Multihop networks for capacity and coverage enhancement in TDD/UTRAN1 J. Vidal1, M. Madueño1, J. R. Fonollosa1, S. Barbarossa2, O. Gasparini3, S. Ponnekanti4, A. Andritsou5, A. Nix6 1
Universitat Politecnica de Catalunya, Spain; 2Università di Roma “La Sapienza”, Italy; 3 Dune, Italy; 4Fujitsu, UK; 5Intracom, Greece; 6University of Bristol, UK email:
[email protected] ABSTRACT 1.
The provision of high capacity, low cost and reliable wireless multimedia communication for bursty packet traffic (as well as voice and delay constrained streaming traffic) continues to be a challenging aspect of future wireless communications systems. In this sense, structured and ad-hoc multihop networks and intelligent relaying are expected to play a significant role in 4G wireless communication systems. These systems have the potential to cost-effectively extend the coverage and/or increase spectral efficiency, thus driving the cost of delivering 4G services lower. Radio Resource Management (RRM) and individual radio link management in such networks will be crucial in ensuring that radio and network resources are utilised efficiently. In parallel, advances in radio transceiver techniques such as multiple antennas architectures, Space Time Coding (STC), cost effective Multi User Detection (MUD) and modulations adapting to channel variations will play an important role in the Relay-Relay link, as well as in the BS-Relay link which will most likely support the bottleneck of deployment of multihop networks. The techniques described here are the fundamentals for the definition and analysis of such systems.
2.
3.
Relay multihop network with infrastructure support, being the relays fixed or mobile (figure 1). Three main benefits could be provided within this architecture: first (1 in figure 1) is the coverage extension of High Bit Rate (HBR) services; second (2 in figure 1) is the service coverage for those users which are beyond the coverage area of the BS; third (3 in figure 1) is the potential interference reduction when any packet connection evaluates the possibility of a multihop versus a single hop link. System internetworking for vertical handover (figure 2), allowing HBR coverage of picocells (as the most promising possibility for 4G systems). Ad-hoc multihop networks (with no infrastructure support), providing connection of close users within picocells.
Figure 1. Applications of a multihop network with unstructured relay users and some infrastructure
2
HBR-UE
3
LBR-UE
I. MULTIHOP SYSTEMS Multihop networks are mobile communication structures dynamic in nature and consist of mobile stations with retransmission capabilities on behalf of other mobile stations. The main reasons for introducing multihop networks in 3G systems are to overcome the capacity-range trade-off of CDMA/TDMA systems, and thus increase and improve the services for packet users, as well as to reduce the cell planning difficulties for operators. The gains thus obtained should compensate the eventual increased complexity of the terminals. Architectures for multihop networks in 3G The multihop architectures that are most effective within 3G systems: 1
No coverage
LBR coverage
HBR coverage
1 HBR-UE
2 Sync info
support. Trade-offs in multihop networks The foremost benefits of the deployment of a multihop network are 1) extended coverage for HBR users and for those users beyond the coverage area of the BS, and 2) reduction in system interference and hence increased network capacity.
This work has been carried out in the framework of the EC-funded project ROMANTIK and partially supported by Spanish Government Grants: TIC2000-1025, FIT-070000-2000-649, Generalitat of Catalunya grant CIRIT 2000SGR 00083.
Breaking a larger path down into a number of smaller hops reduces the aggregate radio path losses due to the non-linearity of the propagation equations, reducing total transmission power and smoothing the distribution of interference. As a direct consequence, the subscriber’s battery life increases and the fears from radiating hazards are eased. The mechanism that allows capacity increase for a multihop CDMA network is threefold: 1) The breakdown of each path into multiple paths will reduce the interference generated into neighbour cells, thus improving the network capacity. 2) The power requirements for the in-cell far users are break down in multiple hops, and cell breathing effect [11] is reduced. This allows a better efficiency in DL channels at the BS. 3) As the DL users are near the BS, the near-far effect is alleviated. This is particularly important for terminals not using multiuser detection at the DL. In this way, better efficiency is observed in DL. Points 2 and 3 are specially important when considering that the very nature of a multihop system will require a high number of channels for BS-relay links. Any action oriented to improve the efficiency of the DL (as developing affordable MUD in BS-relay link) is of foremost relevance. On the other hand, additional links (relay-to-relay and relay-mobile terminal) are required to establish a BSMT connection through multiple hops. Strategies that reduces this increased requirement have to be considered. Spatial reuse of these channels throughout the cell is the solution, in which MAC layer (collision resolution and interference avoidance), DL layer (right packet schedule and ARQ strategies) and physical layer (improved channel use through STC and interference rejection) play the main role.
