Abstract:
Method and apparatus for generating data packets for transmission in an orthogonal frequency division modulated communication system, in which preamble sequence for each packet is generated in the frequency domain or the time domain depending on at least two conditions to save power consumption and enable implementation in a single CMOS chip.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and apparatus for generating data packets for transmission in an OFDM communication system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Orthogonal frequency division multiplexing (OFDM) systems have gained a lot of popularity in recent years partly due to their inherent multi-path resilience properties. A number of standards (such as 802.11a, 802.11g, DVB-T etc.) established in the past few years use OFDM based physical layer (PHY). Most of these standards are for packet based applications such as wireless local area networks (WLANs) and wireless personal area networks (WPANs). In these OFDM systems, the data is transmitted in short bursts (usually in multiple Kbytes). As such, each packet transmission includes fields specifically meant for packet detection and channel estimation. This information is transmitted as preamble at the beginning of each packet. The preamble consists of several symbols which can be derived from one source symbol. As an example, WiMedia has 30-symbol long preamble for standard packets and 18-symbol long preamble for burst packets. In addition, a frequency diversity technique known as time domain spreading is utilised to provide more error protection for data transmission with data rates lower than 320 Mbps. 
         [0003]    For mobile and wireless systems, complementary metal-oxide semiconductor (CMOS) implementation of radio frequency (RF) circuits is becoming more and more important since it can integrate with CMOS digital baseband circuits and thus provide a cheaper solution. To compensate for implementation loss of CMOS RF circuits, some pre-compensation techniques (e.g. subcarrier pre-compensation) are always used in digital baseband. Also, WiMedia devices should not interfere with other fixed services terminals. In the single, near-by interference case, active mitigation techniques in the form of dynamic frequency selection (DFS) can provide sufficient protection for indoor fixed services terminals. 
         [0004]    Conventional WiMedia PHY can provide data rates from 53.3 Mbps to 480 Mbps. It uses a rate −⅓ convolutional coder to encode the scrambled information bits. The encoded data is punctured to obtain different coding rates. Quadrature phase shift keying (QPSK) modulation is used for lower data rate modes (up to 200 Mbps), while dual carrier modulation (DCM) is used for the higher data rates modes. Additional frequency diversity is provided for the lower data rate modes through time domain spreading. 
         [0005]      FIG. 1  shows the WiMedia physical layer convergence protocol frame format. The PLCP frame consists of three portions a) preamble portion  101 , b) header  103  and c) payload portion  105 . The preamble  101  is composed of time domain (TD) training sequence  107  and frequency domain (FD) training sequence  109 . The duration of the TD preamble  107  is either 24 or 12 OFDM symbols depending on the mode of transmission (standard or streaming). The TD preamble  107  is used by a receiver for packet and frame synchronization. The TD preamble  107  is followed by the FD preamble  109 . The FD preamble  109 , consisting of six OFDM symbols, and is used for channel estimation (CE) and therefore the symbols transmitted in this field are referred to as CE symbols CE 1 - 6 ,  111 _ 1 ,  111 _ 2 ,  111 _ 3 ,  111 _ 4 ,  111 _ 5 ,  111 _ 6 . The preamble is followed by 12 header (HDR) symbols  113 _ 1 ,  113 _ 2 , . . . ,  113 _ 11 ,  113 _ 12  and a variable number of pay load symbols  105  having a maximum, for example, of 4095 bytes. The header symbols  103  are transmitted at the base rate (53.3 Mbps), while the payload symbols  105  are transmitted at the specified rate 
         [0006]    Conventional WiMedia communication systems utilise frequency hopping OFDM system in order to provide higher data rates while keeping the system complexity to a reasonable level. In this system, the carrier frequency of OFDM symbol is modified on each hop and is selected from a set of three sub-bands based on the symbol number and the time-frequency will be applied to achieve frequency diversity and thus better error protection. In this case, the spreaded symbol will derive from the symbol just proceeding it. Specifically, for data rates of 53.3 and 880 Mbps, the n th  spreaded symbols in time domain will be as follows: 
         [0000]        S   spreaded ( n )= P ( n )* S   original ( n ) 
         [0000]    where P is a cover sequence. For data rates of 106.7, 160 and 200 Mbps, the n th  spreaded symbol will be as 
         [0000]        S   spreaded ( n )= P ( n )*swap( S   original ( n )) 
         [0000]    where swap is to switch In-phase component and Quanrature component of a complex value. 
         [0007]    The preamble can be generated from one source symbol in the time domain assuming that the preamble symbols are identical except for their sign bits. However, this assumption will not hold in the systems with pre-compensation and/or DFS techniques. With these techniques, different subcarriers are modulated with different magnitude and can sometimes even be nulled out. Moreover, such a kind of modulation can change from time to time depending on operation conditions. In this disclosure, we propose an architecture to originate preamble generation in the frequency domain. Also, we propose a dual time spreading structure. Several operation modes are proposed so that the system can switch among them to maximize the power efficiency. 
