Fast synchronization for frequency hopping systems

The present invention relates to a synchronization method and apparatus for providing synchronization to a received data stream in a frequency hopping transmission system.

Wireless Personal Area Networks (WPANs) have become a ubiquitous technology. Bluetooth, as standardized by the IEEE 802.15 WPAN Working Group (WG) in 1999, combines low cost and ease of use to a new class of wireless applications. Targeting at the market of broadband multimedia applications, e.g., Wireless Universal Serial Bus (WUSB), Wireless IEEE 1394 and High Definition TV (HDTV), the next generation of WPANs is expected to provide much higher data rates as well as better Quality of Service (QoS) support. The IEEE 802.15 Task Group 3a (TG3a) was formed for standardizing a high speed alternative Physical layer (PHY) based on the Ultra- Wideband (UWB) frequency band aiming at data rates up to 480Mb/s. A multiband orthogonal frequency division multiplex (MB-OFDM) solution, as one of the two leading proposals, was proposed and supported by the Multiband OFDM Alliance (MBOA).

Multiband (MB) OFDM PHY uses the well known Orthogonal Frequency Division Multiplexing (OFDM) combining with band hopping technique in the UWB frequency band. In MB-OFDM data is transmitted simultaneously over multiple carriers spaced apart at precise frequencies. Fast Fourier Transform (FFT) algorithms provide nearly 100 percent efficiency in capturing energy in a multi-path environment, while only slightly increasing transmitter complexity. Beneficial attributes of MB-OFDM include high spectral flexibility and resiliency to RF interference and multi-path effects. The overall 7.5GHz frequency bandwidth of UWB is divided into 14 bands, with each having a bandwidth of 528MHz. The 14 bands are further organized into 5 groups, of which the first four groups consist of three bands each and the last group has only two. Frequency hopping is carried out within each band group according to predefined Time-Frequency Codes (TFC) and with the frequency of each hop per OFDM symbol. Frequency hopping between the bands provides enhanced robustness against multi-path and interference. Typically, frequency hopping

occurs after every OFDM symbol. Totally 30 logical channels are provided by using the multiband solution and the frequency hopping scheme.

The proposed timing for MB-OFDM is performed using a preamble in packet data systems. In the MB-OFDM proposal for 802.15.3a a physical layer convergence procedure (PLCP) preamble is added to PLCP frame format. The PLCP preamble aids receiver algorithms related to synchronization, carrier offset recovery and channel estimation. The PLCP preamble consists of three portions comprising a packet synchronization sequence, a frame synchronization sequence, and channel estimation sequence. The packet synchronization sequence is used for packet detection, coarse carrier frequency estimation and coarse symbol timing. The frame synchronization sequence is used to synchronize the receiver algorithm within the preamble. The channel estimation sequence is used to estimate the channel frequency response for fine carrier frequency estimation and fine symbol timing. Fig. 1 shows a typical OFDM receiver part. The OFDM signal is received via an antenna 10 and a pre-selection filter 12 adapted to perform rough selection of the desired frequency band(s). A subsequent low noise amplifier (LNA) 14 amplifies the received

OFDM signal and supplies the amplifies signal to two mixer or multiplier circuits 20, where the amplified OFDM signal is mixed with respective sine and cosine local oscillator signals to generate an in-phase signal I and a quadrature phase signal Q. The I and Q signals are processed in parallel circuit branches by a respective low-pass filter 22, variable gain amplifier (VGA) 24 and analog-to-digital converter (ADC) 26. Based on the digital output signal of at least one of the ADCs 26, a automatic gain control (AGC) circuit 30 generates an AGC control signal supplied at least to the VGA 24 in order to control amplification within the parallel circuit branches to keep the received I and Q signal within a predetermined level range. The above-described blocks 12 to 26 belong to a conventional UWB receiver 60 with AGC setting and frequency hopping control (not shown in Fig. 1).

