Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J13/00—Code division multiplex systems
  • H04J13/0007—Code type
  • H04J13/004—Orthogonal
  • H04J13/0048—Walsh
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J13/00—Code division multiplex systems
  • H04J13/16—Code allocation
  • H04J13/18—Allocation of orthogonal codes

Definitions

• PN codes are quasi-ortnogonal, i.e. they have relatively low but nonzero cross-correlation values with each other.
• ODS-CDMA systems are designed such that all signals are received in tone and frequency synchronism. Thus all users remain orthogonal to each other and, in an ideal world, any user can be recovered with no multiple access noise from other users. This is most practical in a star configured network where a multiplicity of users transmit to and receive from a single Hub Station. This configuration is used in cellular as well as satellite networks.
• each user is assigned a code which is ortnogonal to all of the other user codes (i.e. the ortnogonal codes have a cross-correlation value of zero with each other).
• the orthogonal code period is chosen such that the code repeats an integer number of times in a data symbol time (usually once, since this results in the maximum number of available ortnogonal functions).
• the code epoch is synchronized with the symbol transitions so that no data transitions occur within the code.
• the number of users is limited by the number of orthogonal codes available, which is equal, at most, to the length of the code. Therefore, the chipping rate is equal to the maximum number of orthogonal users times the symbol rate.
• Patent Numoer 5,416,797 “System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System,” incorporated herein by reference, lower data rates than the nominal are supported on the c ⁇ ll-to-mobile link (which is orthogonal), by repeating data symbols of a low rate user a number of times to obtain the nominal symbol rate. This sequence of symbols is then spread by an orthogonal code and transmitted at a lower power proportional to the lower rate.
• This technique has the disadvantage of not being orthogonal code space efficient, i.e. each low rate user uses the same amount of code space as a high rate user.
• An object of this invention is to provide a means by which an ODS-CDMA communications system can function efficiently with data rates that are not all equal. This non-homogeneity in data rates allows different users to communicate with different data bandwidths while the ODS-CDMA communications system signaling rate and orthogonal nature remain the same as in a homogeneous data rate system.
• ⁇ further object of this invention is to provide an ODS-CDMA system which supports a mix of users , with data rates related by 2', which makes efficient use of the available orthogonal code space, i.e. a user with symbol rate R/k uses 1/k of the code space of a user with symbol rate R.
• each user is assigned a code which is orthogonal to all of the other user codes (i.e. the orthogonal codes have a cross-correlation value of zero with each other).
• the orthogonal code period is chosen such that the code repeats an integer number of times in a data symbol time (usually once, since this results in the maximum numoer of available orthogonal functions) and the code epoch is synchronized with the symbol transitions so that no data transitions occur within the code.
• the individual users can be demodulated with no interference from other users.
• the number of users is limited by the number of orthogonal codes available, which is equal, at most, to the length of the code.
• the chipping rate is equal to the maximum number of orthogonal users times the symbol rate. This implies that, for a fixed chipping rate, a system transmitting data at a rate of R/4 should be able to accommodate 4 time ⁇ as many users as a system with rate R
• the novelty of this invention lies in the selection of an orthogonal codebook which enables high rate data to be spread using shorter codewords, and conversely, low rate data to be spread using longer codewords, ail of which remain mutually orthogonal.
• each user transmits a single code that is constant in amplitude and requires no data multiplexing or demultiplexing.
• the disclosed technique results in low receiver and transmitter complexity since no additional code generators or correlators are required to accommodate rates that are higher or lower than the nominal data rate. Further, the constant amplitude nature of the signal allows efficient power amplification.
• FIG. 1 Diagram of multi-rate ODS-CDMA Return Link Structure with M users Figure 2. Diagram of an Exemplary ODS-CDMA Receiver at the Hub Station
• FIG. 1 A generalized structure for an ODS-CDMA communications system is illustrated in Figure 1.
• a Return Link structure is shown to facilitate the invention description, however, the concept can be applied to the Forward Link as well, in the Return Link structure, many users communicate with the Hub Station by means of a modulator and transmitter process that uses some medium for tran ⁇ mission.
• the most common medium for transmission is radio frequency electromagnetic radiation.
• all users in the Return Link transmit on the same carrier frequency using the same type of modulator, it is necessary that the users transmit at time instances such that all signals received at the Hub 6
• the waveform transmitted by each user comprises of MPSK modulated data spread with an orthogonal ODS-CDMA code.
• the length of spreading code for each user is varied.
