Why is synchronization important in communication

In addition to joint timing and carrier synchronization, [ ] considers time domain synchronous TDS -OFDM system which replaces cyclic prefix with a pseudo noise PN and thus proposes PN-correlation-based synchronization, and [ ] considers hardware implementation.

The differences among them are that in addition to joint timing and carrier synchronization, CFO estimation in [ ] applies to a wide CFO range, i. The second group of papers provides an estimation error lower bound [ — ].

In addition to joint timing and carrier synchronization, [ ] considers doubly selective channel, [ ] considers hexagonal multi-carrier transmission system, and [ ] proposes training sequence design.

Timing and carrier synchronization for multi-carrier SISO communication systems is still an ongoing topic of research, as evidenced by the large number of published papers. In particular, there is a major emphasis on accurate CFO estimation for different types of systems and often in conjunction with RF impairments such as phase noise and IQ imbalance.

Few recent research works have considered the practical problem of both estimation and compensation of timing and carrier frequency offsets in the presence of channel impairments [ — ]. The authors in [ ] only consider integer frequency offsets and the particular work may be extended considering both integer and fractional frequency offsets. The works of [ — ] can be considered as a baseline reference for further extension in the relevant system models.

In a multi-antenna wireless communication system, data are transmitted across different channels that are modeled either as quasi-static or time varying. The received signal at an antenna is given by a linear combination of the data symbols transmitted from different transmit antennas. In order to achieve multiplexing or capacity gain, independent data are transmitted from different transmit antennas. A space-time MIMO decoder can be used to decode the signal from multiple antenna streams.

On the other hand, in order to achieve diversity gain, the same symbol weighted by a complex scale factor may be sent over each transmit antenna. This latter scheme is also referred to as MIMO beamforming [ ]. Depending on the spatial distance between the transmit or receive antennas, which may differ for line-of-sight LOS and non-LOS propagation, the antennas may be equipped with either their own oscillators or use the same oscillator.

Depending on the number of antennas at the transmitter and the receiver, multi-antenna systems can be further categorized into MIMO systems, multiple-input single-output MISO systems, or single-input multiple-output SIMO systems. Further, if the antennas at the transmitter side are not co-located at a single device, such a system is referred to as a distributed-MIMO system, i.

In multi-antenna systems, multiple signal streams arrive at the receive antenna from different transmit antennas resulting in multiple timing offsets MTOs. In some special cases, multiple timing offsets actually reduce to a single timing offset, e. If the transmit antennas are fed through independent oscillators, the received signal at the receive antenna is affected by multiple carrier frequency offsets MCFOs because of the existence of independent frequency offset between each transmit antenna oscillator and the receive antenna oscillator.

On the other hand, if the transmit antennas are equipped with a single oscillator, the received signal at the receive antenna is affected by a single frequency offset. Thus, each receive antenna has to estimate and compensate for a single or multiple timing and frequency offsets, depending on the system model assumptions including Doppler fading.

In the case of distributed antenna systems, the receiver has to estimate and compensate for multiple CFOs and multiple TOs because each distributed transmit antenna is equipped with its own oscillator and multiple signal streams arriving at the receive antenna experience different propagation delays. Thus, in practice, the number of distributed antennas may need to be limited to avoid synchronization and pilot overhead associated with obtaining multiple CFOs and TOs.

The summary of the research carried out to achieve timing and carrier synchronization in single-carrier multi-antenna communication systems is given in Table 8 :.

The estimation or compensation of CFO alone is studied in [ — ]. The joint timing and carrier synchronization is studied in [ — ]. They also differ in terms of proposing estimation, compensation, joint channel estimation, or estimation lower bound.

Further details or differences are provided in the last column of Table 4 , which indicates if any additional parameter, e. Synchronization in single-carrier multi-antenna communication systems has not received as much attention compared to synchronization in multi-carrier multi-antenna communication systems. This may not be surprising since the latter is adopted in current wireless cellular standards.

Still, single-carrier multi-antenna communication has got its importance in microwave backhaul links [ ]. The authors assume pilot-free systems to propose blind synchronization. Generally, performance improvement is expected in the presence of training-based estimation and compensation. This, along with hardware implementation of the relevant algorithms, can be the subject of possible future extensions. In multi-carrier multi-antenna systems, the information at each antenna is modulated over multiple carriers.

Similar to single-carrier multi-antenna systems, the signal arriving at the receive antenna can potentially be affected by multiple TOs and multiple CFOs, when the transmit antennas are fed by different oscillators and are distant from one another. The synchronization challenge is to jointly estimate and compensate for the effect of multiple TOs and multiple CFOs in order to mitigate ISI and ICI and decode the signal from multiple antenna streams.

The summary of the research carried out to achieve timing and carrier synchronization in single-carrier multi-antenna communication systems is given in Table 9 :. The estimation or compensation of TO and CFO alone is studied in [ — ] and [ — ], respectively. They also differ in proposing estimation, compensation, joint channel estimation, or estimation lower bound. Further details or differences are provided in the last column of Table 9 , which indicates if additional parameters, e.

Compared to the estimation of single TO and single CFO, estimation of MTOs and MCFOs is more challenging, due to pilot design issues, overhead, pilot contamination problem, complexity, and non-convex nature of optimization problems. Particularly, the authors in [ ] propose a compact TS design. However, both papers, [ , ], do not consider channel estimation as it may help in improving the estimation performance of synchronization impairments, and further, they do not suggest algorithms for compensation of MTOs and MCFOs.

Though joint estimation and compensation of timing and carrier frequency offset has been studied, e. The shortcomings in the above key papers can be the subject of possible future research.

In cooperative communication systems, the information transmission between the two communicating nodes is accomplished with the help of an intermediate relay. Let us assume a general scenario with the presence of multiple relays.

There are two important types of cooperative communication networks:. One-way relaying network OWRN , where information transmission occurs in one direction via intermediate relays. Two-way relaying network TWRN , where information transmission occurs simultaneously in both directions and both nodes exchange their information with the help of intermediate relays. The relays themselves can operate in different modes. The two most common modes are i decode-and-forward DF and ii amplify-and-forward AF operation.

In DF mode, the relays decode the received signal and forward the decoded signal to the intended destination node s. In AF mode, the relays do not decode the received message and simply amplify and forward the received signal. In the following subsections, we review the recent literature that deals with timing and carrier synchronization in single-carrier and multi-carrier cooperative communication systems. Since the relaying operations are different in DF and AF cooperative communication systems, so too are their synchronization methodologies.

In AF, for example, the relays may not be required to convert the received passband signal to baseband and to perform carrier synchronization [ 20 ]. In TWRN, there is self-interference, which affects the way how the synchronization problem is formulated.

The communication generally takes place in two phases. During the first phase, the source transmits the information to the relays. During the second phase, the relays decode the received signal and forward it to the destination. Typically, it is assumed that the direct communication link between the source and the destination is absent or blocked due to some obstacles.

However, in general, there could be a direct communication link between them. In such a case, the destination also hears the source message during the first phase and coherently combines it with the message received during the second phase. In DF-OWRN, during the first phase of the two-phase communication process, the synchronization between the source and the relays or between the source and the destination in the presence of direct link is achieved by estimating and compensating for a single TO and CFO between the source and each relay or between the source and the destination in the presence of direct link.

During the second phase, the synchronization between the relays and the destination is achieved by estimating and compensating for the multiple TOs and multiple CFOs between the multiple relays and the destination. Note that in the case of a single relay, only a single TO and a single CFO are required to be estimated and compensated for at the destination during the second communication phase. Increasing the number of relays raises the challenge of pilot design and estimation overhead.

The summary of the research carried out to achieve timing and carrier synchronization in single-carrier DF-OWRN is given in Table 10 :. Estimation or compensation of timing offsets alone and frequency offsets alone is studied in [ — ] and [ 19 , — ], respectively. Joint timing and carrier synchronization is studied in [ 20 , — ]. TWRNs allow for more bandwidth efficient use of the available spectrum since they allow for simultaneous information exchange between the two nodes.

In TWRNs, it is usually assumed that there is no direct communication link between the two nodes. During the first phase of the two-phase communication process, the information arrives at the relays from the two nodes.

The signals from the two nodes are superimposed at the relays. During the second phase, the relays decode the exclusive OR XOR of the bits from the received superimposed signal and then broadcast a signal constructed from the XOR of the bits back to the two nodes [ ]. Thus, unlike DF-OWRN, the received signal at each relay during the first communication phase is a function of two TOs and two CFOs, which need to be jointly estimated and compensated for in order to decode the modulo-2 sum of the bits from the two nodes.

Another challenge for TWRN is the pilot design in the presence of self-interference at the relay node. There is only one paper in the last 5 years that falls in this category and proposes joint estimation and compensation of timing offsets [ ]. However, instead of DF operation, the relays amplify and forward the source information. This is due to imperfect synchronization during the first communication phase, and the amplified and forwarded signal from the relays is a function of the residual TOs and CFOs between the source and the respective relays.

The summary of the research carried out to achieve timing and carrier synchronization in single-carrier AF-OWRN is given in Table 10 :. Estimation or compensation of timing offset alone and frequency offset alone is studied in [ , , ] and [ 19 ], respectively.

Joint timing and carrier synchronization is studied in [ 20 , , ]. During the second phase, the relays amplify and broadcast the superimposed signal back to the two nodes [ ].

In AF-TWRN, when the relays receive the superimposed signals from the two nodes during the first communication phase, each relay only needs to carry out timing synchronization, i. The reason will be explained shortly. During the second communication phase, the relays amplify and broadcast the time-synchronized version of the superimposed signal.

Each node then needs to estimate and compensate for the MTOs between the relays and itself and the sum of the multiple CFOs from the other node-to-relays-to-itself. Note that each node in this case does not need to estimate and compensate for the multiple CFOs from itself to relays to the other node because the effect of CFOs between itself and the relays during the first communication phase is canceled by the effect of CFOs between the relays and itself during the second communication phase due to the use of the same oscillators [ , ].

Due to this very reason, the authors in [ , ] propose to only perform timing synchronization at the relay nodes during the first communication phase in AF-TWRN. The estimation or compensation of timing offset alone and frequency offset alone is studied by [ — ] and [ , ], respectively. The joint timing and carrier synchronization is studied by [ ].

In single-carrier cooperative communication systems, few recent research works have considered the important problem of both estimation and compensation of MTOs and MCFOs in the presence of channel impairments [ 20 , , , ].

Particularly, synchronization in OWRNs is studied by [ 20 , , ], where [ ] proposes blind synchronization with blind source separation and relay selection and [ 20 , ] proposes training-based synchronization. AA Nasir et al. Finally, synchronization in TWRNs is studied by [ ]. There are still many open research problems to solve in this area, e.

Since most of the papers consider orthogonal frequency division multiplexing OFDM as a special case of multi-carrier communication system, the system model and synchronization challenge below are presented for OFDM systems. The synchronization challenge during the first communication phase between the source and the relays is similar to that presented for SISO multi-carrier systems in Section 2. During the second communication phase, the relays decode the received signal and forward it to the destination.

The summary of the research carried out to achieve timing and carrier synchronization in single-carrier DF-OWRN is given in Table 11 :. Estimation or compensation of frequency offset alone is studied in [ — ]. Joint timing and carrier synchronization is studied in [ — ]. The received signal at each relay during the first communication phase is a function of two TOs and two CFOs, which need to be jointly estimated and compensated. To the best of our knowledge, no paper in the last 5 years falls into this category, since the research in the synchronization of multi-carrier TWRN has considered AF relaying.

The estimation or compensation of timing offset alone and frequency offset alone is studied in [ , — ] and [ ], respectively. Further details or differences are provided in the last column of Table 11 , which indicates if any additional parameter, e. The estimation or compensation of frequency offset alone is studied in [ — ]. The categorized papers differ in the sense that training sequence design is proposed in [ ], joint CFO estimation and compensation with channel estimation is studied in [ ], and CFO estimation alone is proposed in [ ].

In multi-carrier cooperative communication systems, the problem of joint estimation and compensation of MTOs and MCFOs has been considered in very few works, e. Future research investigations may help to achieve efficient algorithms. Moreover, most of the solutions are pilot based. Hence, it is a challenging open problem to design semiblind and blind estimators. In SC-FDMA uplink communication systems, multiple users communicate with a single receiver and the effect of channel distortion is equalized in frequency domain at the receiver.

Disjoint sets of M subcarriers are assigned to each of the K users, and data symbols from each user are modulated over a unique set of subcarriers through an M -point FFT operation.

Next, cyclic prefix is appended at the start of the transmission block to mitigate the multipath channel effect. Due to the presence of an independent oscillator at each transmitting user and due to different propagation delays between each user and the receiver, the received signal in the SC-FDMA uplink communication system suffers from multiple CFOs and multiple TOs. Synchronization is achieved through periodically transmitted primary and secondary synchronization signals from the base station.

Any user who has not yet acquired the uplink synchronization can use the primary and secondary synchronization signals transmitted by the base station to first achieve synchronization in the downlink. Next, compensation for the propagation loss is made as part of the uplink random access procedure [ ]. The research carried out to achieve timing and carrier synchronization in SC-FDMA uplink communication systems is summarized in Table 12 :.

The estimation or compensation of multiple CFOs alone is studied in [ , — ]. Though [ — , — ] consider the same channel model with known CSI in order to propose multiple CFO compensation, they differ in the following characteristics.

Different channel allocation strategies and their effects on interference due to CFO is studied in [ ]. An interference self-cancelation scheme to compensate for the effect of CFO is proposed by [ ]. Joint timing and carrier synchronization is studied in [ ]. Further, [ , ] consider multiple antennas at the users and receiver, [ ] proposes blind beamforming assuming that the multipath delay is greater than the cyclic prefix length, and [ ] proposes MMSE-FDE.

Considering pilot-free transmission, joint estimation and compensation of timing and carrier frequency offsets is studied in [ ].

The work can be extended to consider pilot-based systems in order to improve the estimation accuracy. Further, CRLB derivation in the presence of synchronization impairments and hardware implementation can also be the subject of future work. Unlike OFDM systems, where information of a single user is modulated over all subcarriers, in OFDMA uplink transmitter, each user transmits over a set of assigned subcarriers.

In the end, cyclic prefix is appended and information is transmitted from every user. At the receiver side, following cyclic prefix removal and FFT operation, equalization is carried out to decode the information from the K users. The receiver has to estimate and compensate the effect of multiple TOs and multiple CFOs in order to decode information from each user.

The research to achieve timing and carrier synchronization in OFDMA uplink communication systems is summarized in Table Almost all categorized papers study carrier synchronization only, [ — ], except the authors in [ ] consider ranging scheme to propose timing estimation in OFDMA uplink communication system.

In addition, the authors in [ ] provide solutions for synchronizing both the timing and frequency errors of multiple unsynchronized users in OFDMA-based spectrum sharing system. Further extension may consider the derivation of CRLBs in the presence of impairments and hardware implementation design.

Further details or differences are provided in the last column of Table Code division multiple access CDMA communication systems allow multiple transmitters to send information simultaneously to a single receiver. All users share the same frequency and time resources. To permit this to be achieved without undue interference, CDMA employs spread-spectrum technology, i.

A modulated signal for each user is spread with a unique spreading code at the transmitter. Finally, at the receiver, information for each user is dispread by using the same unique despreading code. All users share the full available spectrum. At each successive time slot of brief duration, the frequency band assignments are reordered. Each user employs a PN code, orthogonal or nearly orthogonal to all the other user codes, that dictates the frequency hopping band assignments.

The design of special spreading and despreading codes that are robust to synchronization errors is also a challenging task. Moreover, since the received signal powers from different users may vary significantly, the correct synchronized despreading may be challenging.

Synchronization also faces multipath delay spread since PN chips have a short time duration. Since spectral spreading does not use a very high hopping frequency but rather a large hop-set, the hop time will be much longer than the DS-CDMA system chip time. Moreover, in FH-CDMA, there is no need for synchronization among user groups, only between transmitter and receiver within a group is required. Table 14 summarizes the research carried out to achieve timing and carrier synchronization in CDMA multiuser communication systems.

All categorized papers consider DS spread spectrum technology and study the carrier synchronization alone [ — ], where as identified in the second column of Table 14 , [ , ] consider single-carrier communication and [ — ] consider multi-carrier communication.

Note that further details or differences are provided in the last column of Table The joint estimation and compensation of carrier frequency offsets has been considered in [ , ]. Their work can be extended to include timing synchronization. Cognitive radio networks allow unlicensed secondary users SUs access to the spectrum of the licensed primary users PUs , without impairing the performance of the PUs. Depending on the spectrum access strategy, there are three main cognitive radio network paradigms [ ]:.

In the underlay cognitive networks , SUs can concurrently use the spectrum occupied by a PU by guaranteeing that the interference at the PU is below some acceptable threshold [ ]. Thus, SUs must know the channel gains to the PUs and are also allowed to communicate with each other in order to sense how much interference is being created to the PUs. In the overlay cognitive networks , there is tight interaction and active cooperation between the PUs and the SUs.

Thus, SUs use sophisticated signal processing and coding to maintain or improve the PU transmissions while also obtaining some additional bandwidth for their own transmission. In interweave cognitive networks , the SUs are not allowed to cause any interference to the PUs. Thus, SUs must periodically sense the environment to detect spectrum occupancy and transmit opportunistically only when the PUs are silent [ ].

In the context of interweave cognitive networks , SUs sense the spectrum to detect the presence or absence of PUs and use the unoccupied bands while maintaining a predefined probability of missed detection.

Different methods are used to detect the presence of PUs such as matched filtering, energy detection, cyclostationary detection, wavelet detection, and covariance detection. In cognitive radio-based communication systems, the presence of timing and frequency offset affects the spectrum sensing performance and may result in false detection by the SUs. However, this is challenging given that SUs do not have access to pilot symbols and may need to estimate these parameters in a blind fashion.

Blind synchronization algorithms are also known to be less accurate, which introduces new challenges to spectrum sensing in the presence of synchronization errors in cognitive radio networks. Recent research in synchronization for cognitive radio-based communication systems is summarized in Table In Table 15 ,.

The carrier synchronization alone is studied in [ — ], where as identified in second column of Table 15 , [ , ] consider single-carrier communication and [ , , — ] consider multi-carrier communication. The joint timing and carrier synchronization is proposed in [ — ], where as identified in second column of Table 15 , [ ] considers single-carrier communication and [ — ] consider multi-carrier communication.

As detailed in Table 15 , the categorized papers differ in channel model, providing joint channel estimation, requiring CSI or training, proposing estimation or compensation, or providing lower bound. If applicable, further details for some paper, e. The important problem of joint estimation and compensation of timing and carrier frequency offsets has been considered in [ , ] for training-based and pilot-free communication systems.

The future work in this area may build upon them to develop more efficient estimation and compensation techniques and hardware implementation. In distributed multiuser communication systems, multiple distributed users try to communicate with a common receiver. Cooperation may exist among the distributed users to transmit the same information, i. On the other hand, each user might send its own data, which can cause MUI at the receiver. The receiver can employ successive interference cancelation to decode information from the desired user.

Both single-carrier and multi-carrier modulation schemes can be employed by the multiple users. Due to the presence of independent oscillator at each transmitting user, the Doppler effect, and the existence of a different propagation delay between each user and the receiver, the received signal may suffer from multiple CFOs and multiple TOs. The receiver has to jointly estimate and compensate the effect of these synchronization impairments in order to decode the desired information.

Table 16 summarizes the research carried out to achieve timing and carrier synchronization in distributed multiuser communication systems. All listed papers [ — ] consider multi-carrier communication. Carrier synchronization alone is studied by [ ]. Joint timing and carrier synchronization is proposed in [ — ], where joint estimation and compensation design is considered by [ , ] and only compensation of timing and carrier frequency offsets is studied in [ , ].

Different from [ , ], [ , ] also considers joint channel estimation. Thus, CoMP is also referred to as multicell cooperation. Note that the transmission from the base stations can take place over single or multiple carriers. Some main points regarding the CoMP techniques are as follows:. In coordinated scheduling and coordinated beamforming , multiple coordinated transmission points only share the CSI for multiple users.

The data packets that need to be conveyed to the users are available only at the respective transmission point to which each user belongs. Thus, every base station coordinates with its cell edge user through beamforming.

In joint transmission , multiple coordinated transmission points share both CSI and the data packets to be conveyed to all users. Thus, the same data is simultaneously transmitted to the intended user from multiple coordinated transmission points with appropriate beamforming weights. Transmission point selection can be regarded as a special form of JT, where transmission of beamformed data for a given user is performed at a single transmission point at each time instance.

In addition, both CSI and the data are assumed to be available at multiple coordinated transmission points. Thus, an appropriate transmission point with access to the best channel conditions for individual users can be scheduled.

While one transmission point coordinates with the scheduled user, other transmission points may possibly communicate in parallel to their respective users. The synchronization between coordinating base stations and the users can be achieved either in uplink or downlink transmission. For downlink CoMP , due to the different oscillators at the base stations and the different propagation delays between each base station and the user, the received signal at the user suffers from multiple CFOs and multiple TOs.

The receiver has to estimate these parameters jointly and compensate for their effects in order to establish successful CB, JT, or TPS scheme among coordinating base stations. The base stations have to compensate for the effect of these multiple synchronization parameters in order to synchronize data transmission to the users during downlink communication by adopting any CoMP scheme. Since CoMP uses the backhaul link for coordination among the base stations, synchronization parameters can be exchanged in order to enhance the synchronization performance.

Considering synchronization in CoMP-based communication systems, Table 17 summarizes the recent research:. The timing synchronization alone is studied by [ — ], where as identified in the second column of Table 17 , [ ] considers single-carrier communication and [ , ] consider multi-carrier communication.

Carrier synchronization alone is studied by [ — ], where as identified in the second column of Table 17 , [ ] considers single-carrier communication and [ — ] consider multi-carrier communication. Joint timing and carrier synchronization considering single- and multi-carrier communication is analyzed in [ ] and [ ], respectively.

Considering the TPS scheme, joint estimation and compensation design of timing and carrier frequency offsets is studied in [ ]. The modification of the proposed algorithm can be studied to achieve joint estimation and compensation of timing and carrier frequency offsets for CB and JT schemes in COMP systems. As presented in Table 17 , the categorized papers differ based on assuming varying channel models, considering channel estimation, assuming the need for CSI or training, proposing either estimation or detection algorithms in the presence of synchronization errors, or driving lower bounds on estimation of TO and CFO.

Further details about the assumed channel models in each paper are provided in the last column of Table In a multicell interference communication network, the received signal is contaminated by intercell interference.

The interference can arise from the neighboring cells, when there is a universal frequency reuse, i. This can cause interference among different tiers in a heterogeneous network, e. The synchronization challenge for the receiver is to achieve timing and frequency synchronization with the desired signal in the presence of this interference. In addition to estimating and compensating TO and CFO between desired user and receiver, it also has to suppress intercell interference in order to decode the desired signal.