The processed I and Q signals are input into a synchronization block 40 where synchronization processing and FFT (Fast Fourier Transformation) processing is done. The processed digital data stream is supplied to a digital processing chain 42 which includes for example processing steps for pilot removal, de-interleaving, Viterbi decoding, and de- scrambling.

Setting up a wireless link based on the above MBOA UWB standard requires synchronization, signal detection and AGC setting for a specific frequency hopping signal. The synchronization block 40 at the receiver will perform those synchronizations based on the PLCP preamble. It is assumed that the FFT size is 128 (128 OFDM carriers), so that N

=128+32+5=165 samples are required to be processed per OFDM block. I.e., 5 samples correspond to the 9.5nsec guard time to switch and 32 samples correspond to the length of a cyclic prefix.

Fig. 2 shows a schematic flow diagram of an example of a conventional synchronization sequence where switching is performed every OFDM symbol.

In step SlOO, N data samples Rec are recorded or stored. Then, in step SlOl, correlation with one OFDM PLCP preamble of size N is calculated or computed at the synchronization block 40, e.g. Test=corr(Rec, PLCP(I :N)). In step S102 a check is made by a peak detector whether the correlation result Test exceeds a predetermined threshold. If not, the procedure jumps back to step SlOO to receive the next N data samples. On the other hand, if it is determined in step S 102 that the predetermined threshold has been exceeded, a peak index is determined and the start of the OFDM symbol is computed. Thus, the different hopping sequences combined with null's in OFDM symbol requires a sophisticated detection and synchronization method over multiple symbols.

It is therefore an object of the present invention to provide an improved synchronization method and apparatus, by means of which simple and fast detection and synchronization of the frequency hopping signal can be achieved . This object is achieved by synchronization apparatus circuit as claimed in claim 1 and by a synchronization method as claimed in claim 12.

Accordingly, fast hopping frequency switching is used to scan multiple bands within a single symbol period, so as to provide fast and simple synchronization to the symbol pattern of the received data stream. Moreover, the scanning sequence and timing can be chosen in such a manner that the correlation output can be used to simultaneously detect the frequency hopping sequence, synchronization timing and interferer and signal amplitude within a minimal amount of symbol periods.

The number of symbols required for synchronization are thus minimized and fast automatic gain control (AGC) setting can be performed, which results in shorter set-up times for wireless links in e.g. an UWB system based on the above MBOA proposal. This can be traded for more accurate estimation of timing and signal and interferer levels.

The correlation means may be arranged to store results of the correlation for a plurality of subsequent time periods included in one symbol period. Thus, the set of

correlation results obtained during a symbol period is readily available for evaluation and can be supplied in parallel to the synchronization control means.

Furthermore, the synchronization control means may be arranged to determine the start of a symbol period of the data stream based on determined ratios between the stored correlation results of one symbol period. This provides the advantage that synchronization can be achieved very fast, due to the fact that correlation results are already available after one symbol period.

Additionally, the synchronization control means may be arranged to use the stored correlation results for estimating at least one of noise power, channel delay spread, timing offset, noise or interference power in adjacent hopping frequency bands, and automatic gain control settings. This combined or simultaneous estimation of several parameters leads to an improved system efficiency. According to a specific example, the timing offset can be estimated based on a correlation result obtained for a zero prefix header portion of the received data stream. In this case, output power of the respective correlation result or output is proportional to the timing offset, in view of the fact that during the zero prefix a zero correlation output would be obtained at perfect synchronization.

In addition thereto, the frequency hopping pattern can be obtained based on subsequent patterns of said stored correlation results of a predetermined number of symbol periods. This predetermined number depends on the total number of hopping patterns which is also determined by the total number of hopping frequency bands. As the several hopping frequency bands are scanned during one symbol period, determination time can be reduced significantly.

The correlation means may further be arranged to classify correlation results into a null output (n) indicating correlation results close to zero, a small output (s) indicating small correlation results which depend on the degree of synchronization, and a big output (B) indicating very high correlation results. Thereby, only three values have to be distinguished for obtaining the above estimations.

