Simultaneous Cell Group and Cyclic Prefix Detection Method, Apparatus and System
A method, and associated apparatus and system, for simultaneous cell group and cyclic prefix (CP) detection, having the steps of determining primary synchronization signal (P-SyS) timing τ using the P-SyS; based on τ, determine a secondary synchronization signal (S-SyS) timing; placing a single Fast Fourier Transform (FFT) window; FFT processing the signal to obtain the frequency domain S-SyS symbols; equalizing the frequency domain S-SyS signal; phase correcting the S-SyS signal; and detecting the cell group and CP length by the correlation giving maximum energy.
This application claims the benefit of U.S. Provisional Application No. 60/945,399 filed Jun. 21, 2007, the disclosure of which is incorporated herein by reference.
TECHNICAL FIELDThe present invention relates generally to communication systems and components and, more particularly, to wireless communication systems an components adapted to use Orthogonal Frequency Division Multiplexing (OFDM) modulation techniques.
BACKGROUNDEvolving mobile cellular standards such as Global System for Mobile Communications (GSM) and Wideband Code Division Multiple Access (WCDMA) will likely require modulation techniques such as OFDM in order to deliver higher data rates. OFDM is a method for multiplexing signals which divides the available bandwidth (BW) into a series of frequencies known as sub-carriers.
In order to ensure a smooth migration from existing cellular systems to high capacity, high data rate systems using existing radio spectrum, new systems must be able to operate on a flexible BW. Third generation Long Term Evolution (LTE) has been proposed as a new flexible cellular system. LTE is intended as an evolution of the WCDMA standard. LTE will likely use OFDM and operate on BWs spanning from 1.25 MHz to 20 MHz. Data rates of up to 100 Mb/s will be possible in the high BW LTE service.
Low rate services such as voice are also expected to use LTE. Because LTE is designed for Transmission Control Protocol/Internet Protocol (TCP/IP), voice over IP (VoIP) will likely be the service carrying speech.
One important aspect of LTE is the mobility function. As a result, synchronization symbols and cell search procedures are of major importance in order for an apparatus, such as a user equipment (UE), to detect and synchronize with other cells.
The proposed cell search scheme for LTE is as follows:
1. Detect symbol timing for new cell using the primary synchronizer signal (P-SyS). Furthermore, because there are three P-SyS, the UE also detects which of the P-SyS have been transmitted from the cell. The index of each P-SyS identifies the cell ID within a group. P-SyS is transmitted every 5 milliseconds (ms).
2. Detect frame timing and cell group using the secondary synchronization signal (S-SyS). The frequency domain representation of P-SyS is used as phase reference and then the S-SyS detection (correlation to different S-SyS sequences) is performed in the frequency domain.
3. From steps (1) and (2), the cell is detected.
4. Read broadcast channel (BCH) to receive cell specific system information
In LTE, there will be a possibility of using a short cyclic prefic (CP) or a long CP length. The short CP length (4.7 microseconds (μ sec)) will be used for small cells and the long CP length (16.7 μsec) will be used for large cells and broadcast services. The intention is that the UE should detect the cell specific CP length blindly. This is preferably performed prior to detecting the frame timing and cell group using the secondary S-SyS (step 2 of the cell search scheme described above). Blind CP detection can be made in the time domain, as seen in the block diagram 100 of
It would be advantageous to have a low complexity blind CP detection method and apparatus that is robust also in scenarios existing in OFDM cellular system like LTE. The present invention provides such a method and apparatus.
The present invention is a method, apparatus and system to simultaneously determine the CP length and the cell group during the cell search step of detecting the frame timing and cell group using the S-SyS in a wireless telecommunications system.
DETAILED DESCRIPTIONThe present invention is a method to simultaneously determine the CP length and the cell group during the cell search when detecting the frame timing and cell group using the S-SyS by time adjusting the Fast Fourier Transform (FFT) window for the S-SyS. The present invention further includes an apparatus and system adapted to implement said method.
When the P-SyS 5 ms timing is detected, the timing for S-SyS can be computed for both for the long CP and short CP length case, that is, the placement of the FFT window for both cases can be determined. In the present invention, the FFT window for S-SyS is set between the estimated timing for the long CP and short CP. Then, the channel in the frequency domain is positive phase shifted for the long CP length and negative phase shifted for the short CP relative the channel determined by the P-SyS. Therefore, prior to the correlation to the S-SyS sequences, the received frequency domain transformed S-SyS signal is positive and negative phase corrected and the S-SyS sequences are correlated to both corrected signals. The SyS sequence and correction giving maximum energy is detected as the cell group and the length of CP. As noted, the CP can be detected in the frequency domain, avoiding the multiple peak problem of the conventional method, while advantageously using only one FFT processing. Hence the method of the present invention is robust and has low complexity.
