Abstract:
A hybrid orthogonal frequency division multiple access (OFDMA) system including a transmitter and a receiver is disclosed. The transmitter includes a first spread OFDMA subassembly, a first non-spread OFDMA subassembly and a first common subassembly. The first spread OFDMA subassembly spreads input data and maps the spread data to a first group of subcarriers. The first non-spread OFDMA subassembly maps input data to a second group of subcarriers. The first common subassembly transmits the input data mapped to the first and second group of subcarriers using OFDMA. The receiver includes a second spread OFDMA subassembly, a second non-spread OFDMA subassembly and a second common subassembly. The second common subassembly processes received data to recover data mapped to the subcarriers using OFDMA. The second spread OFDMA subassembly recovers the first input data by separating user data in a code domain and the second non-spread OFDMA subassembly recovers the second input data.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/673,872 filed Apr. 22, 2005, which is incorporated by reference as if fully set forth. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention is related to wireless communication systems. More particularly, the present invention is related to a hybrid orthogonal frequency division multiple access (OFDMA) system and method. 
       BACKGROUND 
       [0003]    It is expected that future wireless communication systems will provide broadband services such as wireless Internet access to subscribers. Such broadband services require reliable and high throughput transmissions over a wireless channel which is time dispersive and frequency selective. The wireless channel is subject to limited spectrum and inter-symbol interference (ISI) caused by multipath fading. Orthogonal frequency division multiplexing (OFDM) and OFDMA are some of the most promising solutions for next generation wireless communication systems. 
         [0004]    OFDM has a high spectral efficiency since the subcarriers used in the OFDM system overlap in frequency and an adaptive modulation and coding scheme (MCS) may be employed across subcarriers. In addition, implementation of OFDM is very simple because the baseband modulation and demodulation are performed by simple inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT) operations. Other advantages of the OFDM system include a simplified receiver structure and excellent robustness in a multipath environment. 
         [0005]    OFDM and OFDMA have been adopted by several wireless/wired communication standards, such as digital audio broadcast (DAB), digital audio broadcast terrestrial (DAB-T), IEEE 802.11a/g, IEEE 802.16, asymmetric digital subscriber line (ADSL) and is being considered for adoption in third generation partnership project (3GPP) long term evolution (LTE), cdma2000 evolution, a fourth generation (4G) wireless communication system, IEEE 802.11n, or the like. 
         [0006]    One key problem with OFDM and OFDMA is that it is difficult to mitigate or control inter-cell interference to achieve a frequency reuse factor of one. Frequency hopping and subcarrier allocation cooperation between cells have been proposed to mitigate inter-cell interference. However, the effectiveness of both methods is limited. 
       SUMMARY 
       [0007]    The present invention is related to a hybrid OFDMA system and method. The system includes a transmitter and a receiver. The transmitter includes a first spread OFDMA subassembly, a first non-spread OFDMA subassembly and a first common subassembly. The first spread OFDMA subassembly spreads input data and maps the spread data to a first group of subcarriers. The first non-spread OFDMA subassembly maps input data to a second group of subcarriers. The first common subassembly transmits the input data mapped to the first group of subcarriers and the second group of subcarriers using OFDMA. The receiver includes a second spread OFDMA subassembly, a second non-spread OFDMA subassembly and a second common subassembly. The second common subassembly of the receiver processes received data to recover data mapped to the subcarriers using OFDMA. The second spread OFDMA subassembly recovers the first input data by separating user data in a code domain and the second non-spread OFDMA subassembly recovers the second input data. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a block diagram of an exemplary hybrid OFDMA system configured in accordance with the present invention. 
           [0009]      FIG. 2  shows an example of frequency domain spreading and subcarrier mapping in accordance with the present invention. 
           [0010]      FIG. 3  shows another example of spreading and subcarrier mapping in accordance with the present invention. 
           [0011]      FIG. 4  shows an example of time-frequency hopping of subcarriers in accordance with the present invention. 
           [0012]      FIG. 5  is a block diagram of an exemplary time-frequency Rake combiner configured in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    Hereafter, the terminology “transmitter” and “receiver” includes but are not limited to a user equipment (UE), a wireless transmit/receive unit (WTRU), a mobile station, a fixed or mobile subscriber unit, a pager, a Node-B, a base station, a site controller, an access point or any other type of device capable of operating in a wireless environment. 
         [0014]    The features of the present invention may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components. 
         [0015]    The present invention is applicable to any wireless communication system that utilizes OFDMA (or OFDM) and/or code division multiple access (CDMA), such as IEEE 802.11, IEEE 802.16, third generation (3G) cellular systems, 4G systems, satellite communication systems, or the like. 
