I. Field of the Invention
The present invention pertains generally to the field of wireless communications, and more specifically to providing an efficient method and apparatus for retransmitting data frames in a voice-over-data communication system.
II. Background
The field of wireless communications has many applications including cordless telephones, paging, wireless local loops, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. (As used herein, the term xe2x80x9ccellularxe2x80x9d systems encompasses both cellular and PCS frequencies.) Various over-the-air interfaces have been developed for such cellular telephone systems including frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). In particular, IS-95 and its derivatives, such as IS-95A, IS-95B (often referred to collectively herein as IS-95), ANSI J-STD-008, (IS-99, IS-657, IS-707, and others, are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. An exemplary cellular telephone system configured substantially in accordance with the use of the IS-95 standard is described in U.S. Pat. No. 5,103,459 entitled xe2x80x9cSystem and Method for Generating Signal Waveforms in a CDMA Cellular Telephone Systemxe2x80x9d, which is assigned to the assignee of the present invention and incorporated herein by reference. The aforesaid patent illustrates transmit, or forward-link, signal processing in a CDMA base station. Exemplary receive, or reverse-link, signal processing in a CDMA base station is described in U.S. application Ser. No. 08/987,172, filed Dec. 9, 1997, entitled MULTICHANNEL DEMODULATOR, which is assigned to the assignee of the present invention and incorporated herein by reference. In CDMA systems, over-the-air power control is a vital issue. An exemplary method of power control in a CDMA system is described in U.S. Pat. No. 5,056,109 entitled xe2x80x9cMethod and Apparatus for Controlling Transmission Power in A CDMA Cellular Mobile Telephone Systemxe2x80x9d which is assigned to the assignee of the present invention and incorporated herein by reference.
A primary benefit of using a CDMA over-the-air interface is that communications are conducted simultaneously over the same RF band. For example, each mobile subscriber unit (typically a cellular telephone) in a given cellular telephone system can communicate with the same base station by transmitting a reverse-link signal over the same 1.25 MHz of RF spectrum. Similarly, each base station in such a system can communicate with mobile units by transmitting a forward-link signal over another 1.25 MHz of RF spectrum.
Transmitting signals over the same RF spectrum provides various benefits including an increase in the frequency reuse of a cellular telephone system and the ability to conduct soft handoff between two or more base stations. Increased frequency reuse allows a greater number of calls to be conducted over a given amount of spectrum. Soft handoff is a robust method of transitioning a mobile unit between the coverage area of two or more base stations that involves simultaneously interfacing with two or more base stations. (In contrast, hard handoff involves terminating the interface with a first base station before establishing the interface with a second base station.) An exemplary method of performing soft handoff is described in U.S. Pat. No. 5,267,261 entitled xe2x80x9cMobile Station Assisted Soft Handoff in a CDMA Cellular Communications Systemxe2x80x9d which is assigned to the assignee of the present invention and incorporated herein by reference.
Under the IS-99 and IS-657 standards (referred to hereinafter collectively as IS-707), an IS-95-compliant communications system can provide both voice and data communications services. Data communications services allow digital data to be exchanged between a transmitter and one or more receivers over a wireless interface. Examples of the type of digital data typically transmitted using the IS-707 standard include computer files and electronic mail.
In accordance with both the IS-95 and IS-707 standards, the data exchanged between a transmitter and a receiver is processed in frames, or predefined time periods. To increase the likelihood that a frame will be successfully transmitted during a data transmission, IS-707 employs a radio link protocol (RLP) to track the frames transmitted successfully and to perform frame retransmission when a frame is not transmitted successfully. Retransmission is performed up to three times in IS-707, and it is the responsibility of the higher layer protocols to take additional steps to ensure that the frame was received successfully.
