HK1132612B - A code division multiple access user device - Google Patents

Description

CDMA user equipment

The present application is a divisional application of chinese patent application filed on 6/13/2002 under the name of 02814052.4 entitled "heartbeat signal transmission at a level lower than the heartbeat request".

Background

The increasing use of wireless telephones and personal computers has resulted in an increasing demand for advanced communication services. Whereas in the past such services were once seen to be provided only for specific application areas. In the 80's of the 20 th century, wireless voice communication was gaining popularity over cellular telephone networks. Since it is expected that the user cost will be high, it was thought from the outset that such services were targeted to business personnel. The same is true with respect to access to a remote distributed computer network. Just before, only business personnel and large organizations have been able to afford the required computers and wired access devices.

With the popularization of new technologies that people have come to use, people do not satisfy the requirement for accessing networks such as the internet and private intranets in a wired manner, and wish to access networks in a wireless manner. Wireless technology is particularly useful to users of portable computers, laptop computers, handheld digital assistants, etc., who do not want to be constrained by a telephone line.

There is currently no universally viable, satisfactory way to achieve low cost, high speed access to the internet, private intranets, and other networks using existing wireless infrastructure. This is likely to be the result of some unfortunate circumstances. First, the typical manner of providing high-speed data services over a wired network in a business environment cannot easily be adapted to voice-level services available in most homes or offices. For example, such standard high speed data services are not necessarily able to efficiently transmit information via a standard cellular radiotelephone handset, since wireless networks were originally designed to transmit only voice, although some systems, such as CDMA systems, do employ some mode of operation with asymmetric properties for data transmission, e.g., the communication industry association (TIA) specifies a forward traffic channel data rate for IS-95 systems as: rate set 1, incrementally adjustable from 1.2 kbit/s to 9.6 kbit/s; rate set 2, may be incrementally adjusted from 1.8 kbit/s to 14.4 kbit/s. However, the data rate of the reverse link traffic channel is fixed at 4.8 kbit/sec.

Thus, existing wireless systems typically provide only one radio channel at best, and the maximum data rate in the forward link direction can reach 14.4 kbits/sec per second. Such low speed channels cannot be used directly to transfer information at the rates of the currently commonly used 28.8k and even 56.6 kbit/sec inexpensive cable modems, not to mention the higher rates such as 128 kbit/sec that are achievable with Integrated Services Digital Network (ISDN) equipment. These data rates soon become the lowest rates acceptable for activities such as browsing web pages.

Although wired networks have been known when cellular systems have just been developed, in most cases no measures have been taken to enable such wireless systems to provide higher speed ISDN or ADSL level data services over a cellular network architecture.

In most wireless systems, there are many more potential users than radio channel resources. Some sort of demand-based multiple access system is required.

The radio spectrum has characteristics to be shared whether multiple access is provided by conventional Frequency Division Multiple Access (FDMA) techniques, which perform analog modulation of a set of radio frequency carrier signals, or by sharing the frequencies of one carrier signal using time division multiple access, or by code division multiple access. This is very different from the conventional environment supporting data transmission. In which the wired medium is relatively inexpensive and is generally not ready for sharing.

Other factors to be considered in the design of a wireless system are the nature of the data itself. For example, it is contemplated that access to web pages is generally burst oriented, requiring asymmetric rates of data transmission in the reverse and forward directions. It is common practice for a remote client computer user to first specify the address of a web page for a browser program. The browser program then sends the address, typically 100 bytes or less in length, over the network to a server computer. The server computer then gives the answer with the requested web page content. The content may be text, image, audio or video data from 10 kbytes to several megabytes. Thereafter, the user may take several seconds or even minutes to read the web page content and then download another web page.

In an office environment, the habit of most employees working with computers is generally to first view several web pages and then do something else for a longer period of time, such as accessing locally stored data or even completely stopping the use of the computer. Thus, even if these users are connected to the internet or private intranets all day long, the practical use of high-speed data links is generally quite disconcerting.

Optimizing the use of available resources in wireless CDMA communication systems becomes increasingly important if wireless data transfer services supporting internet bonding coexist with wireless voice communications. Frequency reuse and dynamic traffic channel assignment are related to certain aspects that enhance the performance of high performance wireless CDMA communication systems, but there is still a need to more efficiently utilize existing resources.