As an example for easy implementation, the frequency hopping transmission system may comprises three different frequency hopping bands and the predetermined time may then by a quarter of the symbol period, so that four correlation results are obtained during one symbol period. In this exemplary case, the frequency hopping pattern can be distinguished with six preamble symbols by using the suggested technique.

The reference pattern may comprise a synchronization sequence of a frame header, e.g. a PLCP header as described initially, of the frequency hopping transmission system which may be an MB-OFDM system.

Further advantageous modifications are defined in the dependent claims.

In the following, the present invention will be described in more detail based on a preferred embodiment with reference to the accompanying drawings in which:

Fig. 1 shows a schematic block diagram of a conventional OFDM receiver circuit in which the present invention can be implemented;

Fig. 2 shows a schematic flow diagram of a conventional synchronization method where the reception band is switched once for every symbol period;

Fig. 3 shows a schematic block diagram of an improved synchronization apparatus according to the preferred embodiment; Fig. 4 shows a schematic flow diagram of a synchronization method according to the preferred embodiment where the reception band is switched four times during a symbol period;

Fig. 5 shows a table indicating frequency hopping patterns and their corresponding correlation output sequences according to the preferred embodiment; Figs. 6 to 17 show schematic timing diagrams indicating examples of specific frequency hopping patterns and corresponding correlation outputs; and

Fig. 18 shows a schematic timing diagram for a specific frequency hopping pattern and the effect on the corresponding correlation outputs in case of a timing offset.

The preferred embodiment will now be described in connection with a 3 -band receiver device for an UWB OFDM system as proposed by MBOA for WPAN standard IEEE 802.15.3a. Of course, the preferred embodiment is not limited to this system and can be generally used in any frequency hopping communication system. Since such modern standards have a preamble pre-append to the data symbols, data-based systems can be used to perform synchronization based on a correlation. The synchronization block 40 at the receiver of Fig. 1 is suggested to perform this synchronizations in the preferred embodiment based on the PLCP preamble.

It is assumed that six frequency hopping patterns FHP are used for transmission, and an FFT size NfJt= 128 (i.e. 128 OFDM carriers) is used at the synchronization block 40, which means that N =128+32+5=165 samples must be computes or processed per OFDM block. I.e., five samples correspond to the 9.5ns guard time for switching, thirty-two samples correspond to the length of the cyclic prefix. Furthermore, the preample of the OFDM symbols, e.g. the PLCP preamble, is written as [ZP sw Dl sw Dl sw Dl], where "Dl" designates the same data portion repeated three times per OFDMA symbol, and "sw" designates data that will be lost during switching from one hopping frequency band to another hopping frequency band. This principle is already defined in the standard MBOA 802.15.3a vl for use between two consecutive OFDM symbols, but not within one symbol. A similar "sw" time can of course also be provided for switching between two consecutive OFDM symbols (or hopes) as defined in the above standard. Finally, "ZP" designates a zero prefix part which has a length of at least one data portion "Dl".

Fig. 3 shows a schematic block diagram of an improved synchronization apparatus according to the preferred embodiment. This synchronization apparatus can be provided for example as an enhancement in the OFDM receiver of Fig. 1.

In Fig. 3, an UWB receiver 60, which may basically correspond to the conventional UWB receiver 60 of Fig. 1, is arranged to receive an MB-OFDM signal via the shown antenna and output digital I and Q signals. The UWB receiver 60 comprises at least two control inputs for inputting a hopping control signal HC for frequency hopping control and a gain control signal AGC for automatic gain control. The gain control signal AGC serves to control the gain or amplification of amplifiers in the receiving path, e.g. VGA 24 in Fig. 1, to thereby keep the level of the output signals within a predetermined range. Additionally, the frequency hopping control is performed to switch the receiving band in accordance with the selected frequency hopping pattern used for transmitting the received MB-OFDM signal, and to initially scan the different hopping frequency bands for synchronization purposes, as explained later in detail. This switching may be performed by controlling the local oscillator frequency at respective mixer circuits of the UWB receiver 60, e.g. the mixer circuits 20 of Fig. 1. To achieve the above control functions or functionalities, a correlator 70 is provided which performs a correlation processing on at least one of the digital output signals of the UWB receiver 60. In this correlation processing, a predetermined number of samples of the digital output signal, which corresponds to a fraction of the OFDM symbol period is processed to obtain a correlation level with a predetermined reference pattern, e.g. PLCP