In order to track which frames have been received successfully, IS-707 uses an eight-bit sequence number to be included as a frame header in each frame transmitted. The sequence number is incremented for each frame from 0 to 256 and then reset back to zero. An unsuccessfully transmitted frame is detected when a frame with an out-of-order sequence number is received, or an error is detected using CRC checksum information or other error detection methods. Once an unsuccessfully received frame is detected, the receiver transmits a negative-acknowledgment message (NAK) to the transmit system that includes the sequence number of the frame that was not received. The transmit system then retransmits the frame including the sequence number as originally transmitted. If the retransmitted frame is not received successfully, a second retransmission request, consisting of two NAKs, is sent to the transmit system, this time requesting that the frame be transmitted twice. If the frame is still not received successfully, a third retransmission request, consisting of three NAKs, is sent to the transmit system, this time requesting that the frame be transmitted three times. If the frame is still not received successfully after the third retransmission request, no further retransmissions are requested, and the frame is ignored at the receiver for use in reconstructing the original data.
Recently, a need has arisen for transmitting voice information using the data protocols of IS-707. For example, in a secure communications system, voice information which is manipulated by encryption algorithms may be more easily transmitted using a data protocol. In such applications, it is desirable to maintain the use of existing data protocols so that no changes to existing infrastructure are necessary. However, problems occur when transmitting voice using a data protocol, due to the nature of voice characteristics.
One of the primary problems of transmitting audio information, such as voice, using a data protocol is the delays associated with frame retransmissions. Delays of more than a few hundred milliseconds in speech can result in unacceptable voice quality. When transmitting audio-based data, time delays are easily tolerated due to the non real-time nature of data. As a consequence, the protocols of IS-707 can afford to use the frame retransmission scheme as described above, which may result in transmission delays, or a latency period, of more than a few seconds. Such a latency period is unacceptable for transmitting voice information.
Another disadvantage of transmitting audio information using data protocols is that present mobile communication devices are constrained by the way in which they must respond to retransmission requests from, for example, a base station. Presently, a mobile communication device must follow frame retransmission requests from a base station in accordance with IS-707. For example, if frame number 11 was not received correctly by a base station, the base station will send a single NAK to the mobile communication device, requesting a single retransmission of frame 11. In accordance with IS-707, the mobile communication device must respond with a single retransmission of frame 11. If the re-transmitted frame is still not received by the base station, two NAKs are sent to the mobile communication device requesting that frame 11 be re-transmitted twice. The mobile communication device then responds by transmitting frame 11 twice. In certain situations, it would be desirable for the mobile communication device to respond to NAKs in a more intelligent manner.
A related disadvantage of using data protocols to transmit voice information is that base stations are likewise restrained in their ability to respond to NAKs sent by mobile communication devices. Base stations designed for use in IS-95 compliant communication systems can only send one copy of a requested frame for each NAK received. It would be desirable to allow a base station to also respond in a more intelligent manner to NAKs.
What is needed is a method and apparatus for re-transmitting frames received in error, while minimizing the problems caused by the time delays associated with the retransmissions. Furthermore, the retransmission method and apparatus should be backwards-compatible with existing infrastructure to avoid expensive upgrades to those systems.
The present invention is a method and apparatus for re-transmitting data frames containing audio information over an existing data network. The present invention resides entirely within a mobile communication device so that expensive modifications to existing infrastructure can be avoided.
When a frame is received in error by a first wireless communication device, a number of negative-acknowledgement messages (NAKs) are sent from the first wireless communication device to a transmitter which sent the frame, each of the negative acknowledgement messages transmitted in close succession to each other. The NAKs direct the transmitter, located within a second wireless communication device, generally a base station, to transmit multiple copies of the frame which was received in error, thereby increasing the probability that the frame will be correctly received within a short amount of time. Thus, the audio latency is reduced. Alternatively, when a frame is received in error by the first wireless communication device, the first wireless communication device ignores the error and does not transmit any NAKs. This degrades voice quality only slightly, but significantly reduces voice latency by avoiding the retransmission process altogether.
When a NAK is generated and transmitted by the second wireless communication device and received by the first wireless communication device, a number of copies of the frame identified by the NAK is transmitted by the first wireless communication device, each of the copies transmitted in close succession to each other. Again, the probability of a frame being successfully received is increased using multiple frame re-transmissions. Alternatively, when a single NAK is received by the first wireless communication device, the number of copies are transmitted in two stages. One copy of the identified frame is transmitted immediately upon receiving the single NAK. The remaining copies are transmitted only if and when no new data is available to be transmitted.