HBR coverage
LBR coverage
No coverage WLAN picocell
Lampost relay
Figure 2. Application of a multihop network with structured relays and possible vertical handover to WLAN network. Multihop networks facilitate the coverage planning for operators. Even the most sophisticated planning tools
are unable to predict the position of black spots, specially in indoor and intricate urban areas. In this sense, multihop networks are able to organise themselves so as to boost the system performance without costly infrastructure investment nor excessive network optimisation. These perceived benefits are obtained at a certain cost that has to be evaluated. The main disadvantage of multihop networks is related to the increased complexity in the mobile terminal. Independently of the multi-hop configuration, such a system can be seen as a picocell network where relay nodes take the role of a base station, while maintaining lower hardware complexity and lower intelligent capabilities. Nevertheless, relaying nodes should offer more intelligence (and hence complexit) than existing mobile terminals since they are expected to collaborate on network management. An additional concern appears when the relay terminals are not fixed, that is, the gain in coverage is random and depends on the position and the availability of the subscriber’s user equipment acting as relay. At a given time instant, the subscriber may have initiated a communication or the battery may be flat. II. Multihop networks in TDD/UMTS The TDD mode of UMTS is CDMA/TDMA and it has undergone standardisation activities since the beginning of 3GPP activities. This mode of UTRA seems to be the most appropriate choices for multihop wide-area networks, because of the coordinated nature of the interference (due to the TDMA structure) as well as the use of an unpaired band, suitable for the relay-relay link [3]. However, the potential benefits of multihop mechanisms for other access schemes are yet to receive attention in the literature. The relay-specific transport channels needed are: §
A dedicated channel for relay transmission, which will be mapped onto a DPCH.
§
One or more common channels, used for the possible purposes of broadcast of relay-specific information (BCH information-like), carry control information to a specific neighbour (FACH information-like), random access and data packet transmission.
The number of new common channels to be introduced depends strongly on the definition of procedures for exchanging control information between relays and the different information and flow needed to perform those procedures, and it has to be further studied. Physical layer issues The slot allocation within a frame (slow DCA) should reserve at least one time slot for the relay common channels. Potentially any time slot could be used by a
relay-dedicated channel. When working within a cell coverage area of UTRA-TDD, the slots within a frame are allocated either for uplink, downlink or relay-torelay links. With such flexibility, the TDD multi-hop extension can be adapted to different environments. An example of frame configuration is shown below. The timeslots of relay-to-relay links can be used for transmission and reception by a group of mobiles, assuming spatial reuse and efficient resource management.
performance, are aimed at the reduction of the transmitted power within a link. UL
DL R
R MS
MS
DL R
10 ms (15 time slots) MS
As far as TDD is concerned, a careful analysis of the interference situations of multihop networks yields significant differences with respect to conventional TDD/UTRAN interference [4].
No coverage
HBR coverage
WLAN picocell
Sync info
Figure 3. Application of an ad-hoc multihop standalone network (without infrastructure support). Intra-cell and inter-cell interference New possible intra-cell and inter-cell interference situations may occur in multihop-integrated TDD scenarios (figure 4). Multihop-specific interferences are: • • • •
Base station to relay (BS-R) Mobile station to relay (MS-R) Relay to mobile station (R-MS) Relay to relay (R-R).