preample or portion thereof, stored or generated at the correlator 70. The correlation result or output CO is supplied to a controller unit 80 for AGC and synchronization. Based on the correlation output, the controller unit 80 then generates the gain control signal AGC and the hopping control signal HC used for controlling operation of the UWB receiver 60. In following, an example of an operation of the correlator 70 and the controller unit 80 is described with reference to Fig. 4 for a specific exemplary case, where the hopping control signal HC is used to switch the receiving band, e.g. by switching the local oscillator frequency via a corresponding LO pin, four times per OFDM symbol.

Fig. 4 shows a schematic flow diagram of a synchronization method according to the preferred embodiment for the duration of only one OFDM symbol. For synchronization purposes, this procedure will be repeated several times, as required to obtain all information necessary for synchronization, hopping control, AGC or other control purposes.

In step S200, a running variable i used for counting the number of band switchings per OFDM symbol is set to 1 and the local oscillator frequency is set to a first value LoI. In step S201, N/4 samples Rec(l) of the data stream received through the corresponding selected reception band (hopping frequency band) are received and supplied to the correlator 70. At the correlator 70, a correlation of the samples Rec(l) with a quarter portion PLCP(I :N/4) of the OFDM PLCP preamble is calculated in step S202 to obtain a correlation result Test(l)=corr(Rec(l), PLCP(I :N/4)). This correlation result is then stored in step S203. Thereafter, in step S204, it is checked whether all four sample periods of one OFDM symbol have been processed, i.e., whether the running variable has reached a value i=4. If not, the procedure loops back to step S201 and performs the same processing for the remaining ones of the N/4 samples R(I) to R(4) to obtain all four correlations results Test(l) to Test(4) for one OFDM symbol. When all correlation results Test(i) have been calculated, the condition of step

S204 is fulfilled and the procedure proceeds to step S206 where ratios between the correlation results Test(i) are calculated in order to determine the start of the OFDM symbol. The correlation results Test(i) may be forwarded as the controller output CO to the controller unit 80 which then calculates the ratios and determines the start of the OFDM symbol. Using the above described example of four scanning or band switching operations per OFDM symbol, the actually used frequency hopping pattern can be distinguished at the controlling unit 80 based on six OFDM preamble symbols, as explained in the following based on the table of Fig. 5.

Fig. 5 shows a table indicating frequency hopping patterns used in the UWB OFDM system and corresponding sequences or codes of correlation outputs CO supplied from the correlator 70.

The correlation results Test(i) are categorized or quantized into three correlation levels designated by respective symbols. A first symbol "n" is used to designate a null output indicating correlation results close to zero. A second symbol "s" is used to designate a small output indicating small correlation results which depend on the degree of synchronization, and a third symbol "B" is used to designate a big output indicating very high correlation results. In the table of Fig. 5, twelve frequency hopping patterns (FHP) Al to D3 of three different hopping frequency bands "1" to "3" are listed together with six related patterns of correlation outputs CO, each pattern consisting of four correlation symbols per OFDM symbol. As can be gathered from the table, a used frequency hopping pattern can be detected based on six correlation output patterns (i.e. 24 correlator outputs CO) and thus after six OFDM symbol periods.

During a synchronization operation, an actual sequence of received correlation patterns is compared at the controller unit 80 with the table of Fig. 5, which may be stored as a look-up table or in a (programmable) read-only memory, in order to derive the actual frequency hopping pattern. Additionally, correlation results of "s" symbols, which coincide with the zero prefix part of the PLCP preamble, can be used to estimate the noise power. Furthermore, "n"- related correlation results can be used by the controller unit 80 to estimate the power of an interferer(s) or noise in two adjacent hopping frequency bands. In addition thereto, the relations between the various correlator outputs can be used to set up the AGC gain setting by the controller unit 80.

