Multi-hop concepts can be beneficial in the context of the successful deployment of ubiquitous radio system concepts. The very high data rates envisioned for next generation communication systems, such as IMT-Advanced in reasonably large areas do not appear to be feasible with the conventional cellular architecture due to the following two basic reasons.
First, the transmission rates envisioned for next generation communication systems are two orders of magnitude higher than those of third generation systems. This demand creates serious power concerns because for a given transmit power level, the symbol (and thus bit) energy decreases linearly with the increasing transmission rate.
Second, the spectrum that is considered for IMT-Advanced systems (e.g., WINNER) will be located above the 2-GHz band used by third generation systems. The radio propagation in these bands is significantly more vulnerable to non line-of-sight conditions, which is the typical mode of operation in today’s urban cellular communications.
One solution to these two problems is to significantly increase the density of base stations, resulting in considerably higher deployment costs that would only be feasible if the number of subscribers also increased at the same rate. This is cost demanding and environmentally non-friendly. On the other hand, the same number of subscribers will have a much higher demand in transmission rates, making the aggregate throughput rate the bottleneck in future wireless systems. Subscribers might not even be willing to pay the same amount per data bit as for voice bits, another factor that is of economical disadvantage of a drastic increase in the number of base stations.
As more fundamental enhancements, advanced transmission techniques and co-located antenna technologies, together with some major modifications in the wireless network architecture itself can enable effective distribution and collection of signals to and from wireless users. The integration of multi-hop capability into conventional wireless networks is perhaps the most promising architectural upgrade. It allows that the rate can be traded for range of coverage and vice versa.
Multi-hop relay-based concepts can be divided into homogenous multihop concepts based on one radio interface mode only, heterogeneous deployment concepts based on multihop links that interconnect on more than one radio interface mode or two different radio interface technologies. The benefits of relays are that they improve link budgets and may hence ease covering remote (or shadowed) areas. There also exist issues that need to be considered, like in the case of rapidly moving users, which may, due to inherent delay issues, not be able to take advantage of the relay nodes.
It should be known however that scenarios may exist where the deployment of relay nodes may be undesirable due to resulting complicated interference management issues, similar to a problem reported for hierarchical cellular systems where the deployment of overlapping macrocells and microcells is always a source for unpredictable interference and near-far effects.
Making the relays mobile also introduces new challenges beyond those that are seen when dealing only with fixed relays. For example, dealing with interference between mobile relays may be challenging especially in situations where the mobile relay population is dense. Mobile relays, when acting as moving gateways, may also offer benefits by reducing the number of simultaneous handovers in the network (only the moving gateway is involved in the handover) and by allowing terminals to operate under more favorable link conditions. The deployment and operation of next generation ubiquitous networks needs to be easier in order to prove economically viable and stimulate easy adoption. Thus, the operational aspects need to be taken into account in the design, together with requirements such as the support of autoconfiguration and autotuning. These requirements are especially important in relation to relay nodes, as the amount of relay nodes may exceed significantly the number of existing cellular base station sites.
