WO2005022792A2

WO2005022792A2 - Method of constructing wireless network for orthogonal frequency division multiplex and terminal employing ofdma method

Info

Publication number: WO2005022792A2
Application number: PCT/KR2004/002073
Authority: WO
Inventors: Choong-Il Yeh; Hyoung-Soo Lim; Yu-Ro Lee; Dong-Seung Kwon; Seung-Ku Hwang
Original assignee: Electronics And Telecommunications Research Institute; Samsung Electronics Co., Ltd.; Kt Corporation; Sk Telecom Co., Ltd.; Ktfreetel Co., Ltd.; Hanaro Telecom, Inc.
Priority date: 2003-08-27
Filing date: 2004-08-18
Publication date: 2005-03-10
Also published as: KR20050023192A; WO2005022792A3; KR100505968B1

Abstract

Disclosed are a method of constructing a wireless network in an OFDM system using a single carrier and a terminal employing an OFDM method. The terminal discriminates a wireless network using multiple carriers from a wireless network using a single carrier through a downlink preamble, and acquires frame synchronization within a short period of time. In addition, the terminal determines a position of a CCS used by a cell/sector to which the terminal belongs. Furthermore, the invention proposes a pilot allocation method that can be applied to a wireless network using a single carrier, which obtains frequency diversity effects while effectively using resources. In the case where a wireless network operator uses N channels in N divided FFT domains, the actual number of channels is MN.

Description

Description METHOD OF CONSTRUCTING WIRELESS NETWORK FOR ORTHOGONAL FREQUENCY DIVISION MULTIPLEX AND TERMINAL EMPLOYING OFDMA METHOD Technical Field

[1] The present invention relates to a method of constructing a wireless network for orthogonal frequency division multiplexing (OFDM), and a terminal employing an OFDM method. More specifically, the present invention relates to a method of constructing a wireless network for OFDM using a single carrier, which proposes a frame structure, modulation and demodulation methods, and a terminal employing an OFDM method. Background Art

[2] A broadband radio transmission system using a single carrier employs a complicated equalizer in order to remove inter-symbol interference due to multi-path propagation. Orthogonal frequency division multiplexing (OFDM) is widely used in the broadband radio transmission system because it can effectively remove the inter- symbol interference using a simple equalizer having a single tap.

[3] OFDM/TDMA that allocates a broadband to a single user for a predetermined period of time cannot be applied to a macro-cell system, because it requires a large transmission power. Furthermore, OFDM/TDMA allocates a wide frequency band at once so that the performance of a system that employs OFDM/TDMA is deteriorated due to waste of resources when there is a small quantity of data to be transmitted at a time.

[4] The OFDM technique can be operated with a small power because it divides a broadband frequency resource and allocates the divided frequencies to users for a predetermined period of time. Thus, OFDM can be applied to the macro-cell system. Furthermore, the OFDM method can effectively deal with both a large quantity of data and a small quantity of data because its can minutely divide a frequency resource. Accordingly, OFDM can improve the system performance.

[5] FIG. 1 shows a frame structure using the OFDM method. Referring to FIG. 1, the frame includes a downlink, TTG (Tx/Rx Transition), an uplink, and RTG (Rx/Tx Transition). The downlink includes a common control slot (CCS) and a plurality of downlink data slots (DDSs). Here, the CCS is a slot that transmits system-related common information and the DDS is a slot that transmits user data.

[6] The uplink includes a pre-amble for channel estimation, uplink data slots (UDSs) for transmitting user data, and a pre-amble for uplink synchronization acquisition.

[7] The downlink divides frequency and time resources and multiplexes various slots, and the uplink enables user multiple access.

