TWI423607B - Method and apparatus for pilot signal transmission - Google Patents

Description

Method and device for guiding signal transmission

The present invention relates generally to pilot signal transmission and, more particularly, to a method and apparatus for directing signal transmission in a communication system.

A pilot signal, preamble or reference signal is typically used in communication systems to enable the receiver to perform a number of key functions, including but not limited to timing and frequency synchronization acquisition and tracking, for subsequent demodulation and decoding of information material. Estimation and tracking of the required channels, estimation and monitoring of features of other channels for handover, interference suppression, etc. The communication system may utilize a number of priming mechanisms and typically include transmissions of a known sequence at a known time interval. A receiver that only knows the sequence or knows the sequence and time interval in advance uses the information to perform the functions mentioned above.

One of the typical pilot formats used in early orthogonal frequency division multiplexing (OFDM) systems is a "discrete indexing" format, which is distributed over time and frequency based on the expected maximum Doppler frequency and maximum delay spread, respectively. Discrete references can be considered the most extensive presentation format, but they are difficult to elaborate. For example, how to support multiple transmit antennas, how to optimize low user speed but allow extra interpolation to achieve high speed, and how to avoid discrete display edge effects are less clear. In general, channel estimation is more difficult because of the location of the changes in the frame and sub-frame boundaries. For example, some simpler channel estimation techniques are not available and more sets of interpolation filters may be necessary. In addition to being more complex, discrete references also limit the ability to perform Time Division Multiplexing (TDM) to introduce energy saving mechanisms (eg, by using TDM pilots close to the control channel and then turning off if no data is passed to the receiver) The receiver transmits until the next control channel to decode the control channel). Therefore, there is a need for a method and apparatus for signal transmission in a communication system to reduce the above mentioned problems.

To address the above mentioned needs, a method and apparatus for signaling transmission is disclosed herein. In particular, a basic cell transmits a known sequence at a known time interval as part of its downlink transmission. A remote unit known to have the sequence and time interval utilizes the information to demodulate and decode the transmissions. An illustration is shown in which a first pilot sequence is transmitted over a first antenna group over a plurality of subcarriers during a first OFDM symbol period. During a second OFDM symbol period, a second pilot sequence is transmitted over the second antenna group over a plurality of subcarriers. The first antenna group does not transmit the pilot sequence when the second antenna group transmits the pilot sequence. In addition, the second antenna group does not transmit the pilot sequence when the first antenna group transmits the pilot sequence.

The above described derivation mechanism is optimized for low speeds, and high speeds can be handled by simple time domain interpolation by utilizing such indications in the sub-frames on either side of the current sub-frame. For very high speed or subframes that cannot utilize the common indication in an adjacent subframe, additional additions for channel estimation purposes (eg, an additional OFDM symbol near the end of the frame) may be suitable included.

The present invention encompasses a method of pilot transmission for a transmitter having multiple antennas. The method includes the steps of: transmitting a first pilot sequence over a first antenna group over a plurality of subcarriers during a first symbol period, and passing a plurality of subcarriers during a second symbol period on a second day A second instruction sequence is transmitted on the line group. The first antenna group does not transmit the pilot sequence when the second antenna group transmits the pilot sequence and the second antenna group does not transmit the pilot sequence when the first antenna group transmits the pilot sequence. The first and second antenna groups transmit only one pilot sequence every M symbol periods.

The invention further contemplates a transmitter comprising a first antenna group for transmitting a first pilot sequence over a plurality of subcarriers during a first symbol period. The transmitter in turn includes a second antenna group that transmits a second pilot sequence over the plurality of subcarriers during the second symbol period. The first antenna group does not transmit the pilot sequence when the second antenna group transmits the pilot sequence and the second antenna group does not transmit the pilot sequence when the first antenna group transmits the pilot sequence .

