arXiv:1708.03059v1 [cs.IT] 10 Aug 2017 ato ieessetu nyfrMCaeNarrowband are massive MTC a With for [7]. LTE-M only Narrowband and spectrum [6] (NB-IoT) wireless IoT of alloc which part presente solutions are a cellular directions narrowband given future of is and An Examples MTC status [2]. 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Cellular machine-type-aggregator cellular (BS). a to station with underlay system (MT as a machine-type-devices of consider number We large com with wireless systems heterogenous tion in interference of diversity the omnctosa nelyt ellrNetworks Cellular to Underlay as Communications ovninlwrls omnctosssesaede- are systems communications wireless Conventional Abstract I hspprw rsn ipemto oexploit to method simple a present we paper this —In potnsi ceuigo ahn Type Machine of Scheduling Opportunistic .I I. etefrWrls omnctos nvriyo uu Ou Oulu, of University Communications, Wireless for Centre NTRODUCTION aa.l@uufi [email protected].fi samad.ali@oulu.fi, aa l,NnaaRajatheva Nandana Ali, Samad munica- esof rees oring sof ts oT), in d tion ncy ock ces ms D). ate ith d e e f , a oedabcsta a else sfollows: as listed systems be heterogenous conventional can in in that OSO drawbacks cell interference some of for has small concept The proposed and [18]. is networks macro link between primary mitigation transmitt of for of [17] null-space idea in the taken The is transm mitigation. approach primary similar intercell-interference of A values decoder. singular dat and unused transmits precoder of user MIMO direction secondary In the the [16]. and in modified [15], is [14], concept in this channels d tha further interference multiple case is multiple-input (MIMO) in method for wait, publications This other to transmit. in to them veloped delay opportunity for transmit no possible not is is do there it MTDs and that data assume candidate the sensitive we transmit scenario, secondary and of our technique number (CSI) In large this a channel-state-information for of existence of requirements main knowledge The are interference link. no primary is prim the there the of hence, space null and i the beamformer, scenario into receiver’s falls ideal interference by The that link. multiplied is primary OSO being the is of after users beamformer minimum receive secondary sel is from transmit the transmitter interference of user that secondary one such secondary transmission, slot, of for time selected number each large In networ a candidates. radio are cognitive for there [13] where and initi [12] was [11], scheme in o This diversity introduced network. spatial (BS). the the station in of interference base advantage received takes the OSO on of concept interference The MTC of (OSO) ization pa traffic [10]. this MTC communicating Femtocells of to over objective similar is the method an the proposed This communications Our is cellular users. interference main this cellular the mitigating type to human interference link with underlay causes to resources This MTD spectrum. radio assume unlicensed licensed shares which the in use works be cellular other will linkto to MTA to link opposed link the as underlay that spectrum, an means node is This MTC link that communications. assume communication to we MTA paper, capil- data this to collected In machine-type- In forwards network. a [9]. MTA cellular to the [8], then data and networks transmit (MTA) capillary nodes aggregator connec- MTC by providing networks, of is lary methods MTC the for of tivity One MTDs. of number th of efficiency spectral systems. the communication increase wireless to soluti developed novel network, be the should access to trying devices of number epeetteie fopruitcsailorthogonozal- spatial opportunistic of idea the present We massive for solutions providing on focus we paper, this In u Finland lu, -outpu n in ing ected ally per. ons ary ks, on e- s. it n d a e f t • Considering a very large number of secondary users is not realistic in normal human type communications. • Receive beamformer of the primary link changes at each time step. This means that the secondary user’s opportunity to transmit can be lost in the next time step and it might not be able to transmit its data. In our scenario, the above mentioned drawbacks do not exist. First, the assumption of very large number of users is realistic since our solution is for mMTC. Second, MTDs transmit small packets and even a small transmit opportunity is enough for the MTD to transmit its data. The rest of the paper is organized as follows. In section II we present the system model, signal and SINR equations. In section III the proposed method is developed and presented in details. Simulation results are presented in section IV and conclusions are drawn in section V.