Disclosure of Invention

In one application, a flag is transmitted in a time slot over a channel indicating that a corresponding field unit has made a request to begin operation. I.e., the flag is transmitted in a designated time slot, indicating that the field unit is requesting a reverse link traffic channel be assigned to the subscriber in order to transmit the data payload from the field unit to the base station. This indicates that the field unit is currently in a standby state. In turn, the field unit transmits a flag over the other of the pair of reverse link channels indicating that the field unit has not requested to begin operation. For example, field units now do not want to transmit data over the reverse link channel, but rather request to remain inactive but synchronized with the base station in order to quickly transition back to active at any time.

In either case, a wireless communication system employing the principles of the present invention can improve the detection performance of the markers by transmitting the markers at different power levels (e.g., 9dB for one and 11dB for the other), thereby improving the performance of the system. The difference in power levels of the tags allows the base station to identify no request tags with a low probability of error using selectable criteria, where the selectable criteria may include comparing the tags to respective power level (power level) thresholds, monitoring the occupancy of time slots and the occupancy of mutually exclusive code channels, or a combination thereof. For example, in one embodiment, a request tag, generally higher priority, transmitted at a higher power level may increase the probability of detection and decrease the probability of false detection of the request tag.

In one particular example of a CDMA system application, the field unit provides a Heartbeat (HB) channel and a heartbeat with request channel (HB/RQST). The (HB) channel uses a first code in the reverse link to the base station, and the HB/RQST channel uses a second code in the reverse link. In this example of a CDMA application, the field unit may transmit the HB and HB/RQST channels at different power levels in accordance with the principles of the present invention. The HB/RQST is given higher power in the preferred case because it is a high priority signal.

The invention discloses a field unit, comprising: an antenna; a code division multiple access transceiver; the field unit is configured to establish a wireless connection with a base station based on allocating resources on a forward link and a reverse link; the field unit is further configured to transmit a presence indication from the field unit to the base station over the reverse link and to transmit a request from the field unit over the reverse link in an assigned time slot to request reservation of a reverse link traffic channel.

The present disclosure supports I-CDMA and lxEV-DV systems, but is generally sufficient to support those systems that use various other communication protocols used in wired and wireless communication systems. Code Division Multiple Access (CDMA) systems, such as IS-2000, and Orthogonal Frequency Division Multiplexing (OFDM) systems, such as IEEE 802.11a wireless local area networks, may all employ embodiments of the present invention.

Drawings

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments, as illustrated in the accompanying drawings. Identical components in different figures are denoted by the same reference numerals. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

FIG. 1 is a schematic diagram of a communication system in which embodiments of the present invention may be deployed;

fig. 2 is a diagram illustrating a subsystem employed by a base station in the communication system of fig. 1 for determining whether a reverse link signal contains an indication of a request to change communication state based on signal power level;

FIG. 3A is a signal diagram of an lxEV-DV system, wherein the first flag indicates "control hold" and the second flag indicates "request start work";

FIG. 3B is a Code Division Multiple Access (CDMA) set of code channels with a flag in an assigned time slot indicating that a field unit is requesting a change in communication status;

fig. 3C is a signal diagram of an alternative embodiment of a reverse link signal with an indication;

fig. 4 is a graph of signal-to-noise ratio versus probability of detection that may be used to determine the power level indicated within the signal of fig. 3A-3C.

Detailed Description

The following is a description of preferred embodiments of the invention.

In wireless communications, the preferred embodiment of the invention applies to the power transmitted from the handset (or the target received power at the base station terminal (BTS)) to compare the heartbeat signal (HB) with the heartbeat signal with request (HBR, HB/RQST, or directly referred to as the "request" signal). The HB and HB/RQST signals may be transmitted over a maintenance channel, which is a single code channel (rather than a multi-code channel) on the reverse link of a CDMA communication system, as disclosed in U.S. serial No. 09/775, 305. The maintenance channel is a channel divided into time slots, with different users being allocated on different time slots.

A field unit in a wireless communication system transmits a heartbeat signal to maintain timing and/or power control and transmits a presence indication to a BTS. When a terminal needs an assigned reverse link channel, the terminal transmits at least 1 request signal. The signal or signals may be modulated information or even pilot signals without "digits".