The present invention contemplates a method of directing transmission. The method comprises the steps of: after one or more of the plurality of antennas on a sub-carrier transmission of pilot sequence during one OFDM symbol period, wherein the primer sequence is shown in which each OFDM symbol period in a generally K _D sub Transmission on the carrier. The pilot sequence is transmitted on subcarriers located adjacent to a DC subcarrier during the OFDM symbol period, and the pilot sequence transmitted on the adjacent subcarriers repeats the pilot sequence on the DC subcarrier.

The invention also contemplates a method of directing transmission. The method comprises the steps of: after one or more of the plurality of antennas on a sub-carrier transmission of pilot sequence during one OFDM symbol period, wherein the primer sequence is shown during the OFDM symbol period in a substantially every K _D Transmission on subcarriers. A pilot sequence is transmitted on a subcarrier adjacent to a band edge during a previous OFDM symbol period, wherein the sequence of repetitions transmitted on the subcarrier adjacent to the edge of the band should be located just outside the edge of the band The sequence of instructions on the subcarriers.

Turning now to the drawings, where like numerals indicate similar components. 1 is a block diagram of a communication system 100 utilizing pilot transmission. The communication system 100 utilizes next generation OFDM or multi-carrier based architecture. The architecture may also include one or two-dimensional spreads such as multi-carrier code division multiple access (MC-CDMA), multi-carrier direct sequence CDMA (MC-DS-CDMA), orthogonal frequency division, and code division multiplexing ( The use of the spread technology of OFCDM) may be based on a simpler time-sharing and/or frequency division multiplexing/multiple access technology, or a combination of such various technologies. In an alternate embodiment, communication system 100 may utilize other broadband cellular communication system protocols such as, but not limited to, Time Division Multiple Access (TDMA) or Direct Sequence CDMA.

As is known to those skilled in the art, broadband data is transmitted using multiple subcarriers (e.g., 601 subcarriers, 768 subcarriers, etc.) during operation of an OFDM system. This is illustrated in Figure 2. The wide frequency channel as shown in FIG. 2 is divided into a plurality of narrow frequency bands (subcarriers) 201, and data is transmitted in parallel on the subcarrier 201.

Returning to FIG. 1, the communication system 100 includes a base unit 101 and a remote unit 103. The remote unit 103 can also refer to a communication unit, a user equipment (UE) or only one mobile unit, and a base unit 101 can also refer to a communication unit or only Node-B. Base unit 101 includes a transmitter and receiver that serve a number of remote units within a sector. As is known in the art, the entire physical area of a communication network service can be divided into multiple cells, and each cell can include one or more sectors. Base unit 101 can serve each sector using multiple antennas 109 to provide various advanced communication modes (e.g., adaptive beamforming, transmission diversity, transmission domain multiple access (SDMA), multi-stream transmission, etc.). Base unit 101 transmits downlink communication signal 104 to a servo remote unit located on at least a portion of the same resource (time, frequency, or both). Remote unit 103 communicates with base unit 101 via uplink communication signal 106.

It should be noted that while Figure 1 illustrates only one base unit and a single remote unit, typical communication systems are known to be known to those skilled in the art to include many base units that communicate simultaneously with many remote units. It should also be noted that although for purposes of simplicity, the present invention is primarily described in the context of downlink transmission from a base unit to multiple remote units, the present invention is also applicable to uplink transmissions from multiple remote units to multiple base units.

As mentioned above, the pilot assisted modulation is typically used to assist a number of functions such as channel estimation for subsequent demodulation of the transmitted signal. Based on this knowledge, base unit 101 transmits the known sequence as part of its downlink transmission at a known time interval. The sequence and time interval remote unit 103 is known to use the information to demodulate and decode the transmissions. Figure 3 illustrates the pilot transmission mechanism. As shown, base station 101 downlink transmission 300 from a particular subcarrier generally includes a pilot sequence 301 followed by a remaining transmission 302. The same or different sequences may occur one or more times during the remaining transmission 302. Thus, base unit 101 includes a pilot channel circuit 107 that transmits one or more pilot sequences and a data channel circuit 108 that transmits data. It should be noted that the pilot sequence 301 may or may not be mixed with data symbols. It should also be noted that the pilot sequence 301 and the remaining transmission 302 can include a sub-frame whose structure is repeated at other time intervals. For example, a subframe may be composed of M OFDM symbols, where the M OFDM symbols contain a sequence of pilots and data, and the overall structure of the M OFDM symbols is repeated at different time periods.