Thus, system switching during the reception of one OFDM symbol can be used for at least one several control options, e.g., synchronization, symbol to interference (or noise) ratio estimation, automatic gain control, and determination of frequency hopping patterns. Figs. 6 to 17 show schematic timing diagrams indicating examples of specific frequency hopping patterns and corresponding correlation outputs CO for four OFDM symbol periods in a frequency hopping system based on the table of Fig. 5.

In Fig. 6, a case is shown, where the hopping frequency pattern Al (i.e. frequency band sequence "123123") is used in the received OFDM data stream. The vertical

axis of the diagrams indicates frequency bands and the horizontal axis is a time axis. Hence, the upper line relates to a first frequency band "1", the middle line relates to a second frequency band "2", and the lower line relates to a third frequency band "3". The upper diagram schematically indicates OFDM symbol periods, each with an initial zero prefix period, and their respective frequency bands. The middle diagram indicates the switching pattern of the receiving bands, e.g. by suitable control of the local oscillator frequencies. This middle diagram is identical for all twelve examples, as the sequence of the four reception bands within one OFDM symbol period is kept constant in this embodiment, which is however not essential for the present invention. In the present example, the reception bands are switched with according to a pattern "1231" within each OFDM symbol period. When comparing the middle diagram with the upper diagram, the correlation results of the correlator 70, as shown in the lower diagram, may readily be estimated. For example, during the first OFDM symbol period of Fig. 6, the OFDM symbol is transmitted using the first frequency band "1". Thus, within the first correlation period (first quarter of the symbol period) where the reception band of the UWB receiver 60 is switched to this first frequency band "1", a high output would be obtained. However, due to the fact that the OFDM symbol starts with a zero prefix, a low correlation output is obtained, which however depends on the level of synchronization. Thus, the correlation symbol "s" is obtained as correlation output. In the second and third correlation quarter, wrong reception bands "2" and "3" are switched, so that two successive correlation symbols "n" are obtained. However, in the last correlation quarter of the symbol period, the switched reception band "1" again matches with the hopping frequency band "1" and no zero prefix is received, so that a high correlation output (indicated as a dark area in the lower diagram) and thus a correlation symbol "B" is obtained at the correlator output. Consequently, a total correlation pattern "s n n B" is obtained for the first OFDM symbol.

In summary, the following correlation pattern is generated for six OFDM symbols of the frequency hopping pattern Al :

"s n n B, s n n B, s n n B, s n n B, ...".

Figs. 7 to 17 show similar diagrams for the frequency hopping patterns A2 to C3. In general, the present scanning pattern (middle diagram) always leads to the case that the reception frequency of the first correlation quarter in an OFDM symbol period is identical to the reception frequency of the last quarter. Consequently, a correlation symbol "s" is only obtained in the first quarter if a correlation symbol "B" is obtained in the last quarter.

Similar to the explanation of Fig. 6, the examples of Figs. 7 to 17 lead to the following correlation patterns:

Fig. 7 which corresponds to the frequency hopping pattern A2 ("231231" - rotated pattern Al): "n B n n, n B n n, n B n n, n B n n, ..."

Fig. 8 which corresponds to the frequency hopping pattern A3 ("312312" - rotated pattern Al):

"n n B n, n n B n, n n B n, n n B n, ..."

Fig. 9 which corresponds to the frequency hopping pattern Bl ("132132"): "s n n B, n B n n, n n B n, s n n B, ..."

Fig. 10 which corresponds to the frequency hopping pattern B2 ("321321" - rotated pattern Bl):

"s n n B, n B n n, n n B n, s n n B, ..."

