Terrain type is B, having low delay spread and low doppler affect.
Path loss is an attenuative agent , fading is a distortive agent which can be classified in may different types according to invariant and variant channel, wide band and narrowband fading , fast and slow fading and can be explained in both time and frequency domain. In our explanation, we discuss in terms of narrowband and wideband fading. Also called flat and frequency selective in both time and frequency domain. Our case is limited to invariant channel rather than a variant channel where the channel varies with time , we keep the transmission time to be less than the coherence time.
A Narrowband channel makes sure that the signal across its narrow bandwidth receives constant (almost equal attenuation within the range of narrowband) attenuation. It is thus an unavoidable scenario, because we must realize that in practical terms, fading in narrowband is inescapable. Hence, it would be a good idea to realize this type of channel as it gives a good idea of the amount of attenuation or fading in the signal, which does not vary and remains more or less constant. This helps in designing the system in an efficient manner.
Wimax system design is according to this wideband channel, as it operates in the GHz range which is a very wideband channel. It is another thing that actual symbols travel in a narrowband, as explained in the OFDM. narrowband, but we need to take care of the fact that these narrowband channels are actually part of a wideband channel. Wimax system design is according to this wideband channel, as it operates in the GHz range which is a very wideband channel. It is another thing that actual symbols travel in a narrowband, as explained in the OFDM.
The OFDM signal is able to support NLOS performance while maintaining a high level of spectral efficiency, maximizing the available spectrum Superior NLOS performance enables significant equalizer design simplification Supports operation in multi-path propagation environments Scalable bandwidths provide flexibility and potentially reduce capital Expense OFDM is a multi carrier modulation scheme that transmits data over a number of sub-carriers. A conventional transmission uses only a single carrier, which is modulated with all the data to be sent. OFDM breaks the data to be sent in to small chunks, allocating each sub data stream to a sub-carrier. The data is sent in parallel, so that instead of sending just a single bit of information per symbol, many bits are sent per symbol. The symbol rate for OFDM is N times lower than single carrier
. It is an effective technique to squeeze all the multiple, parallel, modulated sub-carriers together, thereby reducing the required bandwidth by inducing orthogonal aspect amongst the different sub-carriers. [ Intel ] Principle objection to use parralel systems is the complexity of the equipment required to implement the systems.Can be greatly reduced by using DFT, provided in OFDM system.
Hence, knowing that fade affects for a smaller period of time, at a particular symbol, it increases the time of the symbol by creating many small bandwidth sized sub carriers making original signal to be spread over the entire bandwidth considering that the fade or the nulls won’t affect the entire bandwidth at a single time. This will result in some carriers being lost rather than the entire signal and with the help of the coding and other recovery techniques; the original signal can be recovered. • This allows the sub channels to be overlapped. • The result is a transmission method that provides very good bandwidth efficiency. – Approaches log2(M) bits/s/Hz, – where M is the number of points in the “premodulation” constellation onto which the data bits are mapped for each sub channel symbol • This is as good or better than narrowband modulation methods and much better than spread spectrum methods, which have typical bandwidth efficiencies of less than 0.2 bits/s/Hz. The same occurs in the frequency domain. As the frequency is changed the wavelength of the RF signal also changes. This results in the phase of the received multipath components changing, and correspondingly the interference between them. Frequency selective fading results in errors in OFDM signals as subcarriers that occur in nulls have a very low SNR. By tracking the nulls in the spectrum it is possible to avoid transmitting in them, saving transmission power and minimising the error rate.
• In a digital system, the delay spread can lead to inter-symbol interference. • This is due to delayed multipath signals overlapping each other. • This can cause significant errors in high bit rate systems. Inter-symbol interference can be reduced by reducing the data rate. • OFDM accomplishes this by dividing the data stream into a number of lower rate data streams and then transmitting each one on a separate sub-
Inter-symbol interference is further reduced by incorporating a cyclic extension to each OFDM symbol. Channel’s delay spread is greater than T m ,then ISI would be introduced. But incase of OFDM system, T m is already greater than individual one bit duration in the original interval, another way of saying id that this guard interval has time= energy to combat energy loss or it is the time to collect the multipath components and lead them to the bin. Copy a piece of the time-domain OFDM symbol from one end to the other. • Adds a guard interval to prevent intersymbol interference caused by multipath delays. • Length of cyclic extension should be governed by the delay spread of the channel. Residual ISI is still left, comabting the mulipath at the second level, improving the robustness in NLOS conditions. OFDM is naturally resistant to the effects of multipath Inter-Symbol interference (ISI) due to the slow symbol rate. The performance can however be further improved by the addition of a guard period to the start of each symbol. This guard period is a copy of the end of the OFDM symbol, and it effectively extends the waveform of the symbol.
