Nanang Ismail Possibilities frequency division time division code division space division (cellular concept) Multiplex Band sharing by directions of signal flow Only frequency and time divisions are common uplink (reverse link): user to network downlink (forward link): network to user E.g. FDD (frequency division duplex), TDD (time division duplex) Link stuff, no switching possible Multiple Access Band sharing by different users E.g. FDMA (frequency division multiple access), TDMA (time division multiple access), CDMA (frequency division multiple access) Network stuff, switching possible Radio Spectrum is divided into frequency bands power emitted outside these bands must be strictly below a certain level to be on the safe side, unused guard bands limit interference between frequency bands receivers use filtering to receive carrier frequency of interest already introduced in 1900 to organize usage of radio transmitters, e.g. on ships usually used in combination with other division techniques Radio Spectrum usage is divided into time slots each sender is assigned a time slot to avoid collisions, all participating entities need to be synchronized network needs to periodically synchronize terminals need to consider finite traveling time of synchronization signal, depending on distance between terminal and synchronizing entity! guard times between slots prevent collisions due to imperfect synchronization usually used in combination with other division techniques Several signals are sent in the same (but wider) frequency band and the same time slot each signal is created by spreading a narrowband signal through the use of a unique user code to a multiple of the original bandwidth (spreading) the receiver correlates the sum of the received signal with the (timeshifted) user code, and thereby reobtains the original narrowband signal (de-spreading) power level of different signal needs to be aligned codes need to be uncorrelated, otherwise interference replanning cells becomes easier with this technique Space Division is today the technique of choice patented in 1972 by Bell Labs instead of each antenna illuminating an area as big as possible (as in 1G Systems), each antenna covers only small area lower transmission power more efficient use of the spectrum => better coverage higher infrastructure costs need technique for switching moving users traversing from a cell to another: “handover“ (handoff) GSM = FDD + TDMA + FDMA Uplink and Downlink use different frequencies: FDD, 25 MHz each Uplink and Downlink frequency bands are subdivided into frequency 124 channels @200 kHz each of these channels is divided into 8 time slots each user sends on a particular frequency band, on a particular time slot cdmaOne = FDD + CDMA Near-far effect Party scenario: a single loud shout could stop all other conversations User UE2 will be received with higher power by the BS De-spreading at receiver works best when power levels of different signals are the same Assuming UE1 and UE2 transmit the same power, and the channel characteristics are the same for both This user could cause high interference to others in the same cell Power control comes into play to alleviate the problem Near-Far Problem in CDMA How to do power control Different Performance for Subscriber Links A Few Subscribers closest to the BTS may contribute too much multiple access interference (MAI) Force all users to transmit the minimum amount of power Reduce the power transmitted by users closest to the BTS; increase the power transmitted by users farthest to the BTS Open Loop vs. Close Loop Open Loop Power Control Subscriber measures the DL power and adjusts its transmission power Close Loop Power Control BS measures the UL power. MS measures the DL power and reports to the BS. BS instructs the user to raise or lower it transmission power (Hard) Handover Only one link of MS to BS is maintained at a time Dropped call is possible Soft Handover Active MS receives a priority list from the network MS searches priority list for new BSs Softer Handover Soft handover between sectors of the same BS Operation as for soft handover Differences only at network implementation level Interfrequency Handover When handover occurs between cells where a different number of carriers have been allocated For handover between cell layers using different carrier frequency (e.g hierarchichal cells) For interoperator handover For intersystem, e.g. UMTS to GSM 7-cell reuse A more realistic case 1G 2G Bunch of global standards, mainly DS-CDMA based systems, several incompatible air interfaces, seamless roaming(?) 