Source: https://patents.google.com/patent/JP2008505587A/en
Timestamp: 2020-01-26 06:37:40
Document Index: 429125128

Matched Legal Cases: ['art 150', 'art 230', 'art 280', 'art 400', 'art 400', 'art 480']

JP2008505587A - IFDMA communication system with frequency hopping - Google Patents
IFDMA communication system with frequency hopping Download PDF
2008-02-21 Publication of JP2008505587A publication Critical patent/JP2008505587A/en
Frequency hopping in IFDMA systems occurs through the use of time-varying IFDMA modulation codes that vary with time. Specifically, the modulator (102) receives a symbol stream and a user specific IFDMA modulation code (b i (t)) (112). The output of the modulator includes a signal (x i (t)) (114) present at a constant frequency (ie, subcarrier). The actual subcarrier utilized by the signal x i (t) depends on the symbol block repetition and the particular IFDMA modulation code (118) utilized.
The present invention relates to wireless communication. In particular, the present invention relates to an improved modulation scheme for wireless systems.
Developing a flexible and efficient physical layer modulation scheme is a challenging aspect in the design of advanced cellular systems. Desirable attributes of the physical layer include reducing the cost, size and power consumption of subscriber equipment power amplifiers with a moderate average-peak power ratio, supporting a wide range of data rates, and the quality of the received signal. Supports data rate changing performance, adaptive modulation, supports wideband channel (eg 20 MHz) frequency diversity when transmitting at “narrowband” data rates, and minimizes intra-cell interference Performance to maintain orthogonality between transmissions by different users in the cell is included.
Interleaved frequency division multiple access (IFDMA) is a multi-carrier modulation scheme that can meet many of the above criteria for the physical layer. IFDMA is an improved modulation that combines some of the desirable characteristics of both orthogonal frequency division multiplexing (OFDM) and single carrier modulation. In IFDMA, the baseband signal starts as a single carrier quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM) symbol stream. Since IFDMA starts as a single carrier scheme (prior to blocking and repetition), its peak-to-average ratio is the same as single carrier modulation and provides frequency diversity. However, in the IFDMA scheme, each user needs to transmit using the same number of subcarriers. In addition, there is no ability to perform frequency hopping. This hinders the use of different data rates by different users and limits IFDMA flexibility in advanced communication systems. Therefore, there is a need for an improved modulation scheme that provides a high degree of data rate flexibility and frequency hopping while retaining orthogonality and low peak-average ratio.
To address the above needs, a method and apparatus for providing a frequency hopped IFDMA communication system is disclosed herein.
Frequency hopping in IFDMA systems occurs through the use of time-varying IFDMA modulation codes that vary with time. Specifically, the modulator receives a symbol stream and a user specific IFDMA modulation code (b i (t)). The output of the modulator includes a signal (x i (t)) present at a constant frequency (ie, subcarrier). The actual subcarrier utilized by the signal x i (t) depends on the repetition of the symbol block and the particular IFDMA modulation code utilized.
The present invention includes a method for frequency hopping in an IFDMA communication system. The method includes receiving a symbol stream, receiving a first IFDMA modulation code, and modulating the symbol stream with the first IFDMA modulation code in a first period to a first set of frequencies. Generating an existing signal. A second IFDMA modulation code is then received, and in a second period, the symbol stream is modulated with the second IFDMA modulation code to generate a second signal that is present in the second set of frequencies.
Furthermore, the present invention includes a method for frequency hopping in an IFDMA communication system. The method includes receiving a symbol stream and frequency hopping the symbol stream by changing an IFDMA modulation code used in each of the first and second IFDMA symbol periods.
The present invention further includes a method for receiving a signal that is frequency hopped in an IFDMA communication system. Receiving a symbol stream over a first plurality of channels; demodulating the symbol stream using a first IFDMA modulation code; receiving a symbol stream over a second plurality of channels; Demodulating the symbol stream using an IFDMA modulation code.
Furthermore, the present invention receives a symbol block repeater that outputs a block repetition symbol stream, a modulation code vector generator that outputs a time-varying IFDMA modulation code, a block repetition symbol stream and a time-varying IFDMA modulation code, and an IFDMA modulation code And an IFDMA modulator that outputs signals present at different frequency sets in response to
Furthermore, the present invention receives a baseband converter that receives an IFDMA signal that is frequency-hopped and outputs a baseband filter signal that exists at a plurality of frequencies, a time-varying modulation code and a plurality of frequencies, and a time-varying And a demodulator that outputs a signal demodulated based on the modulation code.
