多個接入方案允許許多移動用戶同時共享有限的無線頻譜。

Multiple Access Techniques

在無線通信系統中，通常希望允許用戶在接收從基站到移動站的信息的同時，同時從移動站向基站發送信息。

蜂窩系統將任何給定區域劃分爲蜂窩，其中每個蜂窩中的移動單元與基站通信。蜂窩系統設計的主要目的是能夠增加信道的容量，即在給定帶寬內以足夠的服務質量處理儘可能多的呼叫。

有幾種不同的方式允許訪問頻道。主要包括以下幾點：;

• Frequency division multiple-access (FDMA)
• Time division multiple-access (TDMA)
• Code division multiple-access (CDMA)
• Space division multiple access (SDMA)

根據可用帶寬如何分配給用戶，這些技術可分爲窄帶和寬帶系統。

Narrowband Systems

信道比相干帶寬窄得多的系統稱爲窄帶系統。窄帶時分多址允許用戶使用同一個信道，但爲信道上的每個用戶分配一個唯一的時隙，從而在單個信道上及時分離少量用戶。

Wideband Systems

在寬帶系統中，單個信道的傳輸帶寬遠遠大於信道的相干帶寬。因此，多徑衰落對寬帶信道中的接收信號影響不大，頻率選擇性衰落只發生在信號帶寬的一小部分。

Frequency Division Multiple Access (FDMA)

FDMA是先進行動電話業務的基礎技術。FDMA的特點如下。

• FDMA allots a different sub-band of frequency to each different user to access the network.
• If FDMA is not in use, the channel is left idle instead of allotting to the other users.
• FDMA is implemented in Narrowband systems and it is less complex than TDMA.
• Tight filtering is done here to reduce adjacent channel interference.
• The base station BS and mobile station MS, transmit and receive simultaneously and continuously in FDMA.

Time Division Multiple Access (TDMA)

在不需要連續傳輸的情況下，使用TDMA而不是FDMA。TDMA的特點包括以下幾個方面。

• TDMA shares a single carrier frequency with several users where each users makes use of non-overlapping time slots.
• Data transmission in TDMA is not continuous, but occurs in bursts. Hence handsoff process is simpler.
• TDMA uses different time slots for transmission and reception thus duplexers are not required.
• TDMA has an advantage that is possible to allocate different numbers of time slots per frame to different users.
• Bandwidth can be supplied on demand to different users by concatenating or reassigning time slot based on priority.

Code Division Multiple Access (CDMA)

碼分多址技術是多址技術的一個例子，其中多個發射機使用單個信道同時發送信息。其特點如下。

• In CDMA every user uses the full available spectrum instead of getting allotted by separate frequency.
• CDMA is much recommended for voice and data communications.
• While multiple codes occupy the same channel in CDMA, the users having same code can communicate with each other.
• CDMA offers more air-space capacity than TDMA.
• The hands-off between base stations is very well handled by CDMA.

Space Division Multiple Access (SDMA)

空分多址（Space division multiple access，簡稱空分多址）是一種MIMO（multiple input multiple output，多輸入多輸出）技術，主要應用於無線和衛星通信中。它具有以下特點。

• All users can communicate at the same time using the same channel.
• SDMA is completely free from interference.
• A single satellite can communicate with more satellites receivers of the same frequency.
• The directional spot-beam antennas are used and hence the base station in SDMA, can track a moving user.
• Controls the radiated energy for each user in space.

Spread Spectrum Multiple Access

擴頻多址（SSMA）使用傳輸帶寬大於所需最小RF帶寬的信號。

擴頻多址技術主要有兩種類型&負;

• Frequency hopped spread spectrum (FHSS)
• Direct sequence spread spectrum (DSSS)

Frequency Hopped Spread Spectrum (FHSS)

這是一種數字多址系統，其中在寬帶信道中以僞隨機方式改變單個用戶的載波頻率。數字數據被分解成均勻大小的脈衝，然後在不同的載波頻率上傳輸。

Direct Sequence Spread Spectrum (DSSS)

這是CDMA最常用的技術。在直接序列擴頻系統中，信息信號與僞隨機噪聲碼相乘。每個用戶都有自己的碼字，碼字與其他用戶的碼字正交，爲了檢測用戶，接收機必須知道發射機使用的碼字。

稱爲混合的組合序列也被用作另一種擴頻類型。跳時也是另一種很少提及的類型。