3.5G Evolved air interface, packet switched services, limited roaming of packet calls 3G Digital communications, circuit switched, intelligent services, towards global solution (global roaming with GSM) 2.5G Analog communications, circuit switched, basic services, limited compatibility (roaming in Scandinavia with NMT) Evolved radio interface, IP based core network 4G New air interface, very high bit rate services Analog transmission High power transmitters Simple modulation schemes AMPS: Narrowband FM, 30kHz carrier spacing Nationwide, incompatible systems Voice-only systems Digital transmission Low power transmitters Enhanced modulation schemes GSM: GMSK, 200 kHz carrier spacing Semi-compatible systems Voice and low data rate services Enhancement of existing 2G systems Voice and medium data rate services Circuit- and packet-switched Voice and high data rate services, including multimedia Mixture of different services Mainly packet-switched High efficiency modulation schemes Enhanced multiple access techniques International standardisation Co-existence with 2G and 2.5G systems Real convergence of wired and wireless communications Seamless communications consisting of multiple networks Wired Cellulars PANs (personal area networks, e.g. Bluetooth) Wireless LANs ITSs (Intelligent Transportation Systems) Satellite Communications In excess of 100 Mbps full coverage and high mobility Adaptive e.g. user terminal Consumers are 80% of 2G Usage Corporations are supposed to be 80% of early 3G 3G has increased from 1.3% of the wireless market in 2002 to 23% in 2007 Suara digital CDMA lebih jernih. Kapasitas CDMA jauh lebih besar, untuk satu kanal 8 x kapasitas AMPS dan 4 x kapasitas GSM sehingga investasi operator jauh lebih rendah. Pada CDMAOne, kapasitas suara-yang dihitung dalam erlang-per sektor di BTS (base transceiver station) mencapai 52,5 erlang, pada GSM hanya 13,2 sampai 32,8 erlang. Pada CDMA 2000-1X, kapasitas per sektor antara 92,8133,9 erlang. Tiap sektor di BTS GSM dapat digunakan hanya oleh 20-43 pelanggan bersama-sama, pada CDMAOne dapat sampai 63 pelanggan per sektor dan di CDMA 2000-1X jauh lebih besar, antara 105-147 pelanggan. Kemungkinan drop call yang lebih sedikit daripada GSM karena semua frekuensi CDMA yang besarnya 1,25 MHz disebarkan sekaligus oleh tiap BTSnya (spread spectrum), tidak dipecah-pecah dalam frekuensi kecil-kecil seperti di GSM akibat digunakan lagi di BTS lainnya (reuse). Karena kelebihan ini, CDMA dikenal sebagai seluler yang dapat pindah BTS secara halus (soft handsoff). GSM membagi frekuensi menjadi masing-masing 20 kHz dan tiap BTS yang bertetangga memancarkan frekuensi yang berbeda, frekuensi yang digunakan satu BTS digunakan lagi (reuse) di BTS yang berjauhan, dengan tujuan kapasitasnya dapat ditingkatkan. CDMA versi 2000-1X-dalam hitungan sudah sebanding dengan GSM generasi ketigamampu mengirim data dengan kecepatan sampai 153 kilobit per detik, dibandingkan dengan GSM yang maksimal 64 kbps. Bahkan, pada CDMA 2000-1X EVDO (Evolution Data Optimized) mempunyai kapasitas 2,4 Mbps walau dalam praktik yang dicoba oleh Mobile-8, kecepatan yang dicapai sekitar 800-900 kbps. Pada CDMA 2000-1X EVDV (Evolution Data and Video) kapasitas transmisinya dapat sampai 3,1 Mbps. CDMA juga menghadirkan berbagai aplikasi canggih semisal LBS (location based service) pemetaan, mobile Internet kecepatan tinggi, pesan multimedia, permainan (games), gambar, konferensi video, dan banyak lagi yang melebihi kemampuan GSM. Ada kelemahan CDMA, luas cakupan BTS pada CDMA sangat tergantung dari berapa pelanggan yang menggunakannya. Beda dengan GSM, berapa pun yang menggunakan, cakupannya tetap. Ini karena sifat CDMA, seperti paru-paru yang akan mengecil saat bekerja keras meniupkan udara ke luar. Kalau penggunanya sedikit pada waktu bersamaan, cakupan BTS CDMA akan kembali meluas. Pada beberapa kasus pemasangan pengulang (repeater) tidak optimal karena malah mempersempit cakupan. Cakupan CDMA (maksimal) sama dengan GSM, tergantung dari berapa frekuensi yang digunakan. Makin kecil frekuensinya, makin luas cakupannya. Kalau seluler, CDMA atau GSM, menggunakan frekuensi 1.900 MHz, cakupannya hanya sekitar 2 km, dengan 800 MHz bisa sampai 5-6 km. Namun, dengan 450 MHz, seperti yang digunakan PT Mobisel, bisa sampai 30 km, bahkan hingga 120 km dengan antena khusus. GSM berkemampuan roaming. Pemilik GSM dapat menggunakan ponsel di luar domisili atau operatornya, CDMA belum mampu.