The requirements for detection probability and false detection probability for these channels are very different. For example, the probability of detection of HB is relatively low. Its detection rate need only be sufficient to track the timing transitions of the code channels caused by the physical motion of multipath structure changes due to doppler effects within the channels. In this case, the power control continues to operate independently of the probing or without probing.

For example, if the signal is not detected in the case where the received power does not exceed the predetermined threshold but the correlation matches, the power command indicates that the power is too low and the terminal must "power up". A requirement in this particular embodiment is that the frequency of detection should be sufficient to enable the detector to be aligned with the received signal in time.

On the other hand, since the request is an emergency event, the request signal is regarded as a high priority signal, and therefore, the probability of detecting the request signal is likely to be very high. Thus, the request signal will be transmitted at a higher power and the threshold at the BTS will be set in a different way. This results in a higher probability of detection and a lower probability of false detection.

Thus, in accordance with the principles of the present invention, different detection probabilities and false detection probabilities may be used for heartbeat signals, request signals, or any other signal information.

An access terminal may transmit signals with different powers depending on the signal type. The BTS may use different criteria to probe the request indication sent on the signal. For example, in a channel divided into slots or mutually exclusive code channels, some slots are occupied when a request is made, rather than when there is no request. In this case, either the higher power or the presence, or both, may be used as the detection criteria.

Fig. 1 is a diagram of an exemplary communication system 100. The system uses one embodiment of the present invention as described above. As shown, the terminal base station 25 with antenna tower 23 maintains a wireless communication link with each of the field units 42a, 42b, 42c (combined into field unit 42). These wireless links are established based on the allocation of resources on the forward link 70 and reverse link 65 between the base station 25 and the field unit 42. Each link 65 or 70 is typically comprised of several logical reverse link channels 55 and several logical forward link 60 channels, respectively.

As shown, communication system 100 supports wireless communication between interface 50 and network 20. Network 20 is typically a Public Switched Telephone Network (PSTN) or a computer network such as the internet, or an intranet. Interface 50 is preferably matched to a digital processing device, such as a portable computer 12, sometimes referred to as an access device, to provide wireless access to network 20. Thus, the portable computer 12 may access the network 20 on a communication basis that is a combination of wired and wireless data links.

In a preferred embodiment, forward link 60 and reverse link 55 are defined in communication system 100 as Code Division Multiple Access (CDMA) channels. That is, each CDMA channel is preferably defined by encoding and transmitting on the channel with a spreading Pseudorandom Noise (PN) code sequence. The PN coded data is then modulated onto a radio frequency carrier. This allows the receiver to distinguish one CDMA channel from another channel by knowing the particular spreading PN code assigned to that particular channel. According to one embodiment, each channel may occupy a 1.25 MHz band compliant with the IS-95 CDMA standard and the 1xEV-DV standard and may transmit at 38.4 kbit/sec.

One forward link 70 includes at least 4 logical forward link channels 60. As shown, it includes a pilot channel 60PL, a Link Quality Management (LQM) channel 60L, a paging channel 60 PG and a multipath traffic channel 60T.

One reverse link 65 includes at least 5 logical reverse link channels 55. As shown, it includes a heartbeat standby channel 55HS, a heartbeat request working channel 55HRA, an access channel 55A and a multi-way traffic channel 55T. In general, the reverse link channels 55 are identical to the forward link channels 60, except that each reverse traffic link channel 60T can support a variable data rate from 2.4 to a maximum of 160 kbits/sec,

the data transmitted between the base station 25 and the field unit 42a includes encoded data information, such as web page data. A higher data rate may be achieved for a particular link between the base station 25 and the field unit 42a based on the assignment of multiple traffic channels within the reverse link 65 and the forward link 70. However, since multiple field units 42 compete for bandwidth allocation, one field unit 42a must wait until there is free resource available to allocate to the traffic channel to send the data payload.

Before discussing an exemplary detector system (fig. 2) for distinguishing between heartbeat signals and heartbeat signals with requests, a brief description of exemplary signals will be provided with reference to fig. 3A-3C.

In fig. 3A, an lxEV-DV signal 160 that may be transmitted by a field unit has 3 different states. A "control hold" state 165, a "request to start working" state 170, and a data transfer state 165. In the "control hold" state 165, the signal 160 does not contain an indication of "request to start work". In other words, the signal 160 remains in an "idle" or "control hold" state, which indicates that the field unit 42a has not requested a traffic channel. The "request to start operation" state 170 indicates that the field unit is requesting data to be transmitted to the BTS25 over the data traffic channel of the reverse link. In the data transmission state 175, traffic data is transmitted by the field unit to the BTS. After traffic data is transmitted over the reverse link and a "data transmission complete" state (not shown) is sent, signal 160 resumes the "control hold" state.