Advanced link layer techniques are of utmost relevance to improve the performance of the BS-R link, as well as the R-MS and R-R links. In particular, for ad-hoc multihop network, where a non-synchronous use of the TDD spectrum is proposed (figure 3). In this case, operators will issue a strong requirement for a reduced interference level into a licensed frequency band. Indoor propagation conditions and self-limited range extent could only partially provide the required isolation. Power control, adaptive modulation techniques, HARQ, as well as affordable algorithms for MS implementation of MUD are needed. These techniques, rather than taking decisions on the overall system
R
R
Figure 4. Four multihop-specific interference situations for a TDD systems On the other side, medium access schemes resolving collisions and radio resource management (RRM) techniques are responsible for global interference avoidance and an efficient spatial reuse of codes, slots and power. This requires the existence of either centralised or distributed decision algorithms able to reallocate resources due to the mobility of the users. As result, a holistic approach for RRM in multihop networks is required. A layered RRM approach has to be synthesised to support this requirement. This will incorporate several RRM strategies including methods to counter R-R interference, efficient signalling strategies for path finding, smart routing algorithms that distribute relayed traffic uniformly within the cell, and centralised or distributed dynamic channel allocation mechanisms assigning code, time slots and power. While R-R interference may occur indistinctly within a cell or between adjacent cells. Remaining interferences are inter-cell interferences, assuming that within a cell, R-R and R-MS links cannot be set simultaneously with DL or UL connections in the same cell. The more conflictive interferences from base station to relay take place at cell boundaries. This interference type is comparable to base station to mobile station interference (BS-MS) occurring in non-multihop TDD system. Hard handover is a possible solution to avoid BS-MS interference but a more effective, although a less flexible one, is to coordinately allocate the same time slots for multihop ODMA operation in adjacent cells. This solution will alleviate the performance degradations caused by BS-R, R-MS and MS-R interferences, reducing the interferences problem to the R-R one. Nevertheless, it will limit dynamic reconfiguration of cells when the traffic is not uniform. III. PROPAGATION CHANNEL The street level nature of the R-MS or R-R link translates into channel characteristics far different from the UL or DL. Low height mounted antennas and short distances conform a channel in which propagation losses and shadowing statistics have to be characterised
as little literature is available. University of Bristol has recently built a simulator recreating mobile-to-mobile transmission. Figure 5 plots the carrier-to-interference pattern within a city area when 10 low-mounted relays are placed. Using this simulator and smart techniques for lampost positioning, 50 of those relays have been deployed in a cell. For the 50 positions, it has been found that the required transmission power is reduced significantly in around 18 dB (see figure 5b).
make MUD available at the mobile/relay, and on the other hand time adaptive modulations are introduced also for improving link quality stabilisation and duration. Spectral methods for channel equalisation In a first processor a Cyclic Prefix (CP, like in OFDM) is padded in front of each coded symbol transmission. By doing so, the convolution of the transmitted symbol block with the channel impulse response, as seen by the receiver, becomes cyclical. Then, in the spectral domain, convolution is equivalent to multiplication between channel and coded symbols spectra. The channel can be equalised simply by dividing frequencywise the spectral transform of the symbol block by the channel spectral response, then eliminating ISI and MUI jointly. At this point estimation can be performed independently for the symbols of the various users, in the frequency or in the time domain. CP in combination with Space-Time Block Coding (STBC), allows spectral channel equalisation and simple symbol estimation achieving STBC diversity gain and MUI/ISI cancellation [5]. However, the symbol estimator proposed so far requires that when the conjugate of a symbol is transmitted also conjugation of the spectral transform of the code is taken to achieve full STBC diversity. Conjugation of the spectral code transform, which is equivalent to the time reversal of the time code is not 3G standard compliant.
Figure 5. a) Carrier-to-Interference plot for 10 lowmounted relays units. Tx antennas: 60º sectors, EIRP: 10dBm, 2GHz. b) Transmitter power savings for 50 lampost relays distributed in a cell.