[8] The OFDM/TDMA and OFDM methods use broadband channels and they have been used in a single cell environment. To use the OFDM/TDMA and OFDM methods in a multi-cell environment, however, a large number of channels are needed in order to reduce cochannel interference. This requires a very wide frequency band. Disclosure of Invention Technical Problem

[9] It is an advantage of the present invention to provide a method of constructing a wireless network for OFDM, which constructs a wireless network using a single carrier without a waste of resources, provides a frame structure capable of improving the entire system performance while using OFDM without having uplink synchronization, allows a wireless network operator to freely select a wireless network using a single carrier or a wireless network using multiple carriers, and proposes a frame structure easily detected by a terminal and modulation and demodulation methods, and a terminal employing the OFDM method. Technical Solution

[10] When a wireless network is constructed using a single carrier, adjacent cells share different sub-channels because all the cells use the same carrier frequency in c).

[11] Preferably, the method further comprises e) distributing pilots over the entire frequency domain of a channel to estimate the channel when the sub-channels are uniformly allocated to the plurality of cells/sectors.

[12] When a wireless network is constructed using a single carrier, each of the cells/ sectors carries out channel estimation only for a frequency domain allocated thereto.

[13] In another aspect of the present invention, a method of constructing a wireless network for OFDM having a frame structure that includes a downlink including a common control slot (CCS) and multiple downlink data slots (DDSs) and an uplink including uplink and downlink preambles and a user data slot (UDS), comprises a) constructing a preamble in the downlink using repeated multiple OFDM symbols shorter than an OFDM symbol for data transmission when a wireless network is constructed using multiple carriers; b) constructing preambles in downlinks that share an FFT domain using repeated multiple OFDM symbols that are shorter than the OFDM symbol for data transmission and a section during which no signal is transmitted when a wireless network is constructed using a single carrier; and c) a terminal discriminating the wireless network using multiple carriers from the wireless network using a single carrier using the preamble of the downlink and acquiring base station frame synchronization using the preamble of the downlink.

[14] Preferably, the length of the shorter OFDM symbols is identical to a cyclic prefix (CP) length of the OFDM symbol for data transmission.

[15] Preferably, the terminal becomes aware of a location of a CCS used by a base station to which the terminal belongs through peak detection because the CCS that is a slot for common channel transmission is located at an appointed position in cells/ sectors that divide the entire frequency domain of a channel into M regions and use them in the case of construction of a wireless network using a single carrier.

[16] In another aspect of the present invention, a terminal employing an OFDM method comprises a FIFO that receives sequentially input signals and outputs the input signals in the receiving sequence; a correlator that receives a preamble pattern corresponding thereto among downlink preamble patterns including short OFDM symbols having different patterns when a wireless network using multiple carriers and a wireless network using a single carrier transmit the downlink preamble patterns, and correlates the received preamble pattern with the signals output from the FIFO; a peak detector that detects an output peak of the correlator to discriminate the wireless network using multiple carriers from the wireless network using a single carrier; and a reception power estimator that estimates a reception power when the peak detector detects a peak.

[17] The terminal further comprises a pattern selector that selects one of the preamble patterns and outputs the selected preamble pattern to the correlator, the pattern selector being located before the correlator.

[18] The pattern selector outputs a peak value to the correlator when the same preamble pattern as the pattern of the pattern selector is input to the pattern selector.

[19] The pattern selector selects an OFDM symbol having superior autocorrelation and cross correlation characteristics from first and second OFDM symbols, the first OFDM symbol constructing a preamble using repeated multiple OFDM symbols shorter than the OFDM symbol for data transmission in the downlink in the event of construction of a wireless network using multiple carriers, the second OFDM symbol constructing a preamble using repeated multiple shorter OFDM symbols and a section in which no OFDM symbol is transmitted for a period during which an adjacent cell transmits a preamble in the event of construction of a wireless network using a single carrier.

[20] Preferably, the terminal determines the wireless network to be a wireless network using multiple carriers when the peak detector detects a peak with respect to the first OFDM symbol and determines the wireless network to be a wireless network using a single carrier when the peak detector detects a peak with respect to the second OFDM symbol.

[21] The terminal acquires frame synchronization when the peak detector detects a peak.

[22] The terminal becomes aware of a location of a CCS used by a cell/sector to which the terminal belongs when the peak detector detects the peak with respect to the second OFDM symbol.