Although the signal 160 is shown as a single signal, it should be understood that multiple signals may be selectively encoded into mutually exclusive channels using orthogonal or non-orthogonal codes. For example, the "control hold" state 165 may be transmitted over a different channel from the "request to start work" state 170.

Similarly, traffic data transmitted in the data transmission state 175 may also be transmitted on a channel that is independent of the other two states 165, 170. The multi-channel example will not be discussed with reference to fig. 3B and 3C.

Fig. 3B is an exemplary internet code division multiple access (I-CDMA) signal diagram. This jail allocates slots to users 1, 2, 3 … … N, and repeats the allocation of slots in the signal occurrence periods i 177a, i + 1177 b, and so on. The channel is composed of a heartbeat channel 55H, a request channel 55R, and a traffic channel 55T. Each of these three channels has an associated code C1, C2, C3, C4 … … CN, which enables signals to be transmitted through mutually exclusive code channels. Both the transmitting and receiving systems process the information in the channels by using codes to distinguish the information in the channels in a typical CDMA manner.

As shown, the presence of signal 180 in heartbeat channel 55H indicates that users 1, 2, 4, 5, 6 … … n are requesting to remain in an idle state. And subscriber 3 is requesting data to be transmitted over a reverse link based on a signal 185 in the request channel 55R in the first time period i 177 a. In a second time period i 177b, subscriber 3 starts to transmit traffic data 190 over a corresponding traffic channel using code 5.

Fig. 3C is a more detailed signal diagram of the lxEV-DV signal of fig. 3A, indicating a "request to start work" made from the field unit 42a to the base station. In this embodiment, the 1xEV-DV signal is composed of a plurality of signals on different logical channels, namely: a heartbeat channel 55H and a request channel 55R. The heartbeat channel 55H continuously provides timing and other information (e.g., power level, synchronization, etc.) from the field unit 42a to the base station 25. The field unit 42a makes a request (e.g., a digit "1") to the base station 25 using the request channel 55R to transmit data on one of the channels on the reverse link 65.

The sample time periods 195a, 195b … … 195f (combined as 195), indicated by the arrows, represent the time or interval at which the BTS25 samples the request signal 55R slots, during which the heartbeat channel 55H may also determine whether a request for a traffic channel is in progress. It should be noted that sampling may be performed in the entire slot or a subset of the slot. Also, in this particular embodiment the heartbeat channel 55H and the request channel 55R use mutually exclusive codes, so sampling is done on mutually exclusive code channels 55H and 55R in all or a subset of the time slots. In one particular embodiment, the base station 25 samples the mutually exclusive code channels 55H, 55R in the time slots allocated for the request indication, e.g., in the time slots of sampling times 195b, 195d, and 195 f. In these time slots, the heartbeat channel 55H is "off" and the request channel 55R is "on".

As described above, the signal in the "active" request slot may be modulated information or may simply be a pilot signal without "digits". Thus, the detection is based only on the respective power levels of the heartbeat signal and the heartbeat signal with request in the respective time slots within a particular time interval or intervals. In a particular embodiment, the "control hold" state 165 indicates that the signal has a first power level, while the "request to start operation" state 170 has a second power level.

In this particular embodiment, the two states are distinguished by simply measuring the signal(s) and either (i) comparing the power level to at least one threshold and (ii) determining that in mutually exclusive code channels, a request exists during a time slot in which the heartbeat signal is at logic zero. The different power levels of the indicator signal are provided by the duty cycle, frequency, power, and signal transmission structure, etc. of the signal.

To understand how the power level of a signal can be used to improve system performance, we can refer to fig. 4, which is a graph illustrating the selection of a signal transmission requirement based on the following parameters or factors: (i) probability of detection p (d) (x-axis), (ii) signal-to-noise ratio in decibels (y-axis), (iii) probability of false detection p (fd) (curves in the figure). The figure shows the signal-to-noise ratio required at the input terminal of a linear commutation detector as a function of the probability of detection of a single pulse for the calculation of a non-pulsating signal, with the probability of false detection p (fd) as a parameter. It should be noted that different parameters or factors may be selected to establish or determine the transmission power level of the indicator signal.