Fig. 11 which corresponds to the frequency hopping pattern B3 ("213213" - rotated pattern Bl):

"n B n n, n n B n, s n n B, n B n n,..."

Fig. 12 which corresponds to the frequency hopping pattern Cl ("112233"):

"s n n B, n n B n, n n B n, n B n n, ..."

Fig. 13 which corresponds to the frequency hopping pattern C2 ("223311" - rotated pattern Cl):

"n B n n, s n n B, s n n B, n n B n,..."

Fig. 14 which corresponds to the frequency hopping pattern C3 ("331122" - rotated pattern Cl):

"n n B n, n B n n, n B n n, s n n B,..." Fig. 15 which corresponds to the frequency hopping pattern Dl ("113322"):

"s n n B, n n B n, s n n B, n B n n, ..."

Fig. 16 which corresponds to the frequency hopping pattern D2 ("332211" - rotated pattern Dl):

"n n B n, n B n n, n n B n, n B n n, ..." Fig. 17 which corresponds to the frequency hopping pattern D3 ("221133" - rotated pattern Dl):

"n B n n, s n n B, n B n n, s n n B, ..."

Fig. 18 shows a schematic timing diagram for a specific frequency hopping pattern and the effect on the corresponding correlation outputs in case of a timing offset.

Based on the above examples, it can be seen that the correlation results of "s" symbols, which coincide with the zero prefix part of the PLCP preamble, can also be used to estimate the delay spread of a channel (i.e. mount of multipath) when the frame is perfectly synchronized. If the frame is not perfectly synchronize than this "s" part (which indicates "leakage" from the correlator window between two OFDM symbols) can be used to estimate the time offset and therefore resynchronize the frame.

If a timing difference δt between the correlator windows (correlation quarter of the OFDM symbol) and the OFDM symbol occurs, as indicated in Fig. 18, then instead of having a "0" at the output of the correlator when it is calculating during the zero prefix we will get some power (symbol "s") proportional to this timing offset δt. This is represented by different grey shading of the first box or rectangle of the third diagram as compared to the optimal alignment depicted in the fourth (lowest) diagram of Fig. 18. The power related to the correlation symbol "s" can thus be used by the controller unit 80 for timing control. In addition, the grey shading of the "B" correlator output will show an opposite behavior due to less overlapping at bigger timing differences.

In case of a desynchronization of one "correlator window" a different correlator pattern will be obtained.

In summary, an apparatus and method have been described for synchronizing a receiver function to a received data stream in a frequency hopping transmission system, wherein the data stream is received in at least a first frequency hopping band and a second frequency hopping band, and a correlation between data stream samples received in the receiving step and a predetermined reference pattern is calculated during a predetermined time period smaller than a symbol period of the data stream. The receiving step is controlled to switch from the first frequency hopping band to the second frequency hopping band at the end of the predetermined time period, so that correlation results for different frequency hopping bands are obtained during one symbol period. Thereby, multiple frequency hopping bands can be scanned during a single symbol period so that fast synchronization can be achieved, while hopping sequence, synchronization timing and interference and signal amplitudes can be detected simultaneously. Any algorithms related to synchronization, carrier offset recovery and channel estimation can be used with the above approach. For example, maximum likelihood estimates (MLE) or other correlation based techniques can be used.

Moreover, any fractional or integer relation between the duration of the smaller correlation window and the symbol period could be used to obtain a desired increase

in synchronization speed. Moreover, the scanning pattern for switching the reception frequency bands is not restricted to the example of Figs. 6 to 18. Any band sequence can be used as long as all frequency hopping bands are covered at least after a certain number of symbols. The preferred embodiment may thus vary within the scope of the attached claims.

Finally, it is noted that the term "comprises" or "comprising" when used in the specification including the claims is intended to specify the presence of stated features, means, steps or components, but does not exclude the presence or addition of one or more other features, means, steps, components or group thereof. Further, the word "a" or "an" preceding an element in a claim does not exclude the presence of a plurality of such elements. Moreover, any reference sign does not limit the scope of the claims.