         [0008]    Preamble generation and time spreading is conventionally carried out in time domain as shown in  FIG. 2 . 
         [0009]    The transmitter  200  comprises an input terminal  201  for receiving data to be transmitted. The input-terminal  201  is connected to a processor  203  for carrying out processing on the input data signals such as IFFT and time spreading. A preamble generator  205  generates the preamble. The processed signal and generated preamble are fed to a combiner  207  for inserting required prefixes and guard symbols and the completed data packet is output on the output terminal  209  for transmission. 
         [0010]    It assumes that the preamble symbols are identical except for the cover sequence. As such, only a fixed set of symbols need to be stored in time domain. For each packet transmission, the 24 or 12 TD preamble symbols are derived by applying different cover bits to one source symbol. For FD preamble symbols and time-domain spreaded symbols, the same approach is used. 
         [0011]    For wireless systems, one chip solution to replace current multiple-chip solutions has become increasingly popular. In one chip solution, all circuits including baseband and RF are integrated together using CMOS technology. Due to implementation loss from CMOS RF, baseband always needs to perform some pre-compensation before sending the signals to RF. In this case, the preamble may be changed from packet to packet depending on time-varying characteristics of CMOS RF. Also, RF circuits from different vendors have different characteristics. All these factors make it almost impossible to store all pre-compensated symbols in the time domain as before. Real-time loading of time-domain pre-compensated symbols by software is also not viable since it will take quite some time to load time-domain symbols while such a kind of loading may be required very often (e.g. packet by packet). As a result, generating preamble solely from the time domain is not feasible in CMOS RF systems. 
         [0012]    As WiMedia PHY may hop to different band on 1 or 2-symbol basis (depending on TFC code), the spreaded symbol cannot always be solely derived from the original symbol since different band may have different pre-compensation mask. This makes time spreading difficult to implement solely in time domain. 
         [0013]    Many existing systems propose generating preamble in the frequency domain. For example, US 2004/0114504 disclose efficient generation of the preamble in the frequency domain However, generating preamble solely from the frequency domain greatly increases the power consumption of the device. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention seeks to provide method and apparatus for generating data packets for transmission in which preamble generation and time spreading are controlled to minimise power consumption and is feasible in a CMOS RF system. 
         [0015]    This is achieved according to an aspect of the present invention by a method for generating data packets for transmission in an orthogonal frequency division modulated communication system, the method comprising the steps of: generating a plurality of header and payload symbols; generating a preamble sequence in the frequency domain or the time domain depending on at least two conditions; and combining the header, payload and preamble symbols to generate a data packet. 
         [0016]    This is also achieved according to another aspect of the present invention by apparatus for generating data packets for transmission in an orthogonal frequency division modulated communication system, the apparatus comprising: means for generating a plurality of header and payload symbols; means for generating a preamble sequence in the frequency domain or the time domain depending on at least two conditions; combiner for combining the header, payload and preamble symbols to generate a data packet; and transmitting means for transmitting the data packet. 
         [0017]    This is also achieved according to yet another aspect of the present invention by a transmitter for transmitting data in packets in an orthogonal frequency division modulated communication system, the method comprising the steps of: means for generating a plurality of header and payload symbols; means for generating a preamble sequence in the frequency domain or the time domain depending on at least two conditions; combiner for combing the header, payload and preamble symbols to generate a data packet; and transmitting means for transmitting the data packet. 
         [0018]    In this way the preamble is generated either in the frequency or the time domain depending on conditions such as the status of precompensation and dynamic frequency selection, the value of the time frequency code, the type of symbol and data rate. In switching preamble generation in this way power consumption is greatly reduced whilst maintaining feasibility for implementation of the transmitter CMOS. 
         [0019]    Further reduction in power consumption can be obtained by switching time spreading between the frequency and time domain on the basis of these conditions. 
