WO2001037522A1

WO2001037522A1 - System and method for wireless communication incorporating error concealment

Info

Publication number: WO2001037522A1
Application number: PCT/US2000/031846
Authority: WO
Inventors: Uwe Sydon; Juergen Kockmann; Paulus Sastrodjojo
Original assignee: Siemens Information And Communication Mobile Llc
Priority date: 1999-11-19
Filing date: 2000-11-20
Publication date: 2001-05-25
Also published as: AU1922901A

Abstract

A system (10) for wireless communication incorporating error concealment includes a base unit (12) and a mobile unit (14), each having a transceiver (16, 18) operable to receive and transmit communication signals between them. The system also includes an error concealor (36, 38) operable to determine whether the communication signal requires concealment. The system also includes a step attenuator (48, 50) operable to decrease an amplitude of the communication signal if the communication signal requires concealment. The system further includes a low pass filter (44, 46) operable to filter the communication signal if the communication signal requires concealment.

Description

SYSTEM AND METHOD FOR WIRELESS COMMUNICATION INCORPORATING ERROR CONCEALMENT

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to the field of telecommunications and, more particularly, to a system and method for wireless communication incorporating error concealment .

RELATED APPLICATIONS

This application is related to the following co- pending Applications all filed on November 19, 1999:

Serial No. 09/443,999, entitled System and Me thod for Simul taneously Testing Mul tiple Cordless Tel ephones;

Serial No. 09/444,033, entitled System and Method for Testing An Assembled Tel ephone;

Serial No. 09/444,058, entitled Sys tem and Method for Wireless Communication Incorporating Range Warning;

Serial No. 09/443,968, entitled Method and Sys tem for Wirel ess Telecommuni ca tion Between A Mobile Uni t and A Base Uni t;

Serial No. 09/443,931, entitled Method and Sys tem for Avoiding Periodi c Bursts of Interference In Wireless Communication Between A Mobile Uni t and A Base Uni t;

Serial No. 09/444,028, entitled Method and System for Power -Conserving Interference Avoidance in Communication Between A Mobil e Uni t and A Base Uni t In A Wireless Tel ecommuni cation System;

Serial No. 09/444,008, entitled Method and System for Changing Sta tes In A Wireless Telecommunica tion Sys tem; Serial No. 09/443,933, entitled Method and System for Wireless Communica tion Incorpora ting Distinc t System Identifier Bytes to Preserve Multi- frame Synchronization for Systems with Limited Control Channel Bandwidth;

Serial No 09/443,972, entitled System and Method for Wireless Communication Incorporating Synchronization Concept for 2.4 Ghz Direct Sequence Spread Spectrum Cordl ess Tel ephon e Sys t em ;

Serial No. 09/443,166, entitled System And Method For

Wireless Communication Incorporating Overloading

Prevention

Techniques for Multi- frame- synchronized Systems;

Serial No. 09/443,998, entitled System and Method for Wireless Communication Incorporating Preloaded Response Message;

Serial No. 09/444,057, entitled Method and System for a Wireless Communication System Incorporating Channel Selection Algorithm for 2.4 Ghz Direct Sequence Spread Spectrum Cordless Telephone System;

Serial No. 09/443,997, entitled Method and System for Transmitting and Receiving Caller Id Data in a Wireless Tel eph on e Sys t e ;

Serial No. 09/443,937, entitled Method and System for Prioritization of Control Messages In A Wireless Telephone Sys t em ;

Serial No. 09/443,996, entitled Method and System for Wireless Telecommunications Using a Multi frame Control Message;

Serial No. 09/443,936, entitled Method and System for Transmitting Caller Id Information from a Base Station to a Mobile Unit Outside the Context of an Incoming Call; and

Serial No. 09/443,942, entitled Method and System for Da ta Compression .

BACKGROUND OF THE INVENTION

As society grows more complex and operates at an ever accelerating pace, there has been a growing need for better and more flexible communications devices. One area that has experienced substantial development activity is the area of wireless communication. Wireless systems are also known as portable, cordless or mobile telephones. A typical wireless communications system has a base station located at a customer's or user's premises. The base is connected to the Public Switched Telephone Network (PSTN) over a wireline interface and communicates with a mobile unit or handset over an air interface that permits the user to communicate remotely from the base station. While users desire the freedom and flexibility afforded by mobile wireless communications systems, they typically do not want to sacrifice the numerous features, such as caller ID, that are available through the wireline service over the PSTN. In addition, users of wireless systems increasingly demand a voice quality that is as good as the voice quality available over a wireline link.