As each multipath signal arrives at the receiver it causes constructive or destructive interference depending on the time delay and the strength of the reflection. As the amplitude and phase of an OFDM subcarrier is changed from one symbol to the next, it takes time for all of the reflections to update to the new symbol state. As the new symbol state changes arrives for each delayed reflection it results in a transient change in the amplitude and phase seen by the receiver. After a sufficiently long time (longer than the delay spread of the channel) a steady state amplitude and phase is achieved. The corresponds to the combined vector sum of the direct signal and all the multipath signals. If the FFT of the receiver is taken during this steady state period of the symbol then the effects of ISI are eliminated. The function of the guard period is to extend the length of the each transmitted OFDM symbol so that the receiver can throw away the signals during the transient period, but still have a sufficient number of samples to decode the signal. Translates the frequency-domain mapped code-words into time-domain OFDM symbols for RF transmission. • Creates the orthogonal multiplexed sine waves. • Recommend a size of 64 or 256 depending on requirements such as: – Data Rates – Channel Size – Channel delay spread – QoS The remaining question is then what the size of the DFT should be. A higher value of the DFT size allows for higher throughputs and increases the delay spread tolerance. On the other hand, it increases the complexity, it magnifies the Peak-to-Average ratio problem and increases the phase noise sensitivity, hence requiring a more expensive front-end. As both cost and delay spread tolerance (which is larger than in WLAN applications) are an important factors, a DFT size between 64 and 256 probably makes sense.
1)Reed solomon coding is always uses ½ rate coding mode when requesting the access of the n/w, calculated in terms of Block or frame error rate as the data is bursty in nature.Data is passed in a block format and then passed through a convolution encoderzero padding of bits is appended to the data bits and are not scrambled, code word selection : Best solutionOne codeword encompasses entire channel. – All codewords are “equal”. – All of the strong sub carriers can help out the weak ones. Normal solution:if channel response accros the bandwidth is … Not all code words are equal. – Codeword #2 becomes a “weak link”! – Correction Power in Codeword #3 is wasted. 2) Interleaving 2 step process, first step ensures that the adjacent coded bits are mapped onto non-adjacent sub carriers and are mapped alternately onto MSB AND LSB bits avoiding long runs of low reliable bits. 3)Pilots… They are used to undo any phase rotation caused by RF carrier offsets. 4) Random phase generation helps in minimizing the problems of PAPR. 5) Randomization, padding is added at the end of transmisson block is repeated for each new block of data.IS decided by a certain polynomial which decides addition of zeroes.
1) PAPR-> Caused due to non-linearity in the power amplifiers used and is directly proportional to number of sub-carriers. Various Block coding schemes are used to avoid this, phase optimization. 2)Sensitive to synchronisation as orthogonality could be lost if the system is nto synchronised and leads to BER degradation.More because of doppler affect,where the chanel is variant in nature.
Array Gain: This is the gain achieved by using NARROW band channel/flat fading environment and coherence b/w is less,so uncorelateness b/w 2 signals of different signals of different frequencis comes automatically. multiple antennas so that the signal adds In awgn ber improvement requires 10-2 to 10-3 an increase in snr of 2-3 db but in case of coherently. requires an improvement of 10 db. • Diversity Gain: This is the gain achieved by utilizing multiple paths so that the probability that any one path is bad does not limit performance. Effectively, diversity gain refers to techniques at the transmitter or receiver to achieve multiple “looks” at the fading channel. These schemes improve performance by increasing the stability of the received signal strength in the presence of wireless signal fading. Diversity may be exploited in the spatial (antenna), temporal (time), or spectral (frequency) dimensions.
P in one channel then p^n ,in many channels.p-> deep fade probability Mean powers should be equal, if signal in one branch and another have low cross co-relation but different mean powers , then one with low mean power won’t have any contribution. Should have low cross correlation. Space diversity->when the distances between two transmit antennas is increased, then they give rise to low correlation as increase in path length differences averages the phase difference over number of positions. Antennas with spacing limit the use of space diversity but because of presence of various scattering elements due to multipath channel , the model will bring out good diversity, We want the distance between the 2 antennas to be higher so that diversity increaes. Horizontal diversity is commonly applied as because height of the antenna can lead to significant path loss in a or each antenna. Time diversity->Retransmission of a same signal reduces the system capacity and introduces a transmission delay.The principle is applied effectively with error correction techiques.
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