In
As is known from FFT processing of OFDM symbols, a sampling error of −n chips (relative ideal timing) gives a rotation of −2πn/NFFT radians between consecutive sub-carriers, where NFFT is the length of the FFT. The foregoing relationship is true as long as the sampling error is within the CP, and therefore, if n is known it can be perfectly compensated for in the detection process. As can be seen in
Assume sampling at time instant 304 results in a ±n chip sampling error to the ideal timing in the long CP (+) and short CP (−) case. A mathematical model of the frequency domain received S-SyS symbol at sub-carrier k (where Nused sub-carriers are used for S-SyS sequences) can now be written:
YkS-SyS=e±j2π·n·k/N
where + is true if it is a long CP (positive), and − is true if it is a short CP (negative). The channel Hi is estimated using the P-SyS as a phase reference and hence can be equalized, i.e. can determine the CP length as well as the cell group of the received S-SyS. Equalization can be accomplished using a variety of techniques. For example, and without limitation, the following steps can be used to perform the equalization:
Now, two de-rotated versions, each version phase corrected with the phase shift corresponding to the long CP and short CP of the received S-SyS are generated and the two phase corrected versions are correlated to all possible M S-SyS sequences and the correlation giving the highest power is used to determine the CP length as well as the cell group. Mathematically speaking, the following steps are performed:
A flowchart 400 illustrating the method of the present invention is provided in
An apparatus adapted to implement the method of the present invention is provided in
After signal is received at antenna 501 and demodulated at FE RX 502 it is converted into a digital signal at ADC 503. The P-SyS timing τ is determined using the P-SyS, which corresponds to 301 of
The S-SyS timing is derived at S-SyS timing module 505, based on outcome from P-SyS, corresponding to 304 of
Dotted lines 603 show the approximate boundaries of the cell sites 601. The cell sites are shown approximately circular for the purposes of illustration and explanation only. It should be clearly understood that the cell sites often have other irregular shapes, depending on the cell configuration selected and natural and man-made obstructions.
As is well known in the art, cell sites 601 are comprised of a plurality of sectors (not shown), each sector being illuminated by a directional antenna coupled to the base station. The embodiment of
In the wireless network 600, apparatus 602 is located in cell sites 601A, 601B and is in communication with serving cell 601B. Apparatus 602 is also located close to the edge of cell site 601B. Apparatus 602 routinely performs cell searches to detect the base stations of a wireless network in the vicinity of the apparatus 602. Whenever an apparatus is turned on, an initial cell search is performed in order to search for and acquire at least one of the base stations of wireless network. Thereafter, the apparatus continues to perform cell searches in order to determine the strongest base station(s) in the vicinity and to identify available base stations to which the mobile station may be transferred in case it is necessary to perform a handoff. To improve the efficiency of these cell searches, the system of the present invention includes the apparatus of
There have been described and illustrated herein methods, apparatus, and systems to simultaneously determine the CP length and the cell group during the cell search by time adjusting the Fast Fourier Transform (FFT) window for the S-SyS. While particular embodiments of the present invention have been described, it is not intended that the present invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. For example, the method can be used where there are more than two CP length hypotheses present. In such case, the phase de-rotation is proportional to the difference between the used sampling time instant and the ideal sampling instant for each respective CP length hypothesis. Then the steps described herein can be applied. Further, while the apparatus of the invention is shown in block diagram format, it will be appreciated that the block diagram may be representative of and implemented by hardware, software, firmware, or any combination thereof. Moreover, the functionality of certain aspects of the block diagram can be obtained by equivalent or suitable structure. For example, instead of an FFT, other Fourier transform means could be utilized. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as claimed.
Claims
1. A method for simultaneous cell group and cyclic prefix (CP) detection, comprising the step of placing a single Fast Fourier Transform (FFT) window for a secondary synchronization signal (S-SyS) between the estimated timing corresponding to an Orthogonal Frequency Division Multiplex (OFDM) symbol transmitted using a long CP or a short CP.
2. A method for simultaneous cell group and cyclic prefix (CP) detection comprising the steps of:
- placing a single Fast Fourier Transform (FFT) window for a secondary synchronization signal (S-SyS) between the estimated timing for a long CP and a short CP;
- performing a first phase shift of the channel in the frequency domain for the long CP length;
- performing a second phase shift of the channel in the frequency domain for the short CP relative the channel determined by a primary synchronization signal (P-SyS);
- phase correcting the received frequency domain transformed S-SyS signal prior to the correlation to the S-SyS sequences;
- correlating the S-SyS sequences to both corrected signals;
- detecting the S-SyS sequence and correction providing maximum energy as the cell group and the length of CP.
3. The method of claim 2, wherein the timing position is in the middle between the long CP and short CP.
4. The method of claim 2, wherein the first phase shift is a positive phase shift and the second phase shift is a negative phase shift.
5. The method of claim 2, for use in a user equipment (UE).
6. A method for detecting simultaneous cell group and cyclic prefix (CP), comprising the steps of:
- determining primary synchronization signal (P-SyS) timing τ using the P-SyS;
- based on τ, determine a secondary synchronization signal (S-SyS) timing;
- positioning a single Fast Fourier Transform (FFT) window;
- FFT processing the signal to obtain the frequency domain S-SyS symbols;
- equalizing the frequency domain S-SyS signal;
- phase correcting the signal; and
- detecting the cell group and CP length by the correlation giving maximum energy.
