Abstract:
In an orthogonal frequency division multiplex (OFDM) communication network, a node transmits an echo profile probe to other nodes in the network. The echo profile probe is a message that allows characterization of the unique echo profile through the communication channel between each node pair. The echo profile is used to calculate the cyclic prefix length needed for optimum communication from one node to the other.

Description:
RELATED APPLICATIONS 
     This application is a continuation of copending application Ser. No. 13/032,580, filed Feb. 22, 2011, which is a continuation of application Ser. No. 12/473,656, filed May 28, 2009, now U.S. Pat. No. 7,995,459, which is a continuation of application Ser. No. 11/229,297, filed Sep. 16, 2005, now U.S. Pat. No. 7,542,411, which claims the benefit of the filing date of U.S. provisional patent application No. 60/633,257 filed Dec. 3, 2004. Each patent application identified above is incorporated here by reference in its entirety to provide continuity of disclosure. 
    
    
     BACKGROUND 
     1. Field of Invention 
     The invention relates to broadband communication networks. 
     2. Prior Art 
     Orthogonal Frequency Division Multiplexing (OFDM) is a modulation scheme used in communication systems. It is a technique used in multi-antenna broadband systems since it significantly reduces the complexity of the receiver by providing orthogonal sub-channels. 
     Multipath effects in OFDM are greatly reduced by adding a cyclic prefix (CP) to each OFDM symbol. The CP acts as a guard interval between successive OFDM symbols. In conventional OFDM systems, a fixed-length CP is used. If the channel delay spread is less than or equal to the CP, inter-symbol interference (ISI) is prevented. When the receiver is moving from one environment to another, this condition may not be met, ISI will occur, and the system will be impaired. 
     The phase noise in OFDM can cause inter-channel-interference (ICI). The use of a cyclic prefix also reduces ICI. 
     A drawback to OFDM and the use of a fixed-length CP is the reduction in rate due to the CP overhead. If the fixed length CP is not long enough, system performance deteriorates. If the CP length is too long, spectrum efficiency is reduced. 
     Zhang et al., “A novel OFDM transmission scheme with length-adaptive Cyclic Prefix”, J Zhejiang University SCI, 2004, describes a technique applied to a mobile wireless environment to create a variable length CP in order to counter the aforementioned drawbacks of fixed CP length. The paper describes a system where the CP length is reduced when delay spread is small thus reducing overhead. Conversely, the CP length is increased when delay spread is high and ISI eliminated. The disclosed technique utilizes known symbols in the preamble of each packet transmission or pilot sub-carriers to estimate the channel parameters. The channel parameters are used to determine the RMS delay spread and the resulting CP length needed. The technique is inefficient in slowly changing environments because of the preamble overhead or the pilot symbols present in each transmission. Accuracy in the determination of the channel parameters decreases as the number of pilot symbols is reduced to increase spectrum efficiency. In order to compute the delay spread accurately, a long sequence is needed, but the continual transmission of a long sequence uses bandwidth inefficiently. 
     SUMMARY OF THE INVENTION 
     The present invention uses an echo profile probe as part of a technique of determining the channel impulse response (CIR) of a communication channel in order to set appropriate communication parameters including cyclic prefix (CP) length for orthogonal frequency division multiplex (OFDM) communication. Without probing the channel, the CP length would have to be set conservatively and long. The echo profile probe is a known message transmitted by any node to other nodes. The receiving nodes, having a priori knowledge of the probe message, perform a computation on the received message, and determine delay spread and the required CP length to avoid inter-symbol interference (ISI). 
     When a new node enters a network, the nodes need to determine channel characteristics with each other node. The new node broadcasts the echo profile sequence to each node, and each receiving node returns its computation to the new node. Each node also sends an echo profile to the new node for computation and response. The computations include the determination of the minimum cyclic prefix needed. By minimizing the CP length for each packet transmission, overhead due to the CP is reduced and the bandwidth efficiency and performance of the system is optimized. 
     The present invention uses a separate message, a probe message, to determine the channel characteristics and thereby determine the optimum CP length, which overcomes a limitation of other adaptive schemes that use constantly transmitted symbols in each packet to determine the channel characteristics. The use of the probe or unique message of this invention allows more symbols to be used for channel characterization, providing increased accuracy in calculating delay spread and required CP length. Because the cable environment is considered nearly static, the probe is sent infrequently thus reducing the overhead required for optimum channel characterization. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  Echo Profile Probe Structure 
         FIG. 2  Illustration of Delay Spread 
         FIG. 3  New Node Entering Network 
         FIG. 4  Details of Communication between New Node and Existing Node 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , the echo profile probe is a message  100  used to determine the channel impulse response. The message  100  consists of a preamble  101  and a time-domain data payload  104 . The preamble contains information for gain control  102 , and for timing and frequency-offset adjustments  103 . The payload contains the echo profile sequence consisting of 1024 pseudo-random BPSK-modulated bits. This sequence is generated using an m-sequence of degree 10, given by the generator polynomial 2201 8  with an initial seed of 3FF 16 . 
     In the receiver, the received Echo Profile Sequence (rx) is cross-correlated to a local copy of the same sequence (lseq) as shown in Equation 1.
 