It should be appreciated that while FIG. 3 shows a pilot sequence 301 that exists at the beginning of a transmission, in various embodiments of the invention, the pilot channel circuitry can include a pilot sequence that exists anywhere in the downlink transmission 300. 301, and in turn can be transmitted on a separate channel. Remaining transmission 302 typically includes, but is not limited to, transmissions such as information that the receiver needs to know before performing demodulation/decoding (so-called control information) and the actual information (user data) targeted by the user.

As noted above, the typical pilot format used in early OFDM systems is a "discrete indexing" format with an indication that is distributed over time and frequency based on the expected maximum Doppler frequency and maximum delay spread, respectively. Discrete references can be considered the most extensive presentation format, but they are difficult to elaborate.

To solve this problem, in a first embodiment of the present invention, a derivation mechanism is utilized in which a plurality of subcarriers are passed through the first antenna group during a first symbol period (eg, a second period of OFDM symbols) ( For example, antennas 1 and 2) transmit a first sequence of instructions. During a second OFDM symbol period (e.g., the seventh period of the OFDM symbol), a second pilot sequence is transmitted over the second antenna group (e.g., antennas three and four) over a plurality of subcarriers. In general, an antenna group contains only one antenna system possible. The first antenna group does not transmit the pilot sequence when the second antenna group transmits the pilot sequence. In addition, the second antenna group does not transmit the pilot sequence when the first antenna group transmits the pilot sequence. The first and second antenna groups transmit only one pilot sequence during every M symbol periods, one OFDM symbol period (ie, one subframe per pilot sequence). It should be noted that a first pilot sequence may contain two different sequences that are transmitted independently from the first antenna group. Similarly, a second pilot sequence can contain two different sequences that are transmitted independently from the second antenna group.

Figure 4 shows a more detailed view of the transmission mechanism. The format is a TDM format in which one of the OFDM symbols in the subframe contains all of the indicator symbols. More specifically, as is known to those skilled in the art, for a particular secondary channel of one or more subcarriers, an OFDM frame includes a plurality of subframes, each of which contains M OFDM symbols. A resource block contains one or more subcarriers for N symbols. Thus, in Figure 4, data is transmitted to a remote unit using Nc subcarriers. The Nc subcarriers utilize M symbols per subframe (eg, M=7 symbols/subframe). Figure 4 also shows an intermediate or zero or direct current (DC) subcarrier of a data that typically does not contain downlink transmissions. In addition, the first and second symbol periods including the pilot transmissions from the first and second antenna groups are transmitted in each subframe, wherein one subframe includes M OFDM symbols.

The above described derivation mechanism easily accommodates at least four transmit antennas in one OFDM symbol period by (a) dividing the guides into four groups and alternating between the antennas or (b) Passing all antennas on all of the pilot subcarriers, where each transmit antenna transmits the same basic pilot sequence, but a different phase shift sequence separates the channel estimates at the receiver. For the transmission antenna m on the kth subcarrier, the index value is given as follows: s _m ( k ) = x ( k ) e ^{- j 2 π k ( m - 1 ) / P} (1) where x ( k Is a sequence of specific sectors common to all transmit antennas (eg, a sequence of constant magnitudes with good characteristics) and P is a cyclic shift index. For the pilot format of Figure 4 with a pilot symbol on every other subcarrier, P = 8 for four transmit antennas and P = 4 for two transmit antennas. It should be noted that s _m ( k ) is only defined on subcarriers having a pilot sequence. The advantage of method (b) is that the linear phase shift makes the time domain of the channels leading to the plurality of transmission antennas orthogonal in the channel estimation performed by the mobile unit (ie, the linear phase shift in the frequency domain is in the time domain system). Ring time shift).

