1. Field of the Invention
The present invention relates generally to a method and apparatus for transmitting preambles in a wireless communication system, and in particular, to a method and apparatus for transmitting/receiving preambles of a reverse access channel (or random access channel) in a broadband wireless communication system.
2. Description of the Related Art
With the rapid progress of communication technology, mobile communication systems have reached the stage of providing high-speed data services in which not only normal voice call services but also multimedia services are available. A packet data system supporting the high-speed data services can be roughly classified into a synchronous system mainly adopted, for example, in the United States, and an asynchronous system mainly adopted, for example, in Europe and Japan, and different standardization studies are being conducted by standard groups according to a use/nonuse of the synchronous system and the asynchronous system.
The synchronous packet data system proposed by Third Generation Partnership Project 2 (3GPP2), one of the standards groups, is evolving into Code Division Multiple Access (CDMA) 2000 1x currently in service, 1x EVolution Data Only (EV-DO) in which high-speed packet transmission is available, and EVolution of Data and Voice (EV-DV) capable of supporting both voice and packet services. In addition, the asynchronous packet data system proposed by Third Generation Partnership Project (3GPP), is also called Universal Mobile Telecommunication Systems (UMTS), and the Wideband Code Division Multiple Access (W-CDMA) system can be a typical example of the asynchronous packet data system.
Of the channels used in the W-CDMA system, a reverse common channel uses a Random Access CHannel (RACH), as is well known, and a description of the RACH will be made below.
FIG. 1 is a diagram illustrating a communication signal transmission/reception relationship of a reverse common channel in the conventional W-CDMA system.
In FIG. 1, reference numeral 151 indicates a reverse channel, and the reverse channel can be the RACH. Reference numeral 101 indicates a forward channel, and the forward channel can be an Access Preamble Acquisition Indication CHannel (AICH), also known as AP-AICH. In the case of FIG. 1, a mobile terminal has succeeded in preamble transmission by transmitting an Access Preamble (AP) to a base station over an RACH twice.
Referring to FIG. 1, after transmitting a preamble AP0 152 with a predetermined length over the RACH, a mobile terminal waits for a response from a base station over an AICH. If there is no response from the base station for a predetermined time ‘tp-p’ 156, the mobile terminal retransmits a preamble AP1 154, whose transmission power has increased by ΔP 155, to the base station. Upon detecting a preamble transmitted over the RACH, the base station transmits a signature 102 of the detected preamble to the mobile terminal over an AICH of a forward link in response to the preamble. The mobile terminal determines whether there is a signature signal received over an AICH in response to the transmitted preamble. Upon receipt of a signature signal over the AICH, the mobile terminal demodulates the received signature signal. If the signature responsive to the preamble over the AICH is detected as an ACKnowledgement signal (ACK), the mobile terminal sends a message over the RACH, determining that the base station has detected the preamble.
However, even though the mobile terminal has received the AICH signal 102 transmitted by the base station within a time ‘tp-ai’ 103 set in FIG. 1 after transmitting the preamble 152, if the mobile terminal fails to detect its transmitted signature from the AICH signal 102, the mobile terminal retransmits the preamble after the predetermined time ‘tp-p’ 156, determining that the base station has failed to receive the preamble. In this case, the mobile terminal, as described above, increases power of the preamble transmitted in the previous state by about ΔP(dB), retransmits the preamble with the increased power as shown by reference numeral 154, receives an AICH signal transmitted by the base station within a predetermined time, and detects a signal that uses its transmitted signature.
Upon failure to receive an AICH signal using its transmitted signature from the base station after transmitting the preamble, the mobile terminal delays the set time and then repeatedly performs the above operation while increasing transmission power of the preamble. Upon receipt of the signal using its transmitted signature in the process of transmitting a preamble and receiving an AICH signal as stated above, the mobile terminal delays a set time ‘tp-msg’ 158, and then transmits a message 157 of a reverse common channel with the power corresponding to the preamble.
FIG. 2 is a diagram briefly illustrating a reverse access probe. In FIG. 2, reference numeral 201 indicates a preamble, which is the reverse access probe. A mobile terminal transmits a randomly selected signature as a preamble, and control information other than this is not transmitted. All messages can be transmitted after the mobile terminal receives, over an AICH, an ACK signal indicating that a base station has detected the preamble (or signature), i.e. reverse access probe, transmitted by the mobile terminal.
FIG. 3 is a diagram illustrating a signal transmission/reception relationship of a reverse/forward common channel proposed in 3GPP Long-Term Evolution (LTE), and FIG. 4 is a diagram illustrating exemplary reverse RACH allocation, which is taken into consideration in 3GPP LTE. The LTE system recently proposed in 3GPP, which is the standard group for the asynchronous mobile communication system, uses, as a transmission scheme, Orthogonal Frequency Division Multiplexing (OFDM) in a forward link and Single Carrier-Frequency Division Multiple Access (SC-FDMA) in a reverse link.
In the exemplary reverse RACH allocation of FIG. 4, the horizontal axis represents the time domain and the vertical axis represents the frequency domain. In FIG. 4, one SC-FDMA slot is the one reverse RACH slot 401. It is shown in FIG. 4 that in the defined RACH slot 401, an RACH burst 402 is allocated in a predetermined frequency domain before being transmitted. Even for the RACH of the LTE system, like that of the W-CDMA system, if a base station detects a preamble after a mobile terminal transmits the preamble, the base station sends a response to the preamble to the mobile terminal so that the mobile terminal can transmit a data message. Upon receipt of the response transmitted by the base station, the mobile terminal can perform a series of processes for transmitting the data message. However, in the LTE system, because a transmission scheme of its physical channel is not a CDMA scheme, there is a need for design of an appropriate transmission scheme.
Returning to the description of FIG. 3, in the RACH transmission scheme currently discussed in the LTE system, after transmitting a preamble 352 with a predetermined length over a reverse RACH 351, the mobile terminal waits for a response from the base station. If there is no response from the base station for a predetermined time ‘tp-p’ 356, the mobile terminal retransmits a preamble 353, transmission power of which has increased by ΔP(dB) 354, to the base station. Thereafter, upon detecting a preamble over the RACH, the base station transmits, as a response message to the preamble, a forward AICH 301 such as reference numeral 302 of FIG. 3 within a predetermined time ‘tp-ai’ 303. Upon receipt of the response message, the mobile terminal can transmit its desired transmission data using a message shown by reference numeral 353 after a predetermined time ‘tp-msg’ 357. The response message that the base station sends in response to the preamble is called an access grant message.
In FIG. 3, the mobile terminal determines whether the access grant message responsive to the preamble is received from the base station. Receipt/non-receipt of the access grant message can be determined using a signature corresponding to the preamble of the mobile terminal and/or IDentifier (ID) information of the corresponding mobile terminal. Upon detecting the access grant message, the mobile terminal sends a reverse message in, for example, an SC-FDMA scheme, determining that the base station has detected its transmitted preamble. The mobile terminal can adjust a transmission time of the message transmitted in an SC-FDMA scheme, depending on time correction information from control information received over the access grant message.
However, if the mobile terminal fails to detect a signal using a signature responsive to the preamble as the mobile terminal fails to receive the access grant message from the base station within a predetermined time ‘tp-ai’ after transmitting the preamble 352, the mobile terminal retransmits the preamble after a predetermined time, determining that the base station has failed to detect the preamble. In this case, the mobile terminal increases power of the preamble transmitted in the previous state by ΔP(dB) 354, and retransmits the preamble with the increased power. Thereafter, if the base station transmits an access grant message as the base station normally receives the preamble, the mobile terminal receives the access grant message transmitted by the base station within a predetermined time, and detects from the received access grant message a signal that uses a signature responsive to the preamble and/or mobile terminal's ID information.
After transmitting the preamble, if the mobile terminal fails to receive an access grant message using its transmitted signature from the base station, the mobile terminal delays a predetermined time and then repeatedly performs the above operation while increasing the transmission power of the preamble. If the mobile terminal receives a signal using its transmitted signature in the process of receiving an access grant message from the base station after transmitting the preamble, mobile terminal delays a predetermined time ‘tp-msg’ 357 as shown in FIG. 3 and then transmits a message on a reverse RACH with the power corresponding to the preamble.
In the LTE system, the access grant message that the base station receiving the preamble transmits to the mobile terminal can use a coded message transmitted over a particular frequency/time interval of the OFDM system. In addition, the access grant message can include therein time correction information of RACH, ID of RACH, channel allocation information for the reverse channel over which the mobile terminal transmits data, and the like.
A structure of the RACH preamble now under discussion in the LTE system is shown in FIG. 5. A basic unit of reverse transmission is a sub-frame 501 and has a 0.5-ms length. A preamble 510 is transmitted in one sub-frame 501, and time margins are provided before and after the preamble taking into account the initial timing synchronization of an uplink, round-trip delay time, and maximum delay spread time of the channel, i.e., the preamble 510, to prevent interference to/from the previous symbol, is transmitted for a TP time 530 after a lapse of the maximum delay spread time Tds 520 of the channel beginning from a start point of the sub-frame.
In addition, to prevent uncertainty of the timing synchronization of the uplink and prevent interference to/from the next symbol, the transmission of the preamble terminates in advance of an end point of the sub-frame 501 by a sum of the round-trip delay time TGP and the maximum delay spread time Tds of the channel as shown by reference numeral 540. The round-trip delay time TGP is a delay time required when the mobile station receives a signal transmitted by the base station and the base station receives a signal that the mobile terminal has transmitted after acquiring synchronization, and the round-trip delay time TGP is about 6.7 μsec/km.
However, there is a limitation in extending the maximum supportable cell radius with the preamble structure of FIG. 5. This is because even though the maximum transmission power of the mobile terminal is limited, the maximum cell radius supportable by the preamble is limited to the maximum transmission energy used for the preamble. In addition, though the maximum transmission energy of the preamble is proportional to the length of the preamble, the round-trip delay time 540 increases with the cell radius, causing a decrease in the length of the preamble i.e., because there is a trade-off relationship between the cell radius and the preamble length, there is a limitation in extending the maximum cell radius with the preamble structure of FIG. 5.
As described above, because the round-trip delay time is about 6.7 μsec/km, an increase in the cell radius by 1 km causes a decrease in the preamble length by 6.7 μsec, thereby reducing the preamble energy. The reduction in the preamble energy may reduce the preamble detection capability at the base station. Therefore, because there is a limitation in extending the cell radius with the preamble structure of FIG. 5, there is a demand for a preamble transmission scheme capable of preventing a reduction in the preamble detection performance while increasing the cell radius.