Performance based rank prediction for MIMO design

WO 2006/069300

PCT/US2005/046742

PERFORMANCE BASED RANK PREDICTION FOR ΜΙΜΟ DESIGN

BACKGROUND

I. Field

[0001] The present invention relates generally to communications, and more specifically to techniques for determining a distribution of a data stream to be transmitted via a multi-channel, e.g., a multiple-input multiple-output (ΜΙΜΟ), orthogonal frequency division multiplexing (OFDM) communication system.

IL Background

[0002] In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver via a number of propagation paths. The characteristics of the propagation paths typically vary over time due to a number of factors such as fading and multipath. To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used. If the propagation paths between the transmit and receive antennas are linearly independent (i.e., a transmission on one path is not formed as a linear combination of the transmissions on the other paths), which is generally true to at least an extent, then the likelihood of correctly receiving a data transmission increases as the number of antennas increases. Generally, diversity increases and performance improves as the number of transmit and receive antennas increases.

[0003] A multiple-input multiple-output (ΜΙΜΟ) communication system employs multiple (Nt) transmit antennas and multiple (Nr) receive antennas for data transmission. A ΜΙΜΟ channel formed by the Nt transmit and Nr receive antennas may be decomposed into Ns independent channels, with N_s < min {N_T,N_R}. Each of the Ns independent channels may also be referred to as a spatial subchannel (or a transmission channel) of the ΜΙΜΟ channel and corresponds to a dimension. The ΜΙΜΟ system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.

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[0004] For a full-rank ΜΙΜΟ channel, where N_s = N_T < N_R, an independent data stream may be transmitted from each of the N_T transmit antennas. The transmitted data streams may experience different channel conditions (e.g., different fading and multipath effects) and may achieve different signal-to-noise-and-interference ratios (SNRs) for a given amount of transmit power. Moreover, if successive interference cancellation processing is used at the receiver to recover the transmitted data streams (described below), then different SNRs may be achieved for the data streams depending on the specific order in which the data streams are recovered. Consequently, different data rates may be supported by different data streams, depending on their achieved SNRs. Since the channel conditions typically vary with time, the data rate supported by each data stream also varies with time.

[0005] The ΜΊΜΟ design has two modes of operation - the single code word (SCW) and multiple-code word (MCW).

[0006] In MCW mode, the transmitter can encode the data transmitted on each spatial layer independently, possibly with different rates. The receiver employs a successive interference cancellation (SIC) algorithm which works as follows: Decode the first layer, and then subtract its contribution from the received signal after reencoding and multiplying the encoded first layer with an “estimated channel,” then decode the second layer and so on. This “onion-peeling” approach means that each successively decoded layer sees increasing signal-to-noise (SNR) and hence can support higher rates. In the absence of error-propagation, MCW design with SIC achieves capacity. The disadvantage of this design arise from the burden of “managing” the rates of each spatial later - (a) increased CQI feedback (one CQI for each layer); (b) increased ACK/NACK messaging (one for each layer); (c) complications in Hybrid ARQ (HARQ) since each layer can terminate at different transmissions; (d) performance sensitivity of SIC to channel estimation errors with increased Doppler, and/or low SNR; and (e) Increased decoding latency requirements since each successive layer cannot be decoded until prior layers are decoded.

[0007] In the conventional SCW mode design, the transmitter encodes the data transmitted on each spatial layer with “identical data rates. The receiver can employ a low complexity linear receiver such as a Minimum Mean Square Solution (MMSE) or Zero Frequency (ZF) receiver, or non-linear receivers such as QRM, for each tone.

2005319084 20 Oct 2009

The SCW design overcomes the above mentioned implementation hassles of the MCW design. The drawback is that the SCW mode cannot support the MCW rates in spatially correlated channels or line-of-sight (LOS) channels with a high K-factor. Both of these scenarios lead to a loss in channel rank or increase in channel condition number and increased inter-layer interference. This dramatically lowers the effective SNR for each spatial layer. Hence, the data rate supported by each layer is lowered, which lowers the overall data rate.

K-factor is the ratio of the LOS channel power to the non-LOS channel power. Rank is the number of eigen-modes in the channel with non-zero energy. Condition Number is the ratio of the largest eigenvalue to the smallest eigen-value of the ΜΙΜΟ channel.

