US 20040081131 A1 Abstract Techniques to use OFDM symbols of different sizes to achieve greater efficiency for OFDM systems. The system traffic may be arranged into different categories (e.g., control data, user data, and pilot data). For each category, one or more OFDM symbols of the proper sizes may be selected for use based on the expected payload size for the traffic in that category. For example, control data may be transmitted using OFDM symbols of a first size, user data may be transmitted using OFDM symbols of the first size and a second size, and pilot data may be transmitted using OFDM symbols of a third size or the first size. In one exemplary design, a small OFDM symbol is utilized for pilot and for transport channels used to send control data, and a large OFDM symbol and the small OFDM symbol are utilized for transport channels used to send user data.
Claims(50) 1. A method of transmitting data in an orthogonal frequency division multiplexing (OFDM) communication system, comprising:
transmitting a first block of data in a first OFDM symbol of a first size; and transmitting a second block of data in a second OFDM symbol of a second size that is different from the first size. 2. The method of 3. The method of 4. The method of 5. The method of 6. The method of transmitting a pilot in a third OFDM symbol. 7. The method of 8. The method of 9. The method of 10. The method of 11. The method of 12. The method of 13. The method of 14. An apparatus in an orthogonal frequency division multiplexing (OFDM) communication system, comprising:
means for transmitting a first block of data in a first OFDM symbol of a first size; and means for transmitting a second block of data in a second OFDM symbol of a second size that is different from the first size. 15. The apparatus of means for transmitting a pilot in a third OFDM symbol of the first size. 16. A transmitter unit in an orthogonal frequency division multiplexing (OFDM) communication system, comprising:
a transmit (TX) data processor operative to process a first block of data to obtain a first set of modulation symbols and to process a second block of data to obtain a second set of modulation symbols; and a modulator operative to process the first set of modulation symbols to obtain a first OFDM symbol of a first size and to process the second set of modulation symbols to obtain a second OFDM symbol of a second size that is different from the first size. 17. The transmitter unit of 18. A method of transmitting data in an orthogonal frequency division multiplexing (OFDM) communication system, comprising:
transmitting control data in a first time segment with a first OFDM symbol of a first size; and transmitting user data in a second time segment with a second OFDM symbol of a second size that is different from the first size. 19. The method of transmitting user data in the second time segment with a third OFDM symbol of a third size that is different from the second size. 20. The method of transmitting a pilot in a third time segment with a third OFDM symbol. 21. The method of 22. The method of 23. A method of receiving data in an orthogonal frequency division multiplexing (OFDM) communication system, comprising:
receiving a first OFDM symbol of a first size for a first block of data; and receiving a second OFDM symbol of a second size for a second block of data, the second size being different from the first size. 24. The method of 25. The method of 26. The method of receiving a third OFDM symbol for a pilot. 27. The method of processing the third OFDM symbol to obtain a channel estimate for each of a plurality of subbands. 28. The method of interpolating channel estimates for the plurality of subbands to obtain a channel estimate for an additional subband not among the plurality of subbands. 29. An apparatus in an orthogonal frequency division multiplexing (OFDM) communication system, comprising:
means for receiving a first OFDM symbol of a first size for a first block of data; and means for receiving a second OFDM symbol of a second size for a second block of data, the second size being different from the first size. 30. The apparatus of means for receiving a third OFDM symbol for a pilot; and means for processing the third OFDM symbol to obtain a channel estimate for each of a plurality of subbands. 31. The apparatus of means for interpolating channel estimates for the plurality of subbands to obtain a channel estimate for an additional subband not among the plurality of subbands. 32. A receiver unit in an orthogonal frequency division multiplexing (OFDM) communication system, comprising:
a demodulator operative to process a first OFDM symbol of a first size to obtain a first set of received modulation symbols and to process a second OFDM symbol of a second size to obtain a second set of received modulation symbols, wherein the second size is different from the first size; and a receive (RX) data processor operative to process the first set of received modulation symbols to obtain a first block of data and to process the second set of received modulation symbols to obtain a second block of data. 33. The receiver unit of 34. The receiver unit of a controller operative to interpolate channel estimates for the plurality of subbands to obtain a channel estimate for an additional subband not among the plurality of subbands. 35. A method of processing a pilot in a multiple-input multiple-output (MIO) orthogonal frequency division multiplexing (OFDM) communication system, comprising:
receiving a first set of OFDM symbols from a set of antennas for the pilot; processing the first set of OFDM symbols to obtain a channel response matrix for each of a plurality of subbands; and decomposing the channel response matrix for each of the plurality of subbands to obtain a unitary matrix of eigenvectors for the channel response matrix, wherein the decomposition is performed in a manner to avoid arbitrary phase rotations from subband to subband. 36. The method of 37. The method of generating a steered reference based on a particular column of the unitary matrix for each of the plurality of subbands; and transmitting a second set of OFDM symbols from the set of antennas for the steered reference. 38. A method of processing a steered reference in a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) communication system, comprising:
receiving a set of OFDM symbols from a set of antennas for the steered reference; processing the set of OFDM symbols to obtain a steering vector for each of a plurality of subbands; and interpolating steering vectors for the plurality of subbands to obtain a steering vector for an additional subband not among the plurality of subbands. 39. A method of transmitting a data unit having a data unit size in an orthogonal frequency division multiplexing (OFDM) communication system, the method comprising:
selecting a first OFDM symbol size from a set of OFDM symbol sizes, wherein the set of OFDM symbol sizes comprises a large OFDM symbol size and a small OFDM symbol size that is smaller than the large OFDM symbol size; and transmitting a first portion of the data unit in an OFDM symbol having the first OFDM symbol size. 40. The method of 41. The method of 42. The method of selecting a second OFDM symbol size from the set of OFDM symbol sizes; and
transmitting a second portion of the data unit in a second OFDM symbol having the second OFDM symbol size.
43. The method of 44. The method of 45. An apparatus in an orthogonal frequency division multiplexing (OFDM) communication system, comprising
means for selecting a first OFDM symbol size from a set of OFDM symbol sizes, wherein the set of OFDM symbol sizes comprises a large OFDM symbol size and a small OFDM symbol size that is smaller than the large OFDM symbol size; and means for transmitting a first portion of a data unit in an OFDM symbol having the first OFDM symbol size. 46. The apparatus of means for selecting a second OFDM symbol size from the set of OFDM symbol sizes; and means for transmitting a second portion of the data unit in a second OFDM symbol having the second OFDM symbol size. 47. The apparatus of 48. The apparatus of 49. A transmitter unit in an orthogonal frequency division multiplexing (OFDM) communication system, comprising:
a controller operative to select a first OFDM symbol size from a set of OFDM symbol sizes, wherein the set of OFDM symbol sizes comprises a large OFDM symbol size and a small OFDM symbol size that is smaller than the large OFDM symbol size; and a modulator operative to process a first portion of a data unit to obtain an OFDM symbol having the first OFDM symbol size. 50. The transmitter of Description [0001] This application claims the benefit of provisional U.S. Application Serial No. 60/421,309, entitled “MIMO WLAN System,” filed on Oct. 25, 2002, and provisional U.S. Application Serial No. 60/438,601, entitled “Pilot Transmission Schemes for Wireless Multi-Carrier Communication Systems,” filed on Jan. 7, 2003, both assigned to the assignee of the present application and incorporated herein by reference in their entirety for all purposes. [0002] I. Field [0003] The present invention relates generally to data communication, and more specifically to orthogonal frequency division multiplexing (OFDM) communication systems and techniques for providing OFDM symbol sizes to increase wireless efficiency. [0004] II. Background [0005] Wireless communication systems are widely deployed to provide various types of communication services such as voice, packet data, and so on. These systems may utilize OFDM, which is a modulation technique capable of providing high performance for some wireless environments. OFDM effectively partitions the overall system bandwidth into a number of (N [0006] In OFDM, a stream of information bits is converted to a series of frequency-domain modulation symbols. One modulation symbol may be transmitted on each of the N [0007] To combat frequency selective fading in the wireless channel used for data transmission (described below), a portion of each transformed symbol is typically repeated prior to transmission. The repeated portion is often referred to as a cyclic prefix, and has a length of N [0008] The size of the cyclic prefix relative to that of the OFDM symbol may have a large impact on the efficiency of an OFDM system. The cyclic prefix must be transmitted with each OFDM symbol to simplify the receiver processing in a multipath environment but carries no additional information. The cyclic prefix may be viewed as bandwidth that must be wasted as a price of operating in the multipath environment. The proportion of bandwidth wasted in this way can be computed using the formula