• An orthogonal codebook is constructed which enables high rate data to be spread using shorter codewords, and conversely, low rate data to be spread using longer codewords, all of which remain mutually orthogonal.
• the Sylvester construction of a Hadamard matrix is used to generate the codebook.
• the N rows of the Hadamard matrix H(N) define an orthogonal codebook. For convenience, the rows are indexed from 0 to N-1 , starting from the top row. It can be seen from the recursive construction of H(N) that it is composed of Sytvester-Hadamard submatrices of the form ⁇ H(N ⁇ *), where k is a positive integer ranging from 1 to n-7. Since the rows of H(N/2?) are orthogonal, it follows that the codewords of H(N) consist of subsequences of length N/2* that are orthogonal as well.
• this codeword Since this codeword is sub-divided into two N/2 subsequences, the remaining codewords must be orthogonal over each of these two subsequences such that the users at fundamental rate /, and the high rate user at 2f, do not mutually interfere, thereby maintaining orthogonality in the ODS-CDMA system.
• a rate 2f user may be supported by assigning the user a shorter subsequence of length N/2, and removing the codewords which contain this subsequence from the list of aliocabie codewords.
• a multi-rate ODS-CDMA system with a codebook of N Syivester- Hadamard sequences (N ⁇ _? for some integer p), fundamental data symbol rate f$, and a fixed chip rate « ⁇ // here can support users with higher data rates than the fundamental rate.
• the users may request to transmit data at rates Zf detox where r ranges from 0 to p. This is achieved by reducing tne iengtn of the spreading code to N2 such that the chipping rate, «'- s(N/Z)(_ft_) t remains constant
• Subsequences of length N/2f can be generated by sub-dividing an allocated codeword of i ⁇ ngth N into 2? sub-codes.
• any co ⁇ eword of length N is a repeated subsequence of length N/2 (up to a ⁇ sign), which is orthogonal to the other non- identical subsequences of the same length.
• the A* codeword is composed of a repeated subsequence of length N/2 (up to a ⁇ sign)
• tnen the codewords (k+i ( N2y, ma ⁇ H)' or ⁇ i m 1 £.-•>. / -1 and r > 0, are also composed of the same suose ⁇ uence (up to a ⁇ sign), and are no longer available tor allocation, if system orthogonality is to be preserved. Indeed, supporting 1 user at data rate 2f, is effectively equivalent to supporting Queers at rate f, since 2-1 codes are rendered unusable.
• the network controller it ia advantageous for the network controller to have the capability to change orthogonal sequence assignments periodically in order to most efficiently use the code space as the mix of user rates changes, in addition, since the symbol interval of data at rate 2f, is reduced to 12 times the symbol interval of data at rate /» it may be desired, depending on the application, to increase the transmit power tif2 to maintain the same level of performance.
• a multi-rate ODS-CDMA communication system with a codebook of N orthogonal sequences, fundamental data rate /, , and a fixed chip rate " Nf 9 t can support usere with tower data rates than the fundamental rate.
• the users may request to transmit data at rates fj2, where r is a positive integer ranging from Oto ⁇ .
• a tower rate user, at rate f/2, is assigned a longer spreading code of length 2N such that the chipping rate, * (2N)(f ⁇ Z), remains constant
• the long code is constructed from the codeword allocated to this user, c_. k m C,..., ⁇ , as follows.
• an orthogonal function set G(2) of size 2 is constructed. This could be, but la not necessarily, a Syivester-Hadamard matrix.
• the Kronecker product of the f row of G(2) with the codeword c* produces a member L*_ j* C,...,2-1 of a set of 2 mutually orthogonal codes of length 2N.
• Each of these long sequences is orthogonal to codewords a, where i ⁇ k, &a well aa longer code words Lf formed by the Kronecker product of each row of G(2) with the codewords Q.
• users of rate fj can be assigned to a single code word c_ by assigning each user a different row of G(2).
• This code construction technique can be generalized to accommodate a mix of users with different rates of the form f 2, with the maximum aggregate symbol rate bounded by .
• the proposed scheme suggests that the symbol interval of data at rate /2 is increased to ⁇ times the symbol interval of data at rate / « . It may be desired that a user at the tower data rate reduce transmit power by 2 in order to maintain the eam ⁇ symbol-to-noiso energy ratio (Es No).
• the Hub Station receives a superposition of signal waveforms transmitted by the Return Link users.
• the received signal is demodulated and deapread by the multi-rate ODS-CDMA receiver.