D There is therefore a need in the art for techniques to distribute a data stream dynamically to be transmitted via a multi-channel, e.g., a multiple-input multiple-output (ΜΙΜΟ), orthogonal frequency division multiplexing (OFDM) communication system.

SUMMARY

According to a first aspect of the present invention, there is provided a method of rank prediction to maximize spectral efficiency in a multiple-input multiple-output (ΜΙΜΟ) communication system, including:

calculating ΜΙΜΟ channel matrices corresponding to layer transmissions for each tone;

calculating signal-to-noise ratios (SNRs) for each tone based on the M1M0 channel matrices;

) generating effective SNRs based on the calculated SNRs;

selecting a packet format (PF) with an SNR threshold less than the effective SNR for each layer transmission;

selecting a rank based on the selected PFs; and transmitting a ranking to a transmitting side, the ranking indicating the number of layer transmissions to select for transmission, so as to maximize spectral efficiency.

According to a second aspect of the present invention, there is provided a wireless communications device for maximizing spectral efficiency in a multiple-input multiple-output (ΜΙΜΟ) communication system, including:

means for calculating ΜΙΜΟ channel matrices corresponding to layer transmissions for each 30 tone;

means for calculating signal-to-noise ratios (SNRs) for each tone based on the ΜΙΜΟ channel matrices;

means for generating effective SNRs based on the calculated SNRs;

means for selecting a packet format (PF) with an SNR threshold less than the effective SNR 35 for each layer transmission;

means for selecting a rank based on the selected PFs; and means for transmitting a ranking to a transmitting side, the ranking indicating the number of layer transmissions to select for transmission, so as to maximize spectral efficiency.

2005319084 20 Oct 2009

According to a third aspect of the present invention, there is provided a processor programmed to execute a method of rank prediction to maximize spectral efficiency in a multiple-input multipleoutput (ΜΙΜΟ) communication system, the processor configured to:

calculate ΜΙΜΟ channel matrices corresponding to layer transmissions for each tone;

calculate signal-to-noise ratios (SNRs) for each tone based on the ΜΙΜΟ channel matrices;

generate effective SNRs based on the calculated SNRs;

select a packet format (PF) with an SNR threshold less than the effective SNR for each layer transmission;

select a rank based on the selected PFs; and transmit a ranking to a transmitting side, the ranking indicating the number of layer transmissions to select for transmission, so as to maximize spectral efficiency.

According to a fourth aspect of the present invention, there is provided a computer readable media including program codes for a processor performing a method of rank prediction to maximize spectral efficiency in a multiple-input multiple-output (ΜΙΜΟ) communication system, the method including:

calculating ΜΙΜΟ channel matrices corresponding to layer transmissions for each tone; calculating signal-to-noise ratios (SNRs) for each tone based on the ΜΙΜΟ channel matrices; generating effective SNRs based on the calculated SNRs;

selecting a packet format (PF) with an SNR threshold less than the effective SNR for each layer transmission;

selecting a rank based on the selected PFs; and transmitting a ranking to a transmitting side, the ranking indicating the number of layer transmissions to select for transmission, so as to maximize spectral efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and nature of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

FIG. 1 shows a conventional SCW transmitter;

FIG. 2 shows an SCW transmitter with rank prediction in accordance with an embodiment;

FIG. 3 shows circular multiplexing with ^M_r⁼⁴’^M=2·®¹ in accordance with an embodiment;

FIG. 4 shows block-circular multiplexing with Mr = 4,M = 2,B = 4 in accordance with an embodiment; and

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[0021] FIG. 5 shows a block diagram for performance based rank prediction in accordance with an embodiment.

DETAILED DESCRIPTION

[0022] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs.

[0023] The techniques described herein for performance based rank prediction may be used for various communication systems such as a Code Division Multiple Access (CDMA) system, a Wideband CDMA (WCDMA) system, a direct sequence CDMA (DS-CDMA) system, a Time Division Multiple Access (TDMA) system, a Frequency Division Multiple Access (FDMA) system, a High Speed Downlink Packet Access (HSDPA) system, an orthogonal frequency division multiplexing (OFDM)-based system, an Orthogonal Frequency Division Multiple Access (OFDMA) system, a singleinput single-output (SISO) system, a multiple-input multiple-output (ΜΙΜΟ) system, and so on.