Reference is now made to the drawings. In the drawings, like reference numerals refer to like elements. FIG. 1 is a block diagram of an exemplary system 100 in accordance with the present invention. System 100 includes one or more transmitters 102. Each transmitter 102 corresponds to a particular user and communicates with one or more receivers 104 over a communication channel 106. The communication channel 106 can be a radio frequency (RF) channel in a multi-user communication system such as a cellular system, a paging system, a mobile radio system. As shown, multiple users (eg, N users) such as user i, user j, and user k can transmit data simultaneously through channel 106. The transmitter 102 and the receiver 104 may be one or more of a mobile subscriber unit, such as a mobile phone or pager, and a base station. Transmitter 102 and receiver 104 may include one or more suitable combinations of hardware and software components for implementing the modulation scheme of the present invention. As shown, user i's transmitter 102 of FIG. 1 includes a block repeater 108, a cyclic extension device 110, a modulator 112, and a pulse shaping filter 114. As described in more detail below, receiver 104 includes processing for demodulating and equalizing the received signal.
Furthermore, the present specification describes a case in which the data rate of users in an IFDMA type system can be different in the system 100 and there can be frequency hopping. Two coding techniques (block modulation code and phase ramp modulation code) for orthogonal discrimination of different data rates are disclosed. Further, conditions for maintaining orthogonality between different users, that is, conditions under which multiple access interference (MAI) does not occur will be described in detail.
In IFDMA, the baseband signal begins as a single carrier phase shift keying (PSK) or quadrature amplitude modulation (QAM) symbol stream. The symbols are grouped (via repeater 108) into blocks that are repeated L times prior to filtering (typically by a root cosine filter) and transmission. Here, L is an integer. Symbol block repetition results in a non-zero spectrum of the transmitted signal only at a certain subcarrier frequency (ie, every Lth subcarrier). Therefore, the spectrum of the transmission signal before pulse shaping is similar to that seen when data symbols are modulated only on every Lth subcarrier of an Orthogonal Frequency Division Modulation (OFDM) signal. Specifically, this type of OFDM transmission may be referred to as comb-like OFDM. However, the IFDMA spectrum decays rapidly after pulse shaping compared to OFDM. Since the signal occupies only one subcarrier for every L subcarriers, a total of L different users can transmit simultaneously. The present invention is also applicable to different users using different constellation sizes (for example, QPSK, multi-level QAM, 8-PSK).
In IFDMA transmission, 1) different sets of subcarriers are occupied by the user-specific modulation code (b i (t)) of IFDMA, and 2) the cyclic extension (ie guard period) added to the transmission is the channel pulse response. Longer 3) As long as the signal is time synchronized with the receiver 104, it remains orthogonal.
Thus, modulator 112 receives a symbol stream and a user-specific IFDMA modulation code (b i (t)) (sometimes referred to simply as a modulation code). The output of the modulator 112 includes a signal (x i (t)) present at a constant frequency (ie, subcarrier). The actual subcarrier utilized by the signal x i (t) depends on the symbol block repetition and the particular modulation code utilized. Therefore, the subcarrier set is changed by changing the modulation code (b). However, it is noted that even if the subcarriers used for transmission are changed by changing b i (t), the property that the subcarriers are uniformly spaced is maintained. Therefore, in a preferred embodiment of the present invention, the modulation code vector generator 105 periodically outputs a time-varying user-specific modulation code that varies with time. This modulation code varies depending on the user. By periodically changing the modulation code b i , frequency hopping for a specific user is performed. In the preferred embodiment of the invention, b i is changed once per IFDMA symbol. Thus, each IFDMA symbol occupies a different set of subcarriers.
Reference is now made to FIG. FIG. 2 shows a flowchart 150 showing an example of the operation of the transmitter 102 of FIG. For each active user i (i = 1,..., K), a block transmission of Q i data symbols in one cell of the mobile communication system 100, d (i) = [d 0 (i ), D 1 (i),. , D (Qi-1) (i)] is executed.
In step 152, the data symbols for user i are converted from serial symbol format to parallel symbol format and then received by block repeater 108. In step 154, a symbol block is formed by block repetition (L i times) of Q i data (eg, QAM) symbols. A user specific block repetition rate is used. By the block repetition, the bandwidth occupied by the transmission signal is increased by L i times, and the spectrum becomes non-zero only for every L i th subcarrier frequency (subcarrier interval = 1 / Q i L i ). . FIG. 3 shows an example of a repeated symbol block.
In step 156 of FIG. 2, a guard period (cyclic extension including prefix, postfix, or both) is added prior to modulation and pulse shaping of the repeating symbol block. As shown in FIG. 3, the guard interval and the repeated symbol block together form an IFDMA symbol. In the case of FIG. 3, the guard interval is a cyclic prefix. It is also possible to add a guard interval after the modulation of the repetitive block and before the pulse shaping. When a cyclic prefix having a length v is used and time is expressed as −v to Q i L i −1, elements of the repetitive symbol block f i are as follows.
In step 158, a time varying user specific modulation code is applied by the modulator 112. This modulation code can be any suitable code that satisfies the code assignment conditions disclosed herein. After application of the user-specific modulation code, the vector of transmission symbols is
Here, b l i is a modulation code assigned to a user i having a data rate R i (proportional to Q i ).
