1. Field of the Invention
The present invention relates to a communication system using multi-carriers, and more particularly, to a method for allocating pilot signals in a communication system which transmits and receives data by using multi-carriers.
2. Discussion of the Related Art
The basic principle of orthogonal frequency division multiplexing (OFDM) which is one of modulation schemes used in a communication system is to divide a data stream having a high data transmission rate into a plurality of data streams having a low data transmission rate and simultaneously transmit the data streams by using a plurality of carriers. Each of the plurality of carriers is referred to as a sub-carrier. Since orthogonality exists among the plurality of carriers of the OFDM system, a receiving side can detect frequency components of the carriers even if the respective frequency components are overlapped with each other. The data stream having a high data transmission rate is converted into a plurality of data streams having a low data transmission rate through a serial to parallel converter. The converted data streams are multiplied by each of the sub-carriers, and the respective data streams are added to each other, whereby the resultant data streams are transmitted to the receiving side.
The plurality of parallel data streams generated by the serial to parallel converter can be modulated with a plurality of sub-carriers by inverse discrete fourier transform (IDFT). The IDFT can be realized efficiently using inverse fast fourier transform (IFFT).
Since symbol duration of a sub-carrier having a low data transmission rate increases, temporally relative signal dispersion generated by multi-path delay spread is reduced. Meanwhile, a guard interval longer than delay spread of a channel may be inserted between OFDM symbols to reduce inter-symbol interference. Also, if a part of an OFDM signal is copied in the guard interval and then arranged at a start part of the symbol, the OFDM symbol is cyclically extended to be guarded.
Hereinafter, a method for transmitting pilots in a 3 GPP long term evolution (LTE) system which is currently being discussed.
The 3 GPP LTE system which will become a standard of communication for next generation seeks a method for providing service to a user who is moving at high speed. In particular, the standard rules of the 3 GPP LTE system prescribe that communication can be performed even though the user moves at 500 km per hour. However, for the user who are moving at high speed, packet error increases in case of the existing burst type pilots. Although scattered type pilots can be adapted to time based variation, a problem occurs in that a mobile terminal should be maintained in an active mode not an idle mode.
Pilots are signal components used to estimate channel status in a wireless or wired communication, and are realized by transmitting a predetermined sequence which both a transmitting side and a receiving side know when electric waves are propagated through an unknown channel. The pilots may be referred to as training symbols. Accuracy of channel estimation at the receiving side is determined by a type or power of the transmitted pilots.
Pilot transmission in a conventional mobile communication system depends on a timing point of data transmission, i.e., early or later stage of communication. At the early stage of communication, a network of the mobile communication system transmits all pilots through a specific OFDM symbol to estimate radio channels at a time. Meanwhile, after the early stage of communication, pilot symbols are generally inserted to proper positions of the respective OFDM symbols to chase change of the channel. To estimate the channels using the pilots, each mobile terminal estimates the channels using the pilots included in the specific OFDM symbol and then updates the estimated channel values using scattered pilots.
FIG. 1 is a diagram for illustrating a related art method for transmitting time division multiplexing (TDM) type pilots. As shown in FIG. 1, pilot signals are included in a specific OFDM symbol. A method of arranging pilots shown in FIG. 1 can be used to estimate radio channels at a time at an early stage of communication as described above.
FIG. 2 is a diagram for illustrating a related art method for transmitting scattered pilots. As shown in FIG. 2, pilot signals are not concentrated on a specific OFDM symbol but scattered over the full frequency-time region. In other words, the pilot signals are included in at least two OFDM symbols. As described above, the method of FIG. 2 can be used when change of the channel values is updated after the full radio channels are estimated at the early stage of communication.