II. SYSTEM MODEL We consider the uplink of a single cell system with a BS and an underlay machine-type-aggregator (MTA). Cellular user, MTDs and MTA are considered to be single antenna devices. BS has M antennas and the receive beamformer in BS is Fig. 1: System Model M wc ∈ C , which can be selected by different receive methods, for example, in case of one uplink user, it is selected to be In a similar manner the received signal at the MTA can be maximal ratio combining (MRC). The channel between BS written as follows: M and cellular users is hc ∈ C which is acquired at BS at each ym = hkxk + hcsxc + ns (4) transmission period. There are K MTDs that communicate with the MTA, using the same radio resources as uplink of Here hk is the channel between MTD and MTA, and hcs BS. Furthermore, we assume that MTDs are stationary or is the channel between cellular user and MTA. Additive white low mobility users, with channel from MTD k to the the Gaussian noise (AWGN) is MTA is ns. The received SINR at M BS shown by hkb ∈ C . At each transmission time BS MTA can be given by: carries out the beamformer design wc. BS performs radio 2 Pk|hk| resource allocation for the uplink users to communicate with γk = 2 (5) P |hcs| + N0 BS and MTDs to communicate with MTA. Considering the c beamformer wc, for each resource block, BS selects one MTD, Pk and Pc are transmit powers of MTD and cellular user, k ∈ K = {1, ..., K}, such that received singal to interference respectively. We consider that the cellular user is selected such plus noise (SINR) of uplink user is maximized. The system that the interference can be treated as background noise at the model is given in Fig 1. MTA, so the SINR in the equation (5) becomes: The received signal at the BS ybs from cellular user is given 2 Pkkhkk by: γk = (6) I0 + N0 The cellular user selection can be performed by a location ybs = hcxc + hkbxm + nbs (1) based interference limiting method such as [19]. Considering where xc and xm are the transmit symbols of the cellular the maximum transmit power allowed for cellular user, the M user and MTD respectively. hc ∈ C is channel between users that are far enough from MTA are opted for resource cellular user and BS. MTD is selected from K devices with sharing. By using this method, we only focus on interference channels hkb ∈ {h1b, ..., hKb}. White Gaussian noise is minimization on BS and treat the interference at MTA as represented by nbs. background noise. After applying the receive beamformer at BS, receiver has III. PROPOSED SCHEME the following signal: We assume an OFDMA based system such as LTE where yˆbs = wchcxc + wchkbxm + wcN0 (2) radio resources are divided into orthogonal sub-units in time and frequency domain. In LTE, each resource block has 180 From this equation, now we can write the SINR of the kHz bandwidth and one millisecond duration. Several resource cellular user at BS: blocks in the frequency domain can be allocated to one cellular 2 Pc|wchc| user at a given time. We assume that one resource block is γc = 2 2 (3) shared between uplink cellular user and one MTD to MTA Pk|wchkb| + |wc| N0 link. As mentioned earlier, the objective of this paper is to The probability to find an MTD that will satisfy the criteria minimize the interference on the BS. We assume that the in Eq. 10 becomes one when K → ∞. To prove this argument, H 2 |hc hib| BS has a priori information about the devices that have data we assume that 2 ,i ...K are random variables khck = 1 to transmit. This assumption is valid in scenarios such as X1, ..., XK and Xmin is the random variable defined as periodic meter reporting. Since the locations of the MTD mini{X1, ..., XK }. The probability that Xmin is smaller than nodes are fixed, we also assume that the BS has channel a defined threshold δi is: state information of the MTD to BS links [20]. Once the BS applies our proposed scheme and allocates radio resources P (Xmin <δi)=1 − P (Xmin >δi) for an MTD, it can send the transmission permission using a =1 − P (X1 >δi, .., XK >δi|hc)f(hc)dhc signaling scheme called fast uplink grant [21]. In fast uplink Z grant, users skip the scheduling requests and receive uplink K =1 − (1 − Φ(δ )) f(hc)dh grant periodically. Combination of fast uplink grant and our Z i c proposed OSO provide two benefits in MTC systems. One is (11) reducing the signalling overhead and random access channel where Φ(δ ) is the CDF of the probability that all of the random congestion, and the second is increased spectral efficiency by i variables are smaller than δ . Since δ R and δ providing connectivity to the MTD user