Turbo receiver for MIMOCDMA systems employing parity bit selected and permutation spreading
 Alireza Mirzaee^{1}Email author and
 Claude D’Amours^{1}
https://doi.org/10.1186/168714992013266
© Mirzaee and D’Amours; licensee Springer. 2013
Received: 18 December 2011
Accepted: 22 October 2013
Published: 15 November 2013
Abstract
In this paper, a turbo receiver for MIMOCDMA systems employing parity bit selected and permutation spreading is proposed. In such systems, spreading codes used by transmit antennas are dependent on the transmitted data. In the proposed system, convolutional coding is used as an outer code, while the parity bit selected scheme is used as the inner code. Detection and decoding are performed iteratively for each detected bit. When parity bit selected spreading is used, the parity bits generated by a linear block encoder are used to select a spreading code from a set of orthogonal spreading sequences. The selected spreading code is then used to spread the signals in all transmit antennas. In contrast, in permutation spreading, a permutation of orthogonal spreading sequences is used in each transmit antenna. In the proposed receiver, soft information passes between the detector and the decoder on multiple iterations. Detection is performed using the received signal from all receive antennas in combination with the extrinsic likelihood provided by a SISO decoder. The turbo receiver is further extended to a multiple user system, where the MAI is estimated in each iteration and subtracted out from the received signal. Simulations show a significant improvement in BER when a turbo receiver is used in these systems.
Keywords
1 Introduction
In multiple input multiple output (MIMO) systems, where multiple antennas are used at both ends of the wireless channels, significant power gains are achieved by taking advantage of fading diversity of the channel. Using spacetime codes in MIMO systems, a tradeoff between spatial multiplexing and diversity order can be achieved [1]. When code division multiple access (CDMA) is combined with MIMO systems (MIMOCDMA), orthogonal spreading sequences can be used in each transmit antenna in order to allow the receiver to accurately estimate the channel gains which are very crucial in the performance of MIMO systems.
Application of parity bits in selecting the spreading codes in directsequence spread spectrum (DSSS) systems is first introduced in [2]. In spread spectrum systems employing parity bits selected spreading codes (SSPB), instead of appending the parity bits generated from a linear block encoder, they are used to select a spreading code from a set of mutually orthogonal spreading sequences [2]. The extension of this work in MIMOCDMA systems is discussed in [3], where two main techniques to select spreading waveforms are explained. In a MIMOCDMA system employing parity bit selected spreading code, the calculated parity bits are used to choose a spreading waveform from a set of mutually orthogonal spreading codes to be used in all transmit antenna during that signaling interval. In contrast, in a system employing permutation spreading, each block of information bits assigns a different permutation of spreading waveforms to transmit antennas.
The impressive performance of turbo (iterative) processing techniques that soon followed the discovery of turbo codes [4] has inspired many researchers to utilize the turbo principle in a variety of ways in the architecture of the receiver. The application of turbo processing in MIMO systems has been discussed in the literature, and turbo processing techniques have been shown to attain a channel capacity close to the Shannon limit [5, 6].
More recent works such as [7] have investigated the iterative joint channel estimation and data detection for MIMOCDMA systems. In this paper, we introduce a new turbo receiver for MIMOCDMA systems that employ parity bit selected and permutation spreading. To be able to use turbo processing in the receiver, a serially concatenated convolutional encoder is used in the transmitter. In this system, the parity bit selected or permutation spreading techniques act as an inner code without allocating any transmit energy to the additional redundancy provided by these techniques. The receiver implements a turbo processing by iteratively exchange soft information on coded bits between a softinput softoutput (SISO) detector and a SISO decoder. In this system, detection is performed by incorporating the extrinsic information provided by the decoder in the last iteration into the received signal to calculate the likelihood of each detected bits in terms of log likelihood ratios (LLR) which are used as the input for a SISO decoder. In the case of multiple user systems, a turbo multiuser receiver is presented in which multiple access interference (MAI) affecting each user is estimated based on the extrinsic information of all other user in the system. Simulation results presented in this paper show a significant improvement in the performance of MIMOCDMA systems of both parity bit selected and permutation spreading when a turbo receiver is implemented. Similar iterative detection strategies have been studied for a single carrier as well as multicarrier SSPB systems in [8] and [9], respectively.
The rest of the paper is organized as follows. Section 2 contains the system model of MIMOCDMA systems that employ parity bit selected and permutation spreading. The turbo receiver for these systems is described in Section 3, where the SISO detection is explained, and the details are provided for the calculation of LLRs for each detected bit. Turbo multiuser receiver for MIMOCDMA systems employing parity bit selected and permutation spreading is explained in Section 4. In Section 5, the simulation results for the proposed turbo receiver are presented, and finally, we present our conclusion in Section 6.
2 System model
2.1 MIMOCDMA systems employing parity bit selected spreading
Parity bit selected spreading for CDMA systems is first introduced in [1]. In such systems, unlike the conventional systems, the parity bits of a systematic block code (n,k) are used to select one of a set of 2^{(n−k)} spreading sequences, where n is the length of message vector, and (n−k) is the number of parity bits. In this way, depending on the coding scheme, all possible message vectors are divided into groups that are spread by the same spreading waveform.
For example, if ${\underset{\u203e}{m}}^{l}=(0,0,1,0)$, all four transmit antennas use c_{3}(t) as the spreading waveform.
where ${\alpha}_{\mathit{\text{ij}}}^{l}$ is the complex channel gain on the link between transmit antenna i and receive antenna j on l th signaling interval, E_{b} is the averaged received energy per bit, and ${n}_{j,q}^{l}$ is the sampled noise from the q th matched filter of the j th receive antenna which is a zero mean Gaussian random variable with variance N_{0}/2.
2.2 MIMOCDMA systems employing permutation spreading
Spreading permutations for MIMOCDMA systems with four transit antennas
Coset  Message  w_{1}( t)  w_{2}( t)  w_{3}( t)  w_{4}( t) 

vectors  
M _{1}  0000  c_{1}(t)  c_{3}(t)  c_{5}(t)  c_{7}(t) 
1111  
M _{2}  0001  c_{8}(t)  c_{1}(t)  c_{4}(t)  c_{5}(t) 
1110  
M _{3}  0010  c_{2}(t)  c_{4}(t)  c_{3}(t)  c_{8}(t) 
1101  
M _{4}  0011  c_{5}(t)  c_{2}(t)  c_{6}(t)  c_{3}(t) 
1100  
M _{5}  0100  c_{6}(t)  c_{7}(t)  c_{1}(t)  c_{4}(t) 
1011  
M _{6}  0101  c_{3}(t)  c_{6}(t)  c_{8}(t)  c_{1}(t) 
1010  
M _{7}  0110  c_{7}(t)  c_{8}(t)  c_{2}(t)  c_{6}(t) 
1001  
M _{8}  0111  c_{4}(t)  c_{5}(t)  c_{7}(t)  c_{2}(t) 
1000 
where ${\underset{\u203e}{n}}^{l}=({n}_{1,1}^{l},\cdots \phantom{\rule{0.3em}{0ex}},{n}_{j,q}^{l},\cdots \phantom{\rule{0.3em}{0ex}},{n}_{{N}_{\mathrm{r}},Q}^{l})$.
In what follows, for notational simplicity, we drop the signaling interval index l whenever possible.
In both MIMOCDMA systems that employ parity bit selected or permutation spreading, an additional diversity is achieved by using more spreading waveforms based on the transmitting data. In other words, a sort of space code is created without repeating data on different antennas. More theoretical explanations and performance analysis of such systems are presented in [3].
3 Turbo receiver
The second term in (7), denoted by ${\lambda}_{c}^{p}\left({b}_{k}\right)$, represents the a priori LLR of the coded bit b_{ k } calculated by the SISO channel decoder in the previous iteration (as indicated by a superscript p to show the previous iteration). For the first round of detection (i.e., first iteration) when there is no prior information available about the likelihood of coded bits, we assume that all the coded bits can be ‘ +1’ or ‘ −1’ equiprobably. In other words, ${\lambda}_{c}^{p}\left({b}_{k}\right)=0$ for k=1,2,⋯,N_{r}.
The first term in (7), denoted by λ_{ d }(b_{ k }), represents the extrinsic information provided by the SISO detector about the coded bit of the k th transmit antenna, b_{ k }. This extrinsic information is calculated by the SISO detector based on the matched filter outputs, $\underset{\u203e}{z}$, the spreading code selection strategy of the MIMOCDMA system, and the prior information provided by the SISO channel decoder in the previous iteration about the coded bits other than the transmitted bit of the k th transmit antenna, $\{{\lambda}_{c}^{p}\left({b}_{j}\right),j\ne k\}$. This extrinsic information is then passed through a deinterleaver and fed into the SISO channel decoder as a priori information in the next iteration.
where the a posteriori probabilities are conditioned on the extrinsic information from the SISO detector in the last iteration, ${\left\{{\lambda}_{d}^{p}\left({b}_{k}\right)\right\}}_{k=1}^{{N}_{\mathrm{t}}}$, and the coding structure shown by ‘code structure’ in (8) that represents the trellis structure of the outer convolutional encoder [10].
It can be seen in (8) that the a posteriori LLR Λ_{ c }(b_{ k }) is the summation of the prior information ${\lambda}_{d}^{p}\left({b}_{k}\right)$ and the extrinsic information λ_{ c }(b_{ k }), that is, the information gleaned about b_{ k } from prior information of other coded bits based on the trellis structure of the channel encoder. The SISO channel decoder also computes the a posteriori LLR of the data bits Λ_{ c }(d_{ k }) that is used in the last iteration to make decision about the decoded bits. The extrinsic information delivered in this stage λ_{ c }(b_{ k }) is then passed an interleaver and fed back to the SISO detector as the a priori information in the next iteration (i.e., ${\lambda}_{c}^{p}\left({b}_{k}\right)$).
The SISO channel decoder discussed in this paper which provides the a posteriori information for both coded and data bits is based on the algorithm introduced in [10] which is a slight modification of the wellknown BCJR algorithm [11]. In what follows, we explain how the SISO detector of a MIMOCDMA system that employs parity bit selected and permutation spreading uses the matched filter outputs combined with the prior information from the channel decoder in the last iteration to calculate the extrinsic information for each detected bit.
where P(b_{ j }) is the probability that the bit transmitted by the j th antenna equals to b_{ j } for b_{ j }∈{+1,−1}. Equation (11) holds true because the received signal is fully represented by the outputs of the filters matched to Q orthogonal spreading sequences. It is also necessary and sufficient to observe the matched filter outputs of all N_{t} transmit antennas to be able to completely extract the extrinsic information of each coded bit.
is the squared Euclidean distance between the vector of received and noisefree matched filter outputs when $\underset{\u203e}{b}$ is sent.
By having (15), we can summarize the algorithm of the SISO detector in finding the extrinsic information of each coded bit as follows:

First, ${\mathcal{\mathcal{B}}}_{+}^{k}$ and ${\mathcal{\mathcal{B}}}_{}^{k}$ are formed by dividing all the vectors of N_{t} coded bits, $\underset{\u203e}{b}$, transmitted by all antennas into the vectors in which the k th antenna transmits ‘ +1’ and ‘ −1’, respectively.

Based on whether parity bit selected or permutation spreading technique is used in the transmitter, the expected noisefree matched filter outputs of ${\underset{\u203e}{y}}_{\left(\underset{\u203e}{b}\right)}$ are determined for each vector $\underset{\u203e}{b}$.

The squared Euclidean distance, $d{\left(\underset{\u203e}{b}\right)}^{2}$, is calculated for each vector $\underset{\u203e}{b}$ using (13).

By knowing $d{\left(\underset{\u203e}{b}\right)}^{2}$ and ${\lambda}_{c}^{p}\left({b}_{k}\right)$ for each coded bits, the extrinsic LLR in the output of the SISO detector is computed by (15).
The computed extrinsic LLRs λ_{ d }(b_{ k }) are then passed through a deinterleaver and fed into the SISO channel decoder as the a priori information. The SISO channel decoder provides the a posteriori LLRs for both coded and decoded (data) bits. The extrinsic LLRs λ_{ c }(b_{ k }) at the output of the channel decoder are formed by subtracting the the a priori LLRs ${\lambda}_{d}^{p}\left({b}_{k}\right)$ from the a posteriori LLRs of the coded bits Λ_{ c }(b_{ k }). In the last iteration, a posteriori LLRs of the data bits are used to make decision on data bits, in other words, the estimated data bit, $\widehat{d}$, is the sign of λ_{ c }(d_{ k }) in the last iteration.
4 Turbo multiuser receiver for MIMOCDMA systems employing parity bit selected and permutation spreading
All the discussion in this paper so far has been focused on a system with just a single user, where no interference from other users exist. However, one of the major aspects of any CDMA system is the way the interference form other users of the system is handled. In this section, the multiuser scenario of the MIMOCDMA systems that employ parity bit selected and permutation spreading is discussed, and a turbo multiuser receiver is proposed for such systems.
Let us assume an uplink channel of a synchronous MIMOCDMA system with K users, where each user has N_{t} transmit antennas, and the base station has N_{r} receive antennas. To be able to employ the parity bit selected or permutation spreading techniques, every user is assigned with a set of Q mutually orthogonal spreading sequences.
where ${b}_{j}^{\left(k\right)}$ is the k th user’s transmitted bit by the j th transmit antenna, ${w}_{j}^{\left(k\right)}\left(t\right)$ is the spreading sequence used by the k th user on its j th transmit antennas, and c_{ i }(t) is the i th spreading sequence of the set of Q mutually spreading sequences assigned to the k th user. Depending on whether parity bit selected or permutation spreading technique is used, the spreading waveforms used by each transmit antenna is selected.
and R is the K Q×K Q matrix of crosscorrelation whose (i∘j,m∘n)th element is the crosscorrelation between the i th spreading code of the j th user and the m th spreading code of the n th user.
The proposed turbo multiuser receiver is based on iterative soft interference cancellation. Using the extrinsic information provided by the channel decoder for every bit of every user in the previous iteration, MAI caused by all users is estimated and subtracted out from the matched filter outputs.
where $\underset{\u203e}{\stackrel{~}{Z}}$ is the matched filter outputs after MAI estimation and subtraction, and $\overline{I}$ is the K Q×K Q identity matrix. Matrix $\underset{\u203e}{\stackrel{~}{Z}}$ is considered as the matrix of matched filter outputs to be used in the following iteration.
5 Simulation results
The simulations are performed for coded MIMOCDMA systems that employ parity bit selected and permutation spreading. In the transmitter, at first, the data bits are encoded by a 1/2 rate convolutional encoder with the constraint length of 5 and the generators of 23 and 35 in octal notation. The coded bits are randomly interleaved and serialtoparallel converted such that 4 parallel coded bits are transmitted simultaneously on 4 transmit antennas. Depending on the spreading code selection technique, a message vector of length 4 is used to choose spreading waveforms for transmit antennas. If parity bit selected technique is used, using (2), depending on the coset the message vector comes from, a spreading waveform is selected to be used by all transmit antennas. In the case of MIMOCDMA system with permutation spreading, the message vector follows Table 1 to select permutation of spreading waveforms for each transmit antenna. The channel is modeled as a slowlyvarying, frequencynonselective, Rayleigh fading channel, where the channel gains of different transmitreceive links are uncorrelated. We assume that perfect channel estimation is available at the receiver without a power penalty. We consider MIMOCDMA systems with four transmit antennas and four or one receive antennas.
In the case of multiple user system, every user is assigned a different set of mutually orthogonal waveforms. The crosscorrelation between two waveforms taken from different user sets is not zero. For simplicity, we assume that this crosscorrelation between pairs of waveforms from different sets are equal. We use ρ=0.3 in our simulations. The simulations are performed for the MIMOCDMA system with K=4 users, where each user has N_{t}=4 transmit antennas and the base station is equipped with N_{r}=1 or 4 receive antennas.
The improved BER performance in MIMOCDMA systems that employ either of two discussed spreading code selection techniques is achieved by creating dependency between the bits carried by different antennas. In other words, the information bits transmitted by a transmit antennas encountering deep fades can still be recovered with high probability because of the relationship between the spreading codes used and the information it carries.
Although we considered frequencynonselective fading in this paper, the analysis can be extended to the frequencyselective fading case. In [12], the performance of uncoded MIMOCDMA employing permutation spreading is investigated in frequencyselective fading. It is shown that by using RAKE receivers, there is no significant BER degradation despite loss of orthogonality due to the dispersive nature of the channel. The vector used in the LLR calculations would have length N_{r}×Q×N_{p}, where N_{p} is the number of resolvable paths. This would increase the computational complexity of the LLR calculation with respect to the nonselective fading case.
Another way to combat frequencyselective fading is to employ multicarrier techniques, where the signal is divided into many narrowband CDMA signals so that the fading experienced by each carrier can be considered flat. For example, in [9], the authors present a Turbo MCCDMA system employing the parity bit selected spreading technique. The work in [9] can be extended to multicarrier MIMOCDMA systems employing permutation spreading. Assuming that each carrier is modulated by independent data, then the BER performance of such a system would be identical to the BER performance of the system described in this paper.
6 Conclusion
In this paper, we have introduced a novel turbo receiver for MIMOCDMA systems that employ parity bit selected and permutation spreading. An algorithm for the SISO detector in such systems is developed in which the LLRs of each bit are calculated based on the received signal, the a priori information from the previous iteration, and the code structure of the MIMOCDMA system. The proposed turbo receiver consists of a SISO detector for MIMOCDMA systems followed by a SISO channel decoder. The Iteration of LLRs between these two components provides a significant performance improvement compared to the conventional coded MIMOCDMA systems. The BER improvement that resulted from the proposed turbo multiuser receiver can be traded off against additional users in MIMOCDMA systems that employ parity bit selected and permutation spreading.
Declarations
Authors’ Affiliations
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