In the past, the enhanced features and high voice quality demanded by users has been achieved by the use of sophisticated and complex algorithms and methods that require substantial processor resources and large amounts of memory. These processing and memory resources are not only expensive but also place a substantial drain on battery power, therefore shortening the effective use of the mobile unit . Other technical problems associated with the need for using faster and more powerful processors include larger packaging to accommodate the larger sized components and to dissipate the heat generated by such units. Wireless systems have in the past been large, bulky, and have weighed more than what is satisfactory to many users.

While wireless communication devices and methods have provided an improvement over prior approaches in terms of features, voice quality, cost, packaging size and weight, the challenges in the field of wireless telecommunications has continued to increase with demands for more and better techniques having greater flexibility and adaptability.

For example, frequency interference or other noise disturbances associated with the communication signals transmitted between wireless communication devices may generally adversely affect the voice or line quality of the communication signals. As a result of the frequency or noise disturbance and interference, the user of the wireless communication device may detect audible noise, such as popping noises or garbled audio signals. The user of the wireless communication device may also be unaware that similar audible noise is being detected at an opposite end of the telecommunication link.

SUMMARY OF THE INVENTION

Accordingly, a need has arisen for a system and method for wireless communication incorporating error concealment that provides greater flexibility and increased effectiveness. The present invention provides a system and method for wireless communication incorporating error concealment that addresses shortcomings of prior systems and methods.

According to one embodiment of the present invention, a method for wireless communication includes receiving a communication signal transmitted between a base unit and a mobile unit. The method also includes determining whether the communication signal requires concealment . The method also includes activating a step attenuator to decrease an amplitude of the communication signal if the communication signal requires concealment . The method further includes activating a low pass filter to filter the communication signal if the. communication signal requires concealment.

According to another embodiment of the present invention, a system for wireless communication includes a base unit and a mobile unit, each having a transceiver transceiver operable to receive and transmit communication signals between them. The system also includes an error concealor operable to determine whether the communication signal requires concealment. The system also includes a step attenuator operable to decrease an amplitude of the communication signal if the communication signal requires concealment. The system further includes a low pass filter operable to filter the communication signal if the communication signal requires concealment .

The present invention provides several technical advantages. For example, according to one aspect of the present invention, if a communication signal is determined to require concealment, a step attenuator and a low pass filter may be activated to reduce and/or substantially eliminate communication signal noise resulting from transmission errors or other communication signal disturbances. The step attenuator may be configured to gradually decrease the amplitude of the communication signal such that the audio component of the communication signal fades at a predetermined rate. Additionally, the low pass filter may be configured such that a predetermined cut off frequency filters the communication signal to substantially eliminate higher frequency levels that generally result in greater detectable signal disturbances .

According to another aspect of the present invention, a buffer may be used to store a prior communication signal not requiring concealment. Upon detecting a communication signal requiring concealment, the communication requiring concealment may be replaced with the prior communication signal not requiring replacement. Additionally, the step attenuator and low pass filter may be activated to decrease the amplitude and filter the prior communication signal. Thus, the present invention further reduces signal noise associated with the communication signal resulting from transmission errors and other signal disturbance sources.

Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken in connection with the accompanying drawings, in which:

FIGURE 1 is a block diagram illustrating a wireless communication system incorporating error concealment in accordance with an embodiment of the present invention;

FIGURE 2 is a timing diagram illustrating a step attenuator effect on a communications signal in accordance with an embodiment of the present invention;

FIGURE 3 is a timing diagram illustrating a low pass filter effect on a communication signal in accordance with an embodiment of the present invention; and

FIGURE 4 is a flow diagram illustrating a method for wireless communication incorporating error concealment in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention and the advantages thereof are best understood by referring to the following description and drawings, wherein like numerals are used for like and corresponding parts of the various drawings .