[23] The peak detector discriminates a cell and a sector from each other in the case of a wireless network using a single carrier.

[24] The terminal further comprises a sealer that scales the value output from the correlator by a reception power and transmits the resultant value to the peak detector.

[25] In another aspect of the present invention, a method of constructing a wireless network for OFDM having a frame structure that includes a downlink including a CCS and multiple DDSs and an uplink including a preamble for channel estimation and a UDS, comprises a) OFDM symbols that construct the downlink forming a preamble using a CP having a first length and OFDM symbols that construct the uplink forming a preamble using a CP having a length longer than the first length, thereby constructing a frame; and b) carrying out channel estimation/compensation using the preamble of the uplink without having uplink synchronization.

[26] The CP length of the OFDM symbols of the downlink is determined by a delay spread, and the CP length of the OFDM symbols of the uplink is determined by a delay spread and a round trip delay.

[27] The step a) uses the first OFDM symbol of symbols of the uplink as the preamble, uses other OFDM symbols as data symbols, and divides the entire frequency domain into a plurality of frequency domains to use them for random access and for transmission of acknowledge/non- acknowledge and short messages. Brief Description of the Drawings

[28] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the de- scription, serve to explain the principles of the invention:

[29] FIG. 1 shows a frame structure using an OFDM method;

[30] FIG. 2 shows a procedure of generating sub-channels in OFDM;

[31] FIG. 3 shows network constructions according to system cell plans when a plurality of carriers are used;

[32] FIG. 4 shows a conventional pilot allocation procedure for channel estimation in OFDM;

[33] FIG. 5 shows a problem generated when a common pilot is allocated to a cell/ sector when a wireless network is constructed using a single carrier;

[34] FIG. 6 shows a procedure of allocating pilots to cells/sectors after generating subchannels in an OFDM wireless network using a single carrier;

[35] FIG. 7 shows a pilot allocating procedure in a method of constructing a wireless network according to a first embodiment of the present invention;

[36] FIG. 8 shows a preamble pattern of a downlink in a method of constructing a wireless network for OFDM according to a second embodiment of the present invention;

[37] FIG. 9 shows a hardware structure of a terminal that employs an OFDM method according to a third embodiment of the present invention;

[38] FIG. 10 shows an example of a frame structure in which an uplink and a downlink have different cyclic prefix (CP) lengths in a method of constructing a wireless network for OFDM according to a fourth embodiment of the present invention; and

[39] FIG. 11 shows an effective frame structure utilizing a method that controls CP lengths of an uplink and a downlink to be different from each other so as to not require uplink synchronization in a method of constructing a wireless network for OFDM according to the fourth embodiment of the present invention. Best Mode for Carrying Out the Invention

[40] In the following detailed description, only the preferred embodiment of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventor(s) of carrying out the invention. As will be realized, the invention is capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.

[41] Compared to OFDM/TDMA, OFDM has advantages of extending coverage or improving throughput according to power concentration and effectively using frequency resources according to sub-channels to improve throughput. [42] According to the OFDM method, a cell plan is facilitated because adjacent cells or sectors share subcarriers and a wireless network can be constructed using a single carrier. For example, in the case of OFDM having N physical channels, when subcarriers are divided into M groups and allocated to adjacent cells or sectors, the number of available channels is increased to NxM.

[43] The OFDM method can obtain the following advantages as the number of channels is increased as described above.

[44] Firstly, a difficulty in a cell plan, which is caused by variables such as the to pography, radio environments, and so on, can be overcome. Secondly, a maximum data rate provided to subscribers is high because the entire channel band width can be allocated to the subscribers. Thirdly, adjacent cells or sectors can flexibly share radio resources so that cells or sectors that require a small traffic capacity can easily provide the radio resources to cells or sectors that require a large traffic capacity. Fourthly, hand-off is carried out without changing an RF frequency because a single channel is used. Thus, the hand-off is rapidly performed and soft hand-off is also possible.

