The present invention relates to channel estimators in wireless communication systems generally and to such channel estimators that operate on data and pilot signals in particular.
In practical digital communication systems, the frequency response of the underlying channel from the transmitter to the receiver is seldom known at the receiver side. For example, in digital communication over the dial-up telephone network, the communication channel will be different every time a number is dialed, because the channel route will be different. In this example, the characteristics of the channel are unknown a priori. There are other types of channels, e.g. wireless channel such as radio channels and underwater acoustic channels, whose frequency response characteristics are time varying. Thus coherent communications for such channels require the utilization of adaptive algorithms, known as xe2x80x9cchannel estimatorsxe2x80x9d, for tracking/estimating the varying characteristics of the channel.
Traditionally, channel estimators are divided into two categories: data aided and non-data aided (blind) estimators. Data aided channel estimators operate on a pre-specified set of transmitted symbols that are known to the receiver. These symbols do not convey any information and are often called xe2x80x9cpilot symbolsxe2x80x9d or xe2x80x9ctraining sequencesxe2x80x9d. Data aided channel estimators are typically simple to implement and relatively robust. Their major disadvantage is that they lead to an overall reduction in system throughput, since some of the transmitted symbols (the pilot symbols) do not carry any information.
Non data aided channel estimators, on the other hand, do not reduce the system throughput. However, they are typically quite complicated to implement as they are often based on higher order moments/cumulants of the received signal, and they most often suffer from high statistical variability, i.e. they suffer from large estimation errors.
The article xe2x80x9cMaximum A Posteriori Multipath Fading Channel Estimation for CDMA Systemsxe2x80x9d by Mohamed Siala and Daniel Duponteil, Proceedings of Vehicular Technology Conference, Houston, Tex., May, 1999, describes a channel estimation algorithm which combines both approaches. This algorithm uses both pilot and data symbols to construct a channel estimator. However, this algorithm requires that the joint statistical probability distribution of the channel multipaths be known to the receiver. In practice, a complete statistical description of the channel characteristics is seldom known to the receiver. Moreover, these characteristics may be time varying.
An object of the present invention is to provide an improved channel estimator without using any a priori statistical information about the channel. Instead, the present invention uses a priori probabilities of the received symbols, be they pilot, data, power control, etc. symbols.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a channel estimator based on the values of received data and on a priori probabilities only of received symbols. The channel estimator includes a symbol probability generator, a noise variance estimator and a channel tap estimator. The symbol probability generator generates a priori probabilities only of transmitted symbols found in the received signal(s). The noise variance estimator estimates at least one noise variance corrupting the received signal(s). The channel tap estimator generates channel estimates from the received signal(s), the a priori probabilities and the noise variance(s).
Additionally, in accordance with a preferred embodiment of the present invention, the channel tap estimator solves the following equation:             h      ^        ML    =            1              2        ⁢        T              ·                  ∑                  t          =          1                T            ⁢              xe2x80x83            ⁢                                    y            _                    ⁡                      (            t            )                          ·                              z            ⁡                          (                              t                ;                                                      h                    ^                                    ML                                            )                                *                    
Moreover, in accordance with a preferred embodiment of the present invention, the channel tap estimator includes a z-unit, a combiner and a channel tap unit. The z-unit generates a z-value for z(t;ĥML) from the a priori probabilities, the noise variance(s) and the channel estimates. The combiner combines the z-value with the received signal(s). The channel tap unit determines channel tap values from the output of the combiner.