• the despreading operation involves correlating the received waveform with the spreading sequence of the desired user over a symbol interval. Hence, in a multi-rate system that supports users at higher data rates than /* the waveform of a user at rate 2f, is deapread by correlating it 10
• the waveform of a user at rate 1J2 is despread by correlating rt with the user's allocated 2N chip Kronecker-product code.
• the d ⁇ spreading operation requires that the codewords of all users be time aligned at the code boundaries in order to preserve orthogonality, and that the data symbol rates of all users be known at the receiver.
• the row index of G(2), used in forming the Kronecker-product code must be known at the receiver. Appropriate provisions are to be made in a multi-rate ODS-CDMA system to satisfy these requirements.
• FIG. 1 A functional block diagram of the Return Link structure in the proposed multi-rate ODS-CDMA system is shown in Figure .
• M users transmit data to the Hub Station at data symbol rates 2 i» 2*L..., 2"f t , where ⁇ j « 1,...,M, ranges from 0 to p for a system supporting higher rates than from -q to 0 for a system supporting lower rates than / «, and - ⁇ 7 to p for a system supporting both high rate and tow rates.
• the user's code generator If the fix user requests a higher rate ⁇ ⁇ 0), the user's code generator generates a codeword of length N at rate / «, which is equivalent to generating 2 sub-codes of length 2 r ⁇ N in time-division multiplex at rate f + On the other hand, if the yth user requests a lower rate (ij ⁇ 0), the user's code generator generates a Kronecker-product code of length 2*H at rate 2 f f_. Hence, at each symbol interval, 7 ⁇ - 1/(2* fj, the data symbol of the h user is multiplied by the codeword from the code generator, and the resulting chip sequence is modulated and transmitted to the Hub station.
• the Hub Station receives the composite signal waveform of the M Return Link users.
• the functional block diagram of an exemplary multi-rate ODS-CDMA receiver at the Hub Station is shown in Figure 2.
• the receiver demodulates the received waveform and despreads it For the /h user with ⁇ ⁇ 0, d ⁇ spreading involves formatting each block of N chips of demodulated data into subsequences of length Z N, and correlating each of these subsequences with 11
• each block of Z N chips is correlated with the long code from the code generator. In the absence of channel distortions, the correlator output yields the user data.
• the chip sequence transmitted by User 2 within the same time interval is x® s [s ⁇ *t+1 -1] 5 ⁇ +1 -ID, where s_ & and SiP are the current data symbols. Note that with the current allocation scenario, only code c 9 is available for allocation to a new user with symbol rate /, or lower. A user that requests a higher symbol rate cannot be supported unless the codes are reassigned.
• the signal received at the Hub Station (after demodulation) is of the form
• the signal is de-spread by correlating it with the appropriate spreading code. For User 1, this is achieved by computing the following inner product
• the data symbols of User 2 are retrieved by first partitioning the received chip sequence into 2 subsequences, /" ⁇ [/* /i], where ⁇ - ⁇ ⁇ t+l +1] + s « ⁇ t+1 -1] end * « s ⁇ +1 +1 ⁇ + s ⁇ +1 -1] and then computing the following inner products
• An additional user at rate f/2 may be accomodated with the orthogonal code constructed by taking the Kronecker product of row1 of Gr ⁇ ) with C ⁇ to give L»* [+1 -1) ® cy [ cy -Cj], or 2 additional users can be supported at the rate f 4 by taking the Kronecker product of the rows of G(2) with Lijto construct La*- [4*1 • +1)® w « [C -c* cj -cjjj andLw* [+1 -1 J ⁇ iw -l ⁇ * -Cj -Cj cj. Note that the codes L'* and i j are 16 chips long. 13
• the example system described above can support higher rates than of 21,, 4f traffic and lower symbol rates of 1J2, f ⁇ 4, ... fj2.

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• 1999
  • 1999-02-17 US US09/250,845 patent/US6563808B1/en not_active Expired - Lifetime
  • 1999-03-01 CA CA002322543A patent/CA2322543A1/en not_active Abandoned
  • 1999-03-01 WO PCT/US1999/003722 patent/WO1999045669A1/en active IP Right Grant
  • 1999-03-01 AU AU27781/99A patent/AU748010B2/en not_active Ceased
  • 1999-03-01 EP EP99908319A patent/EP1060588A4/en not_active Ceased
  • 1999-03-01 JP JP2000535113A patent/JP4361682B2/ja not_active Expired - Fee Related
  • 1999-03-01 CN CNB998058238A patent/CN1193528C/zh not_active Expired - Fee Related

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