[0009] For example, if N [0010] Orthogonal frequency division multiple-access (OFDMA) can ameliorate the inefficiency due to excess capacity resulting from the use of a large OFDM symbol. For OFDMA, multiple users share the large OFDM symbol using frequency domain multiplexing. This is achieved by reserving a set of subbands for signaling and allocating different disjoint sets of subbands to different users. However, data transmission using OFDMA may be complicated by various factors such as, for example, different power requirements, propagation delays, Doppler frequency shifts, and/or timing for different users sharing the large OFDM symbol. [0011] Existing OFDM systems typically select a single OFDM symbol size that is a compromise of various objectives, which may include minimizing cyclic prefix overhead and maximizing packing efficiency. The use of this single OFDM symbol size results in inefficiency due to excess capacity when transmitting packets of varying sizes. There is therefore a need in the art for an OFDM system that operates efficiently when transmitting packets of varying sizes. [0012] Techniques are provided herein to use OFDM symbols of different sizes to achieve greater efficiency for OFDM systems. These techniques can address both objectives of minimizing cyclic prefix overhead and maximizing packing efficiency. The OFDM symbol sizes may be selected based on the expected sizes of the different types of payload to be transmitted in an OFDM system. The system traffic may be arranged into different categories. For each category, one or more OFDM symbols of the proper sizes may be selected for use based on the expected payload size for the traffic in that category. [0013] For example, the system traffic may be arranged into control data, user data, and pilot data. Control data may be transmitted using an OFDM symbol of a first size, user data may be transmitted using an OFDM symbol of a second size and the OFDM symbol of the first size, and pilot data may be transmitted using an OFDM symbol of a third size (or the first size). The user data may further be arranged into sub-categories such as, for example, voice data, packet data, messaging data, and so on. A particular OFDM symbol size may then be selected for each sub-category of user data. Alternatively or additionally, the data for each user may be transmitted using an OFDM symbol of a particular size selected for that user. For improved packing efficiency, OFDM symbols of different sizes may be used for a given user data packet to better match the capacity of the OFDM symbols to the packet payload. [0014] In general, any number of OFDM symbol sizes may be used for an OFDM system, and any particular OFDM symbol size may be selected for use. In one illustrative design, a combination of two OFDM symbol sizes are used so as to maximize packing efficiency. In the illustrative design, a small or short OFDM symbol size (e.g., with 64 subbands) is used for pilot and control data. User data may be sent within zero or more OFDM symbols having a large or long OFDM symbol size (e.g., with 256 subbands) and zero or more OFDM symbols having the small OFDM symbol size, depending on the payload size. [0015] The processing at a transmitter and receiver (e.g., encoding, interleaving, symbol mapping, and spatial processing) may be performed in a manner to account for the use of OFDM symbols of different sizes, as described below. Various aspects and embodiments of the invention are also described in further detail below. [0016] The features, nature, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: [0017]FIG. 1 shows a block diagram of an OFDM modulator; [0018]FIG. 2 shows OFDM symbols of different sizes and the overhead due to the cyclic prefix; [0019]FIGS. 3A and 3B show the use of OFDM symbols of different sizes to transmit different types of data; [0020]FIG. 4 shows an IFFT unit with S stages for generating OFDM symbols of different sizes; [0021]FIG. 5 shows an illustrative MIMO-OFDM system; [0022]FIG. 6 shows a frame structure for a TDD MIMO-OFDM system; [0023]FIG. 7 shows a structure for a data packet and a PHY frame; [0024]FIG. 8 shows a block diagram of an access point and two user terminals; [0025]FIG. 9A shows a block diagram of a transmitter unit that may be used for the access point and the user terminal; and [0026]FIG. 9B shows a block diagram of a modulator within the transmitter unit. [0027] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs. [0028]FIG. 1 shows a block diagram of an OFDM modulator [0029] For each OFDM symbol period, one modulation symbol may be transmitted on each subband used for data