[45] FIG. 2 shows a procedure of generating sub-channels in OFDM according to an embodiment of the present invention. To share a single channel by a plurality of cells/ sectors in an OFDM system, sub-channels are generated and allocated to the cells/ sectors, as shown in FIG. 2. Specifically, subcarriers are allocated to a pilot in order to transmit the pilot because the pilot is commonly used in a downlink. Then, available subcarriers other than the subcarriers allocated to the pilot are grouped to obtain L subcarrier groups each of which has N adjacent subcarriers. An arbitrary subcarrier is extracted from each of the subcarrier groups to form a sub-channel such that L subchannels are generated. By doing so, the subcarriers constructing the sub-channels are distributed in the entire channel frequency band. Accordingly, frequency diversity effect can be obtained.

[46] An OFDM wireless network constructed using multiple carriers will be called a multi-frequency network (MFN), and an OFDM wireless network constructed using a single carrier will be called a single frequency network (SFN).

[47] FIG. 3 shows network constructions based on system cell plans when multiple carriers are used. In the case of SFN, all the cells use the same carrier frequency so that adjacent cells should share sub-channels as shown in FIG. 3. In FIG. 3, the same subchannel is allocated to cells using the same color. In the case of MFN, adjacent cells do not use the same carrier frequency in order to mitigate co-channel interference.

[48] In FIG. 3, colors of the cells represent different carrier frequencies in the case of MFN. In the case of SFN, colors of cells mean divided subcarriers. That is, the MFN is constructed using three carrier frequencies and the SFN is constructed using a single carrier frequency. In the case of SFN, subcarriers are grouped to form sub-channels and the sub-channels are divided into three groups and allocated to cells/sectors.

[49] A DVB IEEE 802.16a system employing the OFDM method does not use a preamble, that is, an OFDM symbol in which all of subcarriers are used as pilots, but uses a pilot having a unique pattern in order to acquire frame synchronization. At this time, a technique of acquiring frame synchronization using the pilot having a unique pattern is suitable for a circuit-switched connection method because a frame synchronization acquisition speed of the technique is low. On the other hand, a technique of acquiring frame synchronization using the preamble is suitable for a packet- switched connection method because its frame synchronization acquisition speed is high. Accordingly, a WLAN system employing the OFDM method such as IEEE 802.11a and HiperLAN Type 2 uses the preamble in the downlink for frame synchronization acquisition.

[50] FIG. 4 shows a conventional pilot allocation procedure for channel estimation in an OFDM system, and FIG. 5 shows a problem generated when a common pilot is allocated to cells/sectors in the event of constructing a wireless network using a single carrier. FIG. 6 shows a procedure of allocating pilots to cells/sectors after generating sub-channels in an OFDM wireless network using a single carrier.

[51] In the case of SFN, when sub-channels are generated using a conventional method without respect to effective utilization of resources, as shown in FIG. 6, pilots are distributed in the entire frequency domain of a channel to estimate the channel so that a large number of pilots are needed. Thus, the resources cannot be effectively utilized.

[52] The SFN wastes the resources since cells/sectors that share sub-channels cannot commonly use pilots. This is because that a certain channel cannot be estimated when the pilots are commonly allocated to the cells/sectors, as shown in FIG. 5.

[53] As described above, when the sub-channels are generated using a conventional method without respect to effective utilization of the resources, channel estimation should be carried out for all of subcarriers even though only a part of the subcarriers is actually used. Accordingly, a large amount of frequency resources are required for pilot allocation.

[54] When M cells/sectors share subcarriers allocated to one channel in the case of SFN, the amount of frequency resources required for pilot allocation is N times the amount of frequency resources required for the MFN. [55] In the case that three cells/sectors share sub-channels in the SFN, colors represent pilots allocated to the cells/sectors, as shown in FIG. 6.

[56] A method of constructing a wireless network for OFDM according to a first embodiment of the present invention will now be explained with reference to FIG. 7. FIG. 7 shows a pilot allocation procedure in the method of constructing a wireless network according to the first embodiment of the present invention.