IV. LINK LAYER TECHNIQUES Link layer activities are being carried out within ROMANTIK, with the general goal, applicable both to multihop or single hop networks, of: • • • •
Ensure the minimum power is transmitted, Provide additional link quality stability, Be oriented to increase the capacity of every link (specially for the BS-R link). Minimise interference
To theses purposes, on one hand the capability of the mobile/relay to detect messages of multiple users will be enhanced by analysing simple novel methods to
Figure 6 : Performance comparison of RAKE and CPE processor for downlink employing Space Time Orthogonal Block Coding with 2 TX antenna elements and 1 RX antenna element; 24 active users and 32 chip length Walsh Hadamard codes. The presence of the required CP is not compliant with today 3G standards and leads to some loss in transmission efficiency. A modification has been proposed in [6] which elimination of MUI and ISI is obtained through iterative spectral symbol estimation. Performance is slightly lower with respect to CP estimator, but still positive cost-effective. A comparison of BER in a (2,1) Multiple Input-Single Output (MISO)
antenna configuration is shown in figure 6, for spectral processor with and without the use of CP. 24 equal power users are included, using Walsh-Hadamard codes of length 32. Time Adaptive Modulations 4G advanced modulations are being studied beyond the CDMA/TDD, in the scope of smart scheduling/sharing dynamically and adaptively, network resources (time slots, spectrum, power) to the users. This will be done in agreement with user service profiles and channel structures [7]. The dynamic scheduling of adaptive modulations will be based on suitable channel modelling time extrapolation. By using adaptive modulation, the MS-MS paths are likely to last longer with respect to the case of non time varying, not channel based modulations. Uplink
RTS or CTS packets. An additional problem appears in the form of the so-called capture effect. Under some configurations, a pair of terminals may monopolise the use of the channel and do not allow neighbour terminals to get their respective RTS/CTS messages through. This effect may be overcome by the use of back-off (quiet) time windows after each packet transmission. This strategy is set up at the IEEE 802.11 but this conflicts with transport layer timeouts and reduces spectral efficiency if no closeby MT intend to transmit. An additional, but not incompatible, way of improving channel utilisation would consist on achieving multipacket reception (MPR) at the receiver. MPR would enable correct reception of colliding packets, or a set of them, without retransmission, leading to a direct throughput and delay improvement. More over, the extra traffic load due to retransmissions would vanish or diminish, which would additionally decrease the collision frequency.
Downlink
T1-User
T3-User
T4-User
ti Channel for User 1
Channel for User 2
Channel for User 3
a) Figure 7. TDD fine structure and used intervals assigned by CPEAM as a function of a hypothetic user channel quality. V. MAC LAYER TECHNIQUES Simultaneous access by more than one relaying node results in the destruction of all colliding packets, specially in the relay-relay and relay-MS links. Thus, colliding packets have to be retransmitted later, which yields in decreasing performance of the network in terms of throughput, power consumption and delay. Collision avoidance, interference reduction and multipacket reception (MPR) for random access channels have to be setup to reduce these impairments. These techniques might allow, not only to react against collisions, but also to allow multiple users to coexist thus improving the reuse of relay-relay and relay-MT links. When using pure carrier sense (CS) protocols the channel utilisation achieved in wireless networks is severely reduced, because of the well-known hiddenterminal and exposed-terminal problems. Many protocols have been proposed in order to alleviate the problem. For instance, RTS/CTS-based protocols diminish those problems performing a pre-reservation phase. However, under heavy traffic load conditions, by using RTS/CTS-based protocols, a data packet may still collide with probability as high as 60% due to loss of
b)
Figure 8: Modulation-based MPR by means of P=3 spreading codes, and Signal Processing-based MPR via spatial diversity (Q=2 antennas). MPR can be provided at the modulation level and at the signal processing level. MPR at the modulation level is provided when using transmitter-oriented spread spectrum codes (each user is assigned a unique code for transmission) and MUD capabilities at receptor are feasible. MPR at the signal processing level can be achieved when using receiver-oriented codes (each user is assigned a unique code for reception) by means of different techniques: 1) by means of transmit time diversity, 2) through spatial diversity techniques when MIMO structures are available, 3) using a combination of both schemes. Both MPR techniques could be exploited by means of sharing a small number of spreading codes P, both at transmission and reception. MPR at modulation level would enable the distinction of up to P packets per relay. MPR at signal processing level would enable, in each relay, the identification of up to Q packets per code, where Q depends on the number of antennas and the time diversity transmitted. Unfortunately the envisaged increase in performance is not evident, because the P·Q resolvable packets per relay include intended packet transmissions to the relay and those intended to its neighbours (interferers). Moreover, when acceding to the channel at random, the system is not collision free. When a relay is in the range of more than P·Q transmitting nodes, collisions are unavoidable. Logically, using pure CS protocols is not
compatible with signal processing-based MPR schemes. In addition, when trying to exploit modulation-based MPR techniques, the hidden-terminal and exposedterminal problems will lead in a decrease of performance of the channel utilisation when CS protocols are used. Another close related and very important issue to tackle is how to allocate codes to relays when transmitting to get optimum performance (near P·Q packets per user). The channel efficiency gains versus receiver complexity and power consumption will be evaluated. Figure 8 shows two possible situations when taking advantage of modulation-based MPR by means of 3 spreading codes (c1, c2, c3) and signal processing-based MPR via spatial diversity (Q=2 antennas). In figure 8a, R2 cannot hear the nodes on the left side of R1, and R1 cannot hear the nodes on the right side of R2. In this situation R1 and R2 can perfectly receive P·Q=6 packets each one. When acceding at random, if any other relay in the range of R1 or R2 tries to transmit, a collision will occur. In figure 8b, R1 can hear the nodes transmitting to R1, and R2 can hear the nodes transmitting to R1. Solid lines of arrows indicate the intended recipient of different transmitting nodes. Dashed lines point to an interfered relay, which is in the range of an intended transmission. In this situation R2 can receive P·Q=6 packets, while R1 is not able to receive a single packet. If any other relay in the range of R1 or R2 attempts to transmit, a collision will occur.