transmission, and a signal value of zero is provided for each unused subband. An inverse fast Fourier transform (IFFT) unit [0030] A cyclic prefix generator [0031]FIG. 2 illustrates OFDM symbols of different sizes including the fixed overhead due to the cyclic prefix. For a given system bandwidth of W MHz, the size or duration of an OFDM symbol is dependent on the number of subbands. If the system bandwidth is divided into N subbands with the use of an N-point IFFT, then the resulting transformed symbol comprises N samples and spans N sample periods or N/W μsec. As shown in FIG. 2, the system bandwidth may also be divided into 2N subbands with the use of a 2N-point IFFT. In this case, the resulting transformed symbol would comprise 2N samples, span 2N sample periods, and have approximately twice the data-carrying capacity of the transformed symbol with N samples. Similarly, FIG. 2 also shows how the system bandwidth may be divided into 4N subbands with the use of a 4N-point IFFT. The resulting transformed symbol would then comprise 4N samples and have approximately four times the data-carrying capacity of the transformed symbol with N samples. [0032] As illustrated in FIG. 2, since the cyclic prefix is a fixed overhead, it becomes a smaller percentage of the OFDM symbol as the symbol size increases. Viewed another way, only one cyclic prefix is needed for the transformed symbol of size 4N, whereas four cyclic prefixes are needed for the equivalent four transformed symbols of size N. The amount of overhead for the cyclic prefixes may then be reduced by 75% by the use of the large OFDM symbol of size 4N. (The terms “large” and “long” are used interchangeably herein for OFDM symbols, and the terms “small” and “short” are also used interchangeably.) FIG. 2 indicates that improved efficiency (from the cyclic prefix standpoint) may be attained by using an OFDM symbol with the largest size possible. The largest OFDM symbol that may be used is typically constrained by the coherence time of the wireless channel, which is the time over which the wireless channel is essentially constant. [0033] The use of the largest possible OFDM symbol may be inefficient from other standpoints. In particular, if the data-carrying capacity of the OFDM symbol is much greater than the size of the payload to be sent, then the remaining excess capacity of the OFDM symbol will go unused. This excess capacity of the OFDM symbol represents inefficiency. If the OFDM symbol is too large, then the inefficiency due to excess-capacity may be greater than the inefficiency due to the cyclic prefix. [0034] In an illustrative OFDM system, both types of inefficiency are minimized by using OFDM symbols of different sizes. The OFDM symbol sizes used to transmit a unit of data may be selected from a set of available OFDM symbol sizes, which may in turn be selected based on the expected sizes of the different types of payload to be transmitted in the OFDM system. The system traffic may be arranged into different categories. For each category, one or more OFDM symbols of the proper sizes may be selected for use based on the expected payload size for the traffic in that category and possibly other considerations (e.g., implementation complexity). An OFDM symbol may be viewed as a boxcar that is used to send data. One or more boxcars of the proper sizes may be selected for each category of data depending on the amount of data expected to be sent for that category. A unit of data may be sent using multiple boxcars having identical sizes or having varying sizes. For example, if a unit of data consumes 2.1 times the capacity of a “large” boxcar, then the unit of data may be sent using two “large” boxcars and one “small” boxcar. [0035] As an example, the system traffic may be divided into three basic categories—control data, user data, and pilot data. Control data typically constitutes a small fraction (e.g., less than 10%) of the total system traffic and is usually sent in smaller blocks. User data constitutes the bulk of the system traffic. To minimize cyclic prefix overhead and maximize packing efficiency, a short OFDM symbol may be used to send control data and pilot, and a combination of long OFDM symbols and short OFDM symbols may be used to send user data. [0036]FIG. 3A shows the use of OFDM symbols of different sizes to transmit different types of data in an OFDM system. For simplicity, only one OFDM symbol size is used for each category and type of data in FIG. 3A. In general, any number of OFDM symbol sizes may be used for each category and type of data. [0037] As shown in FIG. 3A, pilot data may be transmitted using an OFDM symbol of size N [0038] In general, any number of OFDM symbol sizes may be used for the OFDM system, and any particular OFDM symbol size may be selected for use. Typically, the minimum OFDM symbol size is dictated by the cyclic prefix overhead and the maximum OFDM symbol size is dictated by the coherence time of the wireless channel. For practical considerations, OFDM symbol sizes that are powers of two (e.g., 32, 64, 128, 256, 512, and so on) are normally selected for use