FIGURE 1 is a block diagram illustrating a wireless communication system 10 incorporating error concealment in accordance with an embodiment of the present invention. In this embodiment, wireless communication system 10 comprises a cordless telephone system; however, wireless communication system 10 may also comprise other suitable types of wireless communication systems. Wireless communication system 10 comprises a base unit 12 and a mobile unit 14. Base unit 12 and mobile unit 14 each comprise a transceiver 16 and 18, respectively, coupled to an antenna 20 and 22, respectively, for transmitting and receiving communication signals transferred between base unit 12 and mobile unit 14 over various frequencies .

Base unit 12 and mobile unit 14 also each comprise a processor 24 and 26, respectively, for processing various data transmitted within base unit 12 or mobile unit 14, respectively, and between base unit 12 and mobile unit 14. Additionally, base unit 12 and mobile unit 14 each comprise a burst mode controller (BMC) 28 and 30, respectively, for detecting and concealing communication signals containing transmission errors and/or other signal disturbances. Thus, burst mode controllers 28 and 30 each comprise an error detector 32 and 34 and an error concealor 36 and 38, respectively.

Error detectors 32 and 34 generally detect transmission errors and other signal disturbances in communication signals transmitted between base unit 12 and mobile unit 14. Error detectors 32 and 34 generally determine a line quality value for each communication signal received by base unit 12 and mobile unit 14, respectively. For example, the line quality value may correspond to a bit rate error of the communication signal. However, other suitable methods or systems may be used for determining the general line quality of the communication signals. Additionally, a plurality of line quality values may be used to determine an average line quality value for communication signals transmitted between base unit 12 and mobile unit 14 over a predetermined period of time. Error detectors 32 and 34 transmit the line quality values to error concealors 36 and 38, respectively, for a determination of whether error concealment is required for the communication signals .

Error concealors 36 and 38 each comprise a buffer 40 and 42, a low pass filter 44 and 46, a step attenuator 48 and 50, and a comparator 52 and 54, respectively. Comparators 52 and 54 generally receive the line quality values from error detectors 32 and 34, respectively, and compare the line quality values to a line quality threshold. For example, a line quality threshold may be selected such that line quality values exceeding the line quality threshold generally indicate that the communication signal requires concealment and line quality values equal to or less than the line quality threshold generally do not require concealment. However, the line quality threshold may be otherwise configured for determining whether the line quality values indicate that concealment of the communication signal is required.

Once error concealors 36 and 38 determine that a communication signal requires concealment, low pass filters 44 and 46 and step attenuators 48 and 50 may be activated to substantially reduce or eliminate detectable noise in the communication signal. For example, step attenuators 48 and 50 may be activated to attenuate or decrease an amplitude of the communication signal to a predetermined attenuated audio level to substantially reduce or eliminate noise resulting from transmission errors or other signal disturbances contained in the corresponding communication signals. For example, the predetermined attenuated audio level may be selected such that the amplitude of the communication signal may be decreased to a substantially muted level. Additionally, a predetermined attenuation rate may be selected for step attenuators 48 and 50 to regulate a rate of amplitude attenuation of the communication signals. Thus, the audio level of the communication signals may decrease or fade out at a predetermined rate to the attenuated audio level selected for step attenuators 48 and 50.

The predetermined attenuation rate may also be used to regulate an increase in the amplitude of the communication signal as step attenuators 48 and 50 are deactivated in response to receiving a communication signal not requiring concealment. Thus, the audio level of the communication signal not requiring concealment may be gradually increased or fade in over a predetermined period of time to a non-attenuated audio level.

Low pass filters 44 and 46 may be used to filter the communication signals requiring concealment to remove higher frequency levels of the communication signals. For example, a predetermined cut-off frequency may be selected such that frequency levels of the communication signals exceeding the predetermined cut-off frequency are generally removed. Thus, the higher frequency levels of the communication signals generally associated with detectable audio noise are removed.

