Литература / UMTS-Report

5. Simulations

Within the Epsilon concept group simulations were performed on the ODMA technology. For this purpose a system simulator tool was created such that the environments, traffic and mobility models of UMTS 04.02 could be supported. The simulated ODMA nodes were software kernels taken from a prototype packet radio system in South Africa which is based on the ODMA technology. Simulations using this tool should therefore be close to reality as the simulated nodes are themselves deciding on the best relay paths, the power levels, overall routing etc. A disadvantage of this approach is that simulations have taken a long time to run and the number of simulated nodes has been limited. As a consequence it has been necessary to concentrate on a small subset of simulation tests relevant to ODMA data relaying i.e. LCD 384kbps. The LCD model was used as it was considered a better measure of performance than UDD which was more open to different interpretations.

A representative link level was used during the simulations but this was not WBTDMA/CDMA. However it is expected that future results using WB-TDMA/CDMA would achieve much better results.

Most of the simulations concentrated on extended coverage scenarios using subscriber relay but some initial work was carried out on capacity for a combined ODMA/cellular solution where traffic is concentrated at a number of nodes close to the BTS.

Where subscriber relay is assumed only 1% of the mobiles believed to be within a coverage area are assumed available as relays. (User density estimate based on figures in early versions of UMTS 04.02).

The very wide area coverage simulations were not to prove that such areas should be covered by a single BTS but rather to show the flexibility of systems that support relaying.

When a MO wishes to start a call it makes a conventional RACH access to the WBTDMA/CDMA/FDD BTS. A conventional authentication/call setup will take place but during the negotiation of resource it will be decided to use ODMA mode. Firstly the BTS will send a broadcast wakeup page to the MS relays. The BTS will then ask the MO to send a message to it via ODMA relaying which it then acknowledges. The initial route for these messages will be based on knowledge acquired from the background probing. The transmissions will be monitored by relays not directly involved in the link. These relays then determine connectivity routes between the MO and BTS and are available to make further transmissions more optimum and reliable. Other mobiles will fall asleep using the page-awake rules. A similar procedure is used for MT calls.

6.1 ODMA/WB-TDMA/CDMA Gateway - Last Hop to BTS

The last MS in the relay chain will have direct connectivity to the BTS over a short high rate link. The MS will require 2 buffers i.e.; to fill from ODMA and empty via WB-TDMA/CDMA and vice versa. For example in the case of significant DL traffic the buffer will be filled by a WB-TDMA/CDMA call and at a defined threshold the MS will switch to ODMA mode until the buffer is emptied. Similarly for an UL case ODMA will fill the buffer until a threshold is reached after which a WBTDMA/CDMA call empties the buffer into the BTS. If an ODMA relay is not available as in WB-TDMA/CDMA mode traffic is either backed up toward the source or an alternative last hop MS is chosen.

7. WB-TDMA/CDMA Support for Relaying

This section presents the findings from joint Delta/Epsilon feasibility study which considered the benefits of TD-CDMA for relaying and how and with which additional complexity the radio sub-blocks within the WB-TDMA/CDMA mobile could support relaying.

7.1 Benefits of Using WB-TDMA/CDMA for Relaying

∙WB-TDMA/CDMA communication in ODMA mode implies MS-to-MS communication. This, in turn, implies MS reception and transmission on the same frequency band, i.e. TDD operation. TDD is already included as a key feature of the WB-TDMA/CDMA proposal of the concept group Delta and only a very limited amount of additional features have to be added to a WB-TDMA/CDMA terminal in order to support relaying as will be discussed further in the next section.

∙Within a given spectrum WB-TDMA/CDMA provides for high granularity of orthogonal basic channels which is beneficial for DCA in an uncoordinated mode of operation context.

∙ An evolution to higher data rates is supported by means of higher order modulation schemes and larger carrier bandwidth (cf. future enhancements option for WB-TDMA/CDMA).

∙WB-TDMA/CDMA features a very high hot spot capacity due to low intracell interference by timeslot orthogonality and joint detection. Thus, use of high order modulation schemes (e.g. 16QAM) can be used to effectively increase cell capacity which would not be possible for CDMA without joint detection techniques.

∙Link budget improvement due to static Seeds with directional antennas can be translated into further capacity gains especially for the last hop while coexisting with other mobile users by the same means (e.g. adaption of modulation order).

∙The joint detection receiver in mobile terminals provides an efficient means to resolve collisions in a way that the colliding transmission can be resolved and both used, thus increasing the efficiency and delay times in ODMA.

7.2 Impact of Relaying on Implementation and Complexity of WBTDMA/CDMA

For mobiles supporting both FDD and TDD modes of operation there is negligible impact on hardware with transmissions taking place using the timing context of the serving BTS. As shown in Figure 22 the required receiver components are as for a normal simple WB-TDMA/CDMA mobile.

The full receiver consisting of both monitoring and detection circuit is regularly switched on for reception of normal FDD paging messages originated by the BTS.

During the probing windows (e.g. 0.5 sec every 30 sec) only the monitoring circuit is switched on to listen to probes from neighbouring mobiles. This is performed by evaluating the code active indications from the Channel Estimator circuit. If a codetime slot burst is detected, the detection circuit is switched on to evaluate the data sections of the receive buffer contents (if just one active code is detected this is a

trivial operation, otherwise it can resolve the «collision» and detect all code-time slot burst transmission colliding in one particular time slot). During the wake-up window the same principle is applied.

The monitoring circuit consists of the burst buffer from which the midamble section is extracted and forwarded to the channel estimator. The channel estimator performs on a burst by burst basis a fast correlation with the known midamble sequence. As can be seen in Figure 23 the output of this correlation process is a set of channel estimation vectors for each code, if it is active or not. The code active indication can be obtained from a code's channel estimation by comparing the energy in the channel estimate with a treshold. From this information a power estimation can be derived as well. For short delay spreads introduced by the radio channel a certain jitter in time alignment of more than 5 μsec in each direction can be tolerated with negligible performance degradation.

The fast correlation is depicted in more detail in Figure 24. It consists of a (fast) discrete Fourier transform of the midample samples, followed by an element-by- element multiplication with a precomputed and stored "inverse" of the correlation sequence and a inverse FFT. This operation has to be performed once per burst which is monitored.

Figure 22 WB-TDMA/CDMA - ODMA Enabled Receiver

The block diagrams illustrate that no additional hardware and DSP functionality is required for a simple WB-TDMA/CDMA mobile in order to support ODMA mode of operation as MO or MT and as relay. Thus, the impact on cost and complexity of the ODMA support in mobile terminals is primarily on the higher layers and thus, considered to be low.

For higher throughputs, e.g. for the last hop and/or fixed seeds, more complex transceiver types e.g. the full featured mobile as well as directional antennas to the base station are helpful, of course.

8. Benefits of ODMA to WB-TDMA/CDMA w.r.t. High Level Requirements

The potential benefits of ODMA as a WB-TDMA/CDMA enhancement are listed below with respect to the UTRA high level requirements.

1The specified bit rate will be available throughout the operator’s service area, with the possibility of large cells

2The specified bit rate will be available with complete coverage of a suburban or urban area, using microcells or smaller macrocells

3The specified bit rate will be available indoors and locallised coverage outdoors.