because of the ease in transforming between the time and frequency domains with the IFFT and fast Fourier Transform (FFT) operations. [0039]FIG. 3A shows the transmission of different types of data in different time segments in a time division multiplexed (TDM) manner. Each frame (which is of a particular time duration) is partitioned into multiple time segments. Each time segment may be used to transmit data of a particular type. The different types of data may also be transmitted in other manners, and this is within the scope of the invention. For example, the pilot and control data may be transmitted on different sets of subbands in the same time segment. As another example, all user data may be transmitted in one time segment for each frame. [0040] For a TDM frame structure, such as the one shown in FIG. 3A, the particular OFDM symbol size to use for each time segment may be determined by various manners. In one embodiment, the OFDM symbol size to use for each time segment is fixed and known a priori by both the transmitters and receivers in the OFDM system. In another embodiment, the OFDM symbol size for each time segment may be configurable and indicated, for example, by signaling sent for each frame. In yet another embodiment, the OFDM symbol sizes for some time segments (e.g., for the pilot and control data) may be fixed and the OFDM symbol sizes for other time segments (e.g., for the user data) may be configurable. In the latter configuration, the transmitter may use the fixed-symbol-size control data channel to transmit the OFDM symbol sizes to be used in subsequent user-data OFDM symbols. [0041]FIG. 3B shows the use of two different OFDM symbol sizes of N and 4N for different types of data. In this embodiment, each frame is partitioned into three time segments for pilot, control data, and user data. Pilot and control data are transmitted using an OFDM symbol of size N, and user data is transmitted using an OFDM symbol of size 4N and the OFDM symbol of size N. One or multiple OFDM symbols of size N may be transmitted for each of the time segments for pilot and control data. Zero or multiple OFDM symbols of size 4N and zero or multiple OFDM symbols of size N may be transmitted for the time segment for user data. [0042]FIG. 4 shows an embodiment of a variable-size IFFT unit [0043] The outputs from last butterfly stage [0044] IFFT unit [0045] As described above in FIG. 1, a cyclic prefix generator [0046] OFDM symbols of different sizes may be advantageously used in various types of OFDM systems. For example, multiple OFDM symbol sizes may be used for (1) single-input single-output OFDM systems that use a single antenna for transmission and reception, (2) multiple-input single-output OFDM systems that use multiple antennas for transmission and a single antenna for reception, (3) single-input multiple-output OFDM systems that use a single antenna for transmission and multiple antennas for reception, and (4) multiple-input multiple-output OFDM systems (i.e., MIMO-OFDM systems) that use multiple antennas for transmission and reception. Multiple OFDM symbol sizes may also be used for (1) frequency division duplexed (FDD) OFDM systems that use different frequency bands for the downlink and uplink, and (2) time division duplexed (TDD) OFDM systems that use one frequency band for both the downlink and uplink in a time-shared manner. [0047] The use of OFDM symbols of different sizes in an exemplary TDD MIMO-OFDM system is described below. [0048]FIG. 5 shows an exemplary MIMO-OFDM system [0049] In FIG. 5, access point [0050]FIG. 6 shows an exemplary frame structure [0051] On the downlink, a BCH segment [0052] On the uplink, an RCH segment [0053] The durations of the portions and segments are not drawn to scale in FIG. 6. The frame structure and transport channels shown in FIG. 6 are described in detail in the aforementioned provisional U.S. Patent Application Serial No. 60/421,309. [0054] Since different transport channels may be associated with different types of data, a suitable OFDM symbol size may be selected for use for each transport channel. If a large amount of data is expected to be transmitted on a given transport channel, then a large OFDM symbol may be used for that transport channel. The cyclic prefix would then represent a smaller percentage of the large OFDM symbol, and greater efficiency may be achieved. Conversely, if a small amount of data is expected to be transmitted on a given transport channel, than a small OFDM symbol may be used for that transport channel. Even though the cyclic prefix represents a larger percentage of the small OFDM symbol, greater efficiency may still be achieved by reducing the amount of excess capacity. [0055] Thus, to attain higher efficiency, the OFDM symbol size for each transport channel may be selected to match the expected payload size for the type of data to be transmitted on that transport channel. Different OFDM symbol sizes may be used for different transport channels. Moreover, multiple OFDM symbol sizes may be used for a given transport channel. For example, each PDU type for the FCH and RCH