Additionally, a predetermined filter rate may be selected such that the frequency levels of the communication signals are added or removed at a predetermined substantially constant rate to provide a gradual or fade in and fade out frequency filtering. For example, once a communication signal not requiring concealment is received, low pass filters 44 and 46 may be deactivated. The predetermined frequency rate may be used to gradually decrease the frequency level previously filtered until a substantially unfiltered communication signal is reached. Buffers 40 and 42 may be used to store a communication signal not requiring concealment such that a subsequent received communication signal requiring concealment may be replaced with the communication signal not requiring concealment. For example, buffers 40 and 42 may be configured to automatically store all communication signals received between base unit 12 and mobile unit 14. Once a communication signal requiring concealment is detected, the prior communication signal stored in buffers 40 and 42 that did not require concealment may be repeated, thereby replacing the communication signal requiring concealment. Additionally, subsequent communication signals also requiring concealment may be automatically replaced with the communication signal stored in buffers 40 and 42 until a communication signal not requiring concealment is received. Error concealors 36 and 38 may also activate low pass filters 44 and 46 and step attenuators 48 and 50, respectively, to attenuate and filter the repeated communication signal .

FIGURE 2 is a timing diagram illustrating operation of step attenuators 48 and 50 in accordance with an embodiment of the present invention. As illustrated in FIGURE 2, the vertical portion of the timing diagram generally represents the amplitude or audio level of communication signals transmitted between base unit 12 and mobile unit 14, and is indicated generally at 60. The horizontal portion of the timing diagram represents time, and is indicated generally at 62. Generally, the amplitude of communication signals not requiring concealment is indicated generally at 64. A predetermined attenuated audio level for step attenuators 48 and 50 is indicated generally at 66. Once a communication signal requiring concealment is detected, indicated generally at 68, error concealors 36 and 38 may be used to automatically attenuate or decrease the amplitude of the communication signal to the predetermined attenuated audio level 66. As described above, the attenuated audio level 66 may be selected such that the amplitude of the communication signal is decreased to a substantially muted level .

An attenuation rate, indicated generally at 70, may also be selected for step attenuators 48 and 50 such that the amplitude of the communication signals is increased or decreased at a substantially constant rate, thereby providing a fade in and fade out, respectively, of the communication signal. For example, as illustrated in FIGURE 2, once a communication signal is received not requiring concealment, indicated generally at 72, step attenuators 48 and 50 may be deactivated. Upon deactivation of step attenuators 48 and 50, the predetermined attenuation rate 70 may be used to increase the amplitude of the communication signal at a substantially constant rate until non-attenuated audio level 64 is reached.

FIGURE 3 is a timing diagram illustrating operation of low pass filters 44 and 46 in accordance with an embodiment of the present invention. The vertical portion of the timing diagram generally represents the frequency level of the communication signals, indicated generally at 80. The horizontal portion of the timing diagram generally represents time, indicated generally at 82. A predetermined cut-off frequency may be selected such that the frequency levels of the communication signal exceeding the predetermined cut-off frequency are generally removed, thereby resulting in a filtered audio level, indicated generally at 84, of the communication signal .

In operation, once a communication signal requiring concealment is detected, indicated generally at 86, error concealors 36 and 38 may be used to activate low pass filters 44 and 46 to filter the communication signal to filtered audio level 84. Once a communication signal is detected that does not require concealment, indicated generally at 88, low pass filters 44 and 46 may be deactivated, thereby allowing the frequency levels previously filtered to be returned until the frequency level reaches a substantially unfiltered level, indicated generally at 90.

As described above, a predetermined filter rate, indicated generally at 92, may also be selected to provide frequency filtering at a substantially constant rate. For example, as illustrated in FIGURE 3, upon detection of a communication signal not requiring concealment, indicated generally at 88, low pass filters 44 and 46 may be deactivated to decrease the filtering effect of low pass filters 44 and 46 or fade in the frequencies of the communication signal previously filtered until a substantially non-filtered communication signal, indicated generally at 90, is reached. The predetermined frequency rate may be selected such that the previously filtered frequency levels fade in at a substantially constant rate to provide a generally smooth transition from the communication signal requiring concealment to a communication signal not requiring concealment .

FIGURE 4 is a flow diagram illustrating a method for wireless communication incorporating error concealment in accordance with an embodiment of the present invention. The method begins at step 200, where a communication signal is received by either base unit 12 or mobile unit 14. For ease of illustration, the method illustrated in FIGURE 4 will be directed generally to receiving the communication signal from base unit 12 at mobile unit 14. However, it should be understood that the following method may be applied to communication signals received by base unit 12 from mobile unit 14 in a similar manner.