[0024] OFDM is a multi-carrier modulation technique that effectively partitions the overall system bandwidth into multiple (NF) orthogonal subbands. These subbands are also referred to as tones, subcarriers, bins, and frequency channels. With OFDM, each subband is associated with a respective subcarrier that may be modulated with data. Up to NF modulation symbols may be transmitted on the NF subbands in each OFDM symbol period. Prior to transmission, these modulation symbols are transformed to the time-domain using an NF-point inverse fast Fourier transform (IFFT) to obtain a “transformed” symbol that contains NF chips.

[0025] The SCW design overcomes the drawbacks of MCW design. However, SCW mode cannot support the MCW rates in spatially correlated channels or line-ofsight (LOS) channels with a high K-factor. Both of these scenarios lead to a loss in channel rank or increase in channel condition number and increased inter-layer interference. This dramatically lowers the effective SNR for each spatial layer. Hence, the data rate supported by each layer is lowered, which lowers the overall data rate.

[0026] One way to reduce interlayer interference is to lower the number of spatial layers transmitted in low-rank channels, and trade-off inter-layer interference and

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ΜΙΜΟ gains. For example, lowering the number of layers transmitted from four to three, i.e., decreasing the rank from four to three, can dramatically increase the effective SNRs for the three layers and hence the data rate supported by each layer. The neteffect is that a three-layer transmission can in-fact have a higher spectral efficiency compared to a four-layer transmission.

[0027] In an embodiment, the SCW design effectively trades off the interlayer interference and ΜΙΜΟ gains to maximize overall spectral efficiency. This is achieved via rank prediction, where the receiver feeds back an optimal number of layers for transmission in addition to a Carrier-Quality-to-Interference (CQI) to match the channel rank.

[0028] It would be apparent to those skilled in the art that quality indicators other than CQI may be utilized.

Conventional SCW Transmitter

[0029] Figure 1 shows a conventional SCW transmitter 100. The bits 102 are turboencoded 104 and QAM mapped 106 depending on the packet format (PF) 108, 110, specified by a rate prediction algorithm 112. The encoding is identical to a single-insingle-out (SISO) design. The coded symbols are then de-multiplexed 114 to layers 116, which are then spatially mapped 118 to OFDM modulators 120 and antennas 122. The OFDM processing for each transmit antenna proceeds then in an identical fashion as the SISO, after which the signals are launched into a ΜΙΜΟ wireless channel. The rate prediction algorithm uses a 4-bit CQI feedback 124 from receiver 126 every 5 msec. The CQI is a measure of the effective SNR /spatial layer, measured at the receiver. The spatial mapping is done in manner to ensure that the SNR for each layer are similar. As explained before, the performance of this design suffers in low rank channels.

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SCW Transmitter with Rank Prediction

[0030] In accordance with an embodiment, a single code word (SCW) design with rank prediction is described. Algorithms for robust rank prediction are presented below. For SNR < 15dB (90% of the users), the performance of the SCW design with low complexity MMSE receiver & rank prediction, is similar to the Multiple Code Word (MCW) design with successive interference cancellation (SIC). Without HARQ, SCW is better than MCW since MCQ is more sensitive to channel estimation errors. These factors make SCW attractive for ΜΙΜΟ due to smaller implementation complexity and overhead compared to MCW.

[0031] For SNR between 15 and 20 dB (10% of the users), the performance gap between SCW and MCW is less than 1.0 dB for low K channels, and 2-5 dB for high K channels. For high K channels, the performance degradation at high SNRs is lowered to 1-2 dB, by employing dual polarized antennas. In effect, the SCW design is within two dB of MCW design even at high SNRs. In the absence of HARQ, the performance of MCW is worse than SCW at SNR < 15 dB, due to increased sensitivity of SIC to channel estimation errors.

[0032] Figure 2 shows an SCW transmitter with rank prediction in accordance with an embodiment. The bits 202 are turbo-encoded 204 and QAM mapped 206 depending on the packet format (PF) 208, 210, specified by a rate prediction algorithm 212.

[0033] In an embodiment, the coded symbols are then de-multiplexed 214 to M streams 216 or layers (1-^- Id_T χ _wh_ere M 228 is a 2-bit integer specified by the receiver 226 feedback every 5 m-sec, in addition to a 5-bit CQI 224.