         [0020]    Preferably, the invention can be applied to most packet-based communication systems (wireless, mobile, satellite, wiry . . . ). As an example, it can be applied to IEEE 802.11a, 802.11g and 802.11n systems with integrated CMOS RF. It can also be applied to WiMedia systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    For a more complete understanding of the present invention, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
           [0022]      FIG. 1  illustrates the physical convergence protocol frame format for a typical WiMedia communication system; 
           [0023]      FIG. 2  is a simplified schematic of a conventional transmitter; 
           [0024]      FIG. 3  is a simplified schematic of a transmitter according to a preferred embodiment of the present invention; and 
           [0025]      FIG. 4  illustrates the modes of operation of the transmitter according to the preferred embodiment of the present invention; 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0026]    Although the preferred embodiment is described with reference to the WiMedia PHY, it can be appreciated that the invention can be applied to most packet-based communication systems. 
         [0027]    A transmitter according to a preferred embodiment will now be described with reference to  FIG. 3 . The transmitter  300  comprises a configuration interface  303 . The configuration interface  303  is connected to a data loader and controller  307  and an inverse fast fourier transformer (IFFT)  309 . The interleaver  305  is connected to a mapper and time spreader  311 . The mapper and time spreader  311  is connected to a pre-compensation/DFS processor  313 . The data loader and controller  307  is connected to the pre-compensation/DFS processor  313 . A preamble memory  315  is also connected to the pre-compensation/DFS processor  313 . The output of the IFFT  309  is connected to an output terminal  317 . The interleaver  305  is connected to an input terminal  319 . 
         [0028]    Operation of the transmitter according to the preferred embodiment will be described with reference to  FIGS. 3 and 4 . Data to be processed is input on the input terminal  319  and hence input to the interleaver  305 . The Mapper &amp; Time spreader  311  gets the data from the interleaver  305  and provides the header symbols to pre-compensation/DFS block  313 . The preamble memory  315  stores standard preamble sequences. The pre-compensation/DFS patterns, which modulate the input symbols, are loaded by data loader and controller block  307 . Loading can be in real-time on a per-packet basis from the configuration interface  303 . The preamble can be generated either in the frequency domain (before IFFT block  309 ) or in the time domain (in IFFT  309 ). 
         [0029]    As a result, it is easy to implement pre-compensation/DFS for preamble generation. The pre-compensation/DFS block  313 , in principle, charges or nulls out certain subcarriers. It is easier to implement pre-compensation/DFS in the frequency domain rather than in time domain since the subcarrier concept is only valid in the frequency domain. 
         [0030]    Furthermore, as software loading of pre-compensation/DFS patterns is real-time, the software only needs to inform baseband about current pre-compensation/DFS pattern in the frequency domain, which contains much less data than its time domain representation. For example, the software only needs to pass the subcarrier index, which needs to be nulled out, to pre-compensation/DFS block rather than one entire OFDM symbol. With software rather than hardware to control the pre-compensation/DFS patterns, the system becomes more feasible. 
         [0031]    With a strong support from IFFT buffer, the power consumption of preamble generation can be reduced significantly. As the preamble is originated from frequency domain, it is sometimes inevitable to invoke IFFT datapath (the most power consuming block in the transmitter). In the proposed architecture, IFFT buffer is used to produce the preamble symbols whenever possible so that the invoking of IFFT datapath can be minimized. 
         [0032]    Further the transmitter of the preferred embodiment can perform time spreading in two locations, namely Mapper &amp; Time Spreader block  311  and IFFT  309 . Such a configuration can maximize the power efficiency while maintain system feasibilities. When current operation mode allows time spreading at IFFT buffer (time domain), the system will let IFFT buffer to produce the spreaded symbol. This helps to save power since IFFT datapath only needs to be activated every other symbol. Otherwise, time spreading can be activated at Mapper &amp; Time spreader (frequency domain) and go through IFFT datapath. 
         [0033]    As the preamble originates in the frequency domain, it needs to go through IFFT datapath, which is the most power hungry block in the transmitter chain. To reduce the power consumption, different operation situations are classified so that the system is able to switch among different operation modes based on current operation conditions.  FIG. 4  shows how operation modes are generated to control the preamble generation and time spreading. 
         [0034]    As shown in  FIG. 4 , there are a few configuration signals to control the generation of operation mode. Taking WiMedia PHY as an example, these signals will specify whether pre-compensation/DFS is enabled or not, the data type of current input, TFC for transmission, the data rate of payload for current transmission and preamble type. Based on these configuration signals, a certain operation mode is selected. Ten operation modes are defined for WiMedia systems as shown in Table 1 below. 
         [0000]    
       