may be associated with a suitable OFDM symbol size for that PDU type. A large OFDM symbol may be used for a large-size FCH/RCH PDU type, and a small OFDM symbol may be used for a small-size FCH/RCH PDU type. [0056] For simplicity, an exemplary design is described below using a small OFDM symbol size N [0057] For this exemplary design, the 64 subbands for the small OFDM symbol are assigned indices of −32 to +31. Of these 64 subbands, 48 subbands (e.g., with indices of ±{1, . . . , 6, 8, . . . , 20, 22, . . . , 26}) are used for data and are referred to as data subbands, 4 subbands (e.g., with indices of ±{7, 21}) are used for pilot and possibly signaling, the DC subband (with index of 0) is not used, and the remaining subbands are also not used and serve as guard subbands. This OFDM subband structure is described in the aforementioned provisional U.S. Patent Application Serial No. 60/421,309. [0058] The 256 subbands for the large OFDM symbol are assigned indices of −128 to +127. The subbands for the small OFDM symbol may be mapped to the subbands for the large OFDM symbol based on the following: [0059] where k is an index for the subbands in the short OFDM symbol (k=−32, . . . +31); [0060] i is an index offset with a range of i=0, 1, 2, 3; and [0061] l is an index for the subbands in the long OFDM symbol (l=−128, . . . +127). [0062] For this exemplary design, the system bandwidth is W=20 MHz, the cyclic prefix is N [0063] For this exemplary design, the BCH segment has a fixed duration of 80 μsec, and each of the remaining segments has a variable duration. For each TDD frame, the start of each PDU sent on the FCH and RCH relative to the start of the FCH and RCH segments and the start of the RACH segment relative to the start of the TDD frame are provided in the FCCH message sent in the FCCH segment. Different OFDM symbol sizes are associated with different symbol durations. Since different OFDM symbol sizes are used for different transport channels (and different OFDM symbol sizes may also be used for the same transport channel), the offsets for the FCH and RCH PDUs are specified with the proper time resolution. For the exemplary design described above, the time resolution may be the cyclic prefix length of 800 nsec. For a TDD frame of 2 msec, a 12-bit value may be used to indicate the start of each FCH/RCH PDU. [0064]FIG. 7 illustrates an exemplary structure for a data packet [0065] The same PHY frame structure may be used for a message sent on the BCH or FCCH. In particular, a BCH/FCCH message may be sent using an integer number of PHY frames, each of which may be processed to obtain one OFDM symbol. Multiple OFDM symbols may be transmitted for the BCH/FCCH message. [0066] For the embodiment shown in FIG. 7, one PHY frame of data is sent in each OFDM symbol. Different PHY frame sizes may be used for different OFDM symbol sizes. Each PHY frame of data may be coded based on a particular coding scheme and may further include a CRC value that permits individual PHY frames to be checked and retransmitted if necessary. The number of information bits that may be sent in each PHY frame is dependent on the coding and modulation schemes selected for use for that PHY frame. Table 1 lists a set of rates that may be used for the MIMO-OFDM system and, for each rate, various parameters for two PHY frame sizes for two OFDM symbol sizes of N
[0067] For the exemplary design described above, the small PHY frame and small OFDM symbol are used for the BCH and FCCH. Both small and large PHY frames and small and large OFDM symbols may be used for the FCH and RCH. In general, a data packet may be sent using any number of large OFDM symbols and a small number of small OFDM symbols. If the large OFDM symbol is four times the size of the small OFDM symbol, then a data packet may be sent using N [0068] The OFDM symbol sizes used for data transmission may be provided to a receiver in various manners. In one embodiment, the FCCH provides the start of each data packet transmitted on the FCH and RCH and the rate of the packet. Some other equivalent information may also be signaled to the receiver. The receiver is then able to determine the size of each data packet being sent, the number of long and short OFDM symbols used for that data packet, and the start of each OFDM symbol. This information is then used by the receiver to determine the size of the FFT to be performed for each received OFDM symbol and to properly align the timing of the FFT. In another embodiment, the start of each data packet and its rate are not signaled to the receiver. In this case, “blind” detection may be used, and the receiver can perform an FFT for every 16 samples (i.e., the cyclic prefix length) and determine whether or not a PHY frame was sent by checking the CRC value included in the PHY frame. [0069] For a given pairing of access point and user terminal in MIMO-OFDM system [0070] The MIMO-OFDM system may be designed to support a number of transmission modes. Table 2 lists the transmission modes that may be used for the downlink and uplink for a user terminal equipped with multiple antennas.