The Id streams 216 are then spatially mapped 218 to OFDM modulators 220 and antennas 222.

Spatial Mapping

[0034] The spatial mapper (precoder) 218 is a matrix that maps Id symbols on to antennas, for each OFDM tone, There can be several choices for the precoder. Consider a ΜΙΜΟ channel . The precoder matrices can be chosen so that the equivalent channel matrix has improved frequency

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PCT/US2005/046742 selectivity compared to . The increased frequency selectivity can be exploited by the decoder to obtain frequency diversity gains.

[0035] In an embodiment, a precoder matrix is the following permutation matrix:

^„(*>=-4=11

-JM mod M_T where ^(0),11(1),.....,ΤΪ(Μ_τ 1) _are M_TxM _sut>_p_errnut_ation matrices derived from the M columns of the identity matrix, _anc| B is a p_arameter to control the frequency selectivity of the equivalent channel.

<img file="AU2005319084B2_D0001.tif" />

[0037] For ^-1, this leads to a circular multiplexing strategy with two layers as shown in figure 3 wherein the vertical-lined boxes 302 correspond to symbols from layer one and horizontal-lined boxes 304 correspond to symbols from layer 2. Figure 3 shows circular multiplexing wither ^_ - 2,B - 1 _vertj_caj _axj_s 395 represents antennas. The horizontal axis 308 represents tones.

[0038] For B - 4 , this leads to a block-circular multiplexing strategy with two layers as shown in figure 4 where the vertical-lined boxes 402 correspond to symbols from layer one and horizontal-lined boxes 404 correspond to symbols from layer 2. Figure 4 shows block-circular multiplexing with M_T = 4,M = 2,B = 4. The vertical axis 406 represents antennas. The horizontal axis 408 represents tones.

[0039] An increase in B leads to a reduction in the frequency selectivity of the equivalent channel, which may be desirable when weak codes are employed. Also, the parameter B is sensitive to channel interleaver choice, therefore parameter B may be optimized later on.

[0040] Circular multiplexing improves frequency diversity regardless of the channel delay spread. In the presence of strong turbo codes, the performance of CM (with Af = 1) approaches Space-Time transmit diversity (STTD). However, for very high PFs

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PCT/US2005/046742 or for control channels that employ weak convolutional codes, STTD can out-perfom CM significantly.

[0041] In an embodiment, a precoder matrix is the following generalized delay diversity matrix:

[0042] where is a sub-DFT matrix obtained from the M columns of the Dpp matrix, and j_{s an} M_T xM_T di_ag_Onai matrix, with the ( 7 Γΐ ^Ηvj'jj entry given by

[0043] The parameter ^is the delay-parameter, which also controls the frequency selectivity of the channel, and N is the number-of OFDM tones. We note that for M=l, the above precoding matrix implements the “pure” delay diversity. The performance of delay diversity is strictly worse than circular multiplexing (and STTD), and has poor performance in LOS channel conditions for high PF. The only advantage of using delay diversity is that it benefits from improved SISO channel estimation gains at very low SNRs (SNR < -5 dB) and for high mobility (> 120 kmph). In these channel scenarios, circular multiplexing cannot benefit from SISO channel estimation gains.

[0044] Packet Formats

[0045] A current SISO design uses 7 PFs with spectral efficiencies [0.5, 1, 1.5, 2.0, 2.5, 3.0, 4.0] bps/Hz. In the SCW design employing a one-layer transmission, this granularity in spectral efficiency (SE) should be sufficient. However, when all four layers are used for transmission, this translates to spectral efficiencies of [2,4,6,8,10,12,16] bps/Hz, with a SE granularity on the order of 2-4 bps/Hz. A consequence of this coarse granularity is a loss in data rate, since these users are constrained to transmit at a much lower data rate than their attainable SE. Note that MCW design with SIC does not have this granularity problem, since the rate in each layer can be adjusted independently, resulting in an overall finer spectral efficiency granularity.