Further, in accordance with a preferred embodiment of the present invention, the z-unit calculates the following equation for a quadrature phase shift keying (QPSK) channel:                               z          ⁡                      (                          t              ;                              h                _                                      )                          ≡                  xe2x80x83                ⁢                  [                                                                      p                  1                                ⁡                                  (                  t                  )                                            ⁢                              (                                  1                  +                  j                                )                            ⁢                              ⅇ                                                      2                                          σ                      2                                                        ⁢                  Real                  ⁢                                      {                                                                   less than                                                                               y                            _                                                    ⁡                                                      (                            t                            )                                                                                              ,                                                                                                    h                            _                                                    *                                                 greater than                                                   (                                                      1                            -                            j                                                    )                                                                                      }                                                                        +                                                            xe2x80x83                ⁢                                                            p                2                            ⁡                              (                t                )                                      ⁢                          (                              1                -                j                            )                        ⁢                          ⅇ                                                2                                      σ                    2                                                  ⁢                Real                ⁢                                  {                                                             less than                                                                         y                          _                                                ⁡                                                  (                          t                          )                                                                                      ,                                                                                            h                          _                                                *                                             greater than                                               (                                                  1                          -                          j                                                )                                                                              }                                                              +                                                  xe2x80x83                ⁢                                                            p                3                            ⁡                              (                t                )                                      ⁢                          (                                                -                  1                                +                j                            )                        ⁢                          ⅇ                                                2                                      σ                    2                                                  ⁢                Real                ⁢                                  {                                                             less than                                                                         y                          _                                                ⁡                                                  (                          t                          )                                                                                      ,                                                                                            h                          _                                                *                                             greater than                                               (                                                                              -                            1                                                    -                          j                                                )                                                                              }                                                              +                                                              xe2x80x83                    ⁢                                                    p                4                            ⁡                              (                t                )                                      ⁢                          (                                                -                  1                                -                j                            )                        ⁢                          ⅇ                                                2                                      σ                    2                                                  ⁢                Real                ⁢                                  {                                                             less than                                                                         y                          _                                                ⁡                                                  (                          t                          )                                                                                      ,                                                                                            h                          _                                                *                                             greater than                                               (                                                                              -                            1                                                    -                          j                                                )                                                                              }                                                              ]                xc3x97                                          xe2x80x83                ⁢                  [                                                                      p                  1                                ⁡                                  (                  t                  )                                            ⁢                              ⅇ                                                      2                                          σ                      2                                                        ⁢                  Real                  ⁢                                      {                                                                   less than                                                                               y                            _                                                    ⁡                                                      (                            t                            )                                                                                              ,                                                                                                    h                            _                                                    *                                                 greater than                                                   (                                                                                    -                              1                                                        -                            j                                                    )                                                                                      }                                                                        +                                                            p                  2                                ⁡                                  (                  t                  )                                            ⁢                              ⅇ                                                      2                                          σ                      2                                                        ⁢                  Real                  ⁢                                      {                                                                   less than                                                                               y                            _                                                    ⁡                                                      (                            t                            )                                                                                              ,                                                                                                    h                            _                                                    *                                                 greater than                                                   (                                                                                    -                              1                                                        +                            j                                                    )                                                                                      }                                                                        +                                                                        xe2x80x83                    ⁢                                                                      p                  3                                ⁡                                  (                  t                  )                                            ⁢                              ⅇ                                                      2                                          σ                      2                                                        ⁢                  Real                  ⁢                                      {                                                                   less than                                                                               y                            _                                                    ⁡                                                      (                            t                            )                                                                                              ,                                                                                                    h                            _                                                    *                                                 greater than                                                   (                                                                                    -                              1                                                        -                            j                                                    )                                                                                      }                                                                        +                                                            p                  4                                ⁡                                  (                  t                  )                                            ⁢                              ⅇ                                                      2                                          σ                      2                                                        ⁢                  Real                  ⁢                                      {                                                                   less than                                                                               y                            _                                                    ⁡                                                      (                            t                            )                                                                                              ,                                                                                                    h                            _                                                    *                                                 greater than                                                   (                                                                                    -                              1                                                        +                            j                                                    )                                                                                      }                                                                                ]                          -          1                    
where {overscore (y)}(t) is a vector of the received signal(s), {overscore (h)} is a vector of the channel tap estimates and {overscore (p)}(t) is a vector of the symbol probabilities.
Still further, in accordance with a preferred embodiment of the present invention, the channel tap unit is a summer over a window of received symbols.
Additionally, in accordance with a preferred embodiment of the present invention, when s(t) is a pilot symbol, then {overscore (p)}(t)={overscore (e)}i where the {overscore (e)}i""s are the normal basis vectors having a one (1) in their i-th component and zero otherwise. When s(t) is a data symbol, then
p1(t)=p2(t)=p3(t)=p4(t)=0.25
When s(t) is a transmit power control (TPC) symbols, then
p1(t)=p4(t)=0.5, p2(t)=p3(t)=0
Moreover, in accordance with a preferred embodiment of the present invention, the channel tap unit includes an anchor unit, an averager and an interpolator. The anchor unit determines a pilot anchor ĥp using Np pilot symbols of one time slot n and a data anchor ĥs using Ns data symbols of the time slot n. The averager averages the pilot and data anchors to produce a slot anchor ĥanchor(n) and the interpolator interpolates between adjacent anchors ĥanchor(nxe2x88x921) and ĥanchor(n) to obtain channel estimates for the n-th slot.