with almost no extra i i ∈ Φ( min) ∈ (0, 1) the term (1 − Φ(δ )K )=0 as K → ∞, hence, cost in form of radio resources. min P (Xmin <δmin)=1. A. Minimum Interference on BS B. Allocating more than one radio resource block In this section, we present the solution for selecting one We discussed the method for utilizing one radio resource user for each time frequency resource block. If we assume block, selecting one MTD node to transmit. In can further be that BS has CSI knowledge of the MTD - BS links, at developed for a group of radio resource blocks. If more than each transmission period, the user which causes minimum one LTE resource block is allocated for cellular user with a interference at the BS is selected for resource sharing with frequency selective channel (we assume frequency flat fading cellular user. This is equal to the minimization of the term in each resource block), then the beamformer wc is different 2 Pk|wchkb| which is a fairly easy task to do. Pc can be for each resource block, and more than one MT device can be a constant value, or defined by a transmit power control selected for radio resource sharing with the cellular user. In algorithm. If we assume that the channel between MT and this case, the problem becomes more complicated since we do MTA is not known, MT devices can transmit with a predefined not want to allocate more than one radio resource block for maximum transmit power. BS in this stage simply selects one the same MTD. If we assume that there are N radio resource of the MTDs that causes the minimum interference on the BS. blocks available at each time for sharing, then we need to With this method applied, the SINR equation for cellular user select N out of K MTDs. For each subcarrier, MTD that in BS is: causes the minimum interference is selected. 2 We from the matrix from channels of MTDs to BS with Pc|wchc| N M Hint h1b, ..., hKb C × and matrix of beamformer as γc = 2 2 (7) = [ ] ∈ T M K mini=1,...KPi|wchib| + |wc| N0 Wc = [wc1, ..., wcN] ∈ C × . We denote the matrix of T N K H interferences as Iint = [i1, ..., iN] ∈ C × in which in ∈ If we assume MRC receiver at BS [22], wc = hc and CK assuming MTD k as the one with minimum interference, we is the vector of interferences on the BS from all MTDs on can have the following equation for SINR: resource block n. It is clear that Iint = WH. The objective here is to find the user with the minimum interference on each 2 Pckhck resource block, and also assign this resource block to only one γc = H 2 (8) Pk |hc hkb| MTD. For this, we propose the following simple algorithm. 2 N0 khck + To further evaluate the performance of this scenario, we can Algorithm 1 Matching interferences and beamformers write the probability of outage for the cellular user: 1: Calculate matrix Iint and for each row i find the column 2 index of smallest element P khck c 2: Compare the index of minimum MT devices for all of Pout = P r{ H 2 ≤ δth} Pk|hc hkb| (9) 2 N0 them, if each index is unique, go to 5 khck + 3: If one index is repeated more than one time, then match where δth is maximum allowed outage probability. To avoid the beamformer that receives smaller interference and for outage, the interference term should be smaller than a thresh- the other one, select the second smallest MT devices old, show by δ : int 4: Repeat 2 for second smallest MT device, until End. H 2 5: End Pk|hc hkb| 2 <δi (10) khck With this algorithm, we enable radio resource sharing be- If the above criteria is satisfied, the selected MTD k can tween cellular users and MTDs almost for all of the radio transmit data to the MTA without harmful interference. resource blocks in the system. IV. SIMULATION RESULTS Fixed MTD TX Power In this section numerical results are presented to validate the 10 effectiveness of the proposed method. We use LTE parameters 9.5 in which each resource block is 180 Khz in frequency domain and one millisecond in time domain. Resource blocks are 9 primarily allocated to cellular users and CSI is acquired 8.5 MTD TX Power 0 dBm at the BS to design the beamformer for the cellular user. MTD TX Power 5 dBm 8 For each cellular user, in frequency domain more than one MTD TX Power 10 dBm resource block can be allocated depending on the required data 7.5 rate. Assuming that the coherence bandwidth of the uplink 7 channel for cellular user is approximately 180 kHz, each SINR of Cellular User 6.5 resource block has different fading parameters and the receive beamformer wc is different for each resource block. We fix 6 the location of MTD users, but the cellular user has a different 5.5 location at each time step. Simulation parameters are given in 5 Table I. 20 40 60 80 100 120 140 160 180 200 TABLE I: Simulation Parameters Number of Machine Type Nodes