[0071] For the beam-steering mode, one PHY frame of a selected rate may be generated for each OFDM symbol period for transmission on the best spatial channel. This PHY frame is initially processed to obtain a set of modulation symbols, which is then spatially processed to obtain N [0072] For the spatial multiplexing mode, up to N [0073] The processing at the transmitter and receiver for the beam-steering and spatial multiplexing modes are described in detail in the aforementioned provisional U.S. Patent Application Serial No. 60/421,309. The spatial processing for the beam-steering and spatial multiplexing modes is essentially the same for both the short and long OFDM symbols, albeit with more subbands for the long OFDM symbol. The diversity mode is described below. [0074] In an embodiment, the diversity mode utilizes space-time transmit diversity (STTD) for dual transmit diversity on a per-subband basis. STTD supports simultaneous transmission of independent symbol streams on two transmit antennas while maintaining orthogonality at the receiver. [0075] The STTD scheme operates as follows. Suppose that two modulation symbols, denoted as s [0076] It is desirable to minimize the processing delay and buffering associated with STTD processing for the large OFDM symbol. In an embodiment, the two STTD symbols x [0077] If the transmitter includes multiple antennas, then different pairs of antennas may be selected for use for each data subband in the diversity mode. Table 3 lists an exemplary subband-antenna assignment scheme for the STTD scheme using four transmit antennas.
[0078] For the embodiment shown in Table 3, transmit antennas [0079] The processing at the transmitter and receiver for the diversity mode is described in detail in the aforementioned provisional U.S. Patent Application Serial No. 60/421,309. [0080] 1. Physical Layer Processing [0081]FIG. 8 shows a block diagram of an embodiment of an access point [0082] On the downlink, at access point [0083] Each modulator (MOD) [0084] At each user terminal [0085] An RX data processor [0086] The processing by access point [0087] For the downlink, at each active user terminal [0088] At access point [0089] Controllers [0090] The OFDM symbol size selection may be performed for the downlink and uplink in various manners. In one embodiment, controller [0091] For both the downlink and uplink, the specific combination of large and small OFDM symbols to use for each data packet is dependent on the packet payload size and the OFDM symbol capacity for each of the available OFDM symbol sizes. For each data packet, the controller may select as many large OFDM symbols as needed, and where appropriate select one or more additional small OFDM symbols for the data packet. This selection may be performed as follows. Assume that two OFDM symbol sizes are used (e.g., with 64 subbands and 256 subbands), the data carrying capacity of the small OFDM symbol is T [0092] where the “int” operation on a provides the integer value of a, and the “ceiling” operation on b provides the next higher integer value for b. If m<4, then the number of large OFDM symbols to use for the data packet is N [0093] Controllers [0094]FIG. 9A shows a block diagram of an embodiment of a transmitter unit [0095] An encoder [0096] An interleaver [0097] For the long OFDM symbol, each group of 192 consecutive code bits to be transmitted on a given spatial channel is interleaved across the 192 data subbands for the long OFDM symbol. In particular, the first subgroup of 48 code bits with indices of 0 through 47 may be transmitted on the 48 data subbands with indices l=4k, where k=±{1 . . . 6, 8 . . . 20, 22 . . . 26}, the second subgroup of 48 code bits with indices of 48 through 95 may be transmitted on the subbands with indices l=4k+1, the third subgroup of 48 code bits with indices of 96 through 143 may be transmitted on the subbands with indices l=4k+2, and the last subgroup of 48 code bits with indices of 144 through 191 may be transmitted on the subbands with indices l=4k+3. The same interleaving scheme is thus essentially used for both the short and long OFDM symbols. [0098] A symbol mapping unit [0099] An exemplary design for framing unit [0100] TX spatial processor [0101]FIG. 9B shows a block diagram of an embodiment of a modulator [0102] 2. Pilot [0103] Various types of pilots may be transmitted to support various functions, such as timing and frequency acquisition, channel estimation, calibration, and so on. Table 4 lists four types of pilot and their short description.