attempt to transmit to R2. R1 would then be able to receive as many packets as R2 without collision. It is worth noting that, in order to be able to exploit MPR at signal processing level, two transmitting relays, whose intended recipients are in the range of both transmitting relays and share the same code, are imposed to use different training sequences for channel estimation. In UTRA TDD, training sequences of different users active in the same cell and same time slot are cyclically shifted versions of one cell-specific single midamble code. For the physical random access channel there exists a fixed association between training sequence shifts and canalisation codes. For the purpose of MPR exploitation a fixed association between training sequences and channelization codes is not desirable. Thus, a very important issue to tackle is how to allocate midambles to channelization codes. In the framework of TDD-Mode multi-hop extension, another discussion to deal with referring midamble sequence selection is if it should be receiver-oriented (cell-like) or conversely, common to all users. The former option forces relaying nodes to broadcast their midamble sequences. Under such configuration, two packets will collide only if they use the same code and midamble shift and have the same intended destination. The second option seems to be less suitable, since two packets could collide when using the same code and cyclic shift whatever their intended destinations are. Power controlled medium access Power control in random access channels is an important issue for battery savings as well as for spatial reuse. It has been shown [12] that gains of 150 % are possible in channel utilisation, through the use of busy tones. This strategy has the drawback of wasting battery power and bandwidth. When taking into account the T/CDMA air interface for UMTS, it is possible to build a power controlled access provided that:
a)
b)
1.
The RTS and CTS messages are assigned fixed physical channels (that is a fixed slot and spreading code). This is possible as it is assumed that a centralised BS provides synchronism and distribution of the slots among UL, DL and ODMA. In this way, collisions between RTS and CTS are not possible.
2.
The RTS and CTS messages are not power controlled so their transmission is at the maximum power. Thus, the possible colliding users are aware of the ongoing transmissions.
3.
Information about reception conditions at the receiving terminal (noise+interference) and the transmitted power of the CTS are broadcast (along with the packet length) to the neighbourhood. According to this information, the neighbours may decide if transmission to the intended user may collide with ongoing packet transfers.