At step 202, error detector 34 determines a line quality value corresponding to the communication signal. As described above, the line quality value generally corresponds to a voice or line quality of the communication signal. At step 204, comparator 54 receives the line quality value corresponding to the communication signal and compares the line quality value to a line quality threshold. At decisional step 206, a determination is made whether the line quality value exceeds the line quality threshold, thereby indicating in this embodiment that concealment of the communication signal is required. If the line quality value does not exceed the line quality threshold, the method proceeds to decisional step 208. If the line quality value exceeds the line quality threshold, the method proceeds to step 218.

At decisional step 208, a determination is made whether a prior communication signal not requiring concealment is stored in buffer 42. If a prior communication signal not requiring concealment is not contained within buffer 42, the method proceeds to step 210 where the current communication signal is stored in buffer 42. If a prior communication signal not requiring concealment has been stored in buffer 42, the method proceeds to step 212 where the prior communication signal stored in buffer 42 is replaced with the current communication signal .

At decisional step 214, a determination is made whether another communication signal is received from base unit 12. If another communication signal is not received from base unit 12, the method ends. If another communication signal is received from base unit 12, the method proceeds to step 216 where the next communication signal is received by mobile unit 14. Upon receiving the next communication signal from base unit 12, the method returns to step 202.

At step 218, if the line quality value corresponding to the communication signal exceeds the line quality threshold, the communication signal requiring concealment may be replaced with a communication signal stored in buffer 42. As described above, the communication signal stored in buffer 42 represents a prior communication signal not requiring concealment. At step 220, step attenuator 50 is activated to attenuate the amplitude of the repeated communication signal. Additionally, a predetermined attenuation rate may be selected such that the amplitude of the repeated communication signal is decreased at a substantially constant rate. At step 222, low pass filter 46 is activated to filter the repeated communication signal to a predetermined filtered audio level 84.

At decisional step 224, a determination is made whether another communication signal is received from base unit 12. If another communication signal is not received from base unit 12, the method ends. If another communication signal is received from base unit 12, the next communication signal is received by mobile unit 14 at step 226. At step 228, error detector 34 and comparator 54 may be used to determine whether the next communication signal requires concealment. At decisional step 230, a determination is made whether the next communication signal requires concealment. If the next communication signal received by mobile unit 14 also requires concealment, the method proceeds to step 232, where the communication signal requiring concealment is replaced by the communication signal stored in buffer 42. After repeating the communication signal stored in buffer 42, the method returns to decisional step 224.

If the communication signal received at step 226 does not require concealment, the method proceeds to step 234 where step attenuator 50 may be deactivated. Additionally, a predetermined attenuation rate may be selected such that the amplitude of the communication signal not requiring concealment may be increased at a substantially constant rate until reaching non-attenuated audio level 64. At step 236, low pass filter 46 may also be deactivated. Additionally, a filter rate may be selected such that frequency levels of the communication signal previously filtered fade in at a substantially constant rate from filtered audio level 84 until reaching a substantially unfiltered audio level 90.

At decisional step 238, a determination is made whether another communication signal is received from base unit 12. If another communication signal is received by mobile unit 14, the method returns to step 202. If another communication signal is not received by mobile unit 14, the method ends.

Therefore, the present invention provides greater efficiency and effectiveness of error concealment than prior systems and methods by providing amplitude attenuation and filtering of the communication signal . Additionally, the amplitude and frequency levels of the communication signal may fade in and fade out at a substantially constant rate to reduce or substantially eliminate abrupt audio disturbances resulting from transmission errors and other signal disturbances associated with the communication signals.

Additionally, communication signals requiring concealment may be automatically replaced with previously received communication signals. The repeated communication signals may also be attenuated and filtered to further decrease or substantially eliminate audio disturbances resulting from transmission errors and other signal disturbances associated with the communication signals .

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the present invention as defined by the appended claims .

Claims

WHAT IS CLAIMED IS:

1. A method for wireless communication, comprising: receiving a communication signal transmitted between a base unit and a mobile unit; determining whether the communication signal requires concealment; activating a step attenuator to decrease an amplitude of the communication signal if the communication signal requires concealment; and activating a low pass filter to filter the communication signal if the communication signal requires concealmen .