In a system which transmits the aforementioned TDM type pilots or the scattered pilots, a two-dimensional (2D) wiener filter is generally used to estimate channels. If the wiener filter is used, a filter value corresponding to a value of a channel to which data are only transmitted should be identified. Accordingly, interpolation and filtering are necessarily required. In other words, since every OFDM symbol does not include pilots, channel estimation is performed for a frequency-time region which does not include pilots, through interpolation and filtering,
First, if portions to which pilots are transmitted are repeated per P number of OFDM symbols, the channels are estimated as follows in case of TDM type pilots. A receiving signal is determined as follows at a position where pilot symbols are received.rk(m)=Hk(m)sk(m)+nk(m)   [Equation 1]
In Equation 1, rk(m) is a receiving signal at a position of a kth sub-carrier of an mth OFDM symbol, Hk(m) is channel response at the kth sub-carrier of the mth OFDM symbol, sk(m) is a transmission symbol value at the kth position of the mth OFDM symbol, and nk(m) is a noise value at a corresponding position. These values can be expressed as vector types by Equation 2 below.{right arrow over (r)}(m)=S(m){right arrow over (H)}(m)+{right arrow over (n)}(m)   [Equation 2]
If a position ‘m’ of the symbol is a position to which pilots are transmitted, the channels are estimated as expressed by Equation 3 below.Ĥ(m)=(SH(m)S(m))−1SH(m){right arrow over (r)}(m)  [Equation 3]
If a communication channel can be modeled by L taps, i.e., L multi-paths, interpolation is applied to the above estimated values as expressed by Equation 4 below.ĥ(m)=F−1Ĥ(m){right arrow over (h)}(m)=[ĥ0(m), ĥ1(m), . . . , ĥL(m), 0, 0, 0, . . . , 0]T  [Equation 4]
Equation 4 indicates the estimated result of channel values for a specific OFDM symbol, and a channel model in a frequency region is finally obtained from the estimated result as expressed by Equation 5 below. H(m)=F h(m)   [Equation 5]
Since the channels estimated as expressed by Equation 5 above exist per Pth position where pilots exist, such channel estimated values are used to decode data OFDM symbols between the pilots. In other words, since the channel values have been estimated for one of a total of P OFDM symbols, interpolation and filtering are performed to estimate channel values for the other (P-1) OFDM symbols. It is more appropriate that intermediate data OFDM symbols should be estimated through interpolation between the Pth OFDM symbols than using the channel values of Equation 5 above. A method for predicting channels using interpolation is realized using h(m) of Equation 4 above. Since multi-path components of the channels are independent from one another, interpolation, prediction, and filtering can be applied to each multi-path component ĥk(Pi)(i=0, 1, 2, . . . ) for k equally. At this time, related art Kalman and least square (LS) filters are used.
FIG. 3 is a diagram for illustrating a concept which predicts channels using pilot signals transmitted in a TDM mode according to the related art. Channel values for OFDM symbols which do not include pilots can be estimated using related art pilots included in a specific OFDM symbol. Generally, for more exact channel estimation, prediction and filtering as well as interpolation can be used together.
Hereinafter, the scattered pilots will be described.
If the scattered pilots are transmitted, the receiving side should delay channel estimation until a sufficient number of pilots are collected. If OFDM symbols which include L number of pilots which is the minimum number of pilots for channel estimation are all ensured, channel estimation can be performed in accordance with Equation 1 to Equation 5 above using a set of the ensured pilots.
Meanwhile, in case of the TDM type pilots, since channel estimation is determined based on an OFDM symbol time, channel estimation for channels where rapid change occurs cannot be performed.
On the other hand, in case of the scattered pilots, channels are not estimated per Pth OFDM symbols but updated using accumulated pilots whenever one OFDM symbol is received after a sufficient number of pilots are collected. Because of this feature, rapid channel change can be estimated under the circumstances that channels are rapidly changed.
However, in case of the scattered pilots, a problem occurs in that high power consumption is caused. In other words, in case of the TDM type pilots, since channel estimation is completed in a moment, the mobile terminal can be shifted into an idle mode during the other time period. However, in case of the scattered pilots, the idle mode cannot be supported.
In the existing low speed system, the TDM type pilots or the scattered pilots could be discussed sufficiently in view of advantages. However, to support the mobile terminal which moves at high speed, a method for transmitting pilots at a time like the method for transmitting TDM type pilots fails to appropriately adapt to rapid channel change, and thus channel estimation is failed, whereby communication is cut off. Also, although the method for transmitting the scattered pilots appropriately adapts to channel change, a problem occurs in that the power is consumed as change of high speed continues to be chased.
A packet retransmission scheme, which is recently used in a mobile communication system, is a hybrid automatic repeat and request (HARQ) which controls throughput by increasing and decreasing parity bits through channel coding. Also, synchronous/asynchronous modes are considered as the HARQ scheme.
In the HARQ scheme, the receiving side decodes a packet transmitted from the transmitting side and identifies whether the packet has errors. Then, the receiving side transmits an acknowledgement signal (ACK) or a negative acknowledgement signal (NACK) to the transmitting side depending on the identified result. If the transmitting side receives ACK from the receiving side, it transmits a new packet and its encoded parity part. If the transmitting side receives NACK from the receiving side, it transmits a retransmission packet to which a parity part is added, wherein the parity part is not transmitted through the prior packet. Then, the receiving side combines the retransmission packet with the previously received packet to perform decoding again.