The above described derivation mechanism is optimized for low speeds and can be processed at high speeds by simple time domain interpolation by using the indications in the sub-frames on either side of the current sub-frame. For very high speed or subframes that cannot utilize the common indication in an adjacent subframe, additional additions for channel estimation purposes (eg, an additional OFDM symbol near the end of the frame) may be suitable included. Such additional references are generally limited to resource blocks or sub-frames that require additional citations and will not be used for other sub-frames unless explicitly (dedicated or broadcast control messages) or implicitly known to exist for additional cueing (eg , during a last subframe of one of the radio frames, or due to a signaled UE speed measurement).

It should be noted that this TDM lure format allows for an extremely simplified channel estimation algorithm and provides a sufficiently high indexing density in the frequency dimension to allow for various enhanced estimation algorithms (eg, for improved log-proportional ratio (LLR) generations Tracking the frequency selectivity of other cell interferences, performing "de-noise" at low signal noise plus interference ratio (SINR) by joint limit technique, etc.). In addition, control information may be located in the same (or adjacent) OFDM symbols as the references to allow any mobile unit to estimate the channel very quickly (with a simplification based on finite impulse response (FIR) or fast Fourier transform (FFT) processing) The 1-dimensional channel is estimated) and the control information is decoded immediately, thus reducing latency. Reducing latency is an important requirement for a variety of wireless communication systems. TDM control also allows for the remaining shutdown of a frame, which is especially important for long frames.

Figure 5 shows a more simplified view of the pilot transmission mechanism, with only a subset of the Nc subcarriers being shown for simplicity. In a first embodiment of the invention, the subsequent data/introduction mechanism is transmitted to the mobile unit 103 using four antennas 109. It is known to those skilled in the art that all of the pilot symbols transmitted during a symbol period include a pilot sequence in which the pilot sequence transmitted thereon is less than all of the subcarriers of the symbol period. The pilot sequence may comprise a generalized frequency interference (GCL) sequence, or in short, a cyclic shift sequence, or any pseudo-dispersion (PN) or PN constant amplitude sequence. In addition, the pilot symbols are broadcast only on a particular antenna during a particular symbol period. Thus, for example, four antennas are used at a base station, and only antennas 1 and 2 are used to broadcast pilot symbols P1, P2, and P3. Antennas 3 and 4 do not broadcast any symbols during these symbol periods. Similarly, only antennas 3 and 4 are used to broadcast symbols P4, P5 and P6. Antennas 1 and 2 do not broadcast any symbols during these symbol periods.

More generally, a first pilot sequence is transmitted over the first antenna group over a plurality of subcarriers during a first symbol period. A second pilot sequence is transmitted over the second antenna group over a plurality of subcarriers during a second symbol period. The first antenna group does not transmit the pilot sequence when the second antenna group transmits the pilot sequence. In addition, the second antenna group does not transmit the pilot sequence when the first antenna group transmits the pilot sequence. The first and second antenna groups transmit only one pilot sequence every M symbol periods (ie, once per subframe). Although the above examples give the same number of antennas in the first and second antenna groups, in an alternative embodiment of the invention, one antenna group may be smaller than the other antenna group (ie, contain fewer antennas than the other antenna group) . Additionally, the preferred two antenna groups comprise multiple antennas; however, either or both of the antenna groups may simply comprise a single antenna.

In rare cases, the adjacent subframes are expected but not present, and can be configured (scheduled) so that there is no high speed user in the subframe to maintain high performance. In addition, the modulation may be limited to QPSK or may be added for additional temporary additions (eg, in OFDM symbols adjacent to the two index symbols).