corr_out=Σ i=0   1024   rx ( t−i )* l seq( i )  Equation 1
 
     Referring to  FIG. 2 , the time distance between the earliest significant correlation peak  200  and the latest significant correlation peak  201  is the significant delay spread in the channel response. This delay spread can be used to determine the minimum cyclic prefix that would maintain the orthogonality of the OFDM symbol and eliminate ISI. 
     The time distance between peaks is determined by further processing of the output of the correlation (corr_out from Equation 1). An example of the processing steps are described below: 
     Step 1: The peak value of corr_out is determined. This is found using the following pseudo-code and recorded as MAX_PEAK. The time where the MAX_PEAK occurs is recorded as PEAK_TIME.
         IF (MAX_PEAK&gt;corr_out) MAX_PEAK=corr_out       

     Step 2: Look for the first instance of a peak that exceeds a threshold X dB below the peak and record this time as START_PROFILE. X assumes an initial value determined by system analysis for a particular system and is a function of the echo amplitude expected in the channel. 
     Step 3: Look for the last instance of a peak that exceeds a threshold X dB below the peak and record this time as END_PROFILE. 
     Step 4: Calculate CP_LEN=END_PROFILE−START_PROFILE. 
     Step 5: If CP_LEN is greater than CYC_PREFIX_MAX, reduce X by 1 dB and repeat step 2-5 until CP_LEN is smaller than CYC_PREFIX_MAX. CYC_PREFIX_MAX is the maximum cyclic prefix allowed in the data path and equals to 64 in the current embodiment. 
     Step 6: Reduce X by 1 dB and repeat step 2, 3 and 4. If the CP_LEN is less than R*CYC_PREFIX_MAX (where R is typically a value between 0 and 1) use the newly calculated CP_LEN. Otherwise, use the previously calculated CP_LEN. In this embodiment, step 6 is only done once, but step 6 may also be applied repeatedly with increasingly smaller R until CP_LEN is greater than R*CYC_PREFIX_MAX 
     Step 7: Compute SYSTEM_BIAS_TIME as PEAK_TIME−START_PROFILE. 
     Step 8: Increase the CP_LEN such that CP_LEN=CP_LEN+DSG_MARGIN. DSG_MARGIN is dependent on implementation, for example 25. 
       FIG. 3  illustrates the used of the echo profile probe when a new node  300  enters a network with existing nodes  301 ,  302 , and  303 . New node  300  broadcasts its echo profile probe to each of the existing nodes. Each of the existing nodes returns results of computations to node  300   
       FIG. 4  illustrates the communication between new node  400  and existing node  401 . The new node  400  initially transmits its echo profile probe to the existing node  401 . The existing node  401  receives the message and performs the necessary computations. The existing node  401  then transmits the calculated cyclic prefix length to the new node  400  followed by the transmission of its echo profile probe. The new node  400  receives the probe and performs the needed calculations, and then sends back to the existing node  401  its calculated cyclic prefix length. Existing node  401  transmits an echo profile probe to new node  400 , which performs computations and transmits a cyclic prefix length to existing node  401 . The probe message can be sent multiple times and the results averaged for increased accuracy. 
     In one embodiment of the invention, a cycle master, also called Network Coordinator (NC), controls admission of a new node. A map broadcast by the NC schedules echo profile probe slots along with all other messages. Existing nodes expect a probe message at the slot time designated in the map. 
     The present invention is suitable for use in a network operating over a communication channel formed by interconnecting nodes with coaxial cable and passive signal splitters.