In the following description, the data rate is defined as the ratio of the number of symbols in the symbol block to the total number of symbols in the repeated symbol block, ie:
By appropriately selecting the modulation code and user data rate, it is possible to maintain orthogonality between users with different data rates even in multipath conditions, as long as the channel changes slowly for the repetition symbol block duration. Is possible. In order to maintain orthogonality, the repetition symbol block duration is the same for all users, i.e .:
In step 160, parallel-serial conversion is performed on the symbol vector. The serialized vector is then filtered using pulse shaping filter 114 before being transmitted on channel 106 (step 162).
The present invention provides different data rates for different users by providing multi-rate transmission using user-specific data blocks and repetition sizes and time-varying user-specific modulation codes, and advanced with frequency hopping. Provides data rate flexibility. The present invention also maintains a low peak-average ratio and provides two choices of user specific modulation codes (block modulation code and phase ramp modulation code). These modulation codes retain orthogonality between different data rates even in multipath channels (ie, there is little or no MAI).
Now consider a typical system with bandwidth B and four users with different data rate requirements. Let Rs be the maximum data rate from which a single user occupying the entire bandwidth B can be obtained. Table 180 of FIG. 4 shows user specific data rate requirements. Also shown are user-specific parameters and possible code assignments in the multi-rate IFDMA of the present invention. As shown in table 180, the present invention allows different data requirements to be fully met by assigning different user specific symbol blocks and repetition sizes and user specific modulation codes.
In the frequency domain, the modulation code has the effect of interleaving the spectrum of various multi-rate users of the system 100. FIG. 5 shows an example of interleaving. Specifically, FIG. 5 shows a graph 190 of subcarriers occupied by four users in the modulation schemes disclosed herein. As can be seen in FIG. 5, the higher data rate user (user A) is assigned more bandwidth (subcarrier) than the other lower data rate users (users B, C, D). The present invention is not limited by the number of user or subcarrier allocations shown in the example of FIG.
[Receiver signal processing]
6 and 8 show two alternative receiver architectures that can be used as the receiver 104 of FIG. Each of these architectures is described in further detail below. However, the general operation of the receiver 104 will first be described. The combined composite envelope of the received signals from K users after propagating through the multipath channel 106 and corrupted by additional white Gaussian noise is given by:
Where p l i = t l * h l i * r l is the equivalent baseband channel pulse response (CPR) for user i (dimension = M + 1), h l = equivalent low-pass channel pulse response, t l = equivalent transmitter baseband pulse, r l = equivalent receiver baseband pulse, n l = receiver noise (AWGN). As mentioned above, the received signal is propagated through different channels depending on the current modulation code b l i (t) used by the transmitter.
The receiver 104 can select the last N samples (ie, after removal of the cyclic extension) of the received signal in Equation (4) and demodulate the user data. The demodulation process for user i consists of equalization of the channel pulse response p l i , modulation code association b l i (t), and repetition of L i . Depending on the order of operation, two receiver structures are possible as described below. b l i (t) becomes the same as the current b l i (t) used by the transmitter, it is clear that changes for each frame. In a preferred embodiment of the present invention, it is possible to change one or more of the data rate utilized by the channel and the current frequency by changing b l i (t). Thus, b l i (t) may change during transmission when the data rate changes, or may change from frame to frame in frequency hopping.
[First receiver structure]
In FIG. 6, a first receiver structure 200 is shown. In this architecture, channel equalization is performed prior to associating a signal with a time-varying user-specific modulation code. The first receiver 200 includes an RF-synthesis baseband conversion circuit 204. The RF-synthesis baseband conversion circuit 204 includes an A / D converter / sampling circuit, a baseband filter 206, a cyclic extension remover 208, an equalizer 210, a demodulator 212, a symbol block combining unit 214, and a symbol determination device 216. Is provided. Demodulator 212, symbol block combiner 214, and symbol determination device 216 perform functions for a specific user.
FIG. 7 is a flowchart 230 showing the operation of the first receiver 200. In step 232, the received signal is down-converted to a synthesized baseband signal by the A / D converter / sampling circuit 204, filtered (image removal, adjacent channel removal, aliasing avoidance), and digitized. In step 233, the baseband signal is a baseband filtered signal present at multiple frequencies, typically a matched filter matched to a transmit pulse shaping filter. In step 234, the cyclic extension is removed.
In step 236, the received signals from all users sampled and baseband combined (N samples for 1 × oversampling at the receiver after removal of the cyclic extension) are frequency domain equalized using a frequency domain equalization technique. Equalized to the channel response of each user. Since each user uses an orthogonal set of subcarriers (for user specific block repetition and modulation codes), all users can be equalized simultaneously in the frequency domain using only one N-point transform. However, the equalizer coefficients are different for each user and apply only to the subcarriers occupied by that user. Accordingly, in step 236, a combination of common processing and user-specific processing is performed. Alternatively, based on other techniques such as linear transversal time domain equalizer, decision feedback equalizer, maximum likelihood sequence estimation, iterative equalizer, intersymbol interference (ISI) canceller and turbo equalizer It is also possible to perform equalization.
In step 238, the demodulator 212 then associates the equalized signal sign with each user. At step 240, the associated signals are combined to yield a soft estimate z (i) for Q i transmitted symbols. If error correction coding is used at the transmitter, the soft decision is passed to a forward error correction (FEC) decoder. In step 242, a logic decision is made based on the estimated symbol to determine the value of the symbol. Each symbol may include one or more bits.
[Second receiver architecture]
In FIG. 8, a second receiver structure 260 is shown. In this architecture, prior to channel equalization, an association with a user specific code is performed. The second receiver 260 includes an RF-synthesis baseband conversion circuit 264. The RF-synthesis baseband conversion circuit 264 includes an A / D converter / sampling circuit, a baseband filter 266, a cyclic extension remover 268, a demodulator 270, a symbol block combining unit 272, an equalizer 274, and a symbol determination device 276. Is provided. Demodulator 270, symbol block combiner 272, equalizer 274, and symbol determination device 276 perform functions specific to the user.
FIG. 9 is a flowchart 280 showing the operation of the second receiver 260. FIG. 9 shows receiver signal processing in this type of receiver structure 260. In step 282, the received signal is down-converted to a synthesized baseband signal by A / D converter / sampling circuit 264, filtered (image removal, adjacent channel removal, aliasing avoidance), and digitized. In step 283, the baseband signal is baseband filtered and is typically a match filter matched to a transmit pulse shaping filter. In step 284, the cyclic extension is removed.
At step 286, the demodulator 270 associates each user with the signal from all users sampled and baseband combined (after removal of the cyclic extension). At step 288, the associated signals are combined using user specific block iterations. The resulting signal (1 × Q i samples for receiver oversampling) is then equalized (time domain or frequency domain) resulting in an estimate z (i) of Q i transmit symbols ( Step 290). Using various techniques including linear transversal time domain equalizer, decision feedback equalizer, maximum likelihood sequence estimation, linear frequency domain equalizer, iterative equalizer, ISI canceller and turbo equalizer, etc. Can be performed. When using frequency domain equalization, Q i point transformation is required for each user. In step 292, a logic decision is made based on the estimated symbol to determine the value of the symbol. Each symbol may include one or more bits.
[Receiver analysis]
The following analysis is based on the first receiver 200 of FIG. However, the results of the following analysis are also valid for the second receiver 260 shown in FIG.
The analysis is as follows. Let g l ij be the response of the j-th user channel pulse response p l j to the equivalent i-th user equalization filter e l j . That is, g l ij = p l j × e l l . Thus, the equalized received signal sample for user i is given by:
Assuming no-noise transmission (n l = 0) in order to derive conditions for distinguishing orthogonal users, the following equation is obtained.
Associating the equalized received signal samples with the corresponding i-th user modulation code b l i and combining the information in L i iterations to estimate Q i data symbols transmitted for user i Is obtained as follows.
[Modulation code selection]
This specification describes two types of modulation codes: blocks and phase ramps. The following description derives the conditions and codes necessary to maintain orthogonality between users with different data rates in the system 100. Regardless of the type of modulation code used, it is possible to vary the modulation code of an individual transmitter / receiver over time so that frequency hopping is possible.
[Block modulation code]
The modulation code b l i (t) for user i is of the form:
Here, a channel identification code vector for distinguishing orthogonal users
The characteristics (and values) of are determined as follows.
From equation (8), as shown in block sequence 300 of FIG. 10, in the repetitive symbol block, user i's modulation code b l i takes only L i different values and repeats the l th symbol block.
It proves to be constant for all data symbols (c l i). Therefore, from Equation (2) and Equation (8),
Where (a) b = a mod b
Is the floor function. If equation (8) is used for equation (7), the estimated value of Q i data symbols transmitted by user i using the block modulation code is given by the following equation.
Here, considering the case where the data rate of user i is an integer multiple of the data rate of user j,
Therefore, from equation (12), as shown in FIG. 11, when the repetition symbol block duration (N) is the same for all users, the number of data block repetitions (L i ) for user i is for user j. W ij times smaller than the number (L j ). FIG. 11 shows representative symbol block iterations 310-320 for user i and user j. Using equation (10) and equation (12), it is shown to user j as follows:
Using equation (13) for equation (11) and substituting Δ = −m, an estimate of Q i data symbols transmitted by user i using block modulation codes can be written as:
Here, let u and v be periodic sequences of length L. The periodic cross-correlation between u and v is given by
In equation (16), the sequence u l j is a version of c l j decimated by w ij and δ j is the decimation phase. Here, from the equation (17), the following equation is established under the condition where the MAI does not exist, that is, the condition for distinguishing the orthogonal users.
That is, the zero lag component of the decimated periodic cross-correlation between the block modulation codes of users with different data rates needs to be zero for all decimation phases.
The above analysis assumes that user transmissions are received simultaneously at the receiver (see equation (4)). For asynchronous users (the cyclic extension is long enough that relative signal arrival delays between users and channel pulse response durations are allowed in the worst case), the absence of MAI can result in decimated periodicity. It can be seen that all lags (shifts) of a simple cross-correlation sequence must also be zero. Therefore, in general, under the condition where no MAI exists, the following equation holds.
Based on equation (19), equation (17), which is an estimate of Q i transmitted data symbols for user i using a block modulation code, is reduced to:
In the case of ideal equalization (intersymbol interference, i.e. complete removal of ISI), i.e. no multipath,
Similarly, if the condition of equation (19) is satisfied, it can be seen that there is no MAI from equation (17) of demodulation for user j, which has a lower data rate.
Summarizing Equations (3), (12) and (19), in a multipath channel using a block modulation code defined by Equation (8), a condition for distinguishing orthogonal users, ie, data rate The conditions for the absence of MAI for different users are as follows:
1. All users have the same repetition symbol block duration.
2. All users' data rates must be integer multiples of each other (K) or a fraction of an integer (1 / K).
3. All lags (shifts) of the decimated periodic cross-correlation sequence between the channel identification code vectors c Li, i and c Lj, j of all users are zero.
[Expansion of multiple rational data rates between users]
It is possible to extend the above-described orthogonality condition for block codes to support users with rational data rates, ie, rational symbol block repetition factors as shown in FIG.
Shown in FIG. 12 is an example of symbol block repetition for user i (330-334) and user j (336-340) within a repeating symbol block. By distinguishing between orthogonal users and supporting rational user data rates such as w ij = 3/2 in FIG. 12, modulation flexibility is improved and a wide range of user data rate requirements are supported. A condition for distinguishing orthogonal users in a multipath channel using a block modulation code, that is, a condition in which MAI does not exist in users having different data rates is updated as follows.
2. All users' data rates must be rational numbers of each other.
3. All lags (shifts) of the periodic cross-correlation sequence between the decimated channel identification code vectors c Li, i and c Lj, j of all users are zero in all possible decimation phases. .
The decimation factor for each code vector is of the form
Here, δ i and δ j are decimation phases, and R uv ji (α) is the following equation.
[Block channel identification code]
Referring back to the conditions for distinguishing orthogonal users, a particular family of channel identification codes that satisfy condition 3 of the orthogonality requirement is in the row of the L-dimensional discrete Fourier transform (DFT) matrix (C L ). Based.
Appropriate assignment of channel identification codes c L, k based on DFT allows users with different data rates to satisfy condition 2 and distinguish between orthogonal users in a multipath channel, however, possible assignments There are many. An example of a small number of channel identification code assignments using code tree structures 350 to 380 is shown in FIG.
As can be seen from the example of the code trees 350 to 380 in FIG. 13, in principle, a wide range of data rates is possible by dividing each node of the tree (in a particular stage) into b branches. is there. At this time, the data rate obtained by each subsequent branch is b times smaller than the data rate of its parent. Since all codes at the same depth in the code tree have the same repetition factor, they occupy the same number of subcarriers. Thus, the data rate obtained by using arbitrary codes at the same depth in the code tree is the same. However, subcarriers occupied by any code are orthogonal to subcarriers occupied by any other code at the same depth in the code tree. Thus, it is possible to support users with different data rate requirements by appropriately assigning one of the channel identification codes according to the following rules and constraints.
1. When a particular code is used, it is impossible to use other codes in the path from that code to the root and in the secondary tree (subtree) under that code.
2. Every code at an arbitrary depth in the tree is a set of DFT basis functions (C L = {c L, k }) of that dimension (L).
The complexity of meeting the above constraints increases when the code utilized between the transmitter / receiver varies with time. In the preferred embodiment of the present invention, vector generator 105 changes the specific channel identification code utilized at each transmitter 102 once every IFDMA symbol to ensure that the above constraints are met.
FIG. 14 shows a flowchart 400 outlining the method of channel identification code assignment. In step 401, user data rate requirements are received. In step 402, the depth of the code tree that satisfies the user data rate requirement is determined. In decision step 404, a check is made to determine if there are any unused codes currently available at the requested data rate. If there is an unused code currently available, the user is assigned to the available code, all codes in the path from the assigned code to the root, and all in the subtree below the assigned code Is marked as an unusable code (step 408).
If no code is available at the requested rate, the user is rescheduled for later transmission, or alternatively, the user is assigned a code corresponding to a higher or lower data rate. (Step 406).
[Phase ramp modulation code]
The analysis for the phase ramp modulation code is the same as the analysis for the block modulation code described above. The modulation code b l i for user i to the following format.
Here, the characteristic (and value) of the channel identification code θ i necessary for distinguishing orthogonal users is determined. From equation (24), the phase of user i's modulation code b l i increases linearly throughout the symbol duration. Therefore, from Equation (2) and Equation (24),
Here, (a) b = a mod b.
Using equation (24) for equation (7), an estimate of Q i data symbols transmitted for user i using a phase ramp modulation code can be written as:
Here, similarly to the block modulation code, the case where the data rate of the user i is an integral multiple of the data rate of the user j, that is, the case of the following equation is considered.
Thus, from equation (28), as shown in FIG. 11, when the repetition symbol block duration (N) is the same for all users, the number of symbol block repetitions (L i ) for user i is for user j. W ij times smaller than the number (L j ).
Using equation (26) and equation (28), the following is shown to user j.
Using equation (29) for equation (27) and replacing Δ = −m, an estimate of Q i data symbols transmitted for user i using the phase ramp modulation code can be written as:
From the expression (31), the following expression is established under the condition where the MAI does not exist, that is, the condition for distinguishing the orthogonal users.
From equation (32), the estimated value of Q i transmitted data symbols for user i using a phase ramp modulation code that satisfies equation (31) is reduced to:
As shown by equation (21), in the case of ideal equalization (intersymbol interference, ie, complete removal of ISI), ie, no multipath, the following equation holds:
Similarly, when the condition of Expression (32) is satisfied, it can be seen that there is no MAI due to demodulation of the user j having a lower data rate (Expression (28)).
Therefore, when Equations (3), (28) and (32) are put together, users with different data rates using the phase ramp modulation code defined by Equation (24) are distinguished from each other in the multipath channel. The conditions for doing so, ie, the absence of MAI, are as follows:
3. All user modulation codes (and hence channel identification codes) must satisfy the following equation:
Like block modulation codes, the above orthogonality condition for phase ramp codes is extended to support users with rational data rates, ie, rational symbol block repetition factors as shown in FIG. It is possible. By distinguishing orthogonal users and supporting a rational number of user data rates, modulation flexibility is improved and a wide range of user data rate requirements are supported. A condition for distinguishing orthogonal users in a multipath channel using a phase ramp modulation code, that is, a condition in which MAI does not exist in users having different data rates is updated as follows.
[Phase ramp channel identification code]
A channel identification code satisfying the orthogonality requirement of Condition 3 described above has the following format.
Therefore, by appropriately assigning channel identification codes θ Li, i , users with different data rates satisfy condition 2 and can distinguish between orthogonal users in the multipath channel. However, as with block modulation codes, the number of possible assignments is large. An example of a small number of channel identification code assignments using code tree structures 420 to 450 is shown in FIG.
As can be seen from the example of FIG. 15, a wide range of data rates is possible by further dividing each node of the tree (at a particular stage) into b branches. At this time, the data rate obtained by each subsequent branch is b times smaller than the data rate of its parent branch. Since all codes at the same depth in the code tree have the same repetition factor, they occupy the same number of subcarriers. Thus, the data rate obtained by using arbitrary codes at the same depth in the code tree is the same. However, subcarriers occupied by any code are orthogonal to subcarriers occupied by any other code at the same depth in the code tree. Thus, when a particular code is used, one of the channel identification codes subject to the condition that it is impossible to use the path from that code to the root and any other code in the subtree below that code. By appropriately assigning, it is possible to support users with different data rate requirements. Frequency hopping is supported by changing the code used by the user.
Similar to the use of block channel identification codes, the complexity of meeting the above constraints increases when the codes utilized between the transmitter / receiver change over time. In the preferred embodiment of the present invention, vector generator 105 changes the specific channel identification code utilized at each transmitter 102 once every IFDMA symbol to ensure that the above constraints are met. A flowchart 400 in FIG. 14 shows a channel identification code assignment method. This method is the same for block modulation codes and phase ramp modulation codes.
Further, the condition for distinguishing orthogonal users using the block modulation code and the phase ramp modulation code derived in the above is the operation in the reverse order in the receiver (that is, code association as shown in FIG. 8). It is also effective in In addition, it is possible to equalize all users at once using frequency domain equalization techniques.
[Asynchronous user]
For asynchronous users, an additional guard period longer than the worst case relative signal arrival delay (P D ) between users is required to preserve orthogonality. The receiver then selects an appropriate temporal position (window) for N symbol samples such that there is no multiple access interference (MAI) for all users (assuming 1 × oversampling). . FIG. 16 shows an example of three asynchronous users.
As seen in FIG. 16, a cyclic extension of length v and a reference user (e.g., user A) is for the relative delay / previous delta i of user i, the starting position of the non-existing temporal window of MAI delta following Satisfy the relationship.
Thus from equation (36), when the relative delay is less than the maximum delay P D, it is possible to N symbols sample window that is not a plurality of contamination. The receiver can select any one of these sampling instances for further processing.
FIG. 17 shows a flowchart 480 for temporal window selection processing for an asynchronous user. In step 401, a signal is received by the receiver 104. In step 482, the relative delay of the user compared to the reference user is calculated. From these relative delays, a minimum delay and a maximum delay are determined (step 484). In step 486, a possible temporal starting point of the window without MAI is determined. In step 488, a sample is selected from the window. In step 489, these samples are provided to the equalization and code association function of the receiver, as described in connection with FIGS.
It is possible to use additional redundant samples as side information to improve the received signal-to-noise ratio and thus improve the detector performance at the receiver. It is also noted that user dependent rotation of the demodulated data symbols can be used prior to forward error correction decoding.
One alternative way to enable multi-rate users is to assign more than one code to a particular user and linearly combine the individual code modulated signals. However, such a scheme can be influenced by a large peak-average ratio because the modulation signals are added linearly. In addition, other multiple access protocols such as time division multiple access (TDMA) or code division multiple access (CDMA) protocols can be used with the multirate IFDMA scheme described herein. Alternatively, those protocols can operate on the multi-rate IFDMA scheme described herein.
FIG. 18 shows frequency hopping by changing the user-specific modulation code. For simplicity, FIG. 19 shows four users with the same data rate and different channel identification code assignments. As will be apparent, when frequency hopping is not utilized, the user specific modulation code (b i (t)) remains fixed for each user. For this reason, in all IFDMA symbol periods, the signal transmitted by the transmitter at a constant subcarrier frequency (that is, every 4 subcarriers in the example of FIG. 19) is non-zero. Thus, for each user, the signal spectrum transmitted before pulse shaping occupies the same subcarrier throughout all symbol periods. This signal spectrum is similar to that seen when data symbols are modulated only every four subcarriers of an Orthogonal Frequency Division Modulation (OFDM) signal.
On the other hand, when b i (t) can change over time, specifically, frequency hopping occurs when b i (t) can change in each IFDMA symbol period. Note that in each symbol period, the transmission signal of each transmitter is similar to that seen when data symbols are modulated only every four subcarriers of an orthogonal frequency division modulation (OFDM) signal. . Thus, in each symbol period, user transmissions occupy all available bandwidth. It is clear that by changing b i (t) in each IFDMA symbol period, the transmitter transmission is frequency hopped while still retaining the OFDM nature of the transmission. Specifically, the transmission signal of each transmitter in FIG. 19 occupies every 4 subcarriers (uniformly spaced). By changing the user-specific modulation code (b i (t)) for each IFDMA symbol while appropriately assigning channel identification codes and maintaining orthogonality between users, the transmission signal of each transmitter is changed to the previous IFDMA. Compared to the symbol, it occupies a different set of subcarriers that are equally spaced every 4 subcarriers. Specifically, in FIG. 19, the subcarrier occupied in a specific symbol is a cyclic shift of the subcarrier occupied in the previous symbol. Here, the cyclic shift is one subcarrier.
FIG. 19 is a flowchart showing the operation of the transmitter. The logic flow begins at step 1901. In step 1901, the user's block repetition symbol stream is received by the modulator 112 in a first period. In step 1903, the first IFDMA modulation code is received by the modulator, and in step 1905, the symbol stream is modulated by the first modulation code. As described above, the output of the modulator 112 includes the signal present on the first set of frequencies, i.e., subcarriers. The actual subcarrier utilized by the signal varies depending on the repetition of the symbol block and the particular modulation code utilized.
In step 1907, a second IFDMA modulation code is received in a second period. This symbol stream is modulated with a second modulation code in a second period (step 1909) to produce a signal present in a second set of frequencies, ie, subcarriers. As described above, preferably the first and second periods correspond to first and second consecutive symbol periods. Processing continues throughout the transmission of the symbol stream so that a unique IFDMA modulation code is utilized in each symbol period.
IFDMA channel identification code hopping is beneficial because it increases frequency diversity and improves the interference averaging of other cells to change the occupied subcarrier when different channel identification codes are used. Therefore, additional interference averaging and by mixing different users with channel identification codes at the same level and at one or more of the same level of IFDMA subtrees for each symbol or a small number of symbols, and It is possible to obtain frequency diversity. A user occupies a different set of subcarriers for each hopping.
FIG. 20 is a flowchart illustrating an operation of a receiver that receives an IFDMA signal that is frequency-hopped. The logic flow begins at step 2001. In step 2001, during the first frame, the baseband converter 204 receives signals from user i propagated through the first plurality of channels. As mentioned above, the channels differ depending on the current modulation code b l i (t) used by the transmitter. In step 2003, the signal is demodulated using a first IFDMA modulation code. As explained, the demodulation process for user i consists of equalization of the channel pulse response p l i , the association of modulation codes b l i (t) and the repetition of L i .
In step 2005, a signal is received during the second frame. However, during the second frame, signals are received through the second plurality of channels. Finally, in step 2007, the signal is demodulated using the second IFDMA modulation code.
1 is a block diagram of a system according to the present invention. The flowchart which shows operation | movement of the transmitter of the system shown in FIG. FIG. 3 is a diagram of an exemplary repeating symbol block. 2 is a table showing characteristic values of modulation schemes used by the system of FIG. 2 is a frequency chart showing subcarriers occupied by users of the system of FIG. 1 in a typical operating scenario. FIG. 2 is a block diagram of an exemplary first receiver that can be used in the system shown in FIG. 7 is a flowchart showing the operation of the first receiver shown in FIG. FIG. 2 is a block diagram showing a representative second receiver that can be used in the system shown in FIG. 1. 9 is a flowchart showing the operation of the second receiver in FIG. 8. FIG. 4 is a diagram of an exemplary sequence of symbol blocks transmitted over a channel. FIG. 4 is a diagram of representative symbol block repetition for multiple users of the system. FIG. 4 is a diagram of representative symbol block repetition for multiple users of a system using rational multiple data rates. A code tree indicating a typical channel identification code assignment in a block modulation code. The flowchart of the method of channel identification code allocation. A code tree showing typical channel identification code assignments in a phase ramp modulation code. FIG. 2 shows the relative position in time of an asynchronous transmission received by a receiver that can be included in the system of FIG. 6 is a flowchart illustrating a method for processing asynchronous transmission. The figure which shows the frequency hopping using a time change modulation code. 6 is a flowchart illustrating an operation of a transmitter that performs frequency hopping by using a time-varying modulation code. The flowchart which shows operation | movement of the receiver which receives the signal by which the frequency hopping is carried out.
A method for frequency hopping in an IFDMA communication system comprising:
A symbol receiving step for receiving a symbol stream;
A first modulation code receiving step of receiving a first IFDMA modulation code;
A first symbol modulation step of modulating a symbol stream with a first IFDMA modulation code in a first period to generate a signal present in a first set of frequencies;
A second modulation code receiving step of receiving a second IFDMA modulation code;
A second symbol modulation step of modulating a symbol stream with a second IFDMA modulation code in a second period to generate a second signal present in a second set of frequencies.
The method of claim 1, wherein the first period and the second period include a first symbol period and a second symbol period.
The first modulation code receiving step and the second modulation code receiving step include the steps of receiving the first IFDMA block modulation code and the second IFDMA block modulation code, the first IFDMA phase ramp modulation code, and the second IFDMA phase. The method of claim 1 including one or more of: receiving a ramp modulation code.
A modulation code changing step of frequency hopping the symbol stream by changing the IFDMA modulation code used in each of the first and second IFDMA symbol periods.
5. The method of claim 4, wherein the modulation code changing step includes one or more of changing an IFDMA phase ramp modulation code and changing an IFDMA block modulation code.
A method for receiving a signal that is frequency hopped in an IFDMA communication system, comprising:
Receiving a symbol stream through a first plurality of channels; and
A first symbol demodulation step of demodulating the symbol stream using the first IFDMA modulation code;
Receiving a symbol stream through a second plurality of channels;
And a second symbol demodulation step of demodulating the symbol stream using a second IFDMA modulation code.
The method of claim 6, wherein the first symbol stream and the second symbol stream are received through the first frame and the second frame.
The first symbol demodulation step and the second symbol demodulation step include a step of using the first IFDMA block modulation code and the second IFDMA block modulation code, a first IFDMA phase ramp modulation code, and a second IFDMA phase ramp modulation code. The method according to claim 7, comprising one or more of the following:
A symbol block repeater that outputs a block repetition symbol stream;
A modulation code vector generator for outputting a time-varying IFDMA modulation code;
An IFDMA modulator that receives a block repetition symbol stream and a time-varying IFDMA modulation code and outputs signals present in different frequency sets in accordance with the IFDMA modulation code.
The apparatus of claim 9, wherein the IFDMA modulation code comprises a block modulation code or a phase ramp modulation code.
10. The apparatus of claim 9, wherein the time varying IFDMA modulation code comprises an IFDMA modulation code that changes from frame to frame.
A baseband converter that receives an IFDMA signal that is frequency-hopped and outputs a baseband filter signal present at a plurality of frequencies;
A demodulator for receiving a time-varying modulation code and a plurality of frequencies and outputting a signal demodulated based on the time-varying modulation code.
The apparatus of claim 12, wherein the time varying modulation code comprises one or more of a time varying block modulation code and a time varying ramp modulation code.
13. The apparatus of claim 12, wherein the frequency hopped IFDMA signal is frequency hopped frame by frame.
JP2007520309A 2004-06-30 2005-06-06 IFDMA communication system with frequency hopping Pending JP2008505587A (en)
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