         [0016]      FIG. 1  is a block diagram of an exemplary hybrid OFDMA system  10  including a transmitter  100  and a receiver  200  in accordance with the present invention. The transmitter  100  includes a spread OFDMA subassembly  130 , a non-spread OFDMA subassembly  140  and a common subassembly  150 . In the spread OFDMA subassembly  130 , input data  101  (for one or more users) is spread with a spreading code to generate a plurality of chips  103  and the chips  103  are then mapped to subcarriers. In the non-spread OFDMA subassembly  140 , input bit  111  (for one or more different users) is mapped to subcarriers without spreading. 
         [0017]    The spread OFDMA subassembly  130  includes a spreader  102  and a first subcarrier mapping unit  104 . The non-spread OFDMA subassembly  140  includes a serial-to-parallel (S/P) converter  112  and a second subcarrier mapping unit  114 . The common subassembly  150  includes an N-point inverse discrete Fourier transform (IDFT) processor  122 , a parallel-to-serial (P/S) converter  124  and a cyclic prefix (CP) insertion unit  126 . 
         [0018]    Assuming that there are N subcarriers in the system and that K different users communicate at the same time in the system, among K users, data to K s  users is transmitted via the spread OFDMA subassembly  130 . The number of subcarriers used in the spread OFDMA subassembly  130  and the non-spread OFDMA subassembly  140  are N s  and N o , respectively. The values of N s  and N o  satisfy the conditions that 0≦N s ≦N, 0≦N o ≦N, and N s +N o ≦N. 
         [0019]    The input data  101  is spread by the spreader  102  to a plurality of chips  103 . The chips  103  are mapped to the N s  subcarriers by the subcarrier mapping unit  104 . The spreading may be performed in the time domain, in the frequency domain, or both. For a particular user, spreading factors in the time domain and the frequency domain are denoted by SF t  and SF f , respectively. A joint spreading factor for the user is denoted by SF joint , which equals to SF t ×SF f . When SF t =1, the spreading is performed only in the frequency domain, and when SF f =1, the spreading is performed only in the time domain. A frequency domain spreading for user i is limited to the number of subcarriers allocated to the user i, N s (i). The allocation of subcarriers can be static or dynamic. In the case where N s (i)=N s  for every user i, the spread OFDMA becomes spread OFDM. 
         [0020]    One subcarrier may be mapped to more than one user in the spread OFDMA subassembly  130 . In such case input data  101  of two or more users mapped to the same subcarrier are code multiplexed, and therefore, should be spread using different spreading codes. If spreading is performed both in the time and frequency domain, spreading codes assigned to users may be different in the time domain, in the frequency domain, or both. 
         [0021]      FIG. 2  shows an example of frequency domain spreading and subcarrier mapping in accordance with the present invention. The input data  101  is multiplied with a spreading code  204  by a multiplier  202  to generate a plurality of chips  103 ′. The chips  103 ′ are converted to parallel chips  103  by an S/P converter  206 . Each of the parallel chips  103  is then mapped to one of the subcarriers by the subcarrier mapping unit  104  before being sent to the IDFT processor  122 . 
         [0022]      FIG. 3  shows another example of frequency domain spreading and subcarrier mapping in accordance with the present invention. Instead of multiplying a spreading code by a spreader, a repeater  302  may be used to repeat each input data  101  multiple times at the chip rate to generate chips  103 ′. The chips  103 ′ are then converted to parallel chips  103  by an S/P converter  304 . Each of the parallel chips  103  is mapped to one of the subcarriers by the subcarrier mapping unit  104  before being sent to the IDFT processor  122 . 
         [0023]    Alternatively, when input data is spread in the time domain, each input data is spread by a spreader to generate a plurality of chip streams and the chip streams are mapped to subcarriers. In such case, the time domain spreading may also be performed by simple repetition of the input data without using a spreading code. 
         [0024]    Common pilots may be transmitted on the subcarriers used in the spread OFDMA subassembly  130 . In order to distinguish from other user data, common pilots are also spread. 
         [0025]    Referring again to  FIG. 1 , in the non-spread OFDMA subassembly  140 , input bits  111  of different users are converted to parallel bits  113  by the S/P converter  112 . The subcarrier mapping unit  114  allocates users to one or more subcarriers, such that each subcarrier is used by at most one user and bits from each user are mapped to the allocated subcarriers for the user by the subcarrier mapping unit. In this way, users are multiplexed in the frequency domain. The number of subcarriers allocated to user i is denoted by N o (i), 0≦N o (i)≦N o . The allocation of subcarriers can be static or dynamic. 
         [0026]    In accordance with the present invention, time-frequency hopping may be performed for the non-spread OFDMA subassembly  140  in a pseudo-random way in each cell. With time domain hopping, the users that transmit in a cell change from time to time, (i.e., over one or several OFDM symbols or frames). With frequency domain hopping, subcarriers allocated to users that transmit in a cell are hopping per one or several OFDM symbols or frames. In this way, the inter-cell interference can be mitigated and averaged among the users and cells. 
