A practical two-stage MMSE based MIMO detector for interference mitigation with non-cooperative interferers
© Shah and Daneshrad; licensee Springer. 2011
Received: 4 February 2011
Accepted: 19 December 2011
Published: 19 December 2011
Wireless Multiple Input Multiple Output systems provide system designers with additional degrees of freedom. These can be used to increase throughput, reliability, or even combat spatial interference. The classical Minimum Mean Squared Error (MMSE) solution is the optimal linear estimator for these systems. Its primary drawback is that it requires an estimate of the channel response. This is generally not an issue when interference is absent. However, in environments where interference power is stronger than the desired signal power, this can become difficult to estimate. The problem is even worse in packet-based systems, which rely on training data to estimate the channel before estimating the signal. A strong interference will hinder the receiver's ability to detect the presence of the packet. This makes it impossible to estimate the channel, a critical component for the classical MMSE estimator. For this reason, the classical solution is infeasible in real environments with stronger interferences. We propose a two-stage system that uses practically obtainable channel state information. We will show how this approach significantly improves packet detection, and how the overall solution approaches the performance of the classical MMSE estimator.
KeywordsMIMO MMSE Interference Mitigation
The unlicensed nature of the ISM band has allowed for rapid development and deployment of various wireless technologies such as 802.11 and bluetooth. Since devices are allowed to operate in the same band without pre-determined frequency or spatial planning, they are bound to interfere with each other. There have been several attempts to mitigate this issue via higher layer protocols. Most of these involve some form of cooperative scheduling [1, 2]. Some work has been done to show that time domain signal processing can be used to mitigate the effects of narrowband interference [3–8]. They have shown in simulation how their techniques can suppress interference on the data payload, but have not taken into account how interference affects other parts of the receiver. The primary omission has been with respect to synchronization. This includes tasks such as packet detection, timing synchronization, and channel estimation. Without the ability to perform these tasks, it becomes impossible to build a practical system.
Some work has been done on MIMO-based interference mitigation for cellular systems [9, 10]. These approaches focus on reducing interference from neighboring cells or users by coordinating transmissions either in time, space, or frequency. They do not provide a method for mitigating interference from a non-cooperative external jammer.
The iterative maximum likelihood algorithm described in [8, 11, 12] is very effective, but computationally expensive making it difficult to implement for high datarate systems. They describe a turbo decoder approach to mitigate interference with an array of processors. Turbo decoders have a computational complexity of O(l 2 k ) where l is the block length and k is the constraint length . This method was proven on real systems, but only for low datarates. It also requires the use of a turbo code in order for it to work. The inability to work with an arbitrary FEC or modulation method makes the result specific to the system that was demonstrated. The minimum interference method offers good performance in some scenarios but degrades when the interference becomes weak. They address channel estimation in the presence of interference, but assume ideal packet detection in the presence of this interference.
It is our intention to demonstrate a method that can be practically implemented on a real system. As a design goal, we will ensure that our technique can operate without a priori knowledge of the nature or existence of the interference. We will show how a two-stage MMSE MIMO estimator can be used to facilitate packet detection as well as to provide superior bit error rate performance. The first stage will be a pre-filter that operates on reduced Channel State Information (CSI). This pre-filter will suppress the interference to a level that allows for reliable packet detection and timing synchronization. This will be followed by a secondary detection stage that uses slightly more information to recover the transmitted data. We will demonstrate how this allows the synchronization tasks to be performed and provides similar performance to an ideal MMSE MIMO estimator.
This paper will be organized as follows, Section 2 will describe the system model and provide derivations for the filters that we are proposing. Section 3 will discuss the simulation results. Section 4 will validate some of the basic assumptions on a real-time hardware testbed. Finally, Section 5 will conclude this work.
2 System model
We begin by defining some notation explicitly. We will use the superscript (*) to denote the complex conjugate transpose (Hermitian) of a vector or a matrix. Lowercase boldface symbols (y) will be used to denote vectors and uppercase boldface (W) will be used to denote matrices. The hat will denote estimates of signals, while a tilde will be used to denote residual error signals. The trace operator for a matrix will be denoted as Tr().
First, we will examine Rayleigh flat fading channels, the simplest class of channels. These channels are modeled as a single impulse chosen from a Rayleigh distribution. A new channel will be chosen at random for each packet, but remain constant throughout the duration of that packet. We will discuss the ideal Minimum Mean Squared Error (MMSE) solution and show why it is impractical in high interference scenarios. We will then review the Sample Matrix Inverse (SMI)  as well as Maximal Signal to Interference plus Noise Ratio (MSINR)  algorithms. These are both well suited for use as a pre-filter since neither require first-order information about the channel. Each of these algorithms will use a standard MMSE detector as the secondary filter to demodulate the data. We will then discuss our proposed two-stage solution with its pre-filter and secondary filter. We will show how the combination of these filters is equivalent to the ideal linear MMSE solution. Finally, we will extend each of these methods to cope with Rayleigh frequency selective channels.
2.1 Rayleigh flat fading channels
The time domain received signal y(t) (1) is the linear combination of the received signal of interest, x(t), convolved with its channel, H s , additive white Gaussian noise (AWGN), n(t), and the interference signal, γ(t), convolved with its channel, H i . Since the channel is a single impulse, the convolution of the channel with the signal is the same as multiplication.
The classical MMSE estimator is very powerful, but requires first-order channel state information (CSI) for the signal of interest (H s ). Traditional packet based systems transmit training data which the receiver can use to estimate (H s ). This is fine when there is no interference present allowing packet detection and timing synchronization algorithms to work as expected. It may even work when the interference is cooperative and can be canceled using a cooperative scheme, such as Walsh codes in a CDMA system. If the interference is non-cooperative and stronger than the desired signal, it may be impossible to detect the packet. This will cause the communications system to fail. When the packet cannot be detected and the symbol boundary cannot be determined, the channel cannot be estimated. These practical limitations render the classical approach infeasible in many real scenarios.
We propose a pre-filter based solely on second-order statistics . These statistics can easily be estimated by averaging outer products of received signals at different moments in time. Interference mitigation algorithms that can operate with only these covariance estimates offer greater exibility for communications systems dealing with non-cooperative interferences.
2.1.1 Covariance estimates
We will describe a few alternatives for the pre-filter in the following sections. These will be important for bootstrapping the system using the available measurements (R2 and R1 ).
2.1.2 Sample matrix inverse
The advantage of this algorithm is that the pre-filter only needs knowledge of the covariance of the undesired signal components. This can be particularly useful during the initialization of the communications system. If a strong interference is present, it may not be possible to determine when the signal of interest is being transmitted. This will make it impossible to take an accurate R2 measurement. Instead, the receiver can take several R1 measurements and use the SMI as the pre-filter to improve synchronization performance.
Since the receiver will not know when the desired signal is present, it may still take improper measurements. It is therefore necessary to take consecutive measurements and apply the SMI until the desired signal can be detected by the synchronization algorithm. This equates to a series of Bernoulli trials. We know the likelihood of x consecutive failures decays exponentially with x. The number of trials required is simply a function of the time the desired signal occupies the band. This can easily be adjusted by the system designer to meet the requirements of the communication system. In Section 3, we will show how effective this algorithm is at improving synchronization performance in the presence of very strong interferences. SMI can be used to boot-strap the system. Once a good R1 measurement has been taken, the system will be able to determine whether the desired signal is present or not. It may not be able to estimate the symbol boundary accurately, but this information will make it possible to take an R2 measurement and improve the pre-filter.
2.1.3 Maximal signal to interference and noise ratio
Instead of directly estimating the transmitted signal, this criterion will try to maximize its power relative to the noise and interference. Once again the demodulation can be done with a MMSE based decoder after packet detection, timing synchronization and channel estimation have been completed. This algorithm requires the covariance of the desired signal as well as the information used in the SMI. Once the pre-filter is performing well enough for synchronization to detect packets, the R2 measurement can be taken, and the SMI pre-filter can be replaced with the MSINR pre-filter.
2.1.4 Two-stage MMSE
This operation may introduce spatial interference by mixing the signal components from independent spatial streams. However, if there is only one spatial stream, the result will be a spreading of the desired signal. This is enough to allow many standard detection algorithms to detect and synchronize with an incoming packet. This modified version of the MMSE estimator leads us to our two-stage approach to interference mitigation.
The zero-forcing decoder is used because H s may not be a square matrix. If the matrix is not square, it will not be directly invertible. This will happen anytime there are fewer transmit streams than receive antennas. Equation (14) shows how the application of these two filters in series results in the original MMSE linear estimator. Equations (13) and (14) together show how the MMSE estimator can be broken down into a two-stage process when ideal CSI is available.
2.2 Channel estimation
2.3 Rayleigh frequency selective channels
Once again, the MMSE estimator is very powerful, but requires first-order CSI () for the signal of interest. As shown in the previous sections (4) and (5), we can estimate the second-order statistics by averaging the outer products of the compound received signals (22)-(23). This brings us back to the notion of building pre-filters using only second-order statistics. We will now consider extensions of the previous algorithms for the more complex frequency selective channel.
3 Simulation results
The algorithms described in Section 2 were simulated in MATLAB using a MIMO systems with 4 receive antennas. This included the ideal MMSE solution, SMI, MSINR and the proposed two-stage MMSE solution. The non-cooperative interference source was a single antenna transmission convolved with its own channel. The interference signal was a white Gaussian noise signal, which is essentially a wideband signal. The desired signal was modeled to have 2 or 3 independent spatial streams. The transmission started with a known sequence to be used for packet detection and timing synchronization. We used the standard 802.11a header  with well-known techniques for packet detection, and timing synchronization from [15–17]. This was followed by training data to be used for channel estimation by the receiver. The body of the packet was an uncoded bit stream modulated onto a QPSK constellation. Independent Rayleigh fading channels were generated randomly for each trial for both the desired and undesired signals. These channels remained constant throughout the duration of each trial.
3.1 Rayleigh flat fading channels
Rayleigh flat fading channels are the easiest channels to compensate. They consist of a single impulse and allow us to model the channel as a simple gain and phase adjustment of the transmitted signal. We begin our analysis by considering the original goal of our approach, which is to ensure packet synchronization can be performed. It is necessary to examine this performance before we can investigate the bit error rate (BER). Without packet detection, the communications system will fail. For our system to declare successful synchronization the receiver must correctly detect the presence of the packet, as well as accurately determine the symbol boundary. The symbol boundary is used to determine when the packet started and when each symbol begins and ends. Without this information, the receiver is unable to estimate the channel since it does not know when the training data begins and ends. The estimated channel is used by the receiver to estimate the transmitted signal in the secondary filter.
Legend entry descriptions
Pre-filtering is omitted
The ideal MMSE solution (3) is used in the pre-filter, no secondary filter is required
Equation (11) is used in the pre-filter
Equation (7) is used in the pre-filter
3.2 Rayleigh frequency selective channels
Once again we see how drastic the improvement in synchronization performance becomes with use of our pre-filter (Figure 7). Without the pre-filtering operation, synchronization fails completely. The two-stage MMSE pre-filtering operation improves that success rate to over 99% when the SNR is greater than 10 dB. This is a very significant improvement that contributes to the stability and throughput of the communications system. The alternatives available for the pre-filter are inferior to the proposed two-stage solution. The SMI solution also fails to outperform the two-stage solution in this complex channel.
4 Hardware implementation
The SMI multi-antenna interference mitigation scheme was implemented on a hardware testbed for verification. The purpose of this was to prove that this type of algorithm can work on real hardware in a real environment. Most importantly, it showed that the method described for obtaining the R1 and R2 measurements in Section 2 could be realized in a real system.
4.1 System overview
The estimation of the covariance is a straightforward averaging of the outer product of the incoming signal. The only concern when estimating the covariance is that the signal being received should contain only the interference and noise. This is required in order to compute an accurate R 1 measurement. A controller state machine was designed to enable estimation of the covariance during time periods which are unlikely to contain the signal of interest. The details of this state machine are omitted.
An onboard microprocessor was used to calculate the spatial filtering matrix W based on the input covariance matrix R. This was computed on a microprocessor with double precision floating point arithmetic using well-known matrix inversion algorithms (Cholesky). A simple protocol was developed for passing matrices between the host and FPGA to prevent data corruption. The interval between passing R to the host and receiving a W back was 1 ms.
About a quarter of the way into the trace the filter input changes. This is when the interference signal became active. Unsurprisingly, it prevented the system from receiving packets. The time required for the system to recover and receive packets is a function of the system design parameters. In this case, the most significant source of delay was the matrix inversion required to compute the pre-filter. This computation took a considerable amount of time and dictated the rate at which we could update our pre-filter.
The absolute bottom row on the trace indicates when the controller is computing an R1 covariance estimate. When the system is behaving well and decoding every packet, the estimates are taken immediately after the packet ends. The transmitter guaranteed this time would be silent, which makes it the optimal time to estimate the interference. Note how these estimates become less frequent when the interference turns on.
The controller waits for a stimulus to begin estimation. If it does not receive the stimulus for a pre-defined time, it assumes interference is preventing the receiver from decoding packets. It then switches to a timeout mode where it measures the covariance on a fixed interval. The gap between the last good estimate and the next attempt is a function of this timeout period. In this example, the controller made a couple of failed attempts at covariance estimation while it was in timeout.
Once it makes a good estimate, it is able calculate the pre-filter. The yellow W indicates the time at which the pre-filter is updated with good coefficients. The improvement in performance is immediately visible at the output of the filter. In the second to last trace, the good packet indicators show successful reception of packets, coinciding with the updated pre-filter.
In this example, the system recovered from the onset of interference in 3 ms. This time can be shortened by reducing the timeout period for the controller. Another way to reduce the recovery time is to use a faster processor to compute the pre-filter from the covariance estimate.
This example validates the assumption that the receiver can reasonably make an R1 measurement even in the presence of strong interference.
We have demonstrated a practically realizable two-stage MMSE based approach to interference mitigation and MIMO detection. The advantage of our algorithm is that it enables synchronization tasks such as packet detection, timing synchronization, and channel estimation to be performed in the absence of complete channel state information. The pre-filtering operation uses information that can be easily estimated in the absence of training sequences. The second-stage filter uses information from the pre-filter as well as channel estimates computed during synchronization.
We have shown how the synchronization performance of this algorithm is superior to the classical approach with no pre-filter. We have also shown that the BER performance is within a 0.5 dB of the ideal (yet infeasible) classical MMSE solution. We have demonstrated significant improvement over the existing algorithms for complex transmission schemes and channels. We have also demonstrated how the necessary statistics can be estimated and how the system can be built to achieve good performance. This has not only been done for Rayleigh flat fading channels but for frequency selective channels as well.
Our approach is significant because it lends itself to practically realizable systems. The use of second-order statistics in the pre-filter is something that can easily be implemented on real-time hardware. We have also shown how a system can be designed to make the necessary measurements in the presence of a strong interference. This was demonstrated on a real-time hardware testbed with a non-cooperative interference source.
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