Figure 9: MPR. Using scrambling-oriented receivers (a) vs. not using at all (b). The exploitation of MPR techniques by sharing a small number of spreading codes is likely to be improved by using a receiver-oriented scrambling code (each user is assigned a unique scrambling code for reception, BSlike). In order to use receiver-oriented scrambling, relaying nodes should broadcast to its neighbours, information about the scrambling they use. Figures 8b-9b, show that when scrambling-oriented receivers are not used, R2 can receive P·Q=6 packets, while R1 it is not able to receive a single packet, and when any other relay in the range of R1 or R2 makes an attempt to transmit, a collision will occur. In contrast, when using scrambling-oriented receivers, as illustrated in figure 9a), R1 is ideally isolated from the intended transmissions to R2. In this case, a collision will only occur when any relay, whatever its position is, makes an
VI. ROUTING STRATEGIES
REFERENCES
Whatever the multihop structure in mind, the routing strategy is responsible for the:
[1] T. J. Harrold, A. R. Nix, “Capacity Enhancement Using Intelligent Relaying For Future Personal Communications System”, in Proceedings of VTC-2000 Fall, pp. 2115-2120. [2] T. Rouse, S. McLaughlin, H. Haas, “CoverageCapacity Analysis of Opportunity Driven Multiple Access (ODMA) in UTRA TDD”, in Proceedings of IEE 3G Mobile Communication Technologies, March 2001, pp. 252-256. [3] 3G TR 25.924 V1.0.0 (1999-12) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Opportunity Driven Multiple Access. [4] H. Holma et al., "Interference Considerations for the Time Division Duplex Mode of the UMTS Terrestrial Radio Access", Journal on Selected Areas in Communications, vol. 8, August 2000, pp. 1386-1393. [5] Barbarossa S., F. Cerquetti. “Simple Space-Time Coded SS-CDMA Systems Capable of Perfect MUI/ISI Elimination”, IEEE Comm. Letters, Dec. 2001. [6] Barbarossa S., G. Scutari, D. Scamolla. “An Efficient Space-Time Coding for Wideband CDMA Systems using Variable Length Cyclic Prefix ”, European Wireless, Folrence, Feb. 27th, 2002. [7] S. Barbarossa, A. Scaglione “Time-varying fading channels”, Chapter 1 in Signal Processing Advances in Wireless & Mobile Communications – Trends in Single- and Multi-User Systems, edited by G.B. Giannakis, Y. Hua, P. Stoica, and L. Tong, Prentice-Hall, 2001. [8] T. J. Harrold, A. R. Nix, “Intelligent relaying for future personal communications systems”, IEE Colloquium on Capacity and Range Enhancement Techniques for the Thrid Generation Mobile Communications and Beyond, February 2000. [9] E. Royer, C. Toh, “A Review of Current Routing Protocols for Ad-hoc Mobile Wireless Networks”, IEEE Personal Communications, April 1999, pp. 46-55. [10] MANET, Mobile Adhoc Network Working Group, http://www.ietf.org/html.charters/manetcharter.html [11] H. Holma, A. Toskala, WCDMA for UMTS, John Wiley, 2001. [12] S. Wu, Y. Tseng, J. Sheu, "Intelligent Medium Access for Mobile Ad Hoc Networks with Busy Tones and Power Control", IEEE Journal on Selected Areas in Communications, vol. 18, no. 9, September 2000. [13] Olaf Bratveit Holm, Trond Friiso, Thomas Haslestad, "Improving UTRA Capacity with ODMA", Proc. IST Mobile Communications Summit 2002, Thessaloniki, 17-19 June 2002.
• •
Path search between the transmitter and receiver. Path maintenance, with a low overhead cost in signalling.
In practice, the additional traffic required for route establishment and maintenance may be mitigated by restricting the number of the neighbour terminals, the smart definition of probing strategies and decisiondistributed routing algorithms. The routing strategy for the TDD/UMTS multihop network has to take into account the additional advantage given by the presence of a BS. In this way, the routing algorithm for the DL can be defined in the following way [13]: each terminal has to build a list of paths from its neighbours to the BS. Each path must contain a set of merit figures that allows taking a decision on which routes a downlink packet should take. Among these figures: observed throughput and latency of the relay, available power, stability of the link and needed total power. In DL, when a packet connection is required from the BS to the terminal, the BS requires the assistance of the end user, on which route is the most convenient. Note that QoS preserving routing is possible under this scheme, since one packet may be sent through multiple routes in case the mobility conditions are high. The decision on the numbe of hops per connection is implicit in the route decision. A similar table (in which the entries may not necessarily be the same) has to be built for the UL. Different merit figures arise from the fact that UL and DL connections are assymmetric. Note that routing decision is completely distributed. VI. CONCLUSIONS Multihop networks within 3G systems are able to break the standard coverage-capacity trade-off and permit operators a greater flexibility in system planning by easily extending the coverage to unforeseen black spots. The capacity increase is obtained as a result of a lower interference which, on its turn, is commensurate with the number of hops and to a higher spectral efficiency of the DL. These structures, however, require effective additional signalling overhead and multiple physical channels per packet connection. This burden has to be reduced through a more efficient reuse of channels: layered RRM strategies, MAC advanced techniques incorporating signal processing in transmitters and receivers are current techniques at hand.