Parameter Value Fig. 2: SINR of the cellular user with fixed MTD TX power Number of Antennas in BS (M) 4 Cell Radius 500 m MTD Power Control MTA Radius 250 m 10.2 Number of resource blocks 20 Noise Figure at BS and MTA 2 dB CU to BS Path Loss Model 128.1 + 36.7log(d[km]) 10 CU Target SINR 10 dB Noise spectral density -174 dBm/Hz 9.8 MTD SINR Target 0 dB First, we present the results for a scenario with only one MTD SINR Target 5 dB MTD SINR Target 10 dB resource block to study the possible effect of number of MTDs 9.6 on the SINR of cellular user. For one resource block two transmit power strategies for MTDs are considered. First one 9.4 is a fixed transmit power from MTDs, and second is a transmit SINR of Cellular User power control to satisfy the required SNR at MTA. Since BS has the knowledge of the location of MTDs, it is rational to 9.2 assume that power control by MTDs can be approximated by

BS. Assuming that target SINR for cellular user is 10 dB 9 and sharing the radio resource between MTDs and cellular 20 40 60 80 100 120 140 160 180 200 users, the degradation of SINR of cellular userdue to MTD Number of Machine Type Nodes interference, is presented in Fig 2 for the case with fixed transmit power for MTDs, and in Fig 3 for the scenario of Fig. 3: SINR of the cellular user with MTD TX power control power control by MTDs. In Fig 2 the results show the effect of the target rate line is the throughput of the cellular user. When interference from MTD on SINR of cellular user for a different the resources are shared with 20 MTDs, this throughput is number of MTDs. This figure gives a clear example of how lower if there is no method of interference management. After a large number of MTDs provides the diversity to select one applying our scheme and increasing the number of MTDs, one MTD with very small interference on BS. For transmit power can see that the throughput of the cellular user gets closer of 0 dBm, this interference almost has not effect on cellular to the rate without interference. This means that a total of user SINR. 20 MTDs are also transmitting at almost no extra cost in in In Fig 3 a similar plot is given for the scenario of having spectral domain. Since each of these MTDs are transmitting in power control for MTDs. Since distances among MTDs and a different frequency channel, their data can be independently MTA are small, MTDs transmit with lower powers which leads decoded in MTA. to less interference. However,l if the number of MTDs is small, it can cause high interference on cellular user. With a large V. CONCLUSIONS number of MTDs, there is no interference on cellular user. In this paper, we presented a method for coexistence of For the scenario presented in section III-B we present the MTC and human type communications in cellular networks throughput of cellular user with resource sharing and the by exploiting the diversity of MTD interference. This scheme target rate with SINR of 10 dB in Fig 4. We assume that is similar to multi-user diversity, with the difference that the 20 resource blocks are available in the frequency domain and spatial diversity of interference is exploited. This diversity is they are shared with MTDs. If no resource sharing is done, available due to the very large number of MTDs in the system. Rate comparison of cellular user [10] M. Condoluci, M. Dohler, G. Araniti, A. Molinaro, and K. Zheng, 13 “Toward 5g densenets: architectural advances for effective machine-type communications over femtocells,” IEEE Communications Magazine, 12.5 vol. 53, no. 1, pp. 134–141, 2015. [11] C. Shen and M. P. Fitz, “Dynamic spatial spectrum access with oppor- 12 tunistic orthogonalization,” in Information Sciences and Systems, 2009. CISS 2009. 43rd Annual Conference on. IEEE, 2009, pp. 600–605. 11.5 Rate of CU [12] ——, “Opportunistic spatial orthogonalization and its application in Target rate of CU fading cognitive radio networks,” IEEE Journal of Selected Topics in 11 Signal Processing, vol. 5, no. 1, pp. 182–189, 2011. 10.5 [13] H. Zhou and T. Ratnarajah, “A novel interference draining scheme for cognitive radio based on interference alignment,” in New Frontiers in 10 Dynamic Spectrum, 2010 IEEE Symposium on. IEEE, 2010, pp. 1–6. [14] S. M. Perlaza, N. Fawaz, S. Lasaulce, and M. Debbah, “From spectrum 9.5 pooling to space pooling: opportunistic interference alignment in mimo

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