         [0027]      FIG. 4  illustrates an example of time-frequency hopping where ten (10) subcarriers, s 0 -s 9 , are used for time periods of T 0 -T 6  in accordance with the present invention. As an example, in  FIG. 2 , subcarriers s 3 , s 5 , s 8  are used for spread OFDMA and the remaining subcarriers are used for non-spread OFDMA. For the subcarriers allocated for non-spread OFDMA, subcarriers and time periods allocated to users are hopping in a pseudo-random way. For example, data for user  1  is transmitted via s 9  at T 0 , s 7  at T 1 , s 7  at T 3 , and s 1  and s 9  at T 4 , and data for user  2  is transmitted via s 4  at T 0 , s 6  at T 1 , s 3  at T 2 , s 0  and s 4  at T 4 . Therefore, data to different users is transmitted over different OFDM symbols or frames and inter-cell interference is mitigated. 
         [0028]    Referring again to  FIG. 1 , both the chips  105  and the data  115  are fed into the IDFT processor  122 . The IDFT processor  122  converts the chips  105  and data  115  to time domain data  123 . The IDFT may be implemented by IFFT or an equivalent operation. The time domain data  123  is then converted to a serial data  125  by the P/S converter  124 . A CP, (also known as a guard period (GP)), is then added to the serial data  125  by the CP insertion unit  126 . Data  127  is then transmitted via the wireless channel  160 . 
         [0029]    The receiver  200  includes a spread OFDMA subassembly  230 , a non-spread OFDMA subassembly  240  and a common subassembly  250  for hybrid OFDMA. The common subassembly  250  includes a CP removal unit  202 , a P/S converter  204 , an N-point discrete Fourier transform (DFT) processor  206 , an equalizer  208  and a subcarrier demapping unit  210 . The spread OFDMA subassembly  230  includes a code domain user separation unit  214  and the non-spread OFDMA subassembly  240  includes a P/S converter  216 . 
         [0030]    The receiver  200  receives data  201  transmitted via the channel. A CP is removed from received data  201  by the CP removal unit  202 . Data  203  after the CP is removed, which is time domain data, is converted to parallel data  205  by the S/P converter  204 . The parallel data  205  is fed to the DFT processor  206  and converted to frequency domain data  207 , which means N parallel data on N subcarriers. The DFT may be implemented by FFT or equivalent operation. The frequency domain data  207  is fed to the equalizer  208  and equalization is performed to data at each subcarrier. As in a conventional OFDM system, a simple one-tap equalizer may be used. 
         [0031]    After equalization at each subcarrier, data corresponding to a particular user is separated by the subcarrier demapping unit  210 , which is an opposite operation performed by the subcarrier mapping units  104 ,  114  at the transmitter  100 . In the non-spread OFDMA subassembly  240 , each user data  211  is simply converted to a serial data  217  by the S/P converter  216 . In the spread OFDMA subassembly  230 , data  212  on the separated subcarriers are further processed by the code domain user separation unit  214 . Depending on the way spreading is performed at the transmitter  100  corresponding user separation is performed in the code domain user separation unit  214 . For example, if the spreading is performed only in the time domain at the transmitter  100 , a conventional Rake combiner may be used as the code domain user separation unit  214 . If the spreading is performed only in the frequency domain at the transmitter  100 , a conventional (frequency domain) despreader may be used as the code domain user separation unit  214 . If the spreading is performed in both the time domain and the frequency domain at the transmitter  100 , a time-frequency Rake combiner may be used as the code domain user separation unit  214 . 
         [0032]      FIG. 5  is a block diagram of an exemplary time-frequency Rake combiner  500  configured in accordance with the present invention. The time-frequency Rake combiner  500  performs processing at both time and frequency domains in order to recover data that is spread in both time and frequency domains at the transmitter  100 . It should be noted that the time-frequency Rake combiners  500  may be implemented in many different ways and the configuration shown in  FIG. 5  is provided as an example, not as a limitation, and the scope of the present invention is not limited to the structure shown in  FIG. 5 . 
         [0033]    The time-frequency Rake combiner  500  comprises a despreader  502  and a Rake combiner  504 . Data  212  separated and collected for a particular user by the subcarrier demapping unit  210  in  FIG. 1  for the spread OFDMA subassembly  230  is forwarded to the despreader  502 . The despreader  502  performs frequency-domain despreading to the data  212  on the subcarriers. The despreader  502  includes a plurality of multipliers  506  for multiplying conjugate  508  of the spreading codes to the data  212 , a summer  512  for summing the multiplication outputs  510 , and a normalizer  516  for normalizing the summed output  514 . The despreader output  518  is then processed by the Rake combiner  504  to recover the data of the user by time domain combining. 
         [0034]    Referring again to  FIG. 1 , the transmitter  100 , the receiver  200 , or both may include multiple antennas and may implement hybrid OFDMA in accordance with the present invention with multiple antennas either at transmitter side, the receiver side, or both. 
         [0035]    Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention.