The inventor has recognised that the performance of a Mobile Communications System may be heavily dependent on the quality of the Physical Layer PHY processing. The PHY may provide for delivering coverage and robustness to radio links between nodes that move through hostile propagation conditions such as urban canyons, and areas of high interference.
Mobility, and in particular high speed terrestrial mobility, may induce yet another set of difficulties for the PHY as the reflections off the surrounding buildings, vehicles and other bodies may combine in a time varying manner. In the case of a Private Mobile Radio PMR network, sensitivity to cost and lower densities of users may often be seen relative to complex and expensive 2G and 3G Cellular networks.
Multi-hopping wireless networks are emerging as a network topology of choice for PMR, since they may provide inexpensive and flexible broadband communications. The flexibility may be afforded by the self forming nature of the network and the small form factor of the network nodes. Some wireless multi-hopping network vendors have rolled legacy IEEE Standardization efforts within the IEEE Standards typically do not specify how to receive signals, rather focussing on what signals should be transmitted. The vendors are then responsible for the receiver technology.
Getting the PHY right in wireless multi-hopping networks may be especially important since local access and a degree of backhaul are provided wirelessly. The inventor considers that the problems of delivering reliable high speed access to mobile users may be exemplified in Private Mobile Radio PMR networks, such as those employed by Public Safety end users.
The inventor recognises a strong market pull for reliable mobile broadband access, in order to meet the requirements of applications such as remote databases and the delivery of real-time video.
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Third generation 3G mobile systems may be considered as an alternative, as they offer increased throughput and reliability. However, such networks were designed primarily with circuit switched voice communications in mind. In contrast, modern applications often call for high speed packet data transfer. The heavy infrastructure and licensing costs of 3G networks may also make them less attractive as an alternative.
It has recently been proposed that wireless multi-hopping packet data networks be used for PMR deployment. In contrast to the 3G model, which relies on a base station topology, the topology of the multi-hopping solution may comprise several small wireless nodes. These nodes may form a network through which data packets are passed from a sender to a single receiver, or broadcast to multiple receivers simultaneously. Such a network may be able to reconfigure dynamically when nodes enter or exit the system, making them attractive for applications which allow mobile users.
Moreover, if a single node fails then the network may remain operational, which stands in contrast to an expected catastrophic failure of a base station. With the multi-hopping network model in mind, there becomes a clear requirement for a wireless modem which is able to offer reliable broadband access, while mobile, in, for example, an urban environment.
However, this technique may have been historically applied to the problem of transmitting data in a stationary indoor environment. The outdoor urban environment may contain many obstacles for the radio signal, such as buildings and trees, which are referred to as clutter. Present wireless technology may be able to offer high throughput only at the expense of receiver sensitivity, hence the cluttered urban environment may lead to poor coverage.
Furthermore, the relative mobility between the transmitter and receiver may cause the impact of these obstacles to change in time.
Journal of Optical Communications and Networking
When the effects of mobility and clutter combine, the resulting wireless channel may present a significant challenge to the communications system designer. With further reference to related art identified by the inventor, attaining mobility and receiver sensitivity requires high quality tracking of parameters necessary for improved demodulation. There are parameters which are considered important for the estimation of the radio channel leading to accurate demodulation increasing the sensitivity of the receiver.
In OFDM systems the frequency domain channel model required for the demodulation of the received signal is predominately characterised by the following six parameters.
The complex channel amplitude may be modelled as the combination of three effects, for example, 1, 2, 4, as above. The complex channel amplitude variation may correspondingly be modelled excluding phase and OFDM timing offsets. With reference to FIG. In terms of modelling a packet channel model it may be assumed the phase shift across subcarriers is invariant with time across OFDM symbols. This phase shift is assumed identical for all subcarriers and models effects such as frequency offsets between the transmitter and receiver RF stages and phase noise.
A third effect h m t collects the higher rate phase and amplitude variations such as those induced by frequency selective fading and mobility where the Doppler frequency is a significant fraction of the OFDM symbol rate. This third multiplicative value may vary with time and subcarrier. The inventor has identified that those conventional approaches to tracking these parameters typically:.
Generally, it is considered that there is a need in which increased data rates requires lower latency decoding. Furthermore, it is considered there is high complexity of multi-antenna demodulation in this environment. The complexity of the determination of the time of arrival of a packet should be kept low in practical systems since the process is running continuously. Accordingly, inaccuracies in the time of arrival should be accounted for in subsequent processing stages so it is desirable that the accuracy of this stage be as high as possible while minimising complexity.
Conventional systems may employ a delay and correlate method where the received signal is multiplied by a delayed version of itself and accumulated and normalised, forming a metric. A packet arrival time may then be determined from any peak in the normalised metric above a predetermined threshold.
Multi Carrier Communication Systems With Examples In Matlab A New Perspective
Normalisation of the input signal is ordinarily required and is subject to large fluctuations in received signals experienced in outdoor wireless communications. Signals with similar properties to the preamble of a desired packet eg, Carrier Wave Jammers and DC Offset effects may impact the conventional systems resulting in unacceptable levels of performance in some cases. False acquisitions may lead to wasted receiver processing resources and eventually leading to reduced throughput, and therefore should also be minimised, again while keeping complexity low.
Delay and Correlate algorithms may ordinarily employ a peak search to determine the timing instant. When these algorithms are implemented in low complexity binary signal processors there may be several timing instants with the same maximum value. This may cause uncertainty in the timing instant.
Fine Time synchronisation may be required in receivers that implement coherent demodulation. Typically a known preamble is transmitted enabling the receiver to correlate for the known sequence in the received signal. Correlation can be expensive when the correlation length is long since each sample accumulated requires a complex multiplying step. This problem may be exacerbated when multiple antennae are employed to receive the signal since the correlation may be implemented for each antenna. If M antenna are used the complexity is M-fold.
The timing instant or, arrival time is found by determining the peak of the correlation powers across a set of correlations differentiated by a timing offset. In the case that it is desired to terminate packet processing based on fine timing metric quality a threshold must be defined. However, thresholds are subject to signal variations and it can be difficult to set an appropriate threshold. Signals transmitted across the wireless media may be subject to frequency selective fading. Different levels of interference may also exist on difference frequencies.
Ordinarily, in OFDM wireless communication systems a redundant cyclic prefix may be employed to mitigate the effects of multipath delay spread on a signal. Consequently, pulse shaping at the transmission and receiving sides of the communication further contributes to spreading the communicated signals on top of the spreading caused by multipath propagation. Remote digital resources that form part of and assist in distributed computing and communication systems such as multi access networks may suffer from limited resourcing in their own right given the demands now placed on such systems.
In wireless communications systems the received signal power may fluctuate beyond the dynamic range supported with Analogue to Digital Conversion devices by up to several orders of magnitude.
Conventional automatic gain control AGC algorithms are threshold based where a change in the VGA gain is only made if the received signal power exceeds a first predetermined threshold, Th 1. Thereafter, it may be possible for a second threshold, Th 2 , to be set as a trigger point for the VGA. If there is no threshold exceeded see Th 3 , such as in the case of a weak but still fluctuating signal, then the VGA gain settings do not change. The inventor recognises that, as noted below, unfortunately the threshold determines receiver sensitivity.
Therefore, receiver sensitivity is a very important performance criterion and should not be compromised if possible. For example, if the threshold is set too low conventional systems trigger and unnecessary state changes result which locks the receiver out. Further, in some RF devices a gain change may mean that the receive path is unstable for a significant time period.
Many wireless communication systems may employ direct-conversion RF receiver devices, where the RF signal is mixed down to DC, into it's baseband equivalent in-phase I and quadrature Q signals. In such a device the receive baseband signals may experience significant DC offset due to various processes internal to the RF receiver device.
In such devices, a wide high-pass filter WHPF may be present that can be enabled to remove this DC offset so that baseband signal power measurement can be performed. However the width of the filter may be such that it filters away a significant portion of the centre of the received signal, making its use unsuitable when a valid burst is present on the channel. It is typically recommended that when not receiving a signal, the RF receiver be operated in this WHPF mode, and that once a signal is detected typically by separate RF signal power measure exceeding some threshold the WHPF is switched off, returning the device to the DC-coupled mode.
This change of mode may induce large DC offsets in the I and Q baseband signals. Operating the modem and receiver in this way may also result in significant distortion at the start of a received burst, and if the signal power is very low in the case of weak signals potentially for the entire burst. Conventional automatic gain control AGC algorithms may be threshold based where a change in the RF device gain, and more significantly, disabling the WHPF mode, is only made if the received signal power exceeds a predetermined threshold. If the signal power is very low as in the case of weak signals the threshold may not be exceeded, leaving the WHPF mode active for the duration of the packet.
This may be undesirable as the resulting distortion may further degrade the possibility of reliably detecting and demodulating the packet successfully.
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The inventor recognises that in this case, unfortunately, the threshold determines receiver sensitivity rather than other aspects of the synchronisation and demodulation process.