Further, in accordance with a preferred embodiment of the present invention, Ns=2Np and Np of the data symbols can be taken from before the pilot symbols of the time slot and Np of the data symbols can be taken from after the pilot symbols.
Still further, in accordance with a preferred embodiment of the present invention, the unit for linearly interpolates includes unit for separately interpolates amplitudes and phases of the adjacent anchors.
Additionally, in accordance with a preferred embodiment of the present invention, the channel estimator also includes a unit that averages slot anchors as follows:                     H        ^            anchor        ⁡          (              n        -        1            )        =                    ∑                  k          =                      -            M                          M            ⁢              xe2x80x83            ⁢                                                  h              ^                        anchor                    ⁡                      (                          n              -              1              -              k                        )                          ·                  β          k                                    ∑                  k          =                      -            M                          M            ⁢              xe2x80x83            ⁢              β        k            
where Hanchor is the average value of slot anchors hanchor in a slot ranging between the values xe2x88x92M to M for k, and xcex2k is a user defined weight factor.
Moreover, in accordance with a preferred embodiment of the present invention, the z-unit includes a lookup table unit.
Further, in accordance with a preferred embodiment of the present invention, the at least one received signal is the output of at least one despreader. Alternatively, it could be the downconverted and demodulated output of more than one antenna.
Still further, in accordance with a preferred embodiment of the present invention, the noise variance estimator generates a noise variance {overscore ({circumflex over ("sgr")})}2  as follows:
{circumflex over ("sgr")}k2(n)=(1xe2x88x92xcex1)xc2x7{circumflex over ("sgr")}k2(nxe2x88x921)+xcex1xc2x7{tilde over ("sgr")}k2(n)
where the time index n is in units of slot, xcex1 is a user selectable exponential forgetting factor, and                     σ        ~            k      2        ⁡          (      n      )        ≡                    1                  N          P                    ⁢                        ∑                      t            =            1                                N            P                          ⁢                  xe2x80x83                ⁢                              "LeftBracketingBar"                                                                                                      y                      _                                        k                                    ⁡                                      (                    t                    )                                                                    G                  ⁡                                      (                    t                    )                                                              ·                                                s                  ⁡                                      (                    t                    )                                                  *                                      "RightBracketingBar"                    2                      -                  "LeftBracketingBar"                              1                          N              P                                ⁢                                    ∑                              t                =                1                                            N                P                                      ⁢                          xe2x80x83                        ⁢                                                                                                      y                      _                                        k                                    ⁡                                      (                    t                    )                                                                    G                  ⁡                                      (                    t                    )                                                              ·                                                S                  ⁡                                      (                    t                    )                                                  *                                                    "RightBracketingBar"            2      
where Np is the number of pilot symbols per slot, G(t) is an automatic gain control (AGC) level and {overscore (y)}k(t) is the k-th element of the vector {overscore (y)}(t).
Further, in accordance with a preferred embodiment of the present invention, the channel tap unit is an infinite impulse response (IIR) filter given by:
ĥ(t)=xe2x88x92ĥ(txe2x88x921)
a1xe2x88x92, . . . , xe2x88x92ĥ(txe2x88x92p)a
p+{overscore (y)}(t)xc2x7
z(t;ĥ(txe2x88x921))*b
0+{overscore (y)}(txe2x88x921)xc2x7z(
txe2x88x921;ĥ(t
xe2x88x922))*b1+, . . . , {overscore (y)}(txe2x88x92q)xc2x7
z(txe2x88x92q;ĥ(txe2x88x92qxe2x88x921))*bq
where ai and bi are user-defined parameters as are the filter orders p and q.
Alternatively, there is also provided a channel estimator having two estimators, a first estimator that operates on a continuous pilot channel and a second estimator that operates on a traffic channel with interleaved pilot symbols. The estimator also includes a combiner that combines the output of the first and second estimators. The first estimator is similar to those of the prior art while the second estimator is similar to that described hereinabove.
Finally, the present invention includes the methods performed by the channel estimators disclosed herein.