         
               
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Operation 
                   
               
               
                 Mode 
                 Conditions for entering the mode 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Pre-compensation/DFS is disabled, or pre-compensation/DFS is enabled 
               
               
                   
                 when TFC is 5 or 6 or 7. 
               
               
                   
                 The current symbol is time preamble. 
               
               
                 2 
                 Pre-compensation/DFS is disabled or pre-compensation/DFS is enabled 
               
               
                   
                 when TFC is 5 or 6 or 7. 
               
               
                   
                 The current symbol is frequently preamble. 
               
               
                 3 
                 Pre-compensation/DFS is enabled when TFC code is 3 or 4. 
               
               
                   
                 The current symbol is time preamble. 
               
               
                 4 
                 Pre-compensation/DFS is enabled when TFC code is 3 or 4. 
               
               
                   
                 The current symbol is frequency preamble. 
               
               
                 5 
                 Pre-compensation/DFS is not enabled, or pre-compensation/DFS is 
               
               
                   
                 enabled when TFC is 3 or 4 or 5 or 6 or 7. 
               
               
                   
                 The current symbol is header symbol or payload symbol with a data rate of 
               
               
                   
                 53.3 or 80 Mbps. 
               
               
                 6 
                 Pre-compensation/DFS is not enabled, or pre-compensation/DFS is 
               
               
                   
                 enabled when TFC is 3 or 4 or 5 or 6 or 7. 
               
               
                   
                 The symbol is payload symbol with a data rate of 106.7 or 160 or 200 
               
               
                   
                 Mbps. 
               
               
                 7 
                 Pre-compensation/DFS is enabled when TFC is 1 or 2. 
               
               
                   
                 The current symbol is time preamble. 
               
               
                 8 
                 (a) Pre-compensation/DFS is enabled when TFC is 1 or 2 and 
               
               
                   
                 the current symbol is frequency preamble, or current symbol is payload 
               
               
                   
                 symbol with a data rate above 200 Mbps. 
               
               
                 9 
                 Pre-compensation/DFS is enabled when TFC is 1 or 2. 
               
               
                   
                 The current symbol is header symbol or payload symbol with a data rate of 
               
               
                   
                 53.3 or 80 Mbps. 
               
               
                 10 
                 Pre-compensation/DFS is enabled when TFC is 1 or 2. 
               
               
                   
                 The current symbol is payload symbol with a data rate of 106.7 or 160 or 
               
               
                   
                 200 Mbps. 
               
               
                   
               
             
          
         
       
     
         [0035]    The operation mode control the Mapper/time spreader  311 , pre-compensation/DFS processor  313  and IFFT  309  shown Table 2 below. 
         [0000]    
       
         
               
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Operation 
                   
               
               
                 Mode 
                 Operation of IFFT, pre-compensation/DFS, Mapper/time spreader 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 IFFT performs one symbol calculation and then reads the results from its 
               
               
                   
                 buffer 24 or 12 times (depending on preamble type). The read out symbols 
               
               
                   
                 are modulated by the cover sequence, which is determined by TFC. 
               
               
                   
                 Pre-compensation/DFS reads once from preamble memory and provides 
               
               
                   
                 pre-compensated symbol to IFFT. 
               
               
                   
                 Mapper/time spreader is not activated. 
               
               
                 2 
                 IFFT performs one symbol calculation and then reads the results from its 
               
               
                   
                 buffer 6 times. 
               
               
                   
                 Pre-compensation/DFS reads once from preamble memory and provides 
               
               
                   
                 the pre-compensated symbol to IFFT. 
               
               
                   
                 Mapper/time spreader is not activated. 
               
               
                 3 
                 IFFT performs 12 or 6 symbol calculations (depending on current 
               
               
                   
                 preamble type). After each calculation, the data is read out twice from 
               
               
                   
                 IFFT buffer. The read out symbols are modulated by the cover sequence. 
               
               
                   
                 Pre-compensation/DFS read 12 or 6 times from preamble memory 
               
               
                   
                 depending on preamble type. It provides the pre-compensated preamble 
               
               
                   
                 symbols to IFFT. 
               
               
                   
                 Mapper/time spreader is not activated. 
               
               
                 4 
                 IFFT performs 3 symbol calculations. After each calculation, the data is 
               
               
                   
                 read out twice from IFFT buffer. 
               
               
                   
                 Pre-compensation/DFS reads preamble memory 3 times and provides 3 
               
               
                   
                 pre-compensated symbols to IFFT. 
               
               
                   
                 Mapper/time spreader is not activated. 
               
               
                 5 
                 IFFT reads data from its buffer twice per symbol calculation. The second 
               
               
                   
                 symbol is modulated by pilot sequence. 
               
               
                   
                 Pre-compensation/DFS pre-compensates symbols from Mapper/time 
               
               
                   
                 spreader. 
               
               
                   
                 Mapper/time spreader disables its time spreading functionality. 
               
               
                 6 
                 IFFT reads data from its buffer twice per symbol calculation. The second 
               
               
                   
                 symbol is modulated by pilot sequence and then I/Q swapped. 
               