If an error occurs during channel decoding, the transmitting side and the receiving side undergo a procedure for increasing a log likelihood ratio (LLR) of each symbol so that the packet is decoded in accordance with pre-determined rules. The related art HARQ scheme can be classified into an incremental redundancy (IR) scheme which gradually transmits parity bits which are not transmitted previously and a chase combining or maximum ratio combining scheme which repeatedly transmits previously transmitted data. In case of the IR scheme, a code rate of a corresponding encoded packet is decreased at the receiving side as the number of packet retransmission increases, and decoding can be performed if the number of packet retransmission exceeds a certain threshold value. By contrast, in case of the chase combining or MRC scheme, previously transmitted code bits are re-transmitted so that the receiving side adds the value of the retransmitted bits to the value of the previously received bits. Also, the receiving side performs decoding using a feature of increasing an LLR of each code bit.
FIG. 4 is a diagram for illustrating the related art HARQ scheme. Referring to FIG. 4, an encoded codeword is transmitted by the IR scheme, and the receiving side transmits NACK to the transmitting side if it fails to decode a first packet. At this time, if the transmitting side selects a part, which is not transmitted, from the first packet to generate a retransmission packet and transmits the retransmission packet to the receiving side, the receiving side combines the first packet with the newly received packet to perform packet decoding.
If the packet is decoded, the receiving side transmits ACK to the transmitting side, and transmission of a corresponding codeword is completed. However, if the receiving side still fails to decode the packet the transmitting side collects corresponding parity parts which are not transmitted and transmits them. If there are no parity parts which are not transmitted, the transmitting side re-transmits the previously transmitted parity part. At this time, the receiving side performs MRC based decoding, and adds the newly received parity value to the previously received parity value to generate a new value as expressed by Equation 6 below. The generated value increases an LLR value and improves codeword decoding likelihood.
                              r          ⁡                      (            k            )                          =                                            ∑                              i                =                0                            n                        ⁢                                                  ⁢                                                            h                                      i                    *                                                  ⁡                                  (                  k                  )                                            ⁢                                                r                                                                                                    ⁢                    i                                                  ⁡                                  (                  k                  )                                                                                        ∑                              i                =                0                            n                        ⁢                                                  ⁢                                                            h                                      i                    *                                                  ⁡                                  (                  k                  )                                            ⁢                                                h                  i                                ⁡                                  (                  k                  )                                                                                        [                  Equation          ⁢                                          ⁢          6                ]            
In Equation 6, ri(k) is a value which a kth code symbol is received during the ith transmission, and hi (k) is a channel value of the kth code symbol during the ith transmission. According to this retransmission procedure, packet decoding can be performed after arbitrary retransmission in accordance with Shannon theory.
Among factors which actually determine throughput of a link in a mobile communication system, the powerful factors are a synchronization level between the transmitting side and the receiving side, accuracy of channel estimation, and a signal to noise ratio (SNR) of the received signal. The aforementioned HARQ scheme could be an access for improving throughput of the link by solving a problem of the received SNR.
Meanwhile, in a mobile communication system based on orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiplexing access (OFDMA), a data frame is defined by a two-dimensional plane where a vertical axis is used as a frequency axis and a horizontal axis is used as a time axis. The transmitting side allocates pilot signals to some of the two-dimensional data area during data transmission, allocates desired transmission data to the other data area, and transmits them to the receiving side. The receiving side estimates channels using the pilot signals transmitted from the transmitting side and decodes the transmitted data using the estimated channels.
Accordingly, decoding performance may depend on accuracy of channel estimation based on the pilot signals of the receiving side. Particularly, since the mobile communication system requires sufficient performance for a user who moves at maximum 350 km/h, accuracy of channel estimation is necessarily required under the channel environment which is varied every moment.
Supposing that accuracy of channel estimation is maintained at a certain level, the related art HARQ scheme is focused on a method for transmitting desired packets as soon as possible from a transmitting side to a receiving side, i.e., a method for improving throughput. However, if channel properties are rapidly varied in a mobile communication system due to the presence of a user who moves at high speed, a problem related to packet decoding at the receiving side occurs due to channel decoding performed in a state that channel estimation is not performed exactly. Also, if the SNR of the received signal is low, accuracy of channel estimation may be lowered. In this case, a method for improving accuracy of channel estimation should be considered. However, since the related art considers only an access for transmitting prior packets well, an error in channel estimation is not considered when a packet is actually decoded. For this reason, a problem occurs in that desired throughput is not obtained even in case of decrease of a coding rate through retransmission.