【導(dǎo)讀】意傳播，從而導(dǎo)致更大帶寬的信號的方法。這些技術(shù)用于各種原因包括增加抗自然干。擾和干擾，以防止檢測，并限制功率流密度的安全通信設(shè)立的。斯拉在1900年7月提出的。特斯拉想出了這個想法后，在1898年時展示了世界上第。號控制船是安全的需要。他的專利涉及兩個實現(xiàn)抗干擾能力根本不同的技術(shù)，實現(xiàn)這。兩個功能通過改變載波頻率或其他專用特征的干擾免疫。第一次在為使控制電路發(fā)射。整，必須在作出回應(yīng)。第二個技術(shù)使用由預(yù)定的方式更改傳輸?shù)念l率的一個編碼輪控。其他幾個專利被帶到了20. 統(tǒng)”1942年獲美國第2,292,387專利。制造商在國防會議上了解到這一問題。發(fā)生變化，其旨在使無線電導(dǎo)向魚雷，讓敵人很難來檢測或干擾。該專利來自五零年。雷達(dá)系統(tǒng)的并行研究和一個稱為“相位編碼”的技術(shù)類似概念。對擴(kuò)頻發(fā)展造成影響。“傳播”的無線電信號較寬的頻率范圍內(nèi)若干程度高于最低要求。對于解擴(kuò)的正常運行，發(fā)送和接收序列必須同步。索過程使發(fā)射器的序列與接收器序列同步。

　　

【正文】 (used . as PHY and MAC layer for ZigBee) Frequencyhopping spread spectrum Frequencyhopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. It is utilized as a multiple access method in the frequencyhopping code division multiple access (FHCDMA) scheme. A spreadspectrum transmission offers three main advantages over a fixedfrequency transmission: 1. Spreadspectrum signals are highly resistant to narrowband interference. The process of recollecting a spread signal spreads out the interfering signal, causing it to recede into the background. 2. Spreadspectrum signals are difficult to intercept. An FHSS signal simply appears as an increase in the background noise to a narrowband receiver. An eavesdropper would only be able to intercept the transmission if they knew the pseudorandom sequence. 3. Spreadspectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spreadspectrum signals add minimal noise to the narrowfrequency munications, and vice versa. As a result, bandwidth can be utilized more efficiently. Basic algorithm Typically, the initiation of an FHSS munication is as follows 1. The initiating party sends a request via a predefined frequency or control channel. 2. The receiving party sends a number, known as a seed. 3. The initiating party uses the number as a variable in a predefined algorithm, which calculates the sequence of frequencies that must be used. Most often the period of the frequency change is predefined, as to allow a single base station to serve multiple connections. 4. The initiating party sends a synchronization signal via the first frequenc y in the 11 calculated sequence, thus acknowledging to the receiving party it has correctly calculated the sequence. 5. The munication begins, and both the receiving and the sending party change their frequencies along the calculated order, starting at the same point in time. Technical considerations The overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one carrier frequency. However, because transmission occurs only on a small portion of this bandwidth at any given time, the effective interference bandwidth is really the same. Whilst providing no extra protection against wideband thermal noise, the frequencyhopping approach does reduce the degradation caused by narrowband interferers. One of the challenges of frequencyhopping systems is to synchronize the transmitter and receiver. One approach is to have a guarantee that the transmitter will use all the channels in a fixed period of time. The receiver can then find the transmitter by picking a random channel and listening for valid data on that channel. The transmitter39。s data is identified by a special sequence of data that is unlikely to occur over the segment of data for this channel and the segment can have a checksum for integrity and further identification. The transmitter and receiver can use fixed tables of channel sequences so that once synchronized they can maintain munication by following the table. On each channel segment, the transmitter can send its current location in the table. In the US, FCC part 15 on unlicensed system in the 900MHz and bands permits more power than nonspread spectrum systems. Both frequency hopping and direct sequence systems can transmit at 1 Watt. The limit is increased from 1 milliwatt to 1 watt or a thousand times increase. The Federal Communications Commission (FCC) prescribes a minimum number of channels and a maximum dwell time for each channel. In a real multipoint radio system, space allows multiple transmissions on the same frequency to be possible using multiple radios in a geographic area. This creates the possibility of system data rates that are higher than the Shannon limit for a single channel. Spread spectrum systems do not violate the Shannon limit. Spread spectrum systems rely on excess signal to noise ratios for sharing of spectrum. This property is also seen in MIMO and DSSS systems. Beam steering and directional antennas also facilitate increased system performance by providing isolation between remote radios. 12