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<td rowspan="2"> Packet Format</td><td rowspan="2"> Modulation</td><td rowspan="2"> Code Rate after one Frame</td><td colspan="6"> Spectral Efficiency Per Layer after N frames of transmission</td>

<td> 1</td><td> 2</td><td> 3</td><td> 4</td><td> 5</td><td> 6</td>

<td> 0</td><td> 2</td><td> 1/4</td><td> 0.50</td><td> 0.25</td><td> 0.17</td><td> 0.13</td><td> 0.10</td><td> 0.08</td>

<td> 1</td><td> 2</td><td> 3/8</td><td> 0.75</td><td> 0.38</td><td> 0.25</td><td> 0.19</td><td> 0.15</td><td> 0.13</td>

<td> 2</td><td> 2</td><td> 1/2</td><td> 1.00</td><td> 0.50</td><td> 0.33</td><td> 0.25</td><td> 0.20</td><td> 0.17</td>

<td> 3</td><td> 4</td><td> 5/16</td><td> 1.25</td><td> 0.63</td><td> 0.42</td><td> 0.31</td><td> 0.25</td><td> 0.21</td>

<td> 4</td><td> 4</td><td> 3/8</td><td> 1.50</td><td> 0.75</td><td> 0.50</td><td> 0.38</td><td> 0.30</td><td> 0.25</td>

<td> 5</td><td> 4</td><td> 7/16</td><td> 1.75</td><td> 0.88</td><td> 0.58</td><td> 0.44</td><td> 0.35</td><td> 0.29</td>

<td> 6</td><td> 4</td><td> 1/2</td><td> 2.00</td><td> 1.00</td><td> 0.67</td><td> 0.50</td><td> 0.40</td><td> 0.33</td>

<td> 7</td><td> 4</td><td> 9/16</td><td> 2.25</td><td> 1.13</td><td> 0.75</td><td> 0.56</td><td> 0.45</td><td> 0.38</td>

<td> 8</td><td> 6</td><td> 5/12</td><td> 2.50</td><td> 1.25</td><td> 0.83</td><td> 0.63</td><td> 0.50</td><td> 0.42</td>

<td> 9</td><td> 6</td><td> 11/24</td><td> 2.75</td><td> 1.38</td><td> 0.92</td><td> 0.69</td><td> 0.55</td><td> 0.46</td>

<td> 10</td><td> 6</td><td> 1/2</td><td> 3.00</td><td> 1.50</td><td> 1.00</td><td> 0.75</td><td> 0.60</td><td> 0.50</td>

<td> 11</td><td> 6</td><td> 13/24</td><td> 3.25</td><td> 1.63</td><td> 1.08</td><td> 0.81</td><td> 0.65</td><td> 0.54</td>

<td> 12</td><td> 6</td><td> 7/12</td><td> 3.50</td><td> 1.75</td><td> 1.17</td><td> 0.88</td><td> 0.70</td><td> 0.58</td>

<td> 13</td><td> 6</td><td> 5/8</td><td> 3.75</td><td> 1.88</td><td> 1.25</td><td> 0.94</td><td> 0.75</td><td> 0.63</td>

<td> 14</td><td> 6</td><td> 2/3</td><td> 4.00</td><td> 2.00</td><td> 1.33</td><td> 1.00</td><td> 0.80</td><td> 0.67</td>

<td> 15</td><td> 6</td><td> 17/24</td><td> 4.25</td><td> 2.13</td><td> 1.42</td><td> 1.06</td><td> 0.85</td><td> 0.71</td>

Table 1: Packet Format for SCW Design with Rank Prediction

[0046] Table 1 shows the packet format for SCW design with rank prediction in accordance with an embodiment. Table 1 shows the PFs with SEs targeting the first to sixth transmission. 16 PFs are provisioned with SE-per-layer ranging from 0.5 bps/Hz/layer to 4.25 bps/Hz/layer with 0.25 bps/Hz/layer increments targeting the first transmission. When targeting the third transmission, the maximum attainable SE-perlayer is 1.42 bps/Hz/layer. The SE between 1.42 bps/Hz/layer and 2.13 bps/Hz/layer can be achieved by targeting the second transmission and SE greater than 2.13 bps/Hz/layer can be achieved by targeting the first transmission, where HARQ benefits diminish.

[0047] In another embodiment, more PF#s may be added with SE/layer > 4.25 bps/Hz so that higher SE can be achieved by targeting the third transmission, and benefit from HARQ gains. In such a case, a 6-bit CQI may be needed to ensure that the PF granularity is captured.