[0104] A MIMO pilot may be sent by a transmitter (e.g., an access point) with the short OFDM symbol and used by a receiver (e.g., a user terminal) to estimate the channel response matrices H(k), for subband indices kεK, where K=±{1 . . . 26}. The receiver may then perform singular value decomposition of the channel response matrix H(k) for each subband, as follows: [0105] where U(k) is an (N [0106] Σ(k) is an (N [0107] V(k) is an (N [0108] “ [0109] A unitary matrix M is characterized by the property M [0110] A “wideband” eigenmode may be defined as the set of same-order eigenmodes of all subbands after the ordering. Thus, wideband eigenmode m includes eigenmode m of all subbands. Each wideband eigenmode is associated with a respective set of eigenvectors for all of the subbands. The “principal” wideband eigenmode is the one associated with the largest singular value in each matrix Σ(k) after the ordering. [0111] If the same frequency band is used for both the downlink and uplink, then the channel response matrix for one link is the transpose of the channel response matrix for the other link. Calibration may be performed to account for differences in the frequency responses of the transmit/receive chains at the access point and user terminal. A steered reference may be sent by a transmitter and used by a receiver to estimate the eigenvectors that may be used for spatial processing for data reception and transmission. [0112] A steered reference may be transmitted for wideband eigenmode m by a transmitter (e.g., a user terminal), as follows: [0113] where x [0114] v [0115] p(k) is the pilot symbol for subband k. [0116] The vector X [0117] The received steered reference at a receiver (e.g., an access point) may be expressed as:
[0118] where r [0119] u [0120] σ [0121] n(k) is the noise. [0122] As shown in equation (6), at the receiver, the received steered reference (in the absence of noise) is approximately u [0123] The steered reference is sent for one wideband eigenmode in each OFDM symbol period (without subband multiplexing), and may in turn be used to obtain an estimate of one eigenvector u [0124] The steered reference may be sent using the short OFDM symbol. The receiver is able to process the received steered reference to obtain a steering vector for each short OFDM subband that was used for steered reference transmission. For the above exemplary design, each short OFDM subband is associated with four long OFDM subbands. If the steered reference is sent using the short OFDM symbol, then the steering vectors for the long OFDM subbands may be obtained in various manners. [0125] In one embodiment, the steering vector obtained for short OFDM subband k is used for long OFDM subbands l=4k through l=4k+3. This embodiment provides good performance for low to moderate SNRs. For high SNRs, some degradation is observed when the coherence bandwidth of the channel is small. The coherence bandwidth is the bandwidth over which the channel is essentially constant or flat. [0126] In another embodiment, the steering vectors u [0127] any other pair of unit length vectors e [0128] This phase ambiguity may be avoided by taking some precautions in the computation of the singular value decomposition of H(k). This may be achieved by constraining the solution to the singular value decomposition so that the first element in each column of V(k) is non-negative. This constraint eliminates arbitrary phase rotations from subband to subband when the variations in the eigenvectors are otherwise smooth and the magnitude of the leading element of the eigenvector is not close to zero. This constraint may be enforced by post-multiplying a diagonal matrix R(k) with each of the unitary matrices U(k) and V(k), which may be obtained in the normal manner and may contain arbitrary phase rotations. The diagonal elements ρ ρ [0129] where v [0130] The constrained eigenvectors in R(k)V(k) may then be used for the steered reference, as shown in equation (5). At the receiver, the received vector r [0131] The use of the short OFDM symbol for the MIMO pilot and steered reference reduces the processing load associated with singular value decomposition of the channel response matrices H(k). Moreover, it can be shown that interpolation, with the constraint described above to avoid arbitrary phase rotation from subband to subband, can reduce the amount of degradation in performance due to interpolation of the steering vectors based on steered reference transmission on fewer than all subbands used for data transmission. [0132] The carrier pilot may be transmitted by the access point and used by the user terminals for phase tracking of a carrier signal. For a short OFDM symbol, the carrier pilot may be transmitted on four short OFDM subbands with indices ±{7, 21}, as shown in Table 3. For a long OFDM symbol, the carrier pilot may be transmitted on the 16 corresponding long OFDM subbands with indices ±{28+i, 84+i}, for i=0, 1, 2, 3. Alternatively, the carrier pilot may be transmitted on four long OFDM subbands with indices ±{28, 24}, in which case the other 12 long OFDM subbands may be used for data transmission or some other purpose. [0133] The various types of pilots and their processing at the transmitter and receiver are described in detail in the aforementioned provisional U.S. Patent Application Serial No. 60/421,309. [0134] For simplicity, the techniques for using of OFDM symbols of different sizes have been described for the downlink. These techniques may also be used for the uplink. A fixed OFDM symbol size may be used for some uplink transmissions (e.g., messages sent on the RACH) and OFDM symbols of different sizes may be used for other uplink transmissions (e.g., data packets sent on the RCH). The specific combination of large and small OFDM symbols to use for each uplink data packet may be depending on the packet payload size and may be determined by controller [0135] The techniques described herein for using OFDM symbols of different sizes in OFDM systems may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the elements used to implement any one or a combination of the techniques may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. [0136] For a software implementation, the techniques described herein may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit (e.g., memory units [0137] Headings are included herein for reference and to aid in locating certain sections. These headings are not intended to limit the scope of the concepts described therein under, and these concepts may have applicability in other sections throughout the entire specification. [0138] The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. Referenced by
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