor resetting the counter when a bad frame is detected and a good frame is detected, and transmitting a signal indicative of the condition when the number of consecutive null frames exceeds a threshold;

a message generator coupled to the control processor, the message generator generating a message requiring a base station to respond when receiving the signal; and

and the transmitter is used for transmitting a message requiring the base station to respond.

7. The remote station of claim 6, further comprising an Ec/Io calculator to calculate an accumulated active set pilot Ec/Io by adding pilot signals of the forward link signal to Ec/Io values of pilot signals from other base stations in the active set of the mobile station.

8. A base station, comprising:

an antenna that receives a reverse link signal comprising a request message;

a receiver coupled to the antenna, the receiver downconverting, amplifying and filtering the reverse link signal;

a demodulator, coupled to the receiver, that demodulates the reverse link signal to thereby generate demodulated symbols of the request message;

a decoder, coupled to the demodulator, that decodes the demodulated symbols;

a control processor coupled to the decoder, the control processor determining whether the message is in a transmit queue on the channel, and if not transmitting the request signal;

a message generator, coupled to the control processor, that generates a message to be sent upon receipt of the request signal, the message being an acknowledgement of receipt of the request message.

9. A wireless apparatus for monitoring a channel, comprising:

means for detecting a number of consecutive null frames on the channel; and

means for sending a message when the number of consecutive empty frames exceeds a threshold, the message requiring a base station to respond