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Performance Based Rank Prediction Algorithm

[0048] Figure 5 shows a block diagram for performance based rank prediction in accordance with an embodiment. For the kth tone, H(k)Pi(k) 502 through H(k)P₄(k) 508 are input into MMSE(l) 512 through MMSE(4) 518, respectively. MMSE(l) 512 through MMSE(4) 518 produce SNRi(k) 522 through SNR₄(k) 528, respectively. SNRi(k) 522 through SNR₄(k) 528 are input into Cap Mapper 532 through Cap Mapper 538, respectively. Cap Mapper 532 through Cap Mapper 538 produce EffSNRi 542 through EffSNR₄ 548, respectively. EffSNRi 542 through EffSNR₄ 548 are input into PF Select 552 through PF Select 558, respectively. SNR thresholds at a 1% packet error rate (PER) are input into PF Select 552 through PF Select 558. PF Select 552 through PF Select 558 produce 1 x PF1 562 through 4 x PF4 568, respectively. 1 x PF1 562 through 4 x PF4 568 are input into a decision unit 570. Decision unit 570 produces a rank 572.

[0049] EffSNRi 542 through EffSNR₄ 548 and the rank 572 are input into a select &amp; quantize unit 574. The select &amp; quantize unit 574 produces a five-bit CQI 576.

[0050] In accordance with an embodiment, the performance based rank prediction algorithm works as follows:

[0051] 1. At each tone, the 4x4, 4x3, 4x2 and 4x1 ΜΙΜΟ channel matrices,

H(k)P_l(k) H(k)P₂(k), H(k)P₃(k) _ancj H(k)Pfik) , _corresp_Onding to the {1,2,3,4} layer transmissions, are calculated. Assuming an MMSE receiver, the post-processing

SNRs for {1,2,3,4} layer transmissions,

SNRfk), SNR₂(k), SNRfik), SNRfik)^ calculated for each tone as:

SNR_m (k) =

Μ -1 r ! r — Y diag{ [p_m (k)’ H(k)* H(k)P_M (k)

M \

<img file="AU2005319084B2_D0002.tif" />

V M = [1,4]

If we assume other receivers such as QRM-MLD or IDD, the post processing SNRs will be calculated in a different fashion.

[0052] 2. The SNRs calculated above for the {1,2,3,4} layer transmissions, are equivalent to the per-tone receiver SNRs calculated for the SISO design. An unconstrained-capacity mapping is then employed (as in the SISO design) to generate an effective-SNR averaged over all tones, for the {1,2,3,4} layer transmissions, which

2005319084 20 Oct 2009 are denoted as EffSNR i, EffSNR ₂, EffSNR 3, EffSNR 4. There is no tone-dependency for the effective SNRs.

3. The effective SNRs are compared against a table with SNR thresholds targeting the 1% PER for a S1SO system. The highest packet format (PF) with SNR threshold less than the measured effective SNR are selected for the {1,2,3,4} layer transmissions. The PFs are denoted as ^Pl\ , ^ρρ2, ^{/ r}3, PF 4 ·

4. The optimum rank/layer is chosen so as to maximize the over-all spectral efficiency, i.e.,

M = arg max [Ai x PF_M ]

M=[l 4]

5. A 5-bit CQI is then fed-back, where σρ/(Λΐ) = ρ^ [EffSNR - ].

M

The techniques described herein may be used for a variety of OFDM-based systems as well as other systems. The rank prediction techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the processing units used to perform interference control may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, 3 other electronic units designed to perform the functions described herein, or a combination thereof.

For a software implementation, the interference control techniques may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit and executed by a processor. The memory unit may be implemented within the processor or external to the processor, in which case it can be 25 communicatively coupled to the processor via various means as is known in the art.

A detailed description of one or more preferred embodiments of the invention is provided above along with accompanying figures that illustrate by way of example the principles of the invention. While the invention is described in connection with such embodiments, it should be understood that the invention is not limited to any embodiment. On the contrary, the scope of the 30 invention is limited only by the appended claims and the invention encompasses numerous alternatives, modifications, and equivalents. For the purpose of example, numerous specific details are set forth in the description above in order to provide a thorough understanding of the present invention. The present invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields 35 related to the invention has not been described in detail so that the present invention is not unnecessarily obscured.

Throughout this specification and the claims that follow unless the context requires otherwise, the words 'comprise' and 'include' and variations such as 'comprising' and 'including' will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge of the technical field.