In the first embodiment of the invention, four basic transmission antennas 109 are used in a predetermined deployment. However, if the base can only accommodate 1-2 antennas, the introduction can be reduced as appropriate by removing half of the references in the presentation format, as shown in FIG. The base capability can communicate at most once for each hyperframe (a superframe is a collection of many sub-frames), and therefore the signalling (configuration messages) of such base capabilities is very rare. To implement the function at a very high speed (or when the adjacent subframe does not exist) one of the antennas 1 and 2 the second pilot sequence can be added to the second OFDM symbol period pilot symbol. This embodiment is also shown in Figure 5, in which antennas 1 and 2 transmit pilot sequences on both OFDM symbol period two and OFDM symbol period seven. In other words, antennas 1 and 2 are transmitted on pilot symbols P1, P2, P3, P4, P5 and P6.

The recommended format is TDM (per antenna). If an advanced multi-antenna technique (e.g., cyclic shift diversity) is used for control, the second index symbol can be moved to a position adjacent to the first symbol and TDM control as desired, as shown in FIG. This mechanism is similar to the mechanism described above in Figure 5, except that no data symbols (in time) exist between the pilot symbols.

In addition, other pilot configurations are possible, such as moving the pilots to transmit antennas three and four to the same OFDM symbol period as antennas one and two, but in different subcarrier groups. As shown in Figure 8, the P _{1 2} indicates the introduction of antennas one and two and P _{3 4} indicates the introduction of one antenna three and four. Thus for the pilot transmission mechanism, a first pilot sequence is transmitted over the first antenna group over a first plurality of subcarriers during an OFDM symbol period. Additionally, a second pilot sequence is transmitted over the second antenna group over a second plurality of subcarriers during the OFDM symbol period. The first antenna group does not transmit anything on the second plurality of subcarriers when the second antenna group transmits the pilot sequence. The second antenna group does not transmit anything on the first plurality of subcarriers when the first antenna group transmits the pilot sequence. The first and second antenna groups transmit only one pilot sequence every M symbol periods (ie, once per subframe).

Figure 9 is a simplified block diagram of a base station 101. As shown, the base station 101 includes a secondary channel circuit 903, a switch 905, and an antenna 109 (only one secondary channel circuit and antenna are labeled). A primary channel may contain one or more subcarriers. If a primary channel contains more than one subcarrier, the secondary carriers including the secondary channel may be adjacent or non-contiguous. A logic circuit 901 is provided to control the output to the secondary channel circuit 903 and to control the switch 905. The operation of the base station 101 occurs as shown in FIG.

FIG. 10 is a flow chart showing the operation of the base station 101. The logic flow begins in step 1001, where the logic circuit 901 receives the data and the information. Logic circuit 901 determines the symbol period and directs the data and instructions to the appropriate secondary channel based on the symbol period (step 1003). For example, if a particular secondary channel transmits pilot information for a particular symbol period, the pilot information is directed to the secondary channel during the symbol period. At step 1005, logic circuit 901 instructs switch 905 to pass the pilot or profile information from secondary channel circuit 903 to the appropriate antenna 109. For example, if the pilot information is to be broadcast only on a subset of the antennas, the switch 905 passes the pilot information to the subset.

As described above, in a first embodiment, the first pilot sequence is transmitted over the first antenna group over a plurality of subcarriers during a first OFDM symbol period. A second pilot sequence is transmitted over a second antenna group over a plurality of subcarriers during a second OFDM symbol period. The first antenna group does not transmit the pilot sequence when the second antenna group transmits the pilot sequence. In addition, the second antenna group does not transmit the pilot sequence when the first antenna group transmits the pilot sequence.

In a second embodiment of the invention, a first pilot sequence is transmitted over the first antenna group over a first plurality of subcarriers during an OFDM symbol period. Additionally, a second pilot sequence is transmitted on the second antenna group over a second plurality of subcarriers during the OFDM symbol period. The first antenna group does not transmit the pilot sequence on the second plurality of subcarriers when the second antenna group transmits the pilot sequence. In addition, the second antenna group does not transmit the pilot sequence on the first plurality of subcarriers when the first antenna group transmits the pilot sequence.