               
                   
                 Pre-compensation/DFS pre-compensates symbols from Mapper/time 
               
               
                   
                 spreader. 
               
               
                   
                 Mapper/time spreader disables its time spreading functionality. 
               
               
                 7 
                 IFFT reads data from its buffer once per symbol calculation. For the odd- 
               
               
                   
                 number symbols, they are further modulated by pilot sequence.. 
               
               
                   
                 Pre-compensation/DFS read preamble memory 24 or 12 times (depending 
               
               
                   
                 on preamble type) and provides pre-compensated symbols to IFFT. 
               
               
                   
                 Mapper/time spreader is not activated. 
               
               
                 8 
                 IFFT reads out data from its buffer once per symbol calculation. 
               
               
                   
                 Pre-compensation/DFS reads preamble memory 6 times and provides pre- 
               
               
                   
                 compensated symbols to IFFT in case of FD preamble. Otherwise, it pre- 
               
               
                   
                 compensates input symbols from Mapper/time spreader. 
               
               
                   
                 Mapper/time spreader is activated for payload symbol. 
               
               
                 9 
                 IFFT reads out data from its buffer once per symbol calculation. For the 
               
               
                   
                 odd-number symbols, they are further modulated by pilot sequence. 
               
               
                   
                 Pre-compensation/DFS pre-compensates symbols from Mapper/time 
               
               
                   
                 spreader. 
               
               
                   
                 Mapper/time spreader enables its time spreading functionality and reads 
               
               
                   
                 the same symbol twice from interleaver. 
               
               
                 10 
                 IFFT reads out data from its buffer once per symbol calculation. For the 
               
               
                   
                 odd-number symbols, they are modulated by pilot sequence and then I/Q 
               
               
                   
                 swapped. 
               
               
                   
                 Pre-compensation/DFS pro-compensates symbols from Mapper/time 
               
               
                   
                 spreader. 
               
               
                   
                 Mapper/time spreader enables its time spreading functionality and reads 
               
               
                   
                 the same symbol twice from interleaver. 
               
               
                   
               
             
          
         
       
     
         [0036]    The power efficiency of the different operation modes of Table 1 are summarized in Table 3 below. 
         [0000]    
       
         
               
               
             
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Operation 
                   
               
               
                 Mode 
                 Power Efficiency 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 1200% and 2400% (i.e. 1-symbol calculation of IFFT datapath generates 
               
               
                   
                 12 or 24 symbols). 
               
               
                 2 
                 600% (i.e. 1-symbol calculation of IFFT datapath generates 6 symbols). 
               
               
                 3 
                 200% (i.e. 1-symbol calculation of IFFT datapath generates 2 symbols). 
               
               
                 4 
                 200% (i.e. 1-symbol calculation of IFFT datapath generates 2 symbols). 
               
               
                 5 
                 200% (i.e. 1-symbol calculation of IFFT datapath generates 2 symbols). 
               
               
                 6 
                 200% (i.e. 1-symbol calculation of IFFT datapath generates 2 symbols). 
               
               
                 7 
                 100% (i.e. 1-symbol calculation of IFFT datapath generates 1 symbol). 
               
               
                 8 
                 100% (i.e. 1-symbol calculation of IFFT datapath generates 1 symbol). 
               
               
                 9 
                 100% (i.e. 1-symbol calculation of IFFT datapath generates 1 symbol). 
               
               
                 10 
                 100% (i.e. 1-symbol calculation of IFFT datapath generates 1 symbol). 
               
               
                   
               
             
          
         
       
     
         [0037]    Although the preferred embodiment is with reference to a WiMedia system, the invention can be applied to other packet-based wireless systems like 802.11a/g wireless LAN systems, in which pure CMOS implementation (i.e. CMOS baseband plus CMOS RF) is utilized. As an example for 802.11a Wireless LAN system, the first preamble symbol can be always generated in frequency domain by real-time loading of pre-compensation patterns from software as described before. As the frequency hopping is not supported in this system, the remaining preamble symbols can be generated by reading out IFFT buffer repeatedly as in Mode 1 of Table 1. 
         [0038]    The transmitter of the preferred embodiment is also compatible with conventional multiple chip solution. In which case pre-compensation/DFS, is disable invoking IFFT datapath once per-type of preamble and use IFFT buffer to generate most of preamble and generate spreaded symbol at IFFT buffer. 
         [0039]    Although a preferred embodiment of the present invention has been illustrated in the accompanying drawings and described in the foregoing description, it will be understood that the invention is not limited to the embodiment disclosed but is capable of numerous modifications without departing from the scope of the invention as set out in the following claims.