[57] As shown in FIG. 7, to allocate pilots without wasting frequency resources in an SFN, subcarriers are first grouped to form L subcarrier groups each of which has N adjacent subcarriers. FIG. 7 shows an example of constructing subcarrier groups when N=12.

[58] Subsequently, the subcarrier groups are uniformly allocated to cells/sectors that will share subcarriers constructing one channel such that frequency diversity can be obtained. FIG. 7 shows an example of allocating K subcarrier groups to cells/sectors in an SFN in which three cells/sectors share an FFT domain. Specifically, subcarrier group numbers 1, 3, 6, 10, ..., L-7, L-4, and L are allocated to the first cell/sector of the three cells/sectors, and subcarrier group numbers 2, 5, 8,..., L-9, L-8, and L-2 are allocated to the second cell/sector. In addition, subcarrier group numbers 4, 7, 9, ..., L- 3, and L-l are allocated to the last cell/sector.

[59] Sub-channels are constructed using L subcarriers that are extracted from the subcarriers allocated to the cells/sectors using a method capable of obtaining frequency diversity.

[60] When the sub-channels are generated as described above, the cells/sectors of the SFN, which share the FFT domain, need not carry out channel estimation for frequency domains that are not allocated thereto. Thus, the number of cells/sector can be reduced to 1/M (M is the number of the cells/sectors) to prevent waste of frequency resources in the event of pilot allocation. Furthermore, frequency diversity effects can be sufficiently utilized because the entire FFT domain is used. Moreover, the SFN can flexibly allocate frequency resources to the cells/sectors to improve throughput of the entire system.

[61] A method of constructing a wireless network for OFDM according to a second embodiment of the present invention will now be described with reference to FIG. 8. FIG. 8 shows preamble patterns of a downlink in the method of constructing a wireless network for OFDM system according to the second embodiment of the present invention.

[62] As shown in FIG. 8, the preambles of the downlink in the method of constructing a wireless network for OFDM system allow a wireless network operator to select one of the SFN and MFN when the operator constructs a wireless network. Here, a terminal is required to discriminate the MFN from the SFN using the preambles of the downlink, attain base station frame synchronization within a short period of time, and determine a location of a CCS used by a base station to which the terminal belongs.

[63] The preambles of the downlink use short OFDM symbols, as shown in FIG. 8. The OFDM symbols have a length that is shorter than the length of an OFDM symbol for data transmission, and is usually identical to a CP length of the OFDM symbol for data transmission.

[64] In FIG. 8, A, B, C, and D represent different short OFDM symbols. When it is assumed that the CP length of the OFDM symbol for data transmission is 1/16 of an FFT length, the length of each of the short OFDM symbols becomes 1/16 of the length of the OFDM symbol for data transmission. In FIG. 8, X denotes sections during which OFDM symbols are not transmitted.

[65] The SFN discriminates a cell and a sector from each other using different preamble patterns located at different positions. The preambles of the downlink are constructed using repeated multiple OFDM symbols that are shorter than an OFDM symbol for data transmission and sections during which no signal is transmitted when other cells/ sectors transmit preambles.

[66] There are (the number of divided FFT domains)+l preambles of the downlink.

[67] FIG. 9 shows a hardware structure of a terminal employing an OFDM method according to a third embodiment of the present invention. Referring to FIG. 9, the terminal includes a pattern selector 11, a FIFO 12, a correlator 13, a peak detector 14, a reception power estimator 15, and a sealer 16.

[68] The pattern selector 11 selects one of preamble patterns of the downlink, as shown in FIG. 8. Here, the pattern selector 11 selects a preamble pattern having superior correlation and cross correlation characteristics among the patterns A, B, C, and D of the short OFDM symbols. The pattern selector 11 is used for sharing hardware resources and outputs a peak value when an input signal is identical to the pattern of the pattern selector 11. If there are as many correlators as downlink preamble patterns, the pattern selector 11 is not needed.

[69] The FIFO 12 sequentially receives input signals and outputs the input signals to the correlator 13 in the receiving sequence. The correlator 13 correlates the downlink preamble patterns with the input signal. The peak detector 14 detects a peak of the output value of the correlator 13. At this time, the terminal recognizes the wireless network as an MFN when the peak detector 14 detects a peak with respect to the OFDM symbol of pattern A and recognizes the wireless network as an SFN when the peak detector 14 detects a peak with respect to the OFDM symbol of the pattern B, C, or D.

[70] The preamble of the pattern A includes repeated short OFDM symbols, and the preambles of the patterns B, C, and D include repeated short OFDM symbols and sections during which no signal is transmitted.

[71] When it is assumed that three cells/sectors share the FFT domain, three CCSs are needed. A location of each of the CCSs is appointed. Accordingly, the terminal becαnes aware of a location of a CCS used by a cell/sector to which the terminal belongs using a peak detected by the peak detector 14.

[72] The reception power estimator 15 estimates a reception power when the peak detector 14 detects the peak, and outputs the estimated power to an automatic gain controller and the sealer 16. The sealer 16 divides the value output frαn the correlator 13 by the reception power value output frαn the reception power estimator 15 and transmits the resultant value to the peak detector 14.

[73] The terminal constructed as above can discriminate the MFN frαn the SNF, acquire frame synchronization within a short period of time, and autαnatically recognize a position of the CCS of the cell/sector to which the terminal belongs.

[74] A frame structure of an uplink using different CP lengths in the method of constructing a wireless network for OFDM according to a fourth embodiment of the present invention will now be explained.

[75] When the OFDM method is applied to the uplink, uplink packets transmitted frαn a plurality of users who use the same FFT dαnain must arrive at a base station at the same time. Thus, uplink synchronization becαnes important in this case.

[76] Referring to FIG. 1, the conventional system allocates frequency and time resources such that terminals can transmit preambles for uplink synchronization. A base station must notify the terminals of an uplink preamble measurement result using a downlink. Thus, the conventional system should designate additional resource allocation and procedure. Here, when the preamble of the uplink is used for uplink synchronization, the following problems are generated.

[77] Firstly, a preamble for uplink synchronization is generated by inverse- fast-Fourier-transforming a PN sequence in a frequency dαnain, and a receiver detects a synchronization error in the uplink using a correlator in the frequency domain to notify a terminal of the error. If at least two terminals simultaneously transmit preambles for uplink synchronization using the same PN sequence and a base station detects the preambles to notify the terminals of the detected preambles, the same uplink is allocated to a plurality of subscribers simultaneously which results in collision.

[78] Secondly, perfect power control is impossible when the preamble is transmitted for the purpose of uplink synchronization. Furthermore, the correlator is operated in the frequency dαnain while channel estimation is not carried out so that the number of PN sequences that can be simultaneously used is restricted.

[79] Thirdly, since the preamble for uplink synchronization is transmitted as codes without having information, a new uplink slot should be allocated to a corresponding terminal for transmission of uplink information even if the base station successively detects the preamble.

[80] Fourthly, the preamble for uplink synchronization is transmitted while synchronization is not acquired so that the preamble interferes with a data slot, to thereby deteriorate the system performance.

[81] Fifthly, correlators of as many as the number of codes of the preamble for uplink synchronization are needed which results in cαnplicated hardware.

[82] FIG. 10 shows an example of a frame structure in which the uplink and downlink have different CP lengths in the method of constructing a wireless network for OFDM according to a fourth embodiment of the present invention. FIG. 11 shows an effective frame structure utilizing a method that controls CP lengths of an uplink and a downlink to be different frαn each other so as not to require uplink synchronization in a method of constructing a wireless network for OFDM according to the fourth embodiment of the present invention.

[83] Referring to FIG. 10, in the construction of the frame structure of the uplink, OFDM symbols for constructing the downlink use a short CP. Here, the CP length is determined by a delay spread. OFDM symbols constructing the uplink use a CP longer than the CP of the downlink. Here, the CP length depends on both of a delay spread and a round trip delay.