At the circle point 200, the signal-to-noise ratio is 3dB, p (d) =20%, p (fd) = 1%. In order to increase the detection probability without changing the probability of false detection, the circle point 200 is simply slid up along the same false detection probability curve, which means that the increase in signal-to-noise ratio is used to improve the performance of the system, thus increasing the probability that the request signal will be detected soon.

Before providing an exemplary discussion of the power levels of the exemplary heartbeat standby 55HS and heartbeat request operation 55HRA used by the exemplary communication system 100 (fig. 1), a brief discussion of a processor and detector that may be used by the system will now be provided.

Fig. 2 is a schematic diagram of a request probe processor 110 for determining whether a field unit 42a requests data to be transmitted to the BTS 25. Receiver Rx 35 receives signals 55 including a maintenance channel 55N, a traffic channel 55T, an access channel 55A, a heartbeat standby channel 55HS and a heartbeat request working channel 55 HRA. The reverse link channel 55 is processed such that the heartbeat channel processor 112 receives the heartbeat standby channel 55HS and the request channel processor 114 receives the heartbeat request working channel 55 HRA.

The heartbeat channel processor 112 and the request channel processor 114 contain the same processing elements, so in this particular embodiment, only a brief description of the heartbeat channel processor 112 will be provided.

The heartbeat channel processor 112 receives the heartbeat standby channel 55 HS. The correlator 115 despreads the heartbeat standby channel 55HS using a despreader 120. The integrator 125 is used to combine the heartbeat signals in a coherent manner. By coherent combination of the information, I, Q and integration of its phase results in a shift in the signal phase and an output of signal power.

After the correlator 115, the signal power is adjusted by a rectifier 130 (i.e. the absolute value of the signal squared) and then integrated by a second integrator 135 to calculate the power of the received heartbeat signal. The second integrator 135 provides non-coherent combination of the signals and completes the calculation in a short time interval. If the terminal moves too fast, the non-coherent integration only provides the amplitude, causing an overlap of the 180 phase points, which can lead to ambiguity in determining the signal power without non-coherent combining.

The heartbeat channel processor 112 outputs a heartbeat power level and the request channel processor 114 outputs a request power level. In this embodiment, each power level is fed back to the hypothesis detector 140, which determines whether the heartbeat signal, the request signal, or neither are within the reverse link channel 55 received by the base station 25.

To determine which signal or signals are present, it is assumed that the detector 140 contains logic functions. For example, in the present embodiment, assume that the detector 140 compares a first power level threshold with a first power level (i.e., a heartbeat power level) and a second power level threshold with a second power level (i.e., a requested power level).

The power level thresholds against which the "heartbeat" power level and the requested power level are compared are 9dB and 11dB, respectively. The power level threshold may be dynamically selected, predetermined, or otherwise applied, for example, based on a transmitted power level that may be reported by the field unit to the base station via the "heartbeat" channel 55H. In performing the power level calculation and comparison, the first and second power levels may be based on the time slots occupied in the signal transmission channel used by signal 55, and for this purpose, the power level threshold may be based on a predetermined or specified number "1" of bits used to indicate "request to start operation" or to indicate a request to remain idle.

It is assumed that the output of the detector 140 can be used to change the state of the communication system. For example, if 140 determines that a field unit is sending a "request to start operation" (i.e., data is being sent on the reverse link), the processor (not shown in BTS 25) is responsible for providing a traffic channel 55T to portable computer 12, assuming the detector outputs a signal to the processor. In one embodiment, if it is determined that the detected signal power level is above the second power level threshold, a traffic channel 55T is assigned by the BTS 25. Additionally, if the detector 140 is assumed to determine that the detected power level is below the second power level threshold, the BTS also assigns the traffic channel 55T.

As explained with reference to fig. 3C, the heartbeat channel processor 112, request channel processor 114, and hypothesis detector 140 are configured and designed in such a way that the monitoring and occupation of the time slot is used to indicate a request to change communication status. In one embodiment, as shown in FIGS. 3B and 3C, probing includes monitoring for mutually exclusive code channel occupancy.

A feedback loop (not shown) is used to cause the heartbeat channel processor 112 and the request channel processor 114 to have "adaptive" capabilities. For example, the integration time of the integrators 125, 135 may be adjusted based on the power level received by the heartbeat channel 55H, and the power level threshold assumed to be used by the detector 140 for comparison with the heartbeat and requested power levels may also be adjusted by the feedback loop.

Such a feedback loop may use a command or information to transmit information between the BTS and the field unit that relates to the power level of the heartbeat or heartbeat-with-request signal transmitted by the field unit.

As described above, the first communication state may be a standby communication state, and the second communication state may be a payload communication state. In other systems, and even in the same system, the communication state may also be referred to as other communication states, such as a request to change a base station, a power control signal transmission state, and the like. The methods described herein for using different power levels in signal transmission are applicable to wired, wireless, or optical communication systems. In either case, the communication state may be used in a voice or data communication system.

As also set forth above with reference to fig. 4, the second power level may be based on a probability of a detected target, a probability of a false detection, or a combination of both probabilities. In other words, the field unit may transmit the request signal at a particular power level, or a particular number of pulses within a particular time period, to achieve a corresponding signal-to-noise ratio to ensure a particular target probability of detection, a false positive rate, or both, as previously described with reference to FIG. 4.

An analysis method may be used to set the transmission power or number of transmitted indicator signals, or a feedback mechanism may be used in the communication system to cause the field device to change its behavior so that the received indicator signal power level reaches a predetermined signal-to-noise ratio, thereby providing the desired detection probability and false positive parameters.

Simulation of

Now, simulation is described. Factors that influence the sounding probability and the false positive rate of a Heartbeat (HB) or heartbeat with request (HB/RQST) channel are discussed. Signal-to-noise ratio (SNR) targets proposed for either Heartbeat (HB) or heartbeat with request (HB/RQST) channels are presented. Analytical calculations are also performed to determine the proposed target E/lo to ensure acceptable detection probability and false positive rate.

To allow the reader to understand the simulation in relation to IS-2000 power control, the reader should be directed to the explanation, this simulation using the following parameters:

800 Hz closed loop power control;

the SNR of the i-th user is calculated by SNR = P (i) -P _ interference + processing gain + Er, wherein P _ interference (i) =20 ^ log 10 (10) ^ Sigma j i (l 0 ^ P (j)/20) +10 (P ^ P)_TH/20)), where P (i) is the power received from the ith user, P_THIs the lowest value of thermal noise and is hard set at 120 dBm;

the processing gain was 10 log 64;

the attenuation model adopts a Jakes model;

er = normally distributed random variable with 1 sigma =0.67 dB, which is the SNR estimation error of BTS;

power Control Bit (PCB) error = 3%.

In this particular simulation, the first choice is to select a target SNR for the HB channel. On a 9dB E/lo basis, where E is the total power in the heartbeat information, a 95% probability of detection in additive white Gaussian noise (AWG) and a 0.1% false positive rate are achieved (see Viterbi, A., CDMA: spread spectrum communications principles, Addison Wesley, 1995, P113).

Increasing the detection probability to 99% will produce a false positive rate of well above 1% in AWGN. This false detection rate is noticeable because it reaches a relatively low level, so that when the terminal loses its communication link with the base station, no sounding occurs for a relatively long time.

Generally, the period of time is determined by a timer, and there is a non-detection time of 500 milliseconds to 2 seconds, i.e. 25 to 100 consecutive non-detections. As a benchmark, in a single-path attenuation environment of 9dB E/lo, a 90% detection probability and a 1% false detection probability are theoretically predetermined. For this case, the following discussion considers specific cases related to probability of detection within the attenuated environment.

Now consider the detection of a heartbeat signal with 50 hz power control to compare to the velocity of a field device. The simulation is based on a full rate model in which some modifications are made, e.g., 50 hz power control rate (PC), slot division by standby terminals, no overlap, etc.

While it does not matter that the speed of the terminal exceeds about 2 mph, closed loop power control is believed to be useful to allow attenuation to vary around the average path loss. It is noted that the results are substantially unaffected by a Power Control Bit (PCB) error rate of up to about 40%. Beyond this limit, system performance deteriorates, indicating that some form of closed loop power control is required to maintain average path loss. Therefore, it is useful to perform some form of closed loop power control to control the power of the field unit transmitter (Tx) to an average value appropriate for the field unit to ensure an average path loss to the base station.

Simulations using the above parameters show that if the base station's probe for the "request to start operation" indicator signal is 2dB below the target SNR (as determined above), the average probe time is about 16 milliseconds and the standard deviation is about 14 milliseconds. To achieve lower HB/RQST probe latency, based on the simulated tie-up, the following equation was determined:

target _ SNR (RQST) = target _ SNR (HB) + 2dB (1)

Depending on the detection/false detection rate to be achieved in the QWPN, a target _ SNR of 9dB is selected for the heartbeat information, and a target _ SNR of 11dB is selected for the heartbeat with request (HB/RQST) information. The application of these parameters results in an average probe latency of 15 milliseconds at 20 mph with low false detection probability.

In terms of the probability of misallocation, although the misdetection rate is not calculated very clearly in the simulation, the following pessimistic bounds are given:

Pfd（RQST）=（1 - Pd（HB））* Pfd（HB） （2）

= 5% * 0.1% = 5E - 5，

where Pfd is the false positive rate and Pd is the probability of detection.

The above equation is the result of two conditions: (i) a HB does not detect its presence when it is present, (ii) a HB is erroneously detected when it is not present. This is the most pessimistic bound, since no additional HB/RQST 2dB more transmit power than HB is included in the analysis.

At a HB rate of 50 hz, this will result in an erroneous allocation of one standby user every 400 seconds on average. This probability is linear for the N heartbeat information users, since the events are independent of each other. Thus, for a particular base station, the number of users at full load is 96, and the average error distribution rate is expected to be about 1 every 4 seconds.

Since erroneous allocations can be detected quickly, the situation of erroneous allocations can be corrected relatively quickly. When a false allocation occurs, there are generally 3 cases. First, no information transmission occurs on the assigned reverse channel. Second, the heartbeat signal with request does not appear. If a missed channel assignment occurs, the HB/RQST continues to appear. Third, heartbeat information may appear. The probability that this condition is not detected in one frame is pdf (rqst) = 5E-3%. This will be detected within one or two frames before the channel can be reassigned to a legitimate user. If it is assumed that the probing needs to be done in 2 frames, the reverse capacity will only be reduced by 1% or even less, since the false positive rate of HB/RQST is targeted to 11dB E/lo.

For a signal with no offset between the target _ SNR and the detection threshold, the average detection delay time simulated with a distant user moving at 1 mph and a user moving at 20 mph was 35 milliseconds. For heartbeat signals with requests, the average detection delay time is less than 20 milliseconds and the detection threshold is 2dB less than 11dB of the target SNR. This is possible because the HB/RQST signal is 2dB higher than the transmit (Tx) power of the HB signal.

Simulation results show that assuming 96 users in a Power Control (PC) cycle, the minimum average delay time is close to 10 milliseconds. The expected delay is better than 77 ms 99% of the time.

Simulation results also show that the additional 2dB power added by the HB/RQST signal transmission improves the detection rate and reduces the detection latency to an average of 15 milliseconds. The total co-channel interference for a full-capacity maintenance channel IS estimated to be within 6 dB less than for an IS-2000 fundamental channel (9600 bit/sec reverse traffic channel (R-TCH), 9600 bit/sec reverse dedicated control channel (R-DCCH)).

While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims (6)

1. A field unit, the field unit comprising:

an antenna;

a code division multiple access transceiver;

the field unit is configured to establish a wireless connection with a base station based on allocating resources on a forward link and a reverse link;

the field unit is further configured to transmit a presence indication from the field unit to the base station over the reverse link, and to transmit a request from the field unit over the reverse link within a specified time slot to request that the base station reserve a reverse link traffic channel.

2. The field unit of claim 1, wherein the code division multiple access transceiver is configured to transmit the presence indication and the request at different power levels.

3. The field unit of claim 1, wherein each channel occupies a frequency band of 1.25 megahertz.

4. The field unit of claim 1, wherein a power level of at least one second type of signal requesting at least one reverse link traffic channel is compared to a predetermined power threshold.

5. The field unit of claim 4, wherein each reverse link traffic channel supports variable data rates from 2.4 kbps to 160 kbps.

6. The field unit of claim 1, wherein the code division multiple access transceiver is configured to operate on a plurality of assignable radio frequency channels.