To date, most approaches to synchronization have modeled the interference as an additive term that can be combined with the noise. However, this approach may be suboptimum for synchronization in heterogeneous networks. The summary of the research carried out to achieve timing and carrier synchronization in multicell-interference-based communication systems is given in Table 18 :.

Timing synchronization alone considering single-carrier communication is analyzed in [ ]. Carrier synchronization alone is studied in [ — ], where [ ] considers single-carrier communication and [ — ] consider multi-carrier communication.

Joint timing and carrier synchronization considering multi-carrier communication is analyzed in [ , ]. The information about the physical layer used, e. Finally, as detailed in Table 18 , the categorized papers may also differ with respect to providing joint channel estimation, requiring CSI or training, proposing estimation or compensation, or providing lower bounds on estimation of synchronization parameters.

Further details about the consideration of TS design, cell search, or DoA estimation is provided in the last column of Table The joint estimation and compensation of timing and carrier frequency offsets in interference-limited communication systems is an open research problem.

UWB refers to a radio communication technique based on transmitting very short duration pulses, typically of nanoseconds or less, whereby the occupied bandwidth is very large. UWB communication transmits in a manner that results in little to no interference to narrow band signals that may be operating in the same frequency band. The baseband or carrierless communication system, known as impulse radio IR UWB communication system, employs time-hopping.

In IR-UWB communications, a single data symbol is associated with several consecutive pulses, each located in its own frame. Accordingly, each data symbol is spread by sub-nanosecond pulses. Spreading of these pulses is achieved by time-hopping these low-duty cycle pulses and data modulation is accomplished by additional pulse position modulation.

Pulse width indicates the center frequency of the UWB signal. As IR-based UWB does not use any carrier signal, it is also known as baseband, or carrierless or zero-carrier technology. In a multipath environment, a fine resolution of multipath arrivals occurs due to large transmission bandwidth. This leads to reduced fading for each path because the transmitted data is in the form of pulses and significant overlap is prevented.

Thus, to reduce the possibility of destructive combining [ ]. Rake receivers [ ] are employed to collect the signal energy of the multipath components, achieving much higher processing gain. Due to its significant bandwidth, an IR-based multiple-access system may accommodate many users, even in multipath environments. Multiple access to the channel is made possible by changing the pulse position within a frame according to a user-specific time-hopping code.

It can also be thought of as a combination of frequency hopping with the sub-carriers occupying one band at one time and hopping according to a pre-defined hopping pattern.

Timing errors as small as a fraction of a nanosecond can seriously degrade the system performance. Timing recovery can be viewed as a two-part process.

This is called frame timing. The second part consists of identifying the first symbol of each frame in the incoming frame stream and is referred to as symbol timing. Frequency offset in IR-UWB communication system arises due to the clocks at the transmitter and receiver that run independently at slightly different frequencies, albeit close to a common nominal value.

Time synchronized networks enable accurate time stamping by each of the computers on the network. This is important to properly sort events and transactions into chronological order so that any disturbances or problems in the data can easily be detected and resolved.

Precision time stamping is also important for data that is used in financial audits or as evidence in court during legal cases. Time synchronization may also include phase and frequency synchronization in the scientific and communications fields.

The data is read from the FIFO using the local clock source that is synchronized to the reference clock source. If the write and read frequencies are different, a loss of data occurs that is referred to as data slip. The stratum 2 clock tracks and holds holdover mode the last best estimate of the input stratum 1 clock reference during degradation of the reference clock. The stratum 3 clock also tracks and holds the last best estimate of the input reference clock from the stratum 2 source.

This approach ensures that all local stratum 3 clock sources are accurately synchronized to the external PRS clock in order to avoid data slip between network elements. In holdover mode, the frequency drift over time must be slow enough for a good reference clock to re-establish.

As a result, it leads to high costs for deployment and maintenance.