Scenario and assumptions

A so-called LTE+WiFi offload scenario is considered in the ensuing investigation.

We assume that the OFDMA based LTE system operates on LTE band 40 and in TDD mode. The WiFi system complies with the 802.11g standard–an OFDM system operating on the ISM band close to LTE band 40.

How these two networks interact is illustrated in Fig.2.1 where we consider the case where there is only one macrocell base station (BS) and one access point (AP). The LTE system uplinks and downlinks data streams in different (time) subframes (of 1 ms duration) while the WiFi link is active at the same time. By measuring the signal-to-interference-plus-noise ratio (SINR), we shall determine if a DRX-based IDCI solution should be activated.

The following system parameters are needed in subsequent discourse.

1. User mobility:

The user mobility affects the channel’s fading rate which is often classified into fast fading (> 120km/hr) or slow fading (3km/hr, 30km/hr). If the IDCI user moves

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fast, the threshold for IDCI decision . 2. LTE and WiFi traffic type:

The traffic types include Http, poisson, FTP, VoIP, or full buffer.

3. LTE and WiFi average SINR:

LTE average SINR is related to the distance between eNB and UE, and it will affect LTE average throughput. WiFi average SINR is related to the distance between AP and UE, and it will affect WiFi transmission rate.

4. Different WiFi stations:

The number of WiFi stations will affect throughput 5. LTE and WiFi operating frequency band:

The interference will be different based on operating frequency band at LTE and WiFi.

6. LTE and WiFi throughput constraint:

The DRX parameters must be chosen to ensure that the LTE/WiFi throughput requirements be satisfied.

2.1.1 A LTE and WiFi coexistence model: Operation and In-terference

We assume that other WiFi users always have data to transmit and that they always use RTS/CTS mechanism.

We will introduce the procedure of Fig.2.1 and Fig.2.2 as follows. At first, the network will have some data stream to be transmitted to UE. If eNB receives the data stream from network, it will buffer the data stream and start to schedule downlink resources for UE by previous CQI report on DRX active mode, or it will not transmit data when UE operates on power saving mode until the next on-duration timer is activated. UE

Figure 2.1: Interaction between LTE and WiFi models.

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Figure 2.2: DRX procedure.

will decode PDCCH in the downlink subframe to know the position of resource for UE.

If UE has WiFi denial, then the LTE downlink subframe will not be interfered by WiFi.

However, if UE doesn’t use WiFi denial, then LTE SINR may be interfered by WiFi.

WiFi uplink signal will be RTS (request to send) and uplink data, they comply with RTS/CTS CSMA-CA scheme. When channel is idle, UE will count down the backoff timer after DIFS time interval(34us) and transmit RTS signal when the backoff timer becomes zero. And if backoff timer of other users also becomes zero, AP will receive RTS signals from different users at the same time. This situation will cause collision, so the AP will not transmit CTS signal, and channel becomes idle. If users don’t receive CTS, they will extend backoff timer and reselect the random number from the new backoff timer. The IDC user will measure SINR in every subframe and compute the throughput by SINR mapped to modulation coding scheme for each subframe. At last, UE will average the throughput during measured period and decide whether to trigger IDC solution or not.

Referring to Figs. 2.1 and 2.2, we define the following acronyms:

1. RTS+SIFS+CTS+SIFS+Uplink data+SIFS+ACK: It means that WiFi UE will send RTS to AP when backoff timer is equal to zero, and AP will send CTS after SIFS when AP receives RTS. UE will send uplink data after SIFS when UE receives CTS, and AP will send ACK after SIFS when AP receives uplink data. The period of RTS and Uplink data will interfere with LTE radio.

2. Uplink data+SIFS+ACK: If there is no RTS/CTS scheme, UE will send uplink data directly. Moreover, AP will send ACK after SIFS when AP receives uplink data. Only the period of Uplink data will interfere with LTE radio.

3. RTS: If WiFi UE sends RTS to AP, and meanwhile there are other users sending RTS to AP, it will result in collision. Only the period of RTS will interfere with LTE radio.

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4. Uplink data: If WiFi UE sends data to AP, and meanwhile there are other users sending RTS to AP, it will result in collision. Only the period of Uplink data will interfere with LTE radio.

5. WiFi Uplink time: If WiFi is transmitting data, it will be one of the cases which are mentioned above( 1, 2, 3 and 4). And red parts of these cases represent the period of WiFi uplink time.

6. TDM solution: For LTE+WiFi case, the TDM solution will be DRX based solution with WiFi denial. Therefore, if the TDM solution is activated, eNB will turn on DRX solution and UE will use WiFi denial to avoid interfering with LTE.

7. FDM solution: eNB will shift LTE central frequency away from ISM band. How-ever, this solution can’t be used for some cases. For example, if eNB only has frequency band which is close to ISM band, the interference will be serious even if the resource blocks are allocated away from ISM band. For another example, because FDM solution can’t avoid interference completely, WiFi interference will still cause serious effect for UE which is located at cell edge.

8. Power control solution: If LTE is interfered by WiFi, WiFi will decrease transmis-sion power for diminishing the interference to LTE and vice versa. If WiFi lowers its uplink power for PC solution, WiFi may also decrease the link performance between AP and UE, which will result in increasing the WiFi transmission time.

It is only used in the case that interference is light because of the limit of maximum power reduction.

9. For DRX in the power saving mode, it has inactivity timer which can extend LTE transmission time. During this inactivity timer, if UE receives the paging signal from PDCCH, UE will reset this inactivity timer and receive the data.

10. Interference time to LTE during this DL subframe: We can observe (5) to get

the period of WiFi uplink time during the DL subframe at present. We need to calculate the period of WiFi interference per subframe to know the WiFi activity property.

11. If the IDC user collides with other users, AP will receive RTS or Uplink data from IDC user and other signal from other users. AP will detect whether collision occurs. If so, it will not transmit CTS or ACK to the IDC user. If the IDC user doesn’t receive response from AP after DIFS(DCF inter frame spacing), IDC user will double its backoff timer and contend with other users again.

12. We can use LTE and WiFi traffic to decide the DRX active ratio. Ex. DRX active ratio=active time/DRX cycle=LTE traffic/( LTE+WiFi traffic)

13. When user operates for on-duration timer or Inactivity timer of DRX scheme, it can still receive DL data from eNB, and eNB will allocate the DL resource around the central frequency by previous CQI in downlink subframe.

14. We can calculate SINR by interference time from (10). If interference time is long, the LTE DL subframe will be interfered by WiFi with large probability.