7. The method of claim 6, wherein the equalizing step is in accordance with: Y _ k S - Sys = Y k S - SyS H ^ k ≈ ± j2π · n · k / NFFT S k + e k
8. The method of claim 6, wherein the phase correcting step is in accordance with the formulas:
- {tilde over (Y)}klong CP=e−j2π·n·k/NFFT {tilde over (Y)}kS-SyS
- {tilde over (Y)}kshort CP=ej2π·n·k/NFFT {tilde over (Y)}kS-SyS
9. The method of claim 6, wherein the correlation providing maximum energy is based on the following formula: cell group, CP length = arg ( long / short ), m max ∑ k = 1 N used ( s k m ) · [ Y ~ k longCP, Y ~ k short CP ] 2 m = 1 … M
10. The method of claim 6, for use in a user equipment (UE).
11. A method for simultaneous cell group and cyclic prefix (CP) detection, comprising the steps of:
- determining a primary synchronization signal (P-SyS) timing; based on the timing, determining secondary synchronization signal (S-SyS) timing;
- placing a single Fast Fourier Transform (FFT) window;
- performing a FFT;
- obtaining S-SyS symbols;
- testing the S-SyS sequences to phase corrected S-SyS symbols; and
- obtaining the cell group and CP length based on the best S-SyS correlation match.
12. The method of claim 11, wherein the best S-SyS correlation is based on the correlation providing maximum energy according to the following formula: cell group, CP length = arg ( long / short ), m max ∑ k = 1 N used ( s k m ) · [ Y ~ k longCP, Y ~ k short CP ] 2 m = 1 … M
13. The method of claim 6, for use in a user equipment (UE).
14. An apparatus for simultaneous cell group and cyclic prefix (CP) detection, comprising:
- cell search means for determining primary synchronization signal (P-SyS) timing;
- cell search means for determining secondary synchronization signal (S-SyS) timing based on the P-SyS timing;
- means for placing a single Fast Fourier Transform (FFT) window;
- means for performing a FFT;
- means for obtaining a plurality of S-SyS symbols;
- means for testing the S-SyS sequences to phase corrected S-SyS symbols; and
- means for obtaining the cell group and CP length based on the best S-SyS correlation match.
15. The apparatus of claim 14, in combination with a UE.
16. The apparatus of claim 11, wherein the UE comprises:
- an antenna;
- a front end receiver (Fe RX) having an input coupled to the antenna;
- an analog to digital converter (ADC) having an input coupled to the output of the Fe RX;
- a P-SyS correlation module having an input coupled to the output of the ADC;
- a S-SyS timing module having an input coupled to the output of the P-SyS correlation module;
- a Fast Fourier Transform (FFT) module having an input coupled to the output of the ADC and an input coupled to the output of the P-SyS module and S-SyS timing module:
- a phase correction module having an input coupled to the output of the P-SyS correlation module;
- a channel estimation module having an input coupled to the output of the FFT;
- a detector module having an input coupled to the output of the FFT and the output of channel estimation module;
- a S-SyS detector having an input coupled to the output of the phase correction module and the output of the channel estimation module, wherein the P-SyS timing τ and S-SyS timing are determined using the P-SyS, at P-SYS correlation module, the FFT window is placed and the signal is FFT processed to obtain the frequency domain S-SyS symbols at FFT module and the cell group and CP length detected at S-SyS detector are given by the correlation giving maximum energy in S-Sys detector module.
17. The apparatus of claim 16, for use in an Orthogonal Frequency Division Multiplexing (OFDM) modulation system.
18. An Orthogonal Frequency Division Multiplexing (OFDM) modulation system, comprising:
- a module for determining primary synchronization signal (P-SyS) timing τ using the P-sys;
- a module for determining a secondary synchronization signal (S-SyS) timing based on τ;
- a module for placing a Fast Fourier Transform (FFT) window;
- a FFT module for processing the signal to obtain the frequency domain S-SyS symbols;
- a module for equalizing the frequency domain S-SyS signal;
- a module for phase correcting the signal; and
- a module for detecting the cell group and CP length by the correlation giving maximum energy.
19. The system of claim 18, in combination with a user equipment (UE) for use in a Long Term Evolution system.
20. The OFDM modulation system of claim 18, wherein the correlation providing maximum energy is based on the following formula: cell group, CP length = arg ( long / short ), m max ∑ k = 1 N used ( s k m ) · [ Y ~ k longCP, Y ~ k short CP ] 2 m = 1 … M
Type: Application
Filed: Dec 20, 2007
Publication Date: Dec 25, 2008
Inventors: Leif Wilhelmsson (Dalby), Bengt Lindoff (Bjarred)
Application Number: 11/961,603
International Classification: H04J 11/00 (20060101);