[84] When channel estimation/cαnpensation is carried out using the uplink preamble, as shown in FIG. 10, OFDM is possible in the uplink without having a separate uplink synchronization procedure. An OFDM symbol timing error in the demodulation of the uplink is removed through channel estimation and correction using the uplink preamble.

[85] In general, the length of the OFDM symbol is very long in the OFDM system. Since the round trip delay is 6.6 sec in a cell having a diameter of 2Km, the CP length of the OFDM symbol is increased 3.3% in comparison with the FFT length when a symbol length of 200 sec is considered.

[86] As shown in FIG. 11, small slots are generated using the last two symbols of the uplink and used for randan access and transmission of acknowledge/nonacknowledge for downlink slots and short messages.

[87] The above-described frame structure can generate a large number of small slots in the uplink so that the probability of generating collisions is very low. Furthermore, the frame structure can create a sufficiently large number of small slots and allocate the small slots to cells/sectors of an SFN when the frame structure is applied to the SFN.

[88] Moreover, since information is transmitted from the beginning, one transmission and reception procedure is omitted in the event of resource allocation when uplink synchronization is acquired using a PN code and hardware is simplified. Furthermore, the above-described frame structure can improve the system performance when the OFDM symbol is long and coverage is small.

[89] While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

[90] According to the method of constructing a wireless network for an OFDM system and the terminal employing an OFDM method according to the present invention, the terminal can discriminate a wireless network using multiple carriers frαn a wireless network using a single carrier through a downlink preamble and acquire frame synchronization within a short period of time. In addition, the terminal can determine a position of a CCS used by a cell/sector to which the terminal belongs.

[91] Furthermore, the present invention proposes a pilot allocation method that can be applied to a wireless network using a single carrier, which obtains frequency diversity effects while effectively using resources. In the case where a wireless network operator uses N channels in N divided FFT dαnains, the actual number of channels is MN. Accordingly, the wireless network operator can easily carry out a cell plan and freely construct a wireless network. [92] Moreover, an uplink and a downlink have different CP lengths so that uplink synchronization is not needed. Furthermore, the frame structure in which small slots are constructed in an arbitrary part of the uplink can be effectively used for randan access and transmission of short messages, and improve the system performance

Claims

[1] A method of constructing a wireless network for OFDM having a frame structure including a downlink and an uplink, cαnprising: a) allocating subcarriers to a pilot that is cαnmonly used in the uplink in order to transmit the pilot; b) grouping subcarriers other than the subcarriers allocated to the pilot, to generate L subcarrier groups each of which has N adjacent subcarriers (N>0); c) uniformly allocating the subcarrier groups to a plurality of cells/sector; and d) extracting one arbitrary subcarrier frαn the subcarriers of each of the subcarrier groups, to generate L/M sub-channels (M is the number of cells/ sectors).

[2] The method as claimed in claim 1, wherein, when a wireless network is constructed using a single carrier, adjacent cells share different sub-channels because all the cells use the same carrier frequency in c).

[3] The method as claimed in claim 1, further cαnprising e) distributing pilots over the entire frequency dαnain of a channel to estimate the channel when the subchannels are uniformly allocated to the plurality of cells/sectors.

[4] The method as claimed in claim 3, wherein, when a wireless network is constructed using a single carrier, each of the cells/sectors carries out channel estimation only for a frequency dαnain allocated thereto.

[5] A method of constructing a wireless network for OFDM having a frame structure that includes a downlink including a cαnmon control slot (CCS) and multiple downlink data slots (DDSs) and an uplink including uplink and downlink preambles and a user data slot (UDS), cαnprising: a) constructing a preamble in the downlink using repeated multiple OFDM symbols that are shorter than an OFDM symbol for data transmission when a wireless network is constructed using multiple carriers; b) constructing preambles in downlinks that share an FFT dαnain using repeated multiple OFDM symbols that are shorter than the OFDM symbol for data transmission and a section during which no signal is transmitted when a wireless network is constructed using a single carrier; and c) a terminal discriminating the wireless network using multiple carriers from the wireless network using a single carrier using the preamble of the downlink and acquiring base station frame synchronization using the preamble of the downlink.