2. The method of Claim 1, further comprising: receiving a next communication signal from the base unit ; determining whether the next communication signal requires concealment; deactivating the step attenuator if the next communication signal does not require concealment; and deactivating the low pass filter if the next communication signal does not require concealment .

3. The method of Claim 1, wherein activating the step attenuator comprises decreasing the amplitude of the communication signal to a muted level .

4. The method of Claim 1, further comprising: storing a prior communication signal not requiring concealment, the prior communication signal received before receiving the communication signal requiring concealment ; and replacing the communication signal requiring concealment with the prior communication signal .

5. The method of Claim 4, wherein activating the step attenuator comprises decreasing an amplitude of the prior communication signal, and wherein activating the low pass filter comprises filtering the prior communication signal.

6. The method of Claim 1, further comprising determining a line quality value corresponding to the communication signal, and wherein determining whether the communication signal requires concealment comprises determining if the line quality value is less than or equal to a line quality threshold.

7. The method of Claim 1, wherein the line quality value corresponds to a bit rate error of the communication signal .

8. A system for wireless communication, comprising : a base unit and a mobile unit, each having a transceiver operable to receive and transmit communication signals between them; an error concealor operable to determine whether the communication signal requires concealment ; a step attenuator operable to decrease an amplitude of the communication signal if the communication signal requires concealment; and a low pass filter operable to filter the communication signal if the communication signal requires concealment .

9. The system of Claim 8, wherein the step attenuator is operable to decrease the amplitude of the communication signal to a muted level.

10. The system of Claim 8, further comprising: a buffer operable to store a prior communication signal not requiring concealment, the prior communication signal received before receiving the communication signal requiring concealment; and a processor operable to replace the communication signal requiring concealment with the prior communication signal ; wherein the step attenuator is operable to decrease an amplitude of the prior communication signal; and wherein the low pass filter is operable to filter the prior communication signal .

11. The system of Claim 10, wherein the transceiver is operable to receive a next communication signal after receiving the communication signal requiring concealment, and wherein the error concealor is operable to determine whether the next communication requires concealment, and wherein the processor is further operable to replace the next communication signal with the prior communication signal if the next communication signal requires concealment .

12. The system of Claim 8, further comprising: an error detector operable to determine a line quality value corresponding to the communication signal; and a comparator operable to compare the line quality value with a line quality threshold for determining whether the communication signal requires concealment .

13. The system of Claim 8, wherein the error concealor is further operable to detect whether a next communication signal requires concealment, the next communication signal received after the communication signal requiring concealment, and wherein the error concealor is further operable to deactivate the step attenuator and the low pass filter if the next communication signal does not require concealment.

14. A method for wireless communication, comprising: receiving a first communication signal from a base unit ; receiving a second communication signal from the base unit, the second communication signal received after receiving the first communication signal; determining whether the second communication signal requires concealment; repeating the first communication signal if the second communication signal requires concealment ; activating a step attenuator to decrease an amplitude of the repeated first communication signal; and activating a low pass filter to filter the repeated first communication signal.

15. The method of Claim 14, wherein activating the step attenuator comprises decreasing the amplitude of the repeated first communication signal to a muted level

16. The method of Claim 14, further comprising determining a line quality value corresponding to the second communication signal, and wherein determining whether the second communication signal requires concealment comprises comparing the line quality value to a line quality threshold.

17. The method of Claim 14, further comprising: receiving a third communication signal from the base unit, the third communication signal received after receiving the second communication signal; determining whether the third communication signal requires concealment; and deactivating the step attenuator and the low pass filter if the second communication signal required concealment and the third communication signal does not require concealment .

18. The method of Claim 17, further comprising repeating the first communication if both the second and third communication signals require concealment.

19. The method of Claim 14, wherein activating the step attenuator comprises decreasing the amplitude of the repeated first communication signal to a predetermined amplitude level at a substantially constant rate.

20. The method of Claim 19, further comprising: receiving a third communication signal from the base unit after receiving the second communication signal; determining whether the third communication signal requires concealment; and deactivating the step attenuator if the second communication signal requires concealment and the third communication signal does not require concealment.