A typical receiver will include a single antenna to receive a first pilot sequence over a plurality of subcarriers transmitted over the first antenna group during a first symbol period. The receiver receives a second pilot sequence over a plurality of subcarriers transmitted over the second antenna group over a second symbol period. As discussed above, when the second antenna group transmits the pilot sequence, the first antenna group does not transmit the pilot sequence and the second antenna group does not transmit when the first antenna group transmits the pilot sequence. Show sequence.

Assuming every three subcarriers have an indication that has the indexing format in Figure 5, an indicator symbol can be loaded on the DC (zero) subcarrier. This is undesirable because a typical wireless communication system does not transmit any pilot or data information on the DC subcarrier. When this occurs, to improve channel estimation performance, an indicator symbol is encoded for the DC subcarrier (ie, using the k value of the DC subcarrier, as indicated by k _dc ), as given in equation (1) Then, the pilot symbol is repeated on subcarrier k _dc -1 and k _dc +1 also replaces the data symbol.

Figure 11 shows an example of how an indicator symbol on a subcarrier adjacent to the DC subcarrier is allocated. Figure 5 shows the pilot format. The pilot transmission is similar to the pilot transmission described above, however, the pilot symbols are assigned to every three subcarriers and the allocation strategy ends with a pilot on the DC subcarrier. In a typical OFDM system, no data or instructions are sent to the DC subcarrier, however it is still desirable to have a virtual pilot with a virtual pilot for good performance of the simple channel estimator. To effect this, the / DC should be present in the primer are shown labeled as P _{d c,} and repeated in the near DC subcarrier by subcarrier. The receiver will then share the received data on the adjacent subcarriers adjacent to the DC subcarrier to establish a virtual received indicator at the DC. An example of P _{d c} will be shown by k = k _dc in (1).

Thus to achieve a better estimate of the DC channel, through a plurality of subcarriers transmitted on one or more antennas during one OFDM symbol period shown a primer sequence, wherein the sequence of pilot OFDM symbol (e.g., k _D = 3 During transmission, it is transmitted on every k _D subcarriers. The pilot sequence includes an indicator symbol transmitted on a subcarrier adjacent to a DC subcarrier. The derivation sequence repetition transmitted on adjacent subcarriers adjacent to the DC subcarrier shall be present on the DC subcarrier or a pilot sequence that should have appeared on the DC subcarrier. The derivation sequence can be the value of P _{d c} mentioned above.

Similar to the method just described for improving channel estimation around DC subcarriers, Figure 12 shows a pilot format for channel estimation for improved band edges. For simplicity, the subcarriers located at the edge of the signal bandwidth are labeled as subcarrier 0 (note that the subcarrier indexed in Figure 12 is different from the subcarrier of Figure 4). Each of the three subcarriers has a pilot sequence of antenna one and antenna two, and the subcarrier starts at the end of the subcarrier (also referred to as subcarrier-1) just outside the edge of the signal band or band and passes through the end of the band. However, nothing is transmitted on subcarrier-1 because it is outside the designated band. To establish a virtual representation on subcarrier-1 of antennas one and two, the pilot symbol designed for subcarrier-1 (eg, using k = -1 in (1)) is first in subcarrier 1 The OFDM symbol and the second OFDM symbol located on subcarrier 0 are repeated. The virtually received pilot symbol at the subcarrier-1 is then established by combining the received signals at the two derivation locations (e.g., by using linear interpolation). A similar procedure can be used at the other end of the band for the pilot sequences of antennas three and four. For example, assuming that the last subcarrier is subcarrier K-1, then a sequence of pilots for antennas three and four can be expected just above the signal band and the subcarrier K outside the band edge. To accomplish this, the pilot sequence of subcarrier K (eg, k = K in (1)), the OFDM symbol above OFDM symbol one and subcarrier K-1 above subcarrier K-2 2 is repeated. The virtual received pilot symbol is then established over the subcarrier K by combining the received signals at the two index locations.

The present invention has been shown and described with reference to a particular embodiment thereof, and it is understood by those skilled in the art that various changes in form and detail can be made without departing from the spirit and scope of the invention. Such variations are intended to fall within the scope of the following patent application.

100. . . Communication system

101. . . Base unit/base station

103. . . Remote unit

104, 300. . . Downlink communication signal

106. . . Uplink communication signal

107. . . Pilot channel circuit

108. . . Data channel circuit

109. . . Multiple antennas

201. . . Narrowband/subcarrier

301. . . Pilot sequence

302. . . Remaining transmission

901. . . Logic circuit

903. . . Secondary channel circuit

905. . . Exchanger

Figure 1 is a block diagram of a communication system.

Figure 2 illustrates the division of a wide frequency channel into a plurality of narrow frequency bands (subcarriers).

Figure 3 illustrates the pilot signal transmission of the communication system of Figure 1.

Figure 4 is a detailed view of one of the introduction format designs.

Figure 5 shows a more simplified view of one of the pilot transmission mechanisms.

Figure 6 shows a dual antenna version of one of the pilot transmission mechanisms.

Figure 7 is an alternate embodiment of the pilot transmission mechanism.

Figure 8 is a four antenna representation transmission mechanism view in which all pilot sequences are included in an OFDM symbol.

Figure 9 is a simple block diagram of a base station.

Figure 10 is a flow chart showing the operation of a base station.

Figure 11 shows an implied transmission mechanism for channel estimation around a modified DC subcarrier.

Figure 12 shows an improved presentation transmission mechanism for channel estimation at the edges of the subcarriers.

Claims (8)

A method for pilot transmission in an orthogonal frequency division multiplexing (OFDM) system for a base unit having a plurality of transmission antennas, the method comprising the steps of: one of the first time periods during a first symbol period Transmitting, by a first plurality of subcarriers, a first pilot sequence on a first antenna group by a transmitter in a communication system; and passing through a first time during a second symbol period Two identical or different multiple subcarriers transmit a second pilot sequence on the second antenna group by the transmitter, wherein the first antenna group and the second antenna group are in M OFDM symbol periods Each sub-frame transmits only one pilot sequence; wherein the step of transmitting the first pilot sequence includes transmitting one of the antennas in the first antenna group during the first symbol period a step of the same first pilot sequence; wherein the first antenna group does not transmit the pilot sequence on the second plurality of subcarriers during the second symbol period when the second antenna group transmits the pilot sequence And wherein the first antenna group is transmitting the pilot sequence The antenna group does not transmit the pilot sequence on the first plurality of subcarriers during the first symbol period, and wherein the first symbol period is different from the second symbol period, and the first pilot sequence is different from the second Lead the sequence. The method of claim 1, wherein the first and second symbol periods comprise a first and a second OFDM symbol period. The method of claim 1, wherein the step of transmitting the first pilot sequence comprises, during substantially every K _D subcarriers, from each of the antennas in the first antenna group during the first symbol period The step of transmitting the first pilot sequence, wherein the K _D is greater than one integer of one. The method of claim 3, wherein K _D = 3. A base unit in an orthogonal frequency division multiplexing (OFDM) system, comprising: transmitting a first antenna group of a first pilot sequence over a first plurality of subcarriers during a first symbol period And transmitting, by a second identical or different plurality of subcarriers, a second antenna group of a second pilot sequence during a second symbol period, wherein the first antenna group and the second antenna group Transmitting only one pilot sequence in each subframe of the M OFDM symbol periods; wherein the first antenna group is not in the second during the second symbol period when the second antenna group is transmitting the pilot sequence Transmitting on a plurality of subcarriers, and wherein the second antenna group is not transmitted on the first plurality of subcarriers during the first symbol period when the first antenna group is transmitting the pilot sequence, and wherein the first antenna group A symbol period is different from the second symbol period, and the first instruction sequence is different from the second instruction sequence. The base unit of claim 5, wherein the first and second symbol periods comprise a first and a second OFDM symbol period. The base unit of claim 5, wherein a different sequence from one of all antennas in the first antenna group is transmitted on every K _D subcarriers during the first symbol period, wherein the K _D system is greater than one Integer. The base unit of claim 7, wherein K _D = 3.