[6] The method as claimed in claim 5, wherein the length of the shorter OFDM symbols is identical to a cyclic prefix (CP) length of the OFDM symbol for data transmission.

[7] The method as claimed in claim 5, wherein, in c), the terminal determines a location of a CCS used by a base station to which the terminal belongs through peak detection because the CCS that is a slot for cαnmon channel transmission is located at an appointed position in cells/sectors that divide the entire frequency domain of a channel into M regions and use them in the case of construction of a wireless network using a single carrier.

[8] A terminal employing an OFDM method, comprising: a FIFO that receives sequentially input signals and outputs the input signals in the receiving sequence; a correlator that receives a preamble pattern corresponding thereto among downlink preamble patterns including short OFDM symbols having different patterns when a wireless network using multiple carriers and a wireless network using a single carrier transmit the downlink preamble patterns, and correlates the received preamble pattern with the signals output from the FIFO; a peak detector that detects an output peak of the correlator to discriminate the wireless network using multiple carriers from the wireless network using a single carrier; and a reception power estimator that estimates a reception power when the peak detector detects a peak.

[9] The terminal as claimed in claim 8, further comprising a pattern selector that selects one of the preamble patterns and outputs the selected preamble pattern to the correlator, the pattern selector being located before the correlator.

[10] The terminal as claimed in claim 9, wherein the pattern selector outputs a peak value to the correlator when the same preamble pattern as the pattern of the pattern selector is input to the pattern selector.

[11] The terminal as claimed in claim 9, wherein the pattern selector selects an OFDM symbol having superior autocorrelation and cross correlation characteristics frαn first and second OFDM symbols, the first OFDM symbol constructing a preamble using repeated multiple OFDM symbols that are shorter than the OFDM symbol for data transmission in the downlink in the event of construction of a wireless network using multiple carriers, the second OFDM symbol constructing a preamble using repeated multiple shorter OFDM symbols and a section in which no OFDM symbol is transmitted for a period during which an adjacent cell transmits a preamble in the event of construction of a wireless network using a single carrier.

[12] The terminal as claimed in claim 11, wherein the terminal determines the wireless network to be a wireless network using multiple carriers when the peak detector detects a peak with respect to the first OFDM symbol and determines the wireless network to be a wireless network using a single carrier when the peak detector detects a peak with respect to the second OFDM symbol.

[13] The terminal as claimed in claim 12, wherein the terminal acquires frame synchronization when the peak detector detects a peak.

[14] The terminal as claimed in claim 12, wherein the terminal determines a location of a CCS used by a cell/sector to which the terminal belongs when the peak detector detects the peak with respect to the second OFDM symbol.

[15] The terminal as claimed in claim 8, wherein the peak detector discriminates a cell and a sector frαn each other in the case of a wireless network using a single carrier.

[16] The terminal as claimed in claim 8, further comprising a sealer that scales the output value frαn the correlator by the reception power and transmits the resultant value to the peak detector.

[17] A method of constructing a wireless network for OFDM having a frame structure that includes a downlink including a CCS and multiple DDSs and an uplink including a preamble for channel estimation and a UDS, cαnprising: a) OFDM symbols that construct the downlink forming a preamble using a CP having a first length and OFDM symbols that construct the uplink forming a preamble using a CP having a length that is longer than the first length, thereby constructing a frame; and b) carrying out channel estimation/compensation using the preamble of the uplink without having uplink synchronization.

[18] The method as claimed in claim 17, wherein the CP length of the OFDM symbols of the downlink is determined by a delay spread, and the CP length of the OFDM symbols of the uplink is determined by a delay spread and a round trip delay.

[19] The method as claimed in claim 17, wherein a) uses the first OFDM symbol of symbols of the uplink as the preamble, uses other OFDM symbols as data symbols, and divides the entire frequency dαnain into a plurality of frequency domains to use them for randan access